Accessing adhesive induced risk for BGAs in temperature cycling

BGA components in mobile systems are often found with adhesives applied at the package corners. The primary objective of these adhesives is to improve shock margins to prevent failures of solder joints in the case of sudden drop of the device. Different adhesives with a wide range of thermo-mechanical material properties have been in use with limited understanding and quantification of their impact on the temperature cycling reliability of the BGA. The objective of this study is to comprehend the impact of temperature cycle on adhesives applied in the corner of BGA components and to define a range of acceptable properties of adhesives for appropriate material selection. The study utilizes a computational mechanics based Finite Element Analysis (FEA) and a Response Surface methodology to perform a detailed numerical Design of Experiments (DOE). Results were validated with test data. The results of the study indicate that for many adhesives, accelerated temperature cycling tests can lead to wrong conclusions about adhesive performance in the field. Moreover, the impact of corner glue on SJ reliability in temperature cycling is strongly dependent on assembly parameters (board thickness, substrate thickness, pitch, etc.) which has not been typically accounted for in the past. In addition, impact of any individual glue thermo-mechanical material property (Glass transition temperature, Young's modulus, Coefficient of thermal expansion) cannot be considered independently of other glue properties. All these interdependencies across geometric and material properties, as well as the temperature range the overall assembly is exposed to have been accounted for in defining adhesive property limits that would not compromise BGA performance in temperature cycling.