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148 IEEE TRANSACTIONS ON COMPONENTS AND PACKAGING TECHNOLOGY,
VOL. 33, NO. 1, MARCH 2010
Comparison of Micro-Pin-Fin and MicrochannelHeat Sinks
Considering Thermal-Hydraulic
Performance and ManufacturabilityBenjamin A. Jasperson, Yongho
Jeon, Kevin T. Turner, Frank E. Pfefferkorn, and Weilin Qu
Abstract This paper explores the potential of micro-pin-fin heat
sinks as an effective alternative to microchannel heatsinks for
dissipating high heat fluxes from small areas. Theoverall goal is
to compare microchannel and micro-pin-fin heatsinks based on three
metrics: thermal performance, hydraulicperformance, and cost of
manufacturing. The channels and pinsof the microchannel and
micro-pin-fin heat sinks, respectively,have a width of 200m and a
height of 670m. A comparison ofthe thermal-hydraulic performance
shows that the micro-pin-finheat sink has a lower convection
thermal resistance at liquid flowrates above approximately 60
g/min, though this is accompaniedby a higher pressure drop. Methods
that could feasibly fabricatethe two heat sinks are reviewed, with
references outlining currentcapabilities and limitations. A case
study on micro-end-millingof the heat sinks is included. This paper
includes equations thatseparate the fabrication cost into the
independent variables thatcontribute to material cost, machining
cost, and machining time.It is concluded that, with
micro-end-milling, the machining timeis the primary factor in
determining cost, and, due to theadditional machining time
required, the micro-pin-fin heat sinksare roughly three times as
expensive to make. It is also notedthat improvements in the
fabrication process, including spindlespeed and tool coatings, will
decrease the difference in cost.
Index Terms Micro heat sink, micro-manufacturing,
micro-machining, pin-fin heat sink.
NOMENCLATUREManufacturing VariablesCtotal Total cost of heat
sink [$].CT Total cost of tools [$].CM Total cost of materials
[$].fr Feedrate [mm/min].t Time to fabricate heat sink
[min].Manuscript received November 25, 2008; revised February 19,
2009. First
version published October 13, 2009; current version published
March 10,2010. This work was supported by the National Science
Foundation, GrantCBET-0729693 at the University of
Wisconsin-Madison, and Grant CBET-0730315 at the University of
Hawaii at Manoa. Recommended for publicationby Associate Editor A.
Bhattacharya upon evaluation of the reviewerscomments.
Y. Jeon was with the Department of Mechanical Engineering,
University ofWisconsin-Madison, Madison, WI 53706 USA. He is now
with the HyundaiMotors, Seoul, South Korea (e-mail:
[email protected]).
K. T. Turner, B. A. Jasperson, and F. E. Pfefferkorn are with
the Depart-ment of Mechanical Engineering, University of
Wisconsin-Madison, Madi-son, WI 53706 USA (e-mail:
[email protected];
[email protected];[email protected]).
W. Qu is with the Department of Mechanical Engineering,
University ofHawaii at Manoa, Honolulu, HI 96822 USA (e-mail:
[email protected]).
Color versions of one or more of the figures in this paper are
availableonline at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TCAPT.2009.2023980
tc Chipload [mm].R Operator rate [$/hr].CT Cost per tool [$].w
Width of heat sink for case study [mm].l Length of heat sink for
case study [mm].Ns Spindle speed [rpm].Ntools Number of tools
required for fabrication of one
heat sink.n f Number of flutes.tchange Time to change one tool
[min].tcleaning Time to clean up after machining [min].tmachining
Machining time [min].tsetup Setup time before machining
[min].ttoolchange Total time to change tools [min].dstraight Tool
path to machine straight channel heat sink
[mm].dstgpin Tool path to machine staggered pin fin heat
sink
[mm].
Thermal-Hydraulic Performance VariablesAt Area of heat sink base
surface [m2].Aht Total heat transfer area of microscale
enhancement
structure [m2].Aht, eff Total effective heat transfer area of
microscale
enhancement structure [m2].h Heat transfer coefficient [W/m2
C].Hfin Height of fin [m].Lhs Length of heat sink [m].P Pressure
drop across heat sink [bar].Pdh Pressure drop in developing region
[bar].Pfh Pressure drop in fully-developed region [bar].q eff Heat
flux based on heat sink base area [W/cm2].Rconv Average convection
thermal resistance [C/m].T f Water bulk temperature [C].Tw Fin base
temperature [C].Whs Width of heat sink [m].Wch Width of flow
channel [m].Wfin Width of fin [m].Wt Mass flow rate [g/min].
Greek Symbols Aspect ratio of microchannel.b Viscosity evaluated
at coolant bulk temperature,
[Ns/m2].1521-3331/$26.00 2010 IEEE
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JASPERSON et al.: COMPARISON OF MICRO-PIN-FIN AND MICROCHANNEL
HEAT SINKS 149
Whs = 1.0 cm, Lhs = 3.38 cm
Hfin = 670 m
Wfin = 200 m Wch = 200 m
Flow
(a)
Flow
Wfin = 200 m Wch = 200 m
Hfin = 670 m
Lfin = 200 m
SL = 400 m
Whs = 1.0 cm, Lhs = 3.38 cm
(b)
Fig. 1. Structure and dimension of (a) microchannel heat sink
and (b) micro-pin-fin heat sink.
w Viscosity evaluated at fin base temperature, [N.s/m2].
Density, [kg/m3].
Subscriptsave Average.f Liquid (water).in Inlet.mc Microchannel
heat sink.mpf Micro-pin-fin heat sink.out Outlet.W Wall.
I. INTRODUCTION
THE CEASELESS pursuit of improved performance withsimultaneous
reduction in volume leads to ever-increasingdissipative heat loads
in a wide range of modern micro-electronic devices. It has been
shown that the performanceand reliability of these devices are
strongly affected by theirtemperature as well as immediate thermal
environment. Asa result, there is an increasing demand for highly
efficientthermal management techniques capable of dissipating
highheat fluxes from small areas.
Single-phase liquid-cooled miniature heat sinks, whichhave
internal heat transfer enhancement structures and flowpassages that
are tens to hundreds of micrometers in size,have emerged as one
solution to the aforementioned thermalmanagement challenges. Among
the large variety of possiblemicroscale enhancement structures,
parallel-plate fins havereceived the most attention so far [1][5].
These miniature
heat sinks consist of parallel channels aligned with the
flow[Fig. 1(a)]. Key technical merits of microchannel heat sinks,as
demonstrated by the previous studies, include low thermalresistance
to dissipative heat flux, high heat transfer areato volume ratio,
compact dimensions, and small coolantinventory requirement
[1][5].
Recent advances in microfabrication technologies, however,allows
more complex microscale geometries to be fabricateddirectly into
high-thermal-conductivity solid substrates (e.g.,metals) at low
cost. This makes it possible to explore morecomplex and 3-D
enhancement structures that may be moreeffective in transferring
heat than the aforementioned parallel-plate fins. A possible
configuration is staggered [Fig. 1(b)] oraligned micro-pin-fin
arrays [6][14].
