Recently, I received a number of calls from engineers seeking low void solder paste to replace their existing paste, which is failing due to large volumes of voids at the interface. Solder paste is a great material, used suitably in various components. However, the material decision for a thermal interface is a critical one that should not be made simply because of the engineer's familiarity with the product or ease of application. These were the factors that influenced the engineers' decision to seek a new paste rather than shop for the best interface material to suit their needs.
Solder paste makes a great interface material for some customers, but it is not appropriate for applications involving very large surface areas. One engineer insisted on soldering down a heat sink with a surface area approaching 12 sq. inches. Another design was complicated by intentions of using paste to solder down a heat pipe that wound a distance of 4 feet altogether. Even a paste citing less than 4% voiding and a high metal load will still leave an incredible amount of voiding behind when spread over this large surface area.
Since these designers were already familiar with the performance of metal interfaces, they would not be swayed into synthetic solutions, so it was explained that the best way to move forward was with a fabricated interface material of solid metal. This was the preferred way to minimize voiding and this material was still simple to apply, providing even more consistent results than the paste they were working with.
This may seem like an obvious answer and you may be thinking to yourself, "Who in their right mind would use paste on this device anyway?" My motivation for writing this is to appeal that you ask the question, does this material suit my interface better than all other options, or am I using this because I am unwilling to further develop my process? If the answer is the latter, please take a few minutes and shop around before signing off on your next design revision.