PCB THERMAL MANAGEMENT

8/13/2019 PCB THERMAL MANAGEMENT

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New Materials and Techniques Tackle PCB Thermal

Management

New thermal management materials and techniques are being applied to subsystem PCB designs

to meet heat dissipation demands introduced by new generations of mostly analog and some

digital ICs that dissipate high current and high power. Thermal management focuses on effectively

dissipating heat generated by those high-power designs on high thermal conductivity and on

maintaining low coefficients of thermal e!pansion "CT#$ while managing CT# mismatches between

components their interconnects and the PCB.

Chipma%ers use a variety of pac%aging such as plastic ceramic flip chip leadless chip carrier

"&CC$ and wafer-level pac%ages "'&P$. #ach has a lower CT# than a standard PCB. Consequently

a CT# mismatch occurs between device pac%aging and the PCB. (epending on the application and

associated cost budgets PCB designers hardware engineers and PCB fabricators implement a

variety of different materials and techniques to deal with those CT# mismatches and manage

thermal issues.

'hile chipma%ers are doing their part to improve thermal management for their devices #)*

providers are placing special attention on thermal management issues at the PCB design level. In

these instances remedies range from using applicable board materials to paying special attention

to mounting holes. In between there are several new as well as tried-and-proven materials and

methods to improve thermal conductivity and heat dissipation.

The industry continues to patent new inventions and offers a variety of new thermal management

materials and techniques. There are too many to cover here. +owever those discussed here are

prominent ones in the tool %it of seasoned PCB designers and engineers. They include advanced

thermal modeling software new heat sin% material new in-plane high conductivity carbon

composite material special casing material and edge plating. Proven techniques include copper

thieving increasing trace thic%ness and ultimately e!ploiting even the mounting holes to

dissipate heat.

Thermal Modeling

New advances are being made to PCB thermal modeling and performance prediction software to

give hardware engineers and designers a head start on managing thermal issues. It,s an

invaluable tool to the PCB designer to assist him or her to quic%ly and easily understand the

relationships between components and how their placement affects a PCB,s thermal dynamics.


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Thermal modeling is used at the most basic levels of board layout mapping for investigating

system airflow heat sin% design and other cooling mechanisms. 'ith this tool PCB designers can

e!tend computer-aided design into prototyping and testing thus saving considerable time and

e!pense. lso designers can build a virtual prototype of the system and test the airflow and heat

distribution at both the board and the system level.

#qually importantly thermal modeling gives the PCB designer the critical tool for conducting

thermal fatigue failure analysis. In turn these analyses can be modeled to provide failure

prediction models. 'hile board failures may not occur for a period of time prediction models can

forecast when certain PCB materials will incur thermal fatigue and cause field failures.

Heat Sinks

+eat sin%s have historically been the PCB wor%horse for thermal management. They help %eep

devices at temperatures below their specified ma!imum operating temperature. There are many

versions different designs and various ways of optimiing heat sin%s. /ver time the technology

has progressed with the use of new materials. 0or e!ample carbon fiber and boron nitride are

recent materials applied to heat sin%s. +igh thermal conductivity fiber spreads heat well at 122

watt per meter 3elvin "'4m-3$ in the direction of the fiber. +owever at 2.5 '4m-3 it doesn,t

spread heat up and down very well.

(evelopers have applied boron nitride crystals as a way to efficiently move heat from one fiber ply

to the ne!t. These crystals are used to 6salt7 carbon fiber sheets or prepregs. Two or more sheets

are then laminated together to form the heat sin% material and throughput for up-down directions

has been improved from 2.5 to about 8 '4m-3.

(ue to their high cost however these materials will li%ely find limited use in future PCB fabrication

and may not replace aluminum heat sin%s in many applications. *till carbon fiber heat sin%s may

best be used in systems that don,t use air-cooling. These may include aircraft missile and

spacecraft components automobiles high-end computers and medical equipment.

/n the other hand fin-based aluminum or copper heat sin%s find greater acceptance in many

applications due to their low cost and ideal thermal dissipation characteristics "0igure 9$.

luminum has a highly acceptable :25 '4m-3 thermal conductivity while copper is about twice as

high at about 822 '4m-3. luminum heat sin%s are ine!pensive; copper ones cost more and they

weigh more. Consequently aluminum gets the nod for most cost-effective applications and copper

is used in selected ones where cost isn,t an issue.