Staggered micro-pin-fin heat sinks have the potential toremove a
high heat flux for a given volume of the heatsink and flow rate of
working fluid, and hence improve theperformance of the
heat-generating component. Despite thepotential for improved heat
transfer from micro-pin-fin heatsinks, economics and realistic
microfabrication options willcontinue to play an important role in
whether these devicesare a viable choice over the nearest
alternative (e.g., straightmicrochannel heat sinks). Unlike
microchannel heat sinks,whose thermal-hydraulic performance can be
fairly accuratelydescribed by conventional macrochannel analytical
models [3],[4], reliable analytical or numerical models for
micro-pin-finheat sinks have not been developed yet due to the
complexnature of fluid flow and heat transfer. Existing studies on
liquidsingle-phase heat transfer and pressure drop in
micro-pin-finarrays are mostly empirical. In these previous
studies, specific
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150 IEEE TRANSACTIONS ON COMPONENTS AND PACKAGING TECHNOLOGY,
VOL. 33, NO. 1, MARCH 2010
TABLE IPOTENTIAL MANUFACTURING METHODS FOR MICRO HEAT SINKS
Mfg Method Can Mfg Mass
ProductionSuitability*PrototypingSuitability*
Cost Comparisonof Designs
EDMWire Channel Poor Average NAPlunge Both Poor Average Pin
Channel
EtchingLIGA/Electroforming Both Good Good Pin Channel
Casting Both Very Good Poor Pin Channel
Extrusion Channel Very Good Poor NA
MachiningEnd Mill Both Average Very Good Pin >
ChannelSlot/Form Mill Channel Good Very Good NA
Sintering Both Very Good Poor Pin Channel
micro-pin-fin configurations were tested and new heat
transferand pressure drop correlations proposed [6][14].
The objective of the present paper is to simultaneouslycompare
the thermo-hydraulic performance and manufactura-bility of the
aforementioned two types of miniature heatsinks. Material is
presented in four sections: 1) a review ofmanufacturing techniques
that can be used to make these microheat sinks out of metals; 2) a
thermal-hydraulic analysis ofsingle-phase water cooled copper heat
sinks to explore whetherthe micro-pin-fin design has the potential
to outperform themicrochannel design; 3) a case study of
micro-end-milling todetermine the difference in manufacturing cost
of the two heatsink designs; and 4) a discussion of the
results.
II. REVIEW OF MANUFACTURING TECHNIQUES
A. Scope of AnalysisBecause it is a highly specialized and
emerging area, there is
a need to review the different manufacturing methods that canbe
used to fabricate microscale heat sinks. Due to the higherthermal
conductivity and mechanical performance of metalalloys as compared
to nonmetallic (i.e., silicon) materials,this review focuses on the
fabrication of micro heat sinksout of metal alloys. This paper does
not attempt to predictwhich technique is best suited for making
micro heat sinks,because there are too many production variables
that mustbe considered when making that decision (material,
design,tolerances, quantity, existing equipment, etc.). Instead,
the goalis to critically review a variety of methods that may be
wellsuited for prototyping, low-volume production, or
high-volumeproduction of heat sinks.
An excellent source on the fabrication of heat sinks
withfeatures similar in size to those discussed in this paper is
thepaper by Eugene et al. [15], which discusses the fabricationof
micro-meso heat sinks embedded in turbine blades. Eugeneet al.
conclude that the three most viable candidates formass
manufacturing microscale features inside turbine blades
are micro electrical discharge machining (micro-EDM), microlaser
machining, and micro casting.
B. Potential Fabrication MethodsTable I summarizes the potential
fabrication methods
discussed below, their ability to make the two heat
sinkgeometries, their suitability for mass production and
prototypefabrication, and a comparison of the manufacturing cost
foreach heat sink design.
1) Electrical Discharge Machining: EDM erodes/removesmaterial
when a spark discharges between an electrode (tool)and a workpiece.
Material is removed from the workpiecebecause of the rapid
temperature rise and explosive phasechange resulting from the
concentrated energy released by theelectric arcs [16]. The
electrode does not experience the samerate of material removal
because its high thermal diffusivitydissipates the heat more
rapidly. Repeatedly discharging aspark at high frequencies under
controlled conditions allowsfor bulk material removal around the
tool. Hence, the cavitythat is created takes the inverse shape of
the tool (or wire)that is used as the electrode. Intricate and
microscale designscan be created in electrically conductive
materials withoutimparting large forces or a significant
heat-affected zone [17].The heat-affected zone is minimal because
of the localizednature of the repeated material removal events and
the EDMtool and workpiece are immersed in a dielectric fluid
thatremoves heat and debris while also controlling the arcs
[16].
EDM uses either a thin wire or a shaped electrode asthe tool
[17]. Wire EDM [Fig. 2(a)] uses wires down to adiameter of 20m and
can create straight through-thicknessslots or cuts [18]. Electrodes
are also commonly machined intocylindrical or square cross-section
bars and plunged straightinto the material to drill a hole or moved
laterally to mill outshapes [Fig. 2(b)]. Very complicated
geometries that need tobe created repeatedly (high-volume
production) often use ashaped electrode that is plunged into the
workpiece once in aprocess called die sinking [Fig. 2(c)] [19].
Micro-EDM milling
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JASPERSON et al.: COMPARISON OF MICRO-PIN-FIN AND MICROCHANNEL
HEAT SINKS 151
Workpiece Direction
Wire Direction
(a)
(b)
(c)
Fig. 2. Illustrations of three electrodischarge machining
techniques: (a) wireEDM, (b) die sinking, and (c) EDM milling.
and drilling can utilize electrodes as small as 5 to 10m
indiameter [16], [20], [21] and aspect ratios as large as 20
havebeen obtained [22], [23]. These tools are more than adequatefor
all features in the micro heat sinks being considered.
The cost of EDM is primarily a function of the timerequired to
shape each part due to the relatively low materialremoval rate. Die
sinking is the fastest method of creating theheat sinks; however,
it requires the most complicated (henceexpensive) die to be
manufactured. Dies and all other types ofEDM electrodes wear out
after repeated use [24]. Hence theselection of an EDM method is a
function of the part geometryand volume. There are examples in the
literature of heat sinksthat have been fabricated through these
methods [25].
2) Photolithographic-Based Techniques: Photolithographyis a core
fabrication technique utilized in the manufacturingof integrated
circuits and microelectromechanical systems.Photolithography uses a
transparent mask containing a desireddevice pattern and an exposure
source (e.g., a UV light source)to transfer patterns onto a
photodefinable polymer resist. Thepatterned resist can be used as a
mask for etching a substrateor serve as a mold that can be filled
with a metal [26]. Becauseof the multiple process steps involved
and significant overheadassociated with the facilities and
equipment, photolithography
(a) (b)
(c) (d)
(e) (f)
Fig. 3. Schematic of LIGA process: (a) deposit a conductive seed
layer, (b)spin on a thick layer of photoresist, (c) expose
photoresist to high-energyX-rays through a mask, (d) develop
photoresist removing X-ray exposedmaterial, (e) deposit metal into
photoresist mold, and (f) dissolve photoresistmold.
is best suited to batch production and most economical
forhigh-volume production.
Etching methods, such as deep reactive ion etching [27],[28],
can be employed to create heat sinks out of silicon,but they cannot
generally be used to create high-aspect-ratiostructures in metals
and will not be discussed here. However,lithography can be used to
form metal heat sinks usingelectrodeposition-based techniques such
as the lithographie,galvanoformung, und abformung (LIGA) process.
The originalLIGA process has three main steps (Fig. 3): 1) A thick
layerof X-ray resist, typically poly(methyl methacrylate) (PMMA),is
deposited on a carrier substrate coated with a conductiveseed layer
[Fig. 3(a) and (b)], 2) The resist is exposed tohigh-energy X-rays
through a mask [Fig. 3(c)] and thendeveloped [Fig. 3(d)], yielding
a 3-D mold; 3) A method ofmetal deposition, most commonly
electroplating, is used tofill the mold [Fig. 3(e)]; and 4) The
resist mold is dissolved(i.e., expendable), resulting in the final
free-standing metalcomponent [Fig. 3(f)] [26]. Similar processes
that use thickphotosensitive resists, such as SU-8 and PMMA,
eliminate theneed for an X-ray source and provide the ability to
producesimilar structures with reduced cost [29][31]. SU-8
processescan also be incorporated to create positive molds, which
thencan be used for subsequent metallic device creation [32].
