A simple moisture diffusion model for the prediction of optimal baking schedules for plastic SMD packages

The moisture concentration at the chip surface is the important parameter for the moisture sensitivity of the P-MQFP80 product considered here. When the critical moisture concentration at the die surface is reached, delamination occurs after soldering shock, e.g at 240°C. This critical moisture concentration, which can be determined by experiments conducted at 30°C/60% relative humidity (RH) followed by soldering shock, allows to predict the product's moisture performance at other ambient conditions. In the case studied here, prediction was done at a customer use condition of 30°C/85% RH. Furthermore, this work showed that preconditioning of plastic packages not only induces the onset of delamination at the die surface but it appears to weaken the adhesion at this interface as well. As a result, delamination failure starts to occur earlier (i.e. within shorter moisture exposure time) in the devices tested after subsequent thermal cycling stress test. A simple moisture diffusion analytical model is proposed here for predicting the optimal baking schedules for plastic SMD packages.