If You Can’T Take The Heat Pdf

President/CEO USTEK Incorporated, Columbus Ohio

Technology Commercialization and Specialty Products

Author: Designing for Thermal Conductivity

Contributing Author:

Whittington’s Dictionary of Plastics

Handbook of Filler and Reinforcements

Polymer-Plastics Technology and Engineering

Numerous seminars in the US, Europe, and the Pacific Rim

Dealing with Heat on Printed Circuit Boards

Typical PCBs are built in layers and

are anisotropic:

Their properties vary with direction.

• Flex strength 450 Mpa

• Tensile strength 310 Mpa

• Tg 120°C

• CTE 14 ppm/°C

• S.G. 1.85

• Tc 0.24 W/m-K

Electrical Resistivity Heat Conductivity

Glass 1.0*10+14 M-ohm cm 0.95 W/m*K

Epoxy 1.0*10+13 M-ohm cm 0.23 W/m*K

FR-4 1.0*10+08 M-ohm cm 0.30 W/m*K

Sn-solder 1.1*10-07 M-ohm cm 60 W/m*K

Aluminum 2.8*10-08 M-ohm cm 250 W/m*K

Copper 1.7*10-08 M-ohm cm 400 W/m*K

• Parallel to the copper plane

• Parallel to the glass fibers

• Perpendicular to the laminate mass

• Perpendicular along the PTH/via route

• Arrange vias under components

• Dense grid pattern through the board

• Drill holes 0.2-0.3mm on 0.6-0.7mm center

• Fill with solder, not mask

The time rate of heat transfer through a material is proportional to

negative temperature gradient and to the area at right angles to that

gradient through which the heat is flowing.

Newton’s law of cooling and Ohm’s law are analogues to Fourier’s

The time rate of heat transfer through a material is calculated:

q = -k*T

q = heat flux, k = thermal conductivity, T = temperature gradient

QPCB

= QLYR-1

+ QLYR-2

+ QLYR-3

+ etc.

Q = TL / (k * A)

L = layer thickness

k = thermal conductivity

A = area normal to heat source

(there is an additional resistance at each junction)

Rhs = (DT / Pth) – Rs

Rhs = max thermal resistance (of the heat sink) in °C/watt

DT = temperature drop in °C

Pth = thermal power applied in watts

Rs = thermal resistance of heat source in °C/W

Example:

Component generates heat = 250 W

Component’s internal thermal resistance = 0.5 °C/W

Max allowable temperature increase (DT) from 25°C to 125°C = 100C°

Max heat sink thermal resistance = 0.1 °C/W

• Choose high conductivity elements

• Minimize thickness of resistive elements

• Maximize area exposed to heat producer

• Consider routes for heat dissipation

• Replace epoxy/glass with a metal

• Minimize thickness of dielectric

• Maximize area exposed to heat producer

• Consider routes for heat dissipation

• 0.5-.07mm Al vs 1.5mm epoxy glass

• Increases conductivity of substrate

• Reduces thickness of thermal barrier

• Exposes heat source to convection

• Reduces heat resistance from 700°C/W to 5.3°C/W

• ENIG finish for flat pads

• High power devices on daughter board with stand-offs

• Stand-offs should be solder plated or solid brass

• Stand-offs flush riveted or spun welded if attaching to a

heatsink

• Dense boards not easily de-populated

• Reflow oven profile to control ramp-up and cool-down

• Reflow more uniform than across an FR-4 board

• Oven exit should have chill plate & forced airflow

• H2O wash solder paste and SN100C paste preferred

• In general - no surprises. Learning curve is short.

Thin Arrays

ENIG

Bare Board Post-assembly forming

RoHS Pb-free

Thick Arrays

USTEK supplies FR-4 and variants as well as aluminum substrate PCBs,

and extruded heatsinks. Phone us to discuss design options for

enhancing your product performance and reducing total cost.

Send us your specifications and

Gerbers when you get a-round-to-it:

E-mail: Info @ ustek.com

Phone: 614.538.8000

Fax: 614.538.8002

Mail: 4663 Executive Drive #3, Columbus, OH 43220