LIGA processes are able to produce structures with aspectratios
as large as 60:1 [33] with tolerances on the order ofmicrometers.
This method can make the smallest features ofany technique
described in this paper; however, one mustsacrifice some resolution
(i.e., tolerance) for increased aspectratio [34]. Common metals
used in LIGA and LIGA-likeprocesses include nickel [30], copper
[35], and gold [33].
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152 IEEE TRANSACTIONS ON COMPONENTS AND PACKAGING TECHNOLOGY,
VOL. 33, NO. 1, MARCH 2010
Heating/Cooling Passages
(a) (b) (c)
Fig. 4. Schematic of die casting: (a) metal molds with runner
and cooling passages, (b) molds pressed together with molten metal
being inserted, and(c) separation of molds and removal of part.
In addition, LIGA-like fabrication has been utilized to makeheat
sinks or cooling plates in the past [36], [37]. The abilityto
create complex 2-D shapes, as shown in [30], provides theoption for
creating nonstandard pin shapes and optimizing theheat sinks for
thermal and hydraulic performance.
3) Casting: The process of casting, in its most basic
form,involves pouring a molten metal into a pre-fabricated
mold,allowing the metal to solidify, and then removing the part
fromthe mold [17]. Of the numerous casting methods, only the
twothat produce the finest features and, hence, are most likely
tomake microscale heat sinks will be described here: die castingand
investment casting.
Investment casting utilizes a wax (typically for macroscale)or
plastic (microscale) pattern that defines the shape of thefinal
part. Ceramic powder is poured around the pattern, dried,and then
sintered to increase the strength of the ceramic moldand melt out
the pattern. The mold is filled with molten metalby vacuum die
casting (evacuate the mold and pressurized gasforces metal into it)
or centrifugal casting (forces generated byspinning are utilized),
and after solidification the expendablemold is removed [38].
Important considerations that determine the quality ofa
microcasting include the preheating temperature of themold and the
filling pressure of the mold. Baumeister et al.[38] showed, for a
particle-hardened gold-based alloy andAlbronze microcastings, that
flowlength increases with anincrease in preheating temperature and
filling pressure. Like-wise, grain size increases with increasing
preheating temper-ature due to the slower cooling rates.
In comparison to investment casting,
metal-mold-basedmicrocasting (Fig. 4) offers the ability to reuse
molds, increaseefficiency in production, and greater repeatability
in partproduction [39]. Aspect ratios of 8 or 9 can be achieved
withmicrocasting [40], and casting of features as small as 200min
size is feasible [38][42]. Die casting does have somegeometric
limitations; notably, undercuts cannot be included
in the permanent die as it would be impossible to remove
thesolidified part.
Cast parts only approach the theoretical density of the
metaland, hence, may have a slightly lower thermal conductivitythan
the same part milled out of a forged or extruded billet.Casting has
been the mainstay of high-volume production ofcomplex metal parts.
The cost of the permanent mold wouldnot make this method suitable
for prototyping or low-volumeproduction.
4) Extrusion: Extrusion is a method of producing
constantcross-sectional area parts through the plastic deformation
ofbillets through a die (Fig. 5) [17]. Hence, this method couldmake
straight channel heat sinks [Fig. 1(a)] but not thestaggered
micro-pin-fin design [Fig. 1(b)]. Most macroscalemetal heat sinks
used for cooling computer chips are madevia extrusion. However,
before extrusion can be applied tothe mass production of micro heat
sinks, further research anddevelopment is required. Microextrusion
is an area of activeresearch [43], [44] with the promise of
industrial application inthe not too distant future. Microextrusion
processes encountertwo problems that are not found in their
macroscale counter-parts. Current process limitations include the
precision of thetools used in creation of the dies and the
precision (i.e., back-lash) present in forming machinery [45]. In
addition, the sizeof the final extruded part relative to the grain
size of the billetmaterial has a significant effect on
manufacturing. Krishnanet al. [46] showed that 568m diameter
extruded pins withgrains 211m in size tended to curl due to
inhomogeneousdeformation, while pins with 32m grain size did
not.
5) Sintering: Sintering in the microfabrication realm maytake
the form of micro powder injection molding (PIM). Inthis process, a
metal powder combined with a binder systemis injected into a mold
of the final part shape (Fig. 6). Afterinjection, the binder is
removed (through thermal means orother methods) and the part is
sintered [47]. Fu et al. [48]demonstrated the ability to create
316L stainless steel pillars
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JASPERSON et al.: COMPARISON OF MICRO-PIN-FIN AND MICROCHANNEL
HEAT SINKS 153
Die
Billet
Fig. 5. Schematic of extrusion process.
(a)
(b) (c) (d)
Fig. 6. Schematic of micro powder injection molding, (a) inject
metal-binder mix into mold, (b) heat to remove binder, (c) sinter
metal powder, and(d) remove part.
through PIM that were 100m in diameter, 200m inheight, and had a
200m pitch. A silicon master was createdto serve as the expendable
mold. This process differs fromcasting because the powderbinder
mixture does not have to beinjected at elevated temperature,
allowing for a wider varietyof mold materials to be used. The part
can be sintered inthe mold or after it is removed from the mold.
Similar tocasting, the near-net-shape part shrinks upon cooling,
whichcan induce distortion and stresses in the part, and a
smallamount of porosity must be taken into account.
Other sintering fabrication methods include selective
lasersintering (SLS), where powder is deposited and then
selec-tively sintered layer by layer to form a bulk part.
Macroscalesystems may have a powder-feed cylinder which suppliesthe
powder to the machine, and a part-build cylinder, whichis
incrementally lowered to create each layer. The powderis typically
transferred using a roller [17]. In micro-SLS,a powder deposition
device replaces the roller and selectivelyplaces the powder for the
micro features [49], [50]. Using thisconcept, feature sizes as
small as 100m are reported [49].
(a) (b)
Fig. 7. Schematics of milling: (a) slot milling and (b) end
milling.
6) Machining: a) Slot/form milling: Slot milling is a poten-tial
method of manufacturing the straight channel heat sinks.A single
circular cutter with teeth on the outer portion of thebit or a
cluster of cutters [Fig. 7(a)] can be used. Slotting sawsas thin as
150m are commercially available, sufficient for thefeature sizes on
the micro heat sinks being compared in thispaper.
b) End milling: Micro-end-milling [Fig. 7(b)] refers to
anend-milling process that uses cutting tools between 5 and1000m in
diameter to create microscale features on micro-,meso-, and
macroscale parts [51]. It is a direct method ofcreating true 3-D
shapes in myriad materials, frequently ina single process step. The
fact that the geometry of interestis created by a part program that
controls the movementof the end mill makes this method flexible.
Therefore, it isclearly suited for prototyping metal heat sinks and
low-volumeproduction.
To maintain the same cutting speed as the diameter ofan end mill
decreases, the spindle speed must be increasedproportionally. For
example, to achieve the recommendedcutting speed for wrought
aluminum alloys being end-milledwith a tungsten carbide tool (3.15
m/s [52]) a 200m-diameterend mill requires a spindle speed of 300
000 rpm. Currently,there are 200 000 rpm spindles commercially
available, andongoing research aims to develop spindles that can
achievemore than 1 million rpm [53]. However, most
micro-end-milling is done with spindles between 50 000 and 100 000
rpm,because it is not yet known if the cutting speeds that
decadesof empirical data have shown to work well at the
macroscaleare optimal for micromachining [54], [55].
Micro-end-millsremove small amounts of material with each rotation,
thushigh-speed spindles do not need to be powerful with
costsranging from approximately $5,000 for a 50 000 rpm
air-drivespindle (fixed rpm) to $25 000 for a 200 000 rpm
electric-drivespindle with variable rpm.
