Design Considerations When Using Heat Pipes (Pt. 2)
Now that we’ve determined which heat pipes can effectively carry the required heat load (Pt 1), we need to ensure we select and size the heat exchanger to dissipate the heat into the surrounding air -based on available air flow. As mentioned in the prior post, this article is not intended to provide detailed training/equations on the proper design of heat pipes and heat sinks, but rather to offer guidance on the number and size of heat pipes used as well as to provide some tips for estimating heat sink size and determining attachment methods of the sink to the PCB.
Types of Heat Sinks
There are numerous choices from zipper pack fins to extruded fin stacks, each with their own cost and performance characteristics. While the heat sink choice can markedly affect heat dissipation performance, the biggest performance boost for any type of heat exchanger comes with forced convection.
Estimating Required Heat Sink Volume
By using the simple equation below, we can estimate the overall volume of the heat sink required to cool a heat source.
V= (Q*Rv)/Delta T
Volume is the outer dimensions of the heat sink. Q is power in watts and Rv is the volumetric thermal resistance (cm3-C/W). Table X provided some
Let’s say we have a 50W heat source (Q) who’s specified maximum operating junction temperature (Tjunction) is 100o C. Further, let’s assume we’re designing this device to operate in a max ambient temperature of 70o C – a rugged environment. By subtracting the ambient temperature from the maximum allowable electronics temperature, you arrive at the delta t: in this
If we are designing a rugged computer, for instance, we’ll probably have a semi or fully sealed enclosure meaning that our heat sink needs to be designed for natural convection. The resulting heat sink volume would be:
V = (50*500)/30= 833 cm3 = a really large heat sink of 9.4 x 9.4 x 9.4 cm. Obviously, we wouldn’t have a heat sink with 9.4 cm high fins in a natural convection environment so the length and width dimensions would have to grow substantially. But, we might surmise that this application would probably need to use the chassis lid as the heat sink.
There’s an example of just such an application that I talked about in a prior post on ruggedized electronics and whose heat load, ambient operating
Had this same heat source
Although beyond the scope of this article, the next steps in heat sink design would be some detailed excel modeling followed by CFD analysis. This will enable engineers to fully understand optimal base thickness, fin pitch, fin height, and base/fin material among other things.
Mating Heat Pipes to the Condenser
Whether we’re dealing with a heat exchanger that is local or remote to the heat source, the options for mating a heat pipe(s) to them are identical.
It should go without saying that simply soldering
The heat sink in Figure 2 uses both a local and remote heat sink. The extruded heat exchanger
Had the pipe been completely round at the heat source, a thicker grooved mounting plate would have been required as seen in Figure 3.
If conduction losses due to the base plate and extra TIM layer are still unacceptable, further flatting and machining of the heat pipes
Attaching the Heat Sink to the Heat Source
The primary reason for considering a heat pipe solution is improved performance. As such, I’m not going to talk about the use of thermal tape or epoxy as the primary means of attaching the heat sink to the die. We generally stick with one of several mechanical attachment methods where we can meet Mil or NEBS shock and vibe requirements. Additionally, the thermal resistance of thermal interface material (TIM) improves as
An inexpensive attachment method for small (low mass) heat sinks are stamped mounting plate. Although it requires two PCB holes, this method offers better shock and vibe protection thermal tape or epoxy and some TIM compression – although still only 5 PSI.
Figure 6 shows spring loaded plastic or steel push pins further increase TIM compression up to around 10 PSI. Installation is fast and simple but removal requires access to the back of the PCB. Push pins should not be considered for anything more than light duty shock and vibe requirements.