Effect of potting on the solder joint reliability of QFN packages under thermal cycling load

The potting material acts as a buffer between the component and the external environment, absorbing thermal and mechanical stresses that would otherwise be transmitted directly to the solder joints. This research is aimed to understand the effect of potting on the reliability of solder joints by performing thermal cycling tests on QFN package covered with four different potting materials. CTE mismatch between potting material, package epoxy molding compound (EMC) material and PCB will cause stresses and deformation in the structure and thus inflicting stresses on the QFN package. Tensile stresses on solder joint lead to cracking and complete fracture over time. QFN stress boards consisting of 8 QFN packages for each potting material were designed and manufactured for this study. The purpose of the test vehicle is to evaluate different potting materials and their impact on solder joint under thermal cycling load of −40 °C to 125 °C. Thermal cycling test results showed that potting can have negative impact on the fatigue life of the solder interconnects and QFN packages with two different rather stiff potting material show early failures as well as reduced characteristic life compared to reference QFN without potting. Despite having very high CTE (300 ppm/°C) for silicone gel based potting material, zero failures have been reported for QFN packages encapsulated with silicone-based material. Detailed Finite element models are also developed to understand the effect of various potting materials and to understand the failure mechanism. Modelling results reveal that Gel like behaviour of silicone-based potting material suppresses thermomechanical stresses acting on QFN solder joint and consequently reduces creep strain at critical solder joint. In summary, potting can have either positive or negative effect on the reliability of solder joints under thermal cycling load and it is important to carefully select the potting material by assessing application specific details such as geometry, involved materials and their temperature dependent properties and other environmental conditions.