As will be shown in the case study, the cost of machininga heat
sink is inversely proportional to the time it takes tomachine a
part (productivity), which is mainly a function of thefeed rate
(mm/min) at which a micro-end-mill can be movedthrough the
material. The feedrate fr is the product of thechipload tc number
of flutes (cutting edges) n f , and spindlespeed Ns
fr = tc n f Ns . (1)Hence, doubling the spindle speed or number
of flutes
will double the feedrate and cut the time to machine a
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154 IEEE TRANSACTIONS ON COMPONENTS AND PACKAGING TECHNOLOGY,
VOL. 33, NO. 1, MARCH 2010
CutterRotation
MaterialFeed
Chip Load[ft/tooth]
Fig. 8. Schematic of chip load.
part in half. The third variable that influences the
feedrate(hence, productivity) is the chipload tc (Fig. 8): the
depth ofengagement of a flute in the direction of travel. The
mag-nitude of the chipload for a micro-end-mill is
fundamentallylimited by the strength and flexibility of these small
diametertools [51]. Decreasing the diameter of an end mill
decreasesthe flexural stiffness and the cutting forces it can
withstandwithout bending and/or breaking. The force acting on the
toolis a function of the chipload, depth of cut, and material
beingmachined. Decreasing the depth of cut will enable an
increasein the chipload but requires more passes of the tool to
createa feature of the desired depth.
Micro-end-milling is a viable option for prototyping
andlow-volume production.
III. THERMAL-HYDRAULIC PERFORMANCEThermal-hydraulic performance
of the micro-pin-fin heat
sink was determined experimentally with details provided
inprevious papers [13], [14]. Only a brief overview is
presentedhere. Made of 110 copper, the micro-pin-fin heat sink had
aplatform area of 1.0 cm in width (Whs) by 3.38 cm in length(Lhs).
An array of 1950 staggered micro-pins with 200 200m2 cross section
and 670m height were milled out ofthe top surface (Fig. 9).
Thermal performance of the micro-pin-fin heat sink isrepresented
by an average convection thermal resistance Rconv
Rconv = Tw,ave Tf ,aveq eff At(2)
where At is the total base area of the heat sink
At = Whs Lhs = 1.0 3.38cm2. (3)q eff is the effective input heat
flux, Tw,ave is the average
pin-fin base (wall) temperature, and Tf ,ave is the average
water(fluid) bulk temperature. Hydraulic performance is
representedby the measured pressure drop across the heat sink P
.Details on how to determine q eff , Tw,ave, Tf ,ave, and Pfor the
micro-pin-fin heat sink can be found in previouspapers [13],
[14].
As heat transfer and pressure drop in microchannel heatsinks can
be adequately described by available analytical mod-els developed
for macrochannels [3], [4], a pseudo microchan-nel heat sink is
proposed. Thermal-hydraulic performance of
the microchannel heat sink is determined using the heat
trans-fer and pressure drop models summarized in Table II
[56],[57].
The performance of the two micro heat sink geometriesare
compared assuming identical heat sink substrate
material,single-phase liquid coolant, overall dimensions,
microscalestructure dimensions, and operating conditions. Fig. 1(a)
illus-trates the structure and key characteristic dimensions of
themicrochannel heat sink along side those of the micro-pin-finheat
sink. In particular, the (Wfin, Wch, Hfin) combination forthe
microchannel heat sink is chosen to be (200m, 200m,670m), which is
the same as that for the micro-pin-fin heatsink. Average convection
thermal resistance for the microchan-nel heat sink is similarly
evaluated from (2) using the heattransfer models provided in Table
II. Pressure drop across themicrochannel heat sink P is the sum of
the pressure dropacross the upstream hydrodynamically developing
entranceregion Pdh and the pressure drop across the downstreamfully
developed region Pfh. Analytical models for evaluatingthe two
pressure drop components are provided in Table II.
Fig. 10(a) and (b) compare the average convection
thermalresistance for the micro-pin-fin heat sink and
microchannelheat sink for Tin = 30 and 60 C, respectively. The
solidline and dashed line in these figures are power-law curvesto
best-fit the micro-pin-fin heat sink and microchannel heatsink
data, respectively, and are used to indicate the overalldata trend.
It can be seen from Fig. 10(a) and (b) that Rconvfor the
microchannel heat sink is fairly constant throughout thetotal flow
rate Wt range, while Rconv for the micro-pin-fin heatsink is more
sensitive to Wt and decreases significantly withincreasing Wt . In
the low Wt range, Rconv for the micro-pin-finheat sink is higher
than that for the microchannel heat sink,but becomes lower at a
higher Wt . The comparison indicatesa better micro-pin-fin heat
sink thermal performance at anelevated cooling water flow rate.
Fig. 11(a) and (b) compare the pressure drop across
themicro-pin-fin heat sink and microchannel heat sink for Tin =30
and 60 C, respectively. It can be seen from Fig. 11(a)and (b) that
P in the micro-pin-fin heat sink is significantlyhigher than that
in the microchannel heat sink at all flow ratestested.
IV. CASE STUDY: MICRO-END-MILLINGThe authors have the most
experience with micro-end-
milling [Fig. 7(b)] and have used it to manufacture coppermicro
heat sinks (Fig. 9). In this section, the cost of micro-end-milling
two different heat sink geometries, pin-fin and straightchannel
(Fig. 1), is compared. Only relative differences willbe highlighted
since any productivity improvements that wouldbe applied to
machining of one design would also be appliedto the other. The goal
of this case study is to determine whichheat sink is more expensive
to manufacture by micro-end-milling and why.
In order to compare the geometries directly, the same
basematerial (110 copper), and hence material cost and overallheat
sink geometry (width = 1.0 cm, length = 3.38 cm) areassumed.
Likewise, the pin width and gap (200m) is con-sistent between heat
sinks, and hence the same tool diameter
-
JASPERSON et al.: COMPARISON OF MICRO-PIN-FIN AND MICROCHANNEL
HEAT SINKS 155
(a) (b)
Fig. 9. Photographs of (a) copper heat sink and (b) pin fin
geometry created by micro-end-milling.
TABLE IIANALYTICAL MODELS FOR MICROCHANNEL HEAT SINK [16],
[17]
Heat transfer coefficient
h
For L 0.2 (thermally fully-developed flow),h =
(Nu3
k fdh
) (wb
)0.14
Nu3 = 8.235(1 1.883 + 3.7672 5.8143 + 5.3614 2.05); = ST
WfinHfinL = zRedh Pr fFor L 0.2 (thermally developing flow),h =
{Nu4 + 8.68(103 L)0.506 exp
[(9.9776 ln () 26.379) L]} ( Nu3Nu4
) ( k fdh
) (wb
)0.14
Nu4 = 8.235(1 2.042 + 3.0852 2.4773 + 1.0584 0.1865)
Pressure loss components
Pdh
Pdh =2 fapp,dh f u2f Ldh
dh ; Ldh = (0.06 + 0.07 0.042)Reindh
fapp,dh = 1Re
3.44 (L+dh)0.5 + K ()
/(4L+dh
)+ ffh Re3.44
(L+dh
)0.5
1+C(
L+dh)2
(wb)0.58
ffh Re = 24(1 1.355 + 1.9472 1.7013 + 0.9564 0.2545)L+dh =
LdhRedh ; K () = 0.6740 + 1.2501 + 0.3417
2 0.83583
C = (0.1811 + 4.3488 1.60272) 104
Pfh Pfh =2[
ffh(w
/b
)0.58] f u2f Lfh
dh ; Lfh = L Ldh
is used for both geometries. Assuming the same tool
materialresults in a constant tool cost. Furthermore, if the same
feedrate is used in both cases, the tool life should be the
same.Machining both parts on the same machine with the sameoperator
running means that the tool change time is constant.Since the heat
sinks have the same geometric envelope andsimilar features sizes,
it is assumed that setup and cleaningtimes are the same for both
designs. The remaining variablesthat determine the cost difference
between the two heat sink
geometries are tool path, which dictates the machining time t
,and cost rate R.
