Research Article Process to Improve the Adherences of ...

Research ArticleProcess to Improve the Adherences of Copper to a PTFE Plate

Abel Pérez, Alfonso Torres, and Reydezel Torres

Electronics Department, Instituto Nacional de Astrofısica, Optica y Electronica, San Andres Cholula, PUE, Mexico

Correspondence should be addressed to Abel Perez; [email protected]

Received 19 May 2016; Revised 24 July 2016; Accepted 4 August 2016

Academic Editor: Debdulal Das

Copyright © 2016 Abel Perez et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

A simple low plasma power and roughness free process for improving the adherence of Cu to PTFE is presented. The results showthat low pressure and Ar flow combination are the drivers of this improved adherence. Copper Peel Strength Tensile values up to60 kg/m are obtained which are comparable to those shown in commercial composite dielectrics for high-frequency applicationsPrinted Circuit Boards.

1. Introduction

Polytetrafluoroethylene (PTFE) has an unusual combinationof high thermal stability, chemical inertness, and electricalstability. The properties of PTFE have led to many successfulchemical and electrical applications, such as electrical cable,lining for reactor, and antisticking coatings for kitchenutensil and tapes. However, its poor adhesion has limited itsapplication in many fields. There have been many studies forimproving the surface adherence of the PTFE, such as wetchemical treatment, plasma, and ion bean, but most of theseintroduce roughness in the surface of the dielectric [1].

The tremendous progress that has been made in thepast four decades inminiaturizing and integration transistorsonto silicon has contributed to the continuing increase incomponent, performance, and load density [2]. All progresseshave required that transmission lines find a correspondingway for increasing density of the substrate and reducinglosses. Moreover, modern applications ranging from high-frequency telecommunications systems to high rate of bytes/sin current computers have set new requirements on PrintedCircuit Board (PCB).

There are three characteristics that the PCB shop needsto master successfully process PTFE fabrication boards andif we can meet these three requirements, everything else willbe essentially like processing rigid double sided FR-4; therequirements are

(1) copper surface preparation,(2) drilling PTFE materials,

(3) plated through hole and PTFE surface preparation[3].

One of the most popular waveguides in a PCB is themicrostrip. Among the figures ofmerit that help us determineits performance, we canmention the losses; the line losses (𝛼)are composed of two contributions: dielectric losses (𝛼𝑑) andohmic losses (𝛼𝑐).The 𝛼𝑐 depends onDC resistance (𝑅dc), ACresistance (𝑅ac), and real part of the impedance (𝑅(𝑍0)), andthe resistances are obtained from the following parameters:metal conductivity (𝜎), permeability (𝜇), frequency (𝑓) andphysical dimensions of the line width (𝑤), and thickness ofthe metal (𝑡) [4].

To incorporate the effect of the surface roughness, anempirical roughness factor (𝑘𝑟) is added to the formula for𝑅ac (4).The transition frequency for this increase in resistanceis dependent on the root mean square (RMS) value of theroughness (ℎRMS), which can be measured directly on thecopper foil or estimated from cross sections (see Figure 1).

In a conventional PCB fabrication process, the roughnessis used for promoting adhesion of metals to the substrate.Since the rough copper surface affects current flow, it willaffect also power dissipation and thus the losses. The tra-ditional way to account for surface roughness losses in atransmission-line model is to use Hammerstad coefficient; itis described in (5).

Equations (1) to (5) show the relationship among all theaforementioned parameters. The roughness is an importantfactor in the ohmic losses especially when we need a low-loss PCB. 𝑅ac is directly proportional to 𝑘𝑟; 𝑅ac is the largest

Hindawi Publishing CorporationIndian Journal of Materials ScienceVolume 2016, Article ID 7419584, 4 pageshttp://dx.doi.org/10.1155/2016/7419584

2 Indian Journal of Materials Science

Trace

Skin depth

Ground plane

Figure 1: Realistic conductors used to manufacture transmissionlines exhibit a rough surface called the “tooth structure.” When theskin depth is similar to tooth size, power dissipation increased [11].

contribution of the ohmic losses; the Hammerstad coefficientdepends on roughness. Thus, by reducing the roughness, theohmic losses can be reduced.

𝛼 = 𝛼𝑑 + 𝛼𝑐 Np/leng, (1)

𝛼𝑐 =

1

2

𝑅dc + 𝑅ac𝑅 (𝑍0)

Np/leng, (2)

𝑅dc =1

𝜎𝑤𝑡

Ω/leng, (3)

𝑅ac =𝑘𝑟

𝑤 + 𝑡

√𝜋𝑓𝜇

𝜎

Ω/leng, (4)

𝑘𝑟 = 1 +

2

𝜋

arctan (1.4𝜋𝜇𝜎𝑓ℎ2RSM) . (5)

In order to reduce the ohmic losses, the Teflon is polishedto decrease it and Cu is electrodeposited on its surface asthe conductor in this PCB material proposal. The aim ofthis work is to obtain a good adherence between the Teflonand copper while maintaining the interface Teflon-Cu aspolished as possible, because the roughness decreases greatlythe performance of a PCB. Actually, the losses are not of thePCB but of the devices built in the PCB.

