An Investigation of Reliability of High Density Electronic Package-to-Board Interconnections from the Perspective of Solder Joint Metallurgy

The integration and miniaturization trend of the electronic packaging leads to much finer pitch of the device and package lead terminations. Several reliability concerns and issues that were previously not encountered are now surfacing. The objective of this thesis work is to investigate the reliability of the package-to-board interconnection from the perspective of solder joint metallurgy. It was carried out with several advanced packages such as CSP, WLCSP and leadless ceramic packages on organic laminate PWBs using tin-silver-copper based interconnection materials. The assemblies were subjected to several loading conditions and levels such as thermal, mechanical, and environmental stresses. As expected, the board level reliability (BLR) of electronic assemblies strongly depended on microstructure and morphology of the solder joints. Dispersion strengthening effect of the intermetallic compounds (IMCs), coarsening of the IMC particles, strain rate hardening, solder fatigue, and recrystallization of Sn grains in the highly stressed areas were observed. These were found to directly impact Pb-free solder joint reliability. Appropriate thermal aging can improve joint reliability up to 50% due to coarsening of the IMC particles. In addition, other factors such as dissolution of metals, interfacial reactions, IMC spalling, and cross interaction of surface materials on the two sides of the joints were also observed and discussed. The effects can be expressed as a series of interactive relationships: materials (pad surface materials and solder alloy composition) and/or soldering process lead to microstructure change in bulk solder and/or at interface, which in turn leads to joint reliability variation.