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Using High Frequency PCB Laminates for Improving Thermal Management Issues

John CoonrodRogers Corporation, Advanced Circuit Materials

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• Basics of thermodynamics as it relates to PCB’s

• Laminate properties that are critical to thermal issues

• Thermal management of PCB applications

• Summary

Outline

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Basics of thermodynamics as it relates to PCB’s

• Three general types of heat transfer:

• Radiation

• Convection

• Conduction

• PCB applications typically are most concerned with Conduction, however:

• some PCB designs can have concerns with radiation

• convection can influence PCB thermal issues

• Heat conduction is a transfer of energy arising from temperature difference between adjacent parts of a body

PageBasics of thermodynamics as it relates to PCB’s

• Assuming a thermal steady state, heat flow (H) is defined as:

• Simple example of heat flow with an insulated metallic rod

A is area, k is thermal conductivity, dT/dx is temperature gradient

dT/dx is change in temperature, with a change in distance


• How the simple heat flow example relates to a PCB

• Assuming a microstrip (double sided PCB) with via farm and a heat sink attached

Assumptions for this example:Heat is generated at the signal planeThe ground plane and heat sink are the same temperature


• Other things to consider with the microstrip example:

• Via farms are typically used with a ground plane under an active device

• Generally via farms can not be used on an active signal conductor

• Trace heating of an active signal cools by heat flow through the substrate

• Cooling of trace heating is improved by:

• Substrates with high k(thermal conductivity)

• Minimizing losses

• Using thin substrates

• Smooth copper

Same as previous example, except without via farms

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Laminate properties that are critical to thermal issues

• Thermal conductivity

• Low dissipation factor

• Smooth copper surface profile

• Low dielectric constant, not critical, but can be helpful

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• Thermal conductivity (TC)

• Unit is W/m/K

• Most common PCB circuit materials have low thermal conductivity

• Some high frequency PCB laminates have higher thermal conductivity

Laminate properties that are critical to thermal issues

High Tg FR-4 0.24 W/m/K

Nearly pure PTFE laminate 0.20 W/m/K

Ceramic filled PTFE laminate 0.50 W/m/K

RO4350BTM Laminate 0.62 W/m/K

RO4360TM Laminate 0.80 W/m/K

RT/duroid® 6035HTC laminate 1.44 W/m/K

Typical TC values

PageLaminate properties that are critical to thermal issues

• Dissipation Factor (Df)

• Unit-less

• Df affects dielectric losses

• A smaller Df number is better

• Having lower dielectric loss translates to lower insertion loss

• Lower insertion loss will generate less heat

High Tg FR-4 0.020

Nearly pure PTFE laminate 0.001

Ceramic filled PTFE laminate 0.002

RO4350BTM Laminate 0.0037

RO4360TM Laminate 0.0038

RT/duroid® 6035HTC laminate 0.0013

Typical Df values


• Copper surface profile

• The surface of concern is at the copper-substrate interface of a laminate

• Unit is usually microns and describes the RMS (root mean square)

• Copper surface roughness affects conductor losses

• Smoother copper yields lower conductor loss, which translates to lower insertion loss

• Lower insertion loss will generate less heat

• Conductor losses and the effects of copper roughness are frequency dependent

• Conductor losses are also substrate thickness dependent

Worse Case Scenario: Thin laminate used at very high frequency, with a rough copper surface profile


• Low dielectric constant

• Generally dielectric constant is not directly related to thermal issues

• Low dielectric constant allows for a wider conductor

• A wider conductor will have less conductor losses

• Less conductor losses means less insertion loss and less heat generated

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• Thermal Management of PCB Applications

• Thermal conductivity study using microstrip circuits

• Study setup, circuit designs and test procedures

• Summary of the results

• Power capabilities of circuits based on thermal management

• High frequency effects regarding thermal management

Page• Thermal Management of PCB Applications


• A common termination resistor was used:100 ohm SMT made by ATC part # CS12525TO100G

• Resistor had a heat transfer pad on the bottom side as shown below

• DC power was used to heat the resistor which was soldered to the circuit

Grey is solderable areas



• Heating a component with DC power was done to eliminate the high frequency variables associated with the different laminates

