CP30 Cold Plate
There was an article published recently in Thermal News on metal thermal interface materials (TIMs). The article, which was based on an interview of Amanda Hartnett from Indium Corporation, covered what differentiates metal TIMs from other interface materials, what properties justify the use of metal TIMs, and the importance of pressure when using compressible metal TIMs.
According to Indium’s website, “Thermal interface materials are useful for a variety of applications, but solder thermal interface materials (sTIM) are especially suited to high-end device cooling. To improve package reliability, it is especially important to choose the right alloy. Indium, in particular, should be considered as a sTIM because of its high thermal conductivity, compressibility (SMA-TIM), and ease of application.”[1] The article also noted that indium and indium alloys can have thermal conductivities as high as 86 W/m°K, offering better performance than standard thermal greases. (Note: All of the thermal greases we found on the web cited a thermal conductivity ranging from 0.8 W/m°K to 5.2 W/m°K.) In, InSn, and InAg TIM materials are either in the form of preforms or a “Heat-Spring” material. The preforms, which have flat surfaces, are punched parts in whatever shape and size is required for the application. The Heat-Springs can also be manufactured in virtually any shape and size, but have a textured pattern on the surface that helps compensate for imperfections in the thermal interfaces that are being mated together. The preforms and Heat-Springs both require pressure on them to significantly lower thermal resistance.
When selecting a TIM, there are numerous considerations including thermal performance, chemical compatibility, ease of application, cost, and amount of pressure required. If the TIM’s resistance is a small percent of the total thermal resistance, then the impact a higher performing metal TIM may have might be minimal and not worth the additional cost. Also, it’s important to ensure that your cold plate can handle the amount of pressure required. Lytron’s CP30 cold plate, which is a standard aluminum vacuum brazed cold plate, is aged to a T6 temper. This hardness allows the tapped holes in the cold plate to withstand the forces required to utilize various metal and pre-form TIM materials. For instance, in an IGBT application, an Infineon Prime Pack 3 module utilizes 14 M5 screws to evenly compress the module and TIM layer to the surface of the cold plate. If plain tapped holes are used in the cold plate, the module and TIM layer can be compressed to up to 55 psi. If additional surface pressure is desired, a custom cold plate solution can be designed with helical thread inserts to maximize the strength of the threads.
- Justin, Engineer
[1] Indium Thermal Interface Materials, Product Data Sheet, Form No. 98134 R2, http://www.indium.com/TIM/information/
