Effect of thermal cycling on the solder joints' reliability of ceramic ball grid array

Abstract

Solder joints are known as one of the weakest elements of microelectronics assemblies. The integrity of solder joints in most microelectronics assemblies is of concern in systems where reliability is of great importance since the stresses experienced by the joints during service can lead to premature failure. This is mainly because the solder joints often constraint materials of different coefficient of thermal expansion (CTE) that imposes cyclic strain (fatigue) through the joints when thermal fluctuations are encountered. In addition, the high homologous temperature at which solder operates at constant stress, makes creep predominant in the solder joints. Due to viscoplastic nature of plastic, large creep and plastic deformations accumulate in a solder joint during thermal cycling, which eventually lead to thermal fatigue failures [1, 2]. Therefore, such fatiguecreep interaction effect in microelectronic packages must be considered when evaluating the reliability of solder. The reliability assessment of solder joints under creep-fatigue effect can be carried out by thermal cycling (TC) test. TC imposes cyclic strains over the solder joints of electronic components. Made by surface mount technology (SMT), which may cause the joints to fail by thermo-mechanical fatigue [3]. Pb-Sn solder used in microelectronics is expected to function at relatively high homologous temperatures over a wide range of temperatures. This is especially true for space and avionics applications where the solder joints may be required to endure extreme thermal fluctuations of −62 ◦C and 140 ◦C which correspond to 0.46TM and 0.91TM respectively for eutectic Pb-Sn alloys. The focus of this project is to study the failure characteristics of solder joints in ceramic ball grid array (CBGA) assembly that have been subjected to thermal cycling. Metallographic sections and fracture surfaces of the failed solder joints were investigated using both optical and scanning electron microscopy (SEM). The most likely location to fail first was identified by calculation of the strains imposed upon each solder joint by the following formula: