Heat sink attachment posts are a little like watching paint dry: it’s boring but a necessary evil. What’s to worry about right? You slap your heat sink down and go. It should require as much analysis as choosing the New England Patriots to win the Superbowl this year!
But this kind of approach can actually limit a key part of your thermal management solution. The attachment matters and in some cases may improve your heat sink’s performance 20% or more. That translates into the very important topics of: better system reliability, lower cost of the thermal management solution, and optimal system performance since it can operate at the right speed. This will be part one of a two part series. In this part we’ll cover thermal tape, epoxy and wire form”z-clips”. In part two we’ll cover clips, push pins and stand offs and have a special offer for our readers to get all this information in a handy download. So, let’s get into this.
There are six main methods you can use:
- Thermal Tape
- Epoxy
- Wire Form “Z-Clips”
- Clips
- Threaded Stand-offs (PEMS) and Compression Springs
- Push-Pins and Compression Springs
Let’s talk about the pros and cons of each of these, and provide some photos so, if your unfamiliar with these, you can see what they look like and how they work.
Thermal Tape: This is a pressure sensitive adhesive tape, generally acrylic. The benefits are mainly that it’s generally easy to attach and it’s inexpensive. It’s also the lowest cost approach for aluminum heat sink attachment. But there are drawbacks. For heavier heat sinks, thermal tape generally doesn’t secure a heat sink strongly. That same drawback can effect you if you are going through NEBS testing, tape generally has a problem securing a heat sink well in those environments. Also, you have to ensure the surface of the heat sink and the chip are very clean so that the tape bonds well and can fulfill its dual role of affixing a heat sink securely to your chip AND acting as thermal interface material. Speaking of thermal interface, tape is generally considered a moderate to low thermal conductor.
Thermal Epoxy: This is a material that is either single or dual-part mixed adhesive with thermally conductive fillers like silver. It has outstanding mechanical adhesion: even copper heat sinks will not move off the semiconductors they are attached to (or any other surface for that matter if you place your epoxy laden heat sink down on your bench for some reason). It’s also fairly inexpensive costing a bit more than tape. There are drawbacks. First, it has to be refrigerated. Second, it takes skill to apply it so that you don’t put on too much or too little, after all, you are using this material BOTH to secure your heat sink AND to act as thermal interface material to conduct the heat from your semiconductors to your heat sink. Second, all surfaces must be squeaky clean, like with thermal tape. And finally, hope you never have to rework your board because epoxy is hard to remove and often times will destroy your boards. That’s right. Its glue! One way to loosen that glue up is by heating it using a heat gun or thermal welder, (Kamweld, our sister company, has some nice ones).
Wire-Form “Z-Clips”: Basically this a cheap mechanical attach method that often works well. A stainless steel wire formed clip is used to both compress the heat sink onto the chip and to hold it down. Key? You need to use thermal interface material (TIM) on that bottom of that sink in order to have the right heat transfer from the chip to the heat sink. Why would you use a Z-Clip? For one, it provides a strong and secure mechanical attach. For shock and vibration environments such a strong mechanical attach is a must. Second, this kind of attachment is easy to apply and easy to remove PLUS it’s non destructive to the semiconductors as opposed to epoxy and occasionally tape. Finally, it applies a preload onto the TIM and that preload actually improves thermal performance. All this being said there are few downsides. First, it requires engineering to develop and use, and that can add development cost to your programs development budget. Second, it can take a few steps in assembly, marginally increasing the assembly time.
An advancement in this space can been seen at our colleagues at Alpha Novatech with their “Quick Set” series. In a nutshell, Quick Set requires a small mounting pins be placed onto a PCB. Mounting pins only require 1.8mm[.071″] diameter holes in the PCB. Then a heat sink can be clipped on creating a secure attachment that uses minimum space. It also appears it can handle oversize heat sinks too, a real boon with large footprint semiconductors.
So that’s part 1! In part two we’ll cover “standard” clips, push pins and stand offs and have a special offer for our readers to get all this information in a handy download. To reach part 2 please click here, “How to Choose a Heat Sink Attachment Method to Secure Your Heat Sink and for Optimal Heat Transfer (part 2 of 2)”
If you’re looking for a solution on heat sink attachment and need to bounce this off us, feel free to! ATS sells various heat sinks attachment options, of course, and we’ll talk about that next time, but we’re happy just to discuss your options in your application, so call us at 781-769-2800, email us at sales.hq@qats.com or visit our website at http://qats.com