Indium Alloys, Metal Thermal Interface Materials Posted by Karthik Vijayamadhavan Monday, October 15th, 2007

Usage of In-alloy Thermal Interface materials in the burn-in stage:
Lets define a burn-in application as one where the package is tested for functionality. In this process, every package makes contact with a heat sink/spreader for heat dissipation. Since the package cannot make direct contact with a heatsink, a Thermal Interface Material (TIM) is used to fill the air-gap and increase the rate of heat transfer. Essentially there would be multiple insertions of multiple packages to the TIM. Maximizing the number of package insertions a Thermal Interface Material can withstand directly impacts the costs and time involved. The more the number of insertions the thermal interface material can withstand, the lesser money and time it takes to replace the thermal interface material.

Metal Thermal Interface Choices
Solder Thermal interface materials (STIM) preforms such as In and In-containing alloys that are very soft and with high thermal conductivities of 60-87 W/m.K are good Thermal Interface material choices. Grease apart from low thermal conductivities of 5-15 W/m.K is prone to bleed-out in short period of time with multiple package insertions.

Attaching the thermal Interface to the heatsink
The STIM could be attached to the heatsink with an adhesive / by reflowing it onto the heatsink. The adhesive could be applied to the heatsink/STIM to facilitate STIM attach to the heatsink. Care should be taken when selecting an adhesive so that the thermal resistance does not drastically increase. If a reflow process was used to attach the STIM to the heatsink; an appropriate low-voiding flux, the right metallization for the heatsink (probably Ni/Au or Ni/Sn), inert reflow atmosphere like nitrogen; are all factors that need to be optimized. Another factor to be considered is the ease of replacing the existing STIM (once it has seen the maximum number of package insertions) with a new one. An adhesive may be an easier option here as the existing STIM just needs to be peeled off and a new one can be slapped in place with fresh adhesive. In a reflow process, the Solder Thermal Interface Material forms an intermetallic bond with the heatsink. So it may have to be scraped off. In the case of 100In, a new STIM preform could be cold-welded to the existing one.

Things to look out for during the insertion process when using Solder Thermal Interface Materials – Oxide builds up on the solder surface of STIMs over time. When the package makes contact with the Solder-TIM, an oxide mark is transferred onto the package surface. Even though there is no performance degradation, it is a cosmetic defect. A perfectly good functional package may have to discarded because of this. Appropriate alloys have been used to eliminate this oxide buildup and subsequent transfer. – After multiple insertions of the package with the STIM, a soft alloy like In is going to get squished and accumulate on the perimeter. If the package has components like passives on its surface, and there is contact between the passives and STIM, there are shorts. So the solder dimensions and number of package insertions need to be optimized.

Posted by Karthik Vijayamadhavan at 12:00 PM (October 15th, 2007)

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