The overall cost of the heat sink can be broken down intothe
cost of tools CT , the cost of materials CM , as well asthe product
of manufacturing time t and cost rate R (4). Thechange in Ctotal
between the heat sinks can be calculatedby taking the difference of
each subcomponent of the totalcost
Ctotal = CT + CM + t R. (4)
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156 IEEE TRANSACTIONS ON COMPONENTS AND PACKAGING TECHNOLOGY,
VOL. 33, NO. 1, MARCH 2010
Micro-pin-fin heat sinkMicrochannel heat sink
WaterTin = 30 C
wt[g/min]
R conv[
C/W
]
30 40 50 60 70 80 90
0.16
0.14
0.12
0.10
0.08
0.06
0.04
0.02
(a)
Micro-pin-fin heat sinkMicrochannel heat sink
WaterTin = 60 C
wt[g/min]
R conv[
C/W
]
30 40 50 60 70 80 90
0.16
0.14
0.12
0.10
0.08
0.06
0.04
0.02
(b)
Fig. 10. Comparison of micro-pin-fin heat sink and microchannel
heat sink average convection thermal resistance for (a) Tin = 30 C
and (b) Tin = 60 C.
Micro-pin-fin heat sinkMicrochannel heat sink
WaterTin = 30 C
wt[g/min]
P [b
ar]
30 40 50 60 70 80 90
0.12
0.10
0.08
0.06
0.04
0.02
0.00
(a)
Micro-pin-fin heat sinkMicrochannel heat sink
WaterTin = 60 C
wt[g/min]
P [b
ar]
30 40 50 60 70 80 90
0.12
0.10
0.08
0.06
0.04
0.02
0.00
(b)
Fig. 11. Comparison of micro-pin-fin heat sink and microchannel
heat sink pressure drop for (a) Tin = 30 C and (b) Tin = 60 C.
The cost of the tools CT is a function of the number of
toolsrequired Ntools and the cost per tool CT . The number of
toolsrequired to manufacture one heat sink is a function of tool
lifeand final part geometry, because it dictates the total tool
pathand, along with feed rate, determines the machining time.
Inthis analysis, we are assuming that only one tool design isused.
However, in reality tools with shorter flute lengths willbe used
where possible because they last longer
CT = Ntools CT . (5)Since the cost per tool does not change
between heat sinks, thetotal tool cost only varies with the number
of tools. The toollife and tool geometry are assumed constant
between heatsinks, meaning that the number of tools varies solely
withthe final part geometry of the heat sinks.
The material costs CM are dependent on the volume ofmaterial
required and the per unit cost. The heat sinks
have the same overall dimensions and are made out of thesame
material, so CM does not vary between the two heatsinks.
The final term in (4) is the cost of the processing time.The
cost rate R includes the capital cost of machinery, anyoverhead and
utilities required for operation, additional train-ing required for
machining/setup of process, labor, etc. Thesefactors do not change,
regardless of fabrication geometry, andas such can be excluded from
the current comparison.
The total time t required to manufacture the heat sink is
afunction of the machining time, the amount of time requiredfor
tool changes, setup time, and cleaning time (6). Theamount of time
spent machining is a function of the feedrateand geometry of the
final design. More complex designsrequire more passes with the
tool, resulting in longer machin-ing time (7). Feedrate is a
complex function of multiple para-meters, including material
properties, tool strength/geometry,
-
JASPERSON et al.: COMPARISON OF MICRO-PIN-FIN AND MICROCHANNEL
HEAT SINKS 157
2(wfin)
wfinl
Fig. 12. Illustration of tool path for milling channel heat
sink.
tool coating, and metal working fluids (8)t = tmachining +
ttoolchange + tsetup + tcleaning (6)
tmachining = f (feed rate, part geometry, tool geometry) (7)Feed
rate = f (material, tool strength, tool geometry,
tool coating, metalworking fluid). (8)None of the variables that
feed rate is a function of changesbetween heat sinks, meaning that
the feed rate can be assumedto be constant. Likewise, the tools
used to machine bothgeometries can be 180m in diameter (allowing
for runout)because both designs have the same characteristic
dimension(spacing between pin/wall). Therefore, machining time
isbased solely on geometry.
The time required for tool changes is also related to the
workpiece material. Shaping a material that is harder to
machineresults in shorter tool life, more tool changes, and
longermachining time due to the increased tool changes. The timeper
tool change is a function of the machine being used, and,when
automated, takes less than 1 min.
Since setup time, cleaning time, feed rate, machine used,and
operator are the same, the time to manufacture the heatsinks varies
only with the geometry of the part. The costequation (4) for
comparing the two geometries simplifies to (9)
Ctotal = Ntools CT + (tmachining + ttoolchange) R. (9)The
remainder of this section will focus on the geometric
differences between straight channel and staggered-pin-fin
heatsinks and how to calculate the tool path length. The methodfor
machining a straight channel heat sink in a piece of copperwith
length l and width w is shown in Fig. 12. For analysispurposes,
assume that w and l are multiples of wfin, whichis the width of one
fin. The machining length dstraight that isrequired to fabricate
one layer for a straight channel for thisgeometry is given by
(10)
dstraight = w wfin2(wfin) (l) + w 3(wfin). (10)The tool path for
machining one layer of a staggered pin-finheat sink dstgpin is more
complicated (Fig. 13) and longer sincemore material must be removed
in order to create the extrasurface area that benefits heat
transfer (11)
dstgpin = l wfin2wfin[
2w + 6wfin + 2wfin(
w wfin2wfin
)]. (11)
w
l
(a)
wfin
2(wfin)(b)
(c) (d)
(e) (f)
Fig. 13. Illustration of tool path for milling staggered pin
heat sink: (a) firstpass, illustrating the effect of tool radius on
the corners of the pins, (b) secondpass, which finishes the first
column of pins and makes the first cut on thesecond column, and (c)
through (f) repeating the process to make multiplecolumns of
pins.
Substituting typical numerical values (l = 3 cm, w = 1 cm,w f in
= 200m) into (10) and (11) and comparing themshow that the tool
distance for the staggered-pin heat sinkdesign used in this paper
is approximately three times greaterthan for the straight channel.
Experience has shown that for a200m diameter tool, a depth of cut
of approximately 50mis appropriate. Therefore, each heat sink would
require 12layers, each of length d, to machine the 600m-deep
pins.Fig. 14 shows the total machining distance as a function ofthe
pin/wall width.
V. DISCUSSION
When comparing the manufacturability of pin-fin versusstraight
channel heat sinks, the geometries shown in Fig. 1are assumed. The
width of and distance between the pinsor channel walls [Fig. 1(a)]
are 200m and the aspect ratiois 3:1, making the height of the
pins/walls 600m. For thestaggered pins [Fig. 1(b)], the distance
between rows of pins is200m with alternating patterns. This
geometry was also usedin the thermal-hydraulic performance analysis
as well as themicro-end-milling case study, to allow for a direct
comparisonbetween all the different aspects of the study.
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158 IEEE TRANSACTIONS ON COMPONENTS AND PACKAGING TECHNOLOGY,
VOL. 33, NO. 1, MARCH 2010
Mac
hini
ng D
istan
ce [m
]
140
Staggered PinsStraigth Pins
120
100
80
60
40
20
00
Pin/Wall Width, wfin [microns]100 200 300 400 500 600
Fig. 14. Total machining distance (tool path as a function of
pin/wall widthfor a 1 cm 3.38 cm area).
