CSP BOARD LEVEL RELIABILITY TESTING OF PB-FREE SN-AG-X (X=CU, IN) AND POLYMER-CORE SOLDER BALL
Authors: Seung Wook Yoon and Ik Seung Park Company: Hyundai Electronics Industries Date Published: 2/13/2001
Pan Pacific Symposium
Abstract: Chip scale packages (CSP) have essential solder joint quality problems and so board level reliability is a key issue in design and development of the CSP type packages. And because of the toxicity of lead, there has been an effort to eliminate it from solder. To evaluate the various solder balls in application of CSP packages, Pb-free Sn-Ag-X (X=In,Cu) and polymer-core solder balls were investigated in the fields of melting behavior, phases, interfacial reaction and solder joint reliability. In order to study the interfacial reaction between solders and UBM (Under Bump Metallurgy) systems, various UBMs were prepared by electroplating and electroless plating. Solder balls were attached on the various UBM-plated substrates and characterized by solder joint strength after environmental tests such as temperature cycles ( T/C, -65~150oC). After T/C test, Sn-Ag-In solder was partially corrosied and its shape was distorted. This phenomenon was also observed in Sn-Ag-In-Cu solder system. Their fractured surface, microstructure of solder joint interface and of bulk solder ball, was examined and analyzed by optical microscopy, XRD, SEM and EDX. To simulate the real surface mounting condition and evaluate the solder joint reliability on board level, daisy chain test samples using LF-CSP packages were prepared with Sn-Ag, Sn-Ag-Cu solders and polymer-core ball then, T/C test (-65 ~ 150oC) was performed. Sn-3.5Ag-0.7Cu solder showed 35% superior solder joint reliability than Sn-36Pb-2Ag solder ball And polymer-coer ball showed the 3 times longer solder joint life time than Sn-36Pb-2Ag. After T/C test, microstructure such as their crack initiation site and propagation path were studied and analyzed. Major crack was found at the CSP package side and crack propaged through solder adjacent to interface for all studied solders.