Heat Sink1r

8/13/2019 Heat Sink1r

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Heat Sinks

Heat sinks have historically been the PCB workhorse for thermal management. They help keep

devices at temperatures below their specified maximum operating temperature. There are many

versions, different designs and various ways of optimizing heat sinks. Over time, the technology

has progressed with the use of new materials. For example, carbon fiber and boron nitride are

recent materials applied to heat sinks. High thermal conductivity fiber spreads heat well at 800

watt per meter Kelvin (W/m-K) in the direction of the fiber. However, at 0.5 W/m-K, it doesnt

spread heat up and down very well.

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

to the next. These crystals are used to salt carbon fiber sheets or prepregs. Two or more sheets

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

has been improved from 0.5 to about 4 W/m-K.

Due to their high cost, however, these materials will likely find limited use in future PCB fabrication

and may not replace aluminum heat sinks in many applications. Still, carbon fiber heat sinks may

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

spacecraft components, automobiles, high-end computers and medical equipment.

On the other hand, fin-based aluminum or copper heat sinks find greater acceptance in many

applications due to their low cost and ideal thermal dissipation characteristics (Figure 1).

Aluminum has a highly acceptable 205 W/m-K thermal conductivity, while copper is about twice as

high at about 400 W/m-K. Aluminum heat sinks are inexpensive; 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 isnt an issue.


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Most heat sinks are finned to provide a simple way of increasing surface area for heat radiation

and conduction. Some vendors claim their special aluminum fin material is 15 percent more

conductive than fin material used in competitive heat sinks. They assert the overall performance of

the bonded fin part increases and compensates for the minor conductivity loss from an epoxy

joint.

Carbon Composite Material

The industry is creating a buzz over the new carbon composite material that is thermally and

electrically conductive and primarily aimed at a PCBs ground planes. Its objective is to improve in-

plane thermal conductivity with its 3 to 6 parts per million per degree centigrade (ppm/C) CTE. It

compares favorably to traditional epoxy-glass and polyimide-glass-based materials with in-plane

CTEs ranging from 15 to 20 ppm/C.

This type of material has multiple benefits. For thermal conductivity, it is ideal for conduction

cooling and reducing PCB hot spots. Its low CTE permits the PCB designer to tailor the PCBs CTE

to match that of ceramic or flip chip packaging. Carbon composite material is rigid, thus

eliminating the need for stiffeners. Lastly, it is as light in weight as typical glass fiber composite

material.


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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. Since it is located close to the PCBs 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 locks.

Figure 2 shows the stack-up of two different boards. One is based on two-core construction and

the other on three-core. Layers number 2 and 9 in the two-core construction are thermally

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

number 3, 8 and a middle one designated electrically non-functional (NF) 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. Heat 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. Here, 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 5% - 10% greater PCB area dedicated to heat dissipation. Further, carbon

composite-based thermal vias can be used in conjunction with edge plating, discussed below, to

increase heat dissipation even more.


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

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

enclosure materials into an overall thermal management strategy. However, with todays drive to

portable electronics and more powerful chips, it is vital for EMS providers to forge all thermal

aspects of an OEMs design.

Thermoplastic materials represent a forerunner in this respect. They are making significant

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

cell phones, notebooks and other portable gear. Recently, various fillers have been used to further

improve thermoplastics with thermal conductivities in the highly acceptable area of 1 to 10 W/m-

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

Edge Plating

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

current. An 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. As

shown in Figure 3, the PCBs 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.

Edge 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. Moreover, edge

plating calls for experienced PCB fabricators who are equipped to implement special techniques to

plate a PCBs edge.

Copper Thieving

Aside 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. An example 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.

Adding copper to the digital side dissipates heat more effectively. For example, Figure 4 shows

thieving applied to such a PCB. In this instance heat dissipation increased by 20% to 25% after

thieving was applied.


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Implementing thicker copper traces on the PCB is another proven technique to spread and

dissipate heat. In these cases, regular 6 mil traces are increased to 8 - 10 mil traces, for example,

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

dont have impedance requirements. Copper traces on some applications can go even thicker, for

instance, 5 mil traces can be doubled to 10 mils as long as the PCB application allows for it and

there are no impedance control restrictions.

Finally, the seasoned PCB designer investigates even the least likely candidates to squeeze out as

much heat as possible. Two examples are tantalum capacitors and mounting holes. A tantalum

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

also takes advantage of the mounting holes on the board for heat spreading and dissipation. Thats

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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Heat Sink1r