A Study of Inter-Metallic Compounds (IMC) Formation and Growth in Ultra-Fine Pitch Sn-Ag-Cu Lead-Free Solder Joints

Solder joints play an important role in electronics packaging, serving both as electrical interconnections between the components and the board, and as mechanical support for components. As the trend towards further miniaturisation of pocket and handheld consumer electronics products continue apace, the requirements for even smaller solder joints continue. With further reductions in the size of solder joints, the reliability of solder joints becomes more and more critical to the long-term performance of these electronic products. One of the most important factors which are known to influence solder joint reliability is the intermetallic compound (IMC) layer formed between the solder and the substrate. Although the formation of an IMC layer signifies good bonding between the solder and substrate, its main disadvantage is that it is also known to be the most brittle part of the solder joint. Thus as the miniaturisation trend continues and solder joints become even smaller in size, the nature and impact of IMCs on solder joint reliability becomes even more of a concern with the introduction of new lead-free solder alloy systems. Although the IMC formation in Sn-Pb alloys have been studied extensively and a number of empirical relationships have been developed, there are very few reports on the Sn-Ag-Cu alloy systems and there is wide agreement that the existing empirical kinetic relationships developed for Sn-Pb alloy can not be extrapolated to fit the new Sn-Ag-Cu alloy systems. The objectives of the study were therefore to study the effect of solder volume on IMC layer formation and growth rate, the effect of reflow soldering profile on Pb-free solder joint microstructure and IMC formation, and the effect of the rework process on the Pb-free solder joint microstructure and IMC formation. This study on the formation and growth of IMC in the new Sn-Ag-Cu lead-free solder alloy system is divided into three main parts. The first part concerns the study of the effect of solder volume on IMC formation and growth rate. The second part of the study concerns the experimental investigation of the effect of reflow soldering profile on solder joint microstructures and IMC formation. The focus of the final part of the study is the investigation of the effect of the rework process on the Pb-free solder joint microstructure and IMC formation. The key research questions explored in the study include the following: what is the effect of solder volume (size) on the IMC layer? What is the effect of primary and secondary rework reflow profile parameters on IMC formation? The results of the experimental work conducted show that the solder volume has very little influence on the growth pattern of the IMC, suggesting that the IMC layer thickness is not very volume sensitive. Further work is however needed, given continuing trend towards further miniaturisation of hand held and pocket consumer electronic products - the size of the solder joint might in future be limited by the IMC layer thickness. With further reductions in joint size, with IMC layer thickness remaining the same, the ratio of the IMC layer thickness to solder joint size increased and adversely impact the joint reliability