Evaluation of Direct Bond Aluminum Substrates for Power Electronic Applications in Extreme Environments

Power packages that require large current capacities typically employ some form of thick conductive traces attached to a thermally conductive ceramic material to create a suitable package substrate. The most common substrate currently used in high power applications is Direct Bonded Copper (DBC). Though this is a well established, reliable, and commonly used substrate, DBC suffers from poor long term mechanical reliability when exposed to extreme temperature excursions. In an attempt to improve on this technology, substrate materials such as Active Metal Bond / Braze (AMB) and Direct Bonded Aluminum (DBA) are being investigated. Previous work has shown that the accelerated aging / thermal shock lifetimes of DBC and AMB are significantly shorter than that of DBA substrates. Though DBA substrates last longer, they still have some issues that require attention before it can be accepted as an improved alternative to DBC substrates in these types of applications. The main issues that have been observed are DBA's increase in surface roughness during aging and aluminum's poor solderability when compared to copper or nickel. The emphasis of this paper is to investigate the dramatic increase in DBA's surface roughness and its' possible causes due to thermal cycling as well as present a thermal cycling lifetime comparison of the three different substrate. To evaluate this, a DBA sample with one side raw aluminum and one side electroless nickel plated (high phosphorous) was thermally shocked from −40°C to 200°C with surface roughness measurements preformed every 300 cycles. Another batch of samples was thermally shocked to 6000 cycles and lifetimes were compared. One nickel plated DBA sample shocked to 4000 cycles was cross-sectioned and analyzed with SEM and EDAX to evaluate any changes in the metal. The grain structure of a thermally cycled sample was also examined with a Scanning Electron Microscope (SEM).