TCoB reliability for Epad LQFP 176 in automotive application

Exposed pad in QFP packages were introduced in the late 1980s and become more and more popular in Automotive industry because of their excellent thermal performance and relative lower cost with higher pin count. However the Thermal cycling on Board (TCoB) reliability for exposed pad for QFP had been a challenge as the requirement become more and more stringent in Automotive industry especially in safety device application. Currently, there are not many studies are available on QFP platform as the Gullwing lead on QFP can perform better than solder ball (BGA platform) & Leadless (QFN platform) due to the Gullwing lead profile provide the flexibilty (mechanical spring effect) to obsorb the thermal mismatch better during TCoB stress. But this scenario had been change as the Automotive TCoB requirement had been significant increase in safety device application. In this work, 3 different low alpha mold compound type are selected for assessment based on the knowleage and understand in order to meet stringent customer requirement. Prior to actual experiment, the TCoB performance for each mold compound type had been simulate and predict by an establish simulation model. The simulation model is based on Robert Darveaux´s model [1] for BGA solder fatigue life prediction using full 3D FEA modelling. The experiment was conducted on LQFP176 Epad package with the body size of 24 × 24 × 1.4 mm³.[see fig 1 & 2] Along with the experimental run, a TCoB solder joint failure for LQFP176 Epad was found before the targeted TCoB performance, this results does not correlate with the simulation result; Extensive failure analysis was carried out to confirm the solder joint failure mechanism. Technical hypothesis was generated to identify the key influencing factor and their impact on the package TCoB solder joint failure. The simulation mode was then carried out to understand the mechanical degradation in the solder joint failure based o the Failure ana- ysis result. The investigation indicate that beside the well know critical factor, the TCoB lifetime is also very sensitive to the package die pad delamination, when serious die pad delamination is present, the thermal mechanical stress can break the Lead frame connecting tie bars and lead to lateral thermal expansion mismatch at solder joint to increase where majority of the stress will transfer to solder joint. As TCoB stress is a long cycle stress, it is recommended to optimize delamination performance before subject to actual TCoB stress, this is able to mitigate the risk and save a lot of resources and time. Lastly, the TCoB reliability design guidelines for exposed pad package are proposed to improve the solder joint reliability in order to meet customer stringent requirement.