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)ost heat sin%s are finned to provide a simple way of increasing surface area for heat radiation

and conduction. *ome vendors claim their special aluminum fin material is 95 percent more

conductive than fin material used in competitive heat sin%s. They assert the overall performance of

the bonded fin part increases and compensates for the minor conductivity loss from an epo!y


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Carbon composite material also performs as a built-in heat spreader and moves heat away from a

hot spot to colder areas of the PCB. *ince it is located close to the PCB,s surfaces there is a short

thermal path from the heat source to this material. The material then enables heat to move from

the heat source to the nearest carbon composite layer and from there to the chassis via mounting

holes or wedge loc%s.

0igure : shows the stac%-up of two different boards. /ne is based on two-core construction and

the other on three-core. &ayers number : and @ in the two-core construction are thermally

conductive layers using carbon composite material. In the three-core construction board layers

number = 1 and a middle one designated electrically non-functional "N0$ use carbon composite

material and serve as thermally conductive layers for heat dissipation.

If carbon composite material is implemented as multicore it effectively reduces PCB hot spots by

two to three times compared to its use as a single core. +eat is thus dissipated not only from the

component and solder sides but also through those layers discussed above. Instead of the

conventional two avenues with traditional in-plane material heat is now dissipated through four

different paths.

The material can also be used to augment PCB mounting holes to further dissipate heat from the

PCB surface through to the chassis to the ambient. It can also be used for thermal vias to re-direct

heat from hot spots to cooler PCB areas. +ere the material is used to plate vias shut thus

creating more volume to dissipate heat. Instead of leaving an air gap in between it is closed with

a thermally conductive material allowing heat to dissipate from the top of the PCB to the bottom.

The result is a 5A - 92A greater PCB area dedicated to heat dissipation. 0urther carbon

composite-based thermal vias can be used in con


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*pecial casing or enclosure materials are complementing those used in PCBs to further manage

heat dissipation. In some instances for various reasons PCB designers don,t factor system

enclosure materials into an overall thermal management strategy. +owever with today,s drive to

portable electronics and more powerful chips it is vital for #)* providers to forge all thermal

aspects of an /#),s design.

Thermoplastic materials represent a forerunner in this respect. They are ma%ing significant

headway by replacing or augmenting metal casings for small and increasingly thermally intense

cell phones noteboo%s and other portable gear. ecently various fillers have been used to further

improve thermoplastics with thermal conductivities in the highly acceptable area of 9 to 92 '4m-

3. The three general classes of filler used are carbon metallic and ceramic.

Edge Plating

*ome PCB applications are highly populated with components that generate high voltage or high

current. n effective thermal management technique is to connect them to the outside chassis

acting as ground. In these cases edge plating provides the best way to manage thermal issues. s

shown in 0igure = the PCB,s edge is connected through the chassis thus the entire chassis is used

as ground as well as a means to dissipate heat. The larger the area of edge plating the greater

will be the heat dissipation.

#dge plating while highly effective for dissipating heat is limited to certain military aerospace

and industrial applications that permit a large chassis to be used as ground. )oreover edge


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plating calls for e!perienced PCB fabricators who are equipped to implement special techniques to

plate a PCB,s edge.

Copper Thieving

side from these new methods and materials copper thieving is a well-proven PCB thermal

management technique. Thieving adds copper to board sections sparsely populated with copper as

a way to dissipate heat besides balancing the surface density. Its use is limited to large boards

with considerable copper density on one side and minimal on the other. n e!ample is a board

with analog circuitry and considerable copper on one side of the PCB and digital ICs and little

copper on the other side.

dding copper to the digital side dissipates heat more effectively. 0or e!ample 0igure 8 shows

thieving applied to such a PCB. In this instance heat dissipation increased by :2A to :5A after

thieving was applied.


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Implementing thic%er copper traces on the PCB is another proven technique to spread and

dissipate heat. In these cases regular > mil traces are increased to 1 - 92 mil traces for e!ample

by depositing larger amounts of copper. This technique is highly acceptable in applications that

don,t have impedance requirements. Copper traces on some applications can go even thic%er for

instance 5 mil traces can be doubled to 92 mils as long as the PCB application allows for it and

there are no impedance control restrictions.

0inally the seasoned PCB designer investigates even the least li%ely candidates to squeee out as

much heat as possible. Two e!amples are tantalum capacitors and mounting holes. tantalum

capacitor has resin fused through the lead frame that conducts and dissipates heat. The designer

also ta%es advantage of the mounting holes on the board for heat spreading and dissipation. That,s

done by using the mounting screws so that the heat on these outer planes can be spread around

to the outer casing or chassis. This provides a larger surface area for heat dissipation into the

ambient.

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PCB THERMAL MANAGEMENT