The tool path for one layer of a straight channel
andstaggered-pin-fin heat sink with 200m feature size (wfin)is
approximately 830 and 2600 mm, respectively. The totalmachining
distances for micro-pin-fin and microchannel heatsinks of this size
(12 layers) are approximately 10 and 31.25 m,respectively. With a
feedrate of approximately 100 mm/minfor 200m diameter end mills
shaping pure copper, totalmachining times of 100 and 312.5 min
result. If the tool lifeis approximately 2 meters, and each tool
change takes 1 min,then the straight channel can be made with 5
tools and the pinfin requires 16. As a first-order estimation, the
cost of eachtool is approximately the same as the hourly rate.
Examiningthese numbers in (9) shows that tmachining R is an orderof
magnitude greater than any other costs that differ betweenthe two
heat sink designs being considered. Therefore, thecost of machining
the heat sinks will scale with the machin-ing distance (Fig. 14).
Hence, the pin-fin heat sink will beapproximately three times as
expensive to make if the materialcosts, setup time, and cleaning
time are much smaller than themachining time. As improvements in
productivity (i.e., feedrate) are made by developing new tool
coatings and usinghigher speed spindles, the percentage of the part
cost related tomachining time will decrease. This also means that
the differ-ence in cost between the two heat sink designs will
decrease.
An appealing option that would minimize the differencein
manufacturing cost between the two miniature heat sinkswould be to
fabricate a mold with micro-end-milling tofacilitate casting of the
final part. If casting were used forhigh-volume production of micro
heat sinks, the difference inunit cost between microchannel and
micro-pin-fin heat sinkswould be small. Approximately the same
amount of materialis used, and the tolerances and dimensions are
similar. Theonly significant cost difference would be in the mold,
whichwould be more complex for the micro-pin-fin heat sink.
Returning to the discussion on the thermal-hydraulicperformance,
the average convection thermal resistance Rconvfor the two heat
sinks can be written as
Rconv = 1have Aht, eff (12)
where have represents the average heat transfer coefficient,
andAht,eff represents the total effective heat transfer area of
themicroscale enhancement structures. Equation (12) indicates
that an improved thermal performance (low Rconv) can beachieved
by either enhancing heat transfer (increasing have)or increasing
total effective heat transfer area Aht,eff .
Total heat transfer area Aht for the micro-pin-fin heat sinkis
approximately 13.3 cm2, and for the microchannel heat sink13.0 cm2.
Total effective heat transfer area Aht,eff assumesvalues lower than
those of Aht due to the fin effect. Aht,effranges from 12.0 to 12.9
cm2 for the micro-pin-fin heat sink,and is 12.5 cm2 for the
microchannel heat sink. This showsthat the pin-fin geometry that
was chosen does not have asignificant area advantage over the
chosen microchannel heatsink geometry.
The better thermal performance for the micro-pin-fin heatsink at
high flow rate can therefore be attributed to enhancedheat
transfer. The data trend as shown in Fig. 10(a) and (b)may be
explained by the nature of water flow in the micro-pin-fin array.
At low flow rate, flow in the micro-pin-fin arrayis dominated by
laminar flow, and vortices in the wake arerelatively weak. As a
result, the downstream faces as well asa substantial portion of the
side faces of the square micro-pin-fins are not exposed to the main
flow, which leads to aless efficient use of the total heat transfer
area. As flow rateincreases, flow in the micro-pin-fin array is
more tortuous,and vortices in the wake become stronger, which
enhancesheat transfer through reducing boundary layer thickness
andactivating a larger portion of pin-fin surface areas.
The higher pressure drop across the micro-pin-fin heat sinkas
shown in Fig. 11(a) and (b) is a result of the drag forcepresented
by each and every pin-fin. Because a staggeredmicro-pin-fin
configuration was used, every pin-fin sees a flowimpinging on its
upstream face. The pressure drop may bedecreased, while maintaining
the same surface area, by usingmicro-pin-fins with airfoil cross
sections [6].
VI. CONCLUSION
After comparing copper microchannel and micro-pin-finheat sinks
(same characteristic dimensions; single-phase waterflow) using
thermal performance, hydraulic performance, andcost of
manufacturing as metrics, it is concluded that neitherdesign is
better for all applications.
The average convection thermal resistance decreases
withincreasing flow rate for the micro-pin-fin, but it does not
varysignificantly with flow rate for the microchannel heat
sink.Below a flow rate of approximately 60 g/min, the
micro-pin-finheat sink has a higher thermal resistance than the
microchannelheat sink. Above 60 g/min, the micro-pin-fin heat sink
has alower thermal resistance. This variation in thermal
resistanceis attributed to the more tortuous flow and strong
vortices inthe wake at high flow rate. Therefore, the micro-pin-fin
heatsink would be chosen for its better thermal performance atflow
rates above 60 g/min.
The pressure drop across the micro-pin-fin heat sink
isapproximately twice as large as that across the microchannelheat
sink at low rates. The difference in pressure drop increaseswith
increasing flow rate for the range of flow rates evaluatedin this
paper. Therefore, the improved thermal performance athigh flow
rates comes with a significant increase in pressure
-
JASPERSON et al.: COMPARISON OF MICRO-PIN-FIN AND MICROCHANNEL
HEAT SINKS 159
drop. Other pin designs (diamond, circular, airfoil, etc.)
thatare being studied have the potential to provide the same
ther-mal performance without the same increase in pressure
drop.
Multiple manufacturing methods exist for creating heatsinks out
of metal. Casting and extrusion are the most eco-nomical choices
for mass production; micro-end-milling andmicro-EDM are ideal for
prototyping. If casting a significantvolume of heat sinks, the cost
per unit would be similar forboth heat sink designs. The tool path
needed to end-mill themicro-pin-fin heat sinks is approximately
three times greaterthan for the microchannel heat sinks. The
machining time isdirectly related to the length of the tool path,
and becauseof the limited feed rates available at this time, the
cost tomicromachine heat sinks is primarily a function of
machiningtime. Therefore, the cost to micro-end-mill a
micro-pin-finheat sink is approximately three times greater than
that for amicrochannel heat sink. As ongoing research enables
increasesin spindle speeds and feedrates, the total cost to machine
theseheat sinks will decrease and the difference in cost between
thetwo designs will decrease.
REFERENCES[1] T. M. Harms, M. J. Kazmierczak, and F. M. Gerner,
Developing
convective heat transfer in deep rectangular microchannels, Int.
J. HeatFluid Flow, vol. 20, no. 2, pp. 149157, 1999.
[2] K. Kawano, K. Minakami, H. Iwasaki, and M. Ishizuka,
Microchannelheat exchanger for cooling electrical equipment,
presented at Proc.ASME Int. Mech. Eng. Congr. Expo., Fairfield, NJ,
1998.
[3] P.-S. Lee, S. V. Garimella, and D. Liu, Investigation of
heat transfer inrectangular microchannels, Int. J. Heat Mass
Transfer, vol. 48, no. 9,pp. 16881704, 2005.
[4] W. Qu and I. Mudawar, Experimental and numerical study of
pressuredrop and heat transfer in a single-phase microchannel heat
sink, Int. J.Heat Mass Transfer, vol. 45, no. 12, pp. 25492565,
Jun. 2002.
[5] D. B. Tuckerman and R. F. W. Pease, High-performance heat
sinkingfor VLSI, IEEE Electron. Devices Lett., vol. 2, no. 5, pp.
126129,May 1981.
[6] A. Kosar, C.-J. Kuo, and Y. Peles, Hydoroil-based
micro-pin-fin heatsink, in Proc. 2006 ASME Int. Mech. Eng. Congr.