For instance, in a coplanar waveguide, there are threetypes of losses: dielectric, ohmic, and radiation/surface wave[4]. In most cases, the radiation losses are neglected; then,we will concentrate on the dielectric and ohmic losses. Thedielectric losses (𝛼𝑑) depend on loss tangent (tan 𝛿) andrelative permittivity (𝜀𝑟) and the dimension of the line:thickness of dielectric (ℎ), width of line (𝑤), and thickness ofmetal (𝑡).The loss tangent and the permittivity are inherent tothe dielectric material. Teflon (PTFE) is selected as substratematerial because it has one of the smallest values for both 𝜀𝑟(2.0–2.3) and tan 𝛿 (0.00025) [5]. Equations (6) and (7) showthe relationship of 𝜀𝑟 and tan 𝛿 with dielectric losses.

𝛼𝑑 =

1

2

𝜔

√𝜀eff

𝑐

tan 𝛿 Np/leng, (6)

where 𝜔 = 2𝜋𝑓 and 𝑐 = light velocity.

Woolen clothPTFE

Polishing

Figure 2: Polishing system.

𝜀eff =

𝜀𝑟 + 1

2

𝜀𝑟 − 1

2

𝐹 − 𝐶,

𝐹

=

{{{{

{{{{

{

1

1.71

(1 + 12

ℎ

𝑤

)

−1/2

+ 0.04 (1 −

𝑤

ℎ

)

2

, (

𝑤

ℎ

) ≤ 1,

1

1.71

(1 + 12

ℎ

𝑤

)

−1/2

, (

𝑤

ℎ

) ≥ 1,

𝐶 =

𝜀𝑟 − 1

7.9

𝑡

ℎ

√ℎ

𝑤

.

(7)

2. Materials and Methods

The process begins by polishing Teflon; this process is amodification of the common glass polishing process, and thusthe average surface roughness decreases from 500 to 30 nmRMS.

The polishing procedure is mechanical and performedby using aluminum oxide powder (about 1𝜇m diameter) inwater.TheTeflon is polished because of the friction between itand the polishing mixture, illustrated in Figure 2 that depictsthe polishing system used in this work.

After the polishing, the roughness of Teflon surface ismeasured with Atomic Force Microscope (AFM) and animage of the surface is shown in Figure 3. As can be seenin it, the average roughness is around 30 nm measured afterthe plasma treatment described below. The polishing stepis necessary because when the roughness decreases at theinterface metal dielectric, the Hammerstad coefficient alsodecreases.

In Figure 4, we have plotted the behavior of the Hammer-stad coefficient as a function of the values of the roughness forfrequencies up to 10GHz. As a comparison, Teflon withoutpolishing (ℎRMS = 0.5 𝜇m) is also showed, Teflon polishedwith our process (ℎRMS = 30 nm) and a typical value ofthe commercial PCB (ℎRMS = 1.8 𝜇m). With a roughness of30 nm, 𝑘𝑟 is almost 1.0 in the whole frequency range and itscontribution to the ohmic losses will be negligible.

To improve the Cu adherence, the next step is an argonplasma treatment on the polished PTFE surface.This process

Indian Journal of Materials Science 3

86.48nm

0.00 nm

Figure 3: Surface of the Teflon after polish.

0 2 4 6 8 100.91.01.11.21.31.41.51.61.71.81.92.0

Frequency (GHz)

1.8 𝜇m ℎRMS

30nm ℎRMS

0.5 𝜇m ℎRMS

kr

Figure 4: Plot of theHammerstad coefficient using (5) with differentvalues of roughness.

was done using a Reactive Ion Etching (RIE) system ofparallel plates; in Table 1 are shown the different conditions(Ar flow, pressure, and RF power) to obtain better adherenceto the polished PTFE surface. The whole process takes8 minutes. Then, a 0.5-micrometer-thick copper layer isdeposited on the PTFE by E-bean evaporation. Subsequently,the copper is thickened by electroplating. In this process, astandardCu solution is used and the 0.5microns of copper onthe Teflon is the anode and a copper plate is used as cathode.The current density is 0.94 amperes per square decimeter(A/dm2), and the Cu thickness increase to 25 microns afterone hour [5].

In this step of the process, one test is done, which iscopper peel strength testing. It is worth mentioning that forthe lowest flow (10 SCCM) in all cases there was not observedany adherence to the polished Teflon surface.

Table 1: Process conditions at the RIE for Cu adherence to thepolished Teflon.

Pressure (mTorrs) Power (watts) Flow (SCCM∗)30 250 3010 250 305 250 3030 250 2010 250 205 250 2030 250 1010 250 105 250 1030 150 3010 150 305 150 3030 150 2010 150 205 150 2030 150 1010 150 105 150 1030 100 3010 100 305 100 3030 100 2010 100 205 100 2030 100 1010 100 105 100 10∗SCCM is standard cubic centimeters per minute, a flowmeasurement termindicating cm3/min at a standard temperature and pressure.