• Heating a discrete component simplifies the thermal models

• Two simple microstrip circuit designs were used

• One design with via farms (top circuit) and one without



• Microstrip circuits were fabricated using several different laminates

• All circuits had:

• the same design features

• 20mil laminates

• 1/2oz copper plated up to approximately 2mils

• The circuits were then laminated to a 1/8” thick aluminum plate

• The lamination used a thermally and electrically conductive adhesive

• The SMT resistor was then soldered to the circuit

• The 1/8” aluminum block was a mounting plate to be used with a water cooled heat sink


Mounting plate with circuit laminated to it

Water cooled heat sink


• Overall testing configuration, without the heat sink shown


High Tg FR-4

RO4350BTM Laminate

RT/duroid® 6035HTC laminate

229ºF109ºC

180ºF82ºC

143ºF62ºC

84ºC rise above ambient



Summary of Results

Thermal Imaging

Circuits with No Via Farm

Page• Thermal Management of PCB Applications Summary of Results

Thermal Imaging

Circuits with No Via Farm


Thermal Imaging

Comparison: Circuit with No Via Farm and with Via farm

Circuit using RO4350BTM laminate with No via farm @ 5 Watts

202ºF 109ºF

Circuit using RO4350BTM laminate with via farm @ 5 Watts


Thermal Imaging

Circuits with Via Farm

• Even though via farms improve the thermal management significantly, there is still benefit of materials with high TC


• Power capabilities of circuits based on thermal management

• Circuits with higher TC material can have higher power capabilities

• MOT (Maximum Operating Temperature) of a PCB is the maximum temperature that the circuit can be operated for an indefinite period of time

• Circuit MOT ratings are based on:

• Circuit materials RTI (Rated Thermal Index)

• Circuit construction

• Circuit processing

• Circuit facility

• The following example will assume a PCB with a MOT of 105ºC

• And assuming ambient temperature of 25ºC, the circuit should not be allowed to heat more than 80ºC

Page• Thermal Management of PCB Applications• Power capabilities of circuits based on thermal managementCircuit models were 20mil thick microstrip, 50 ohm transmission line using 2oz copper, at 800 MHz

• The circuit with the higher TC can be used at higher power without violating the MOT

This chart is theoretical and based on a very conservative thermal model

Page• Thermal Management of PCB Applications• Power capabilities of circuits based on thermal managementCircuit models were 20mil thick microstrip, 50 ohm transmission line using 2oz copper, at 2 GHz

• Frequency does play a critical role

• A higher frequency will develop more heat at a given RF power level

•The circuit with the higher TC can be used at higher power without violating the MOT

This chart is theoretical and based on a very conservative thermal model


• Summary

• PCB material plays a significant role in thermal management issues

• Important material properties:

• Thermal Conductivity (TC)

• Dissipation Factor (Df)

• Copper profile roughness

• The overall insertion loss has an important impact on the thermal management

• Via farms improve thermal management but are limited to ground features

• Trace heating can be improved with materials that have high thermal conductivity and low loss

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The information in this presentation is intended to assist you in working with Rogers' High-Frequency Materials. It is not intended to and does not create any warranties, express or implied, including any warranty of merchantability or fitness for a particular purpose or that any results show in this presentation will be achieved by a user for a particular purpose. The user is responsible for determining the suitability of Rogers' High Frequency Materials for each application.

Prolonged exposure in an oxidative environment may cause changes to the dielectric properties of hydrocarbon based materials. The rate of change increases at higher temperatures and is highly dependent on the circuit design. Although Rogers’ high frequency materials have been used successfully in innumerable applications and reports of oxidation resulting in performance problems are extremely rare, Rogers recommends that the customer evaluate each material and design combination to determine fitness for use over the entire life of the end product.

This presentation may not be reproduced, copied, published, broadcast, transmitted or otherwise distributed without the written approval of Rogers Corporation.

©2010 Rogers Corporation. All rights reserved

RO4350B, RO4360 and RT/duroid are licensed trademarks of Rogers Corporation

The world runs better with Rogers. and the Rogers' logo are licensed trademarks of Rogers Corporation

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