Expo., (IMECE06)Microelectromech. Syst., Chicago, pp. 563570.
[7] A. Kosar, C. Mishra, and Y. Peles, Laminar flow across a
bank oflow aspect ratio micro-pin-fins, J. Fluids Eng., Trans.
ASME, vol. 127,no. 3, pp. 419430, 2005.
[8] A. Kosar and Y. Peles, Thermal-hydraulic performance of
MEMS-basedpin fin heat sink, J. Heat Transfer, vol. 128, no. 2, pp.
121131, 2006.
[9] Y. Peles and A. Kosar, Convective flow of refrigerant
(R-123) across abank of micro-pin-fins, Int. J. Heat Mass Transfer,
vol. 49, no. 1718,pp. 31423155, 2006.
[10] Y. Peles, A. Kosar, C. Mishra, C.-J. Kuo, and B. Schneider,
Forcedconvective heat transfer across a pin fin micro heat sink,
Int. J. HeatMass Transfer, vol. 48, no. 17, pp. 36153627, 2005.
[11] R. S. Prasher, Nusselt number and friction factor of
staggered arraysof low aspect ratio micro-pin-fins under cross flow
for water as fluid,J. Heat Transfer, vol. 129, no. 2, pp. 141153,
2007.
[12] A. Siu-Ho, W. Qu, and F. Pfefferkorn, Experimental study of
pressuredrop and heat transfer in a single-phase micro-pin-fin heat
sink, Trans.ASME. J. Electron. Packaging, vol. 129, no. 4, pp.
479487, 2007.
[13] W. Qu and A. Siu Ho, Liquid single-phase flow in an array
of micro-pin-fins. Part I. Heat transfer characteristics, J. Heat
Transfer, vol. 130,no. 12, pp. 122402-1122402-11, Dec. 2008.
[14] W. Qu and A. Siu Ho, Liquid single-phase flow in an array
of micro-pin-fins. Part II: Pressure drop characteristics, J. Heat
Transfer, vol. 130,no. 12, pp. 124501-1124501-4, 2008.
[15] J. Eugene, F. Xi, B. Tan, and B. A. Jubran, An overview on
micro-mesomanufacturing techniques for micro heat exchangers for
turbine bladecooling, Int. J. Manufacturing Res., vol. 3, no. 1,
pp. 326, 2008.
[16] K. H. Ho and S. T. Newman, State of the art electrical
dischargemachining (EDM), Int. J. Mach. Tools Manufacture, vol. 43,
no. 13,pp. 12871300, 2003.
[17] S. Kalpakjian and S. R. Schmid, Advanced manufacturing
processes,in Manufacturing Engineering and Technology, 5th ed.
Upper SaddleRiver, NJ: Pearson Education, Inc., 2006, ch. 27, pp.
846850.
[18] C. S. Lin, Y. S. Liao, and S. T. Chen, Development of a
novel microwire-EDM mechanism for the fabricating of micro parts,
presented atInt. Conf. Advanced Manufacture, Taipei, Taiwan,
2005.
[19] X. Cheng, Development of ultraprecision machining system
withunique wire EDM tool fabrication system for
micro/nano-machining,CIRP Ann. Manufacturing Technol., vol. 57, no.
1, pp. 415420,2008.
[20] D. T. Pham, S. S. Dimov, S. Bigot, A. Ivanov, and K. Popov,
Micro-EDM recent developments and research issues, presented at
14th Int.Symp. Electromach., Edinburgh, Scotland, 2004.
[21] A. B. M. A. Asad, T. Masaki, M. Rahman, H. S. Lim, and Y.
S. Wong,Tool-based micro-machining, J. Materials Process. Tech.,
vol. 192193, pp. 204211, 2007.
[22] M. M. Sundaram, S. Billa, and K. P. Rajurkar, Generation of
highaspect ratio micro holes by a hybrid micromachining process,
presentedat ASME Int. Conf. Manufacturing Sci. Eng., Atlanta, GA,
2007.
[23] W. Ehrfeld, Micro electro discharge machining as a
technology inmicromachining, in Proc. SPIE Int. Soc. Optical Eng.,
vol. 2879.Austin, TX, 1996, pp. 332337.
[24] M. Sundaram and K. Rajurkar, Toward freeform machining by
microelectro discharge machining process, Trans. North Amer.
ManufacturingRes. Inst., vol. 36, pp. 381388, 2008.
[25] J. S. Coursey, J. Kim, and K. T. Kiger, Spray cooling of
high aspectratio open microchannels, J. Heat Transfer, vol. 129,
no. 8, pp. 10521059, 2007.
[26] M. J. Madou, Lithography, in Fundamentals of
Microfabrication, 2nded. Boca Raton, FL: CRC Press, 2002, ch. 1,
pp. 110, 4950.
[27] R. Muwanga and I. Hassan, Flow boiling oscillations in
microchannelheat sinks, presented at 9th AIAA/ASME Joint
Thermophysics HeatTransfer Conf., San Francisco, CA, 2006.
[28] S. Stefanescu, M. Mehregany, J. Leland, and K. Yerkes,
Micro jet arrayheat sink for power electronics, presented at Proc.
IEEE Micro ElectroMechn. Syst., Orlando, FL, 1999.
[29] X. Wei, C. H. Lee, Z. Jiang, and K. Jiang, Thick
photoresists forelectroforming metallic microcomponents, J. Mech.
Eng. Sci., vol. 222,no. 1, pp. 3742, 2008.
[30] C. H. Ho, K. P. Chin, C. R. Yang, H. M. Wu, and S. L. Chen,
UltrathickSU-8 mold formation and removal, and its application to
the fabricationof LIGA-like micromotors with embedded roots,
Sensors and ActuatorsA (Physical), vol. 102, no. 12, pp. 130138,
2002.
[31] H. Guckel, High-aspect-ratio micromachining via deep X-ray
lithogra-phy, Proc. IEEE, vol. 86, no. 8, pp. 15861593, Aug.
1998.
[32] J. Li, D. Chen, J. Zhang, J. Liu, and J. Zhu, Indirect
removal of SU-8photoresist using PDMS technique, Sensors and
Actuators A (Physical),vol. 125, no. 2, pp. 586589, 2006.
[33] L. T. Romankiw, Path: From electroplating through
lithographic masksin electronics to LIGA in MEMS, Electrochimica
Acta, vol. 42, no. 2022, pp. 29853005, 1997.
[34] R. K. Kupka, F. Bouamrane, C. Cremers, and S. Megtert,
Microfabri-cation: LIGA-X and applications, Appl. Surface Sci.,
vol. 164, no. 14,pp. 97110, 2000.
[35] R. Engelke, Complete 3-D UV microfabrication technology on
stronglysloping topography substrates using epoxy photoresist SU-8,
Microelec-tron. Eng., vol. 7374, pp. 456462, 2004.
[36] A. J. Pang, Design, manufacture and testing of a low-cost
microchannelcooling device, presented at 6th Electron. Packaging
Technol. Conf.,Piscataway, NJ, 2004.
[37] L. S. Stephens, K. W. Kelly, D. Kountouris, and J. McLean,
A pinfin microheat sink for cooling macroscale conformal surfaces
under theinfluence of thrust and frictional forces, J.
Microelectromech. Syst.,vol. 10, no. 2, pp. 222231, 2001.
[38] G. Baumeister, R. Ruprecht, and J. Hausselt, Microcasting
of partsmade of metal alloys, Microsyst. Technol., vol. 10, no. 3,
pp. 261264,2004.
[39] B. S. Li, M. X. Ren, C. Yang, and H. Z. Fu, Microstructure
of Zn-Al4alloy microcastings by micro precision casting based on
metal mold,Trans. Nonferrous Metals Soc. China, vol. 18, no. 2, pp.
