Investigation of compliant interconnect for ball grid array (BGA)

Ball grid array (BGA) carriers use solder balls as interconnects between package and underlying substrate. Stress will be introduced to the solder joints during thermal excursions when the ceramic BGA carrier is interconnected to a standard FR-4 printed circuit board (PCB) substrate due to mismatch in coefficient of thermal expansion (CTE). This has traditionally been overcome by added solder joint heights achieved by high-melting solder balls and/or solders columns. This was feasible when SnPb solder alloys were the standard in use. High density electronic systems demands fine pitch and small bump area while stand off height should be maintained. Transition to lead free alloys introduces new reliability aspects for fine pitch BGA. Lead free alloys exhibit a high Young's modulus and high solidus temperature. Sn-Ag-Cu (SAC) alloys do not mix well with other solders to form a uniform and strong solder structure, hence adequate stand off height is difficult to realize. Introduction of polymer-core solder balls can offer compliant interconnects combined with enhanced stand-off height after reflow. Less thermal and mechanical stress will be introduced due to the improved material match thereby increasing the reliability of the interconnects compared to lead free BGAs. In this paper, initial characterization of 240 µm polymer-core solder balls will be presented. A low temperature co-fired ceramic (LTCC) test vehicle with pad sizes ranging from 250 µm – 300 µm has been designed to investigate pad size and solder-joint geometry for BGA carriers. Result from this investigation is presented and compared to results from finite element modelling and simulation. Light microscopy has been performed on samples after assembly and on cross-sectioned samples, and will be presented together with results from destructive die shear test.