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Discussion of two ways to pull heat from a circuit board.
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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