327332, 2008.
[40] G. Baumeister, K. Mueller, R. Ruprecht, and J. Hausselt,
Production ofmetallic high aspect ratio microstructures by
microcasting, Microsyst.Technol., vol. 8, no. 23, pp. 105108,
2002.
[41] G. Baumeister, R. Ruprecht, and J. Hausselt, Replication of
LIGAstructures using microcasting, Microsyst. Technol., vol. 10,
no. 6,pp. 484488, 2004.
-
160 IEEE TRANSACTIONS ON COMPONENTS AND PACKAGING TECHNOLOGY,
VOL. 33, NO. 1, MARCH 2010
[42] S. Chung, S. Park, H. Jeong, I. Lee, and D. Cho,
Replication techniquesfor a metal microcomponent having real 3-D
shape by microcastingprocess, Microsyst. Technol., vol. 11, no. 6,
pp. 424437, 2005.
[43] J. Cao and N. Krishnan, Recent advances in microforming:
Science,technology and applications, in Proc. Materials Sci.
Technol. 2005,Pittsburgh, PA, pp. 225234.
[44] N. Krishnan, J. Cao, and K. Dohda, Study of the size
effects on frictionconditions in microextrusion Part I:
Microextrusion experiments andanalysis, J. Manufacturing Sci. Eng.,
Trans. ASME, vol. 129, no. 4,pp. 669676, 2007.
[45] M. Geiger, M. Kleiner, R. Eckstein, N. Tiesler, and U.
Engel, Micro-forming, CIRP Ann. Manufacturing Technol., vol. 50,
no. 2, pp. 445462, 2001.
[46] N. Krishnan, J. Cao, B. Kinsey, S. A. Paraslz, and M. Li,
Investigationof deformation characteristics of micro-pins
fabricated using microex-trusion, presented at ASME Int. Mech. Eng.
Congr. Expo., Orlando,FL, 2005.
[47] S. G. Li, Dimensional variation in production of
high-aspect-ratiomicro-pillars array by micro powder injection
molding, Appl. PhysicsA-Materials Sci. Process., vol. 89, no. 3,
pp. 721728, 2007.
[48] G. Fu, Injection molding, debinding and sintering of 316L
stainlesssteel microstructures, Appl. Physics a-Materials Sci.
Process., vol. 81,no. 3, pp. 495500, 2005.
[49] J. Chen, J. Yang, and T. Zuo, Micro fabrication with
selective lasermicro sintering, presented at 1st IEEE Int. Conf.
Nano Micro Eng.Molecular Syst., Zhuhai, China, 2006.
[50] X. Li, H. Choi, and Y. Yang, Micro rapid prototyping system
for microcomponents, Thin Solid Films, vol. 420421, pp. 515523,
2002.
[51] Y. Jeon, and F. Pfefferkorn, Effect of laser preheating the
workpieceon micro-end-milling of metals, presented at Proc. ASME
Int. Mech.Eng. Congr. Expo., Orlando, FL, 2005.
[52] E. Oberg, F. D. Jones, H. L. Horton, and H. H. Ryffel,
Aluminumalloys, in Machinerys Handbook, 28th ed. New York:
Industrial Press,2004, p. 1014.
[53] S. Jahanmir, Ultra-high-speed micro-milling spindle,
PosterMSECICMP2008-72573 presented at ASME Int. Conf.
ManufacturingSci. Eng., Evanston, IL, Oct. 710, 2008.
[54] T. Ozel and T. Altan, Process simulation using finite
element methodprediction of cutting forces, tool stresses and
temperatures in high-speed flat end milling, Int. J. Mach. Tools
Manufacture, vol. 40, no. 5,pp. 713738, 2000.
[55] M. P. Vogler, R. E. DeVor, and S. G. Kapoor, On the
modeling andanalysis of machining performance in micro-end-milling,
Part I: Surfacegeneration, Trans. ASME. J. Manufacturing Sci. Eng.,
vol. 126, no. 4,pp. 685694, 2004.
[56] R. D. Blevins, Applied Fluid Dynamics Handbook, 1st ed. New
York:Van Nostrand Reinhold Company, 1984, ch. 6, pp. 38123.
[57] R. K. Shah and A. L. London, Laminar Flow Forced Convection
inDucts: A Source Book for Compact Heat Exchanger Analytical
Data,Supl. 1, New York: Academic Press, 1978, ch. 7, pp.
196222.
Benjamin A. Jasperson received the B.S. degreein mechanical
engineering from the University ofWisconsin, Madison in May 2008.
He is currentlypursuing masters degree in mechanical engineeringat
the University of Wisconsin, under the supervisionof Professors
Pfefferkorn and Turner.
His research areas include micro heat flux sensors,micro end
milling and micro fabrication.
Mr. Jasperson is a member of Tau Beta Pi(Wisconsin Alpha).
Yongho Jeon received the B.S. degree in mechanicalengineering
from Ajou University, South Korea,and the Illinois Institute of
Technology, in 2003.He received the M.S.M.E. and Ph.D. degrees
fromthe University of Wisconsin, Madison, in 2005 and2008,
respectively.
He is currently the Manager of fundamental manu-facturing
engineering development team at HyundaiMotors, Seoul, South
Korea.
Kevin T. Turner received the B.S. degree inmechanical
engineering from Johns Hopkins Uni-versity, Baltimore, MD, in 1999,
and the S.M. andPh.D. degrees in mechanical engineering from
theMassachusetts Institute of Technology, Cambridge,in 2001 and
2004, respectively.
Since 2005, he has been a Faculty Member in theDepartment of
Mechanical Engineering, Universityof Wisconsin, Madison. His
primary research inter-ests are the mechanics and design of MEMS
andsemiconductor manufacturing processes.
Dr. Turner is a member of American Society of Mechanical
Engineers andthe Materials Research Society. In 2008, he received
the ASEE FerdinandP. Beer and E. Russell Johnston, Jr. Outstanding
New Mechanics EducatorAward.
Frank E. Pfefferkorn received the B.S.M.E. degreefrom the
University of Illinois, Urbana-Champaign,in 1994, and the M.S.M.E.
and Ph.D. degreesin mechanical engineering from Purdue
University,West Lafayette, IN, in 1997 and 2002, respectively.
He has been a Faculty Member in the Departmentof Mechanical
Engineering, University of Wiscon-sin, Madison, since 2003. His
primary research inter-est is in developing a science-based
understandingof manufacturing processes, including heat
transferproblems, micro end milling, friction stir welding,
thermally-assisted manufacturing, and laser micro-polishing.Dr.
Pfefferkorn is a member of the American Society of Mechanical
Engineers and the Society of Manufacturing Engineers. He is the
recipient ofa Research Initiation Award and the 2007 Kuo K. Wang
Outstanding YoungManufacturing Engineer Award from the Society of
Manufacturing Engineers.
Weilin Qu received the B.E. and M.S. degreesin engineering
thermophysics in 1994 and 1997,respectively, both from Tsinghua
University, Beijing,China, and the Ph.D. degree in
mechanicalengineering in 2004 from Purdue University,West
Lafayette, IN.
He joined the Department of Mechanical Engineer-ing, University
of Hawaii at Manoa, Honolulu as anAssistant Professor in 2004,
where he established theMicroScale Thermal/Fluid Laboratory. His
researchhas been focused on microscale thermal/fluid trans-
port processes, boiling and two-phase flow, high-heat-flux
thermal manage-ment, and electronic cooling. His doctoral research
involved experimentalstudy, theoretical modeling, and numerical
analysis of the various transportphenomena associated with
single-phase liquid flow and forced convectiveboiling in
microchannels.
Dr. Qu is a member of the American Society of Mechanical
Engineers.