Backward compatibility of solder alloys with chip scale package

Transition to Pb-free has necessitated the requirement for study on the reliability of mixed assemblies. Mixed assemblies are bound to be used during the transition period and beyond the deadline as well. Processing and reliability issues with Pb-free manufacturing have forced the exemption of certain product categories from the legislations banning the use of lead. Hence, the use of mixed assemblies in electronics manufacturing is inevitable, wherein some assemblies will continue to use traditional eutectic Sn-Pb solder paste while others will use Pb-free solder paste. Beyond the deadline the assemblies that have been exempted from the lead-ban will continue to use the eutectic Sn/Pb paste while their component manufacturers may not be able to provide Sn/Pb components, the manufacturing lines will be either forced to completely shift to lead free or use lead-free components with Sn/Pb paste. The complete transition to lead-free may not be possible also due to temperature limitations of the device.So the industry is forced to use backward-compatible assemblies, whose reliability also needs to be established. The primary objective of the study presented in this paper is to qualify the reliability of mixed assemblies by comparing them to the conventional Sn-Pb assembly and completely Pb-free assembly. The study investigates both forward, traditional Sn/Pb and backward compatibility in electronic assemblies using a design of experiments (DOE) approach. The investigation utilized a test vehicle containing an area array component (BGA, VBK048).The assemblies were subjected to isothermal aging and thermal shock tests as per IPC/JEDEC standards. In order to simulate an intermetallic failure in isothermal aging, the performance of the ball grid array (BGA) solder joint during thermal shock testing was quantified in terms of the number of cycles to failure. Also, the ductile nature of the Sn-Ag-Cu (SAC) alloy provides the joints a better fatigue life. With the area array com- - ponents, Pb-free assembly joints outlived the traditional Sn/Pb joints when subjected to thermal shock loading. Among the mixed assemblies, backward-compatible assemblies (Pb-free sphere and Sn/Pb solder paste) showed comparative performance to traditional eutectic Sn/Pb assembly process. The use of mixed assemblies in electronics manufacturing is inevitable; wherein some assemblies will continue to use traditional eutectic Sn-Pb solder paste while others will use Pb-free solder paste. The complete transition to lead-free may not be possible also due to temperature limitations of the device. Hence, the industry is forced to use backward-compatible assemblies, whose reliability also needs to be established. This study aims at establishing joint-level reliability in mixed assemblies. The analysis results need not be restricted to the package used in this experiment, but can also be generalized for joints with similar compositions and sizes, provided the process conditions are similar.