3. Results and Discussion

Figure 5 shows the resulting Copper Peel Strength Tensile(CPST) on the polished Teflon surface after the different Arplasma treatments.

The test of copper peel strength was done with XLW (PC)Auto Tensile Tester; we can see this system in Figure 6 and thebest result was 3.93 pounds/inch (about 70 kg/m), which is asgood as some commercial PCB’s or better [6, 7]. This test isuseful for knowing the integrity of structures built in a PCB,but the minimum of pressure.

We can observe fromFigure 5 that the largest CPST valuesare obtained at the lowest pressures for all the treatmentconditions here used, and the best value for CPST is when thepower applied to the plasma is 250W. This applied power isthe same at whichH-HChien [8] observed the largest wettingangle on PTFE; the aforementioned author also notice thatlarger power applied to the plasma results in decrease in thewetting angle, whichmeans the surface of the PFTE is turninghydrophobic; the later means that there is a decrease in Cuadhesion. Another important fact to note is that in [9, 10] it isreported that larger treatment times (larger than 10 minutes)

4 Indian Journal of Materials Science

10 20 30 40

10

100

CPST

(Kg/

m)

Pressure (mTorr)

100 W, 20 sccmW, 20 sccm 100 W, 30 sccm

150W, 20 sccm

150

250

250W, 30 sccm

W, 30 sccm

Figure 5: Copper Peel Strength Tensile (CPST) on the polishedTeflon surface after the different Ar plasma treatments.

Figure 6: System to measure copper peel strength.

and higher applied power will result in increase of the averageroughness of the PTFE, property that in this work is keptconstant after the mechanical polishing and plasma processand is the main result here obtained. As a result, a betterperformance and minimum losses are the characteristics ofthe waveguides and devices built on this PTFE substrate.

4. Conclusions

In this work, a simple and low power plasma process forimproving the adherence of Cu to polished Teflon withoutincreasing the roughness is demonstrated. This result is incontrast to many current plasma processes for the samepurpose of increasing the adherence of Cu to PTFE that relyon roughening the substrate surface.

The best result of CPST here obtained is comparable tosome commercial substrates that use a composite dielectric;moreover, it was performed with a final average roughness ofonly 30 nm.

The best conditions for improving adherence are lowpressure and low plasma power and only 8 minutes of plasmaprocess is enough for achieving good results.

Competing Interests

The authors declare that there are no competing interestsregarding the publication of this paper.

Acknowledgments

The authors express their deep sense of gratitude to SvetlanaSejas Garcia and Isola Group for the measurements of theCPST.

References

[1] X. Liao, Surface modification of polytetrafluoroethylene (PTFE)with Vacuum UV radiation from helium plasma to enhance theadhesion of sputtered copper [M.S. thesis], Rochester Institute ofTechnology, 2004.

[2] R. K. Ulrich and L. W. Schaper, Integrated Passive ComponentTechnology, Wiley/IEEE Press, 2003.

[3] M. Hodsong and C. Guiles, “Everything you ever needed toknow to process PTEF Microwave and RF Printed CircuitBoards (Without local Anesthesia),” ARLON Technology Ena-bling Innovation, http://www.arlon-med.com/index.cfm?fuse-action=portfolio.sub-category&portfolioCategoryID=40002,https://imageserv10.team-logic.com/mediaLibrary/303/Every-thing you wanted to know about PTFE.pdf.

[4] M. Cauwe and J. de Baets, “Broadbandmaterial parameter char-acterization for practical high-speed interconnects on printedcircuit board,” IEEE Transactions on Advanced Packaging, vol.31, no. 3, pp. 649–656, 2008.

[5] C. F. Coombs Jr., Printed Circuits Handbook, McGraw-Hill, NewYork, NY, USA, 5th edition, 2001.

[6] P. Brooks, K. Johal, and C. Sparing,Can Peel Strength Predict theStructural Integrity of the Adhesive Bond between the Copper andLaminate in PCB? CPCA, 2004.

[7] 370HR Data Sheet, Isola Group 2014.[8] H. H. Chien, K.-J. Ma, P.-M. Chung, and C.-L. Chao, “The

study of surface modification of e-PTFE materals and theirapplications in micro-arrayed chips,” Chung Hua Journal ofScience and Engineering, vol. 6, no. 2, pp. 45–51, 2008.

[9] S.-R. Kim, “Studies on the surface changes and adhesion ofPTFE by plasma and ion beam treatments,” Korea PolymerJournal, vol. 7, no. 4, pp. 250–258, 1999.

[10] S. M. Pelagade, N. L. Singh, S. Mukherjee, U. P. Deshpande,and V. Ganesan, “Investigation of surface free energy for PTFEpolymer by bipolar argon plasma treatment,” Journal of SurfaceEngineered Materials and Advanced Technology, vol. 2, pp. 132–136, 2012.

[11] S. H. Hall and H. L. Heck, Advanced Signal Integrity for High-Speed Digital Designs, John Wiley & Sons, New York, NY, USA,2009.

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