Seth Homer: As we've discussed, there are three attach levels of peak concern in the IGBT stack up. At Indium Corporation, we're redefining how we use solder at the die-attach, substrate, and baseplate to heat-sink levels, so we can achieve a more reliable IGBT that can perform increasingly higher standards. If you missed the first two videos in this series, be sure to check them out at www.indium.com/IGBT.
Today we're focusing on the substrate level of the stack up. The substrate can have multiple die attached to it, making the solder bond extremely critical. This level comes with its own specific challenges to overcome so that performance and reliability are preserved. Similar to the challenge of bottom termination components, the substrate is prone to voiding. Many of these substrates are larger than 45 millimeters in the X and Y. To ensure the best result, many assemblers opt for a fluxless vacuum reflow solution and a reducing atmosphere to achieve the lowest void result; however, even in this case solder quality can adversely affect the voiding.
Alloy purity and percentages are very important, as well. In cases where fluxless vacuum reflow is not available a flux might be needed. This is tricky, due to the fact that volatiles can also attribute to voiding. It is for this reason that a highly engineered flux coating is the best approach. In addition to voiding, bondline co-planarity is also critical. Given the larger component liquidous solder, may not be able to support the components weight evenly during the reflow process. This can result in an uneven bondline, which will increase areas of stress during cycling.
So, how can you achieve superior performance at the DBC substrate to baseplate level? The answer is to add a reinforcement matrix to the solder to act as a stand off. This creates constancy in the bondline thickness and adds strength laterally, resulting in enhanced thermal cycling survivability. Indium Corporation’s InFORMS® are reinforced solder preforms that are specifically designed to preserve the bondline co-planarity and add strength to the joint, increasing reliability to the process.