Lead-free Solder Joint Interface Compound Layer Growth And Element Diffusion Behavior

With the trends of miniaturization, lighter weight in today's electronic industry and the increasing need of a large number of I/O interconnectors, the size of solder joint is more and more smaller. So, more and more concerns are focused on the reliability of solder joints recently. However, the joints reliability is dependent on the interface behaviour. The research on element interdiffusion , reaction and microstructure at the interface of solder joints is vital to the study of lead free solder joints reliability.In this paper it is investigated that the interface intermetallic compound (IMC) growth behavior and element diffusion path of four solder joints during soldering and aging. They are Sn-3.5Ag/Cu , Sn-37Pb/Cu, Sn-9Zn-3Bi/Cu and Sn-3.5Ag/Ni/Cu respectively. The results are shown as follows:(1) The IMC morphology at the interface of Sn-3.5Ag/Cu , Sn-37Pb/Cu exhibits a scallop structure after soldering. Nano-size Ag3Sn particles are formed at the IMC grain surface of Sn-3.5Ag/Cu. Experimental data for IMC growth of both Sn-3.5Ag/Cu and Sn-37Pb/Cu indicated that the IMC thickened with increasing soldering time. In fact, the thickness of the intermetallic layer is determined by a combined process including simultaneous dissolution and growth of the intermetallic compound. The growth rate is bigger than dissolution rate resulting in the IMC thickening with increasing soldering time. Although Sn-3.5Ag solder has a higher Sn content, the thickness of IMCs is smaller comparing with that of Sn-37Pb/Cu. This is mainly because the IMC dissolution amount of Sn-3.5Ag /Cu is bigger than that of Sn-37Pb /Cu. During aging the IMC layers thickness versus the square root of aging time shows a linear dependence ofd = d0 + Kt. By calculation, the activation energies values for the growth of the totalIMC and Cu6Sn5 IMC are 75.16 kJ/mol and 58.59 kJ/mol for Sn-3.5Ag while 82.19 kJ/mol and 73.11 kJ/mol for Sn-37Pb respectively.(2) The morphology of intermetallic compound (IMC) formed at the interface of Sn-3.5Ag/Ni/Cu shows a planar structure after soldering and nano-size Ag3Sn particles are also observed on the surface. The growth kinetic of IMC layer during soldering versus t1/6 shows a linear dependence while the growth kinetic of IMC layer during aging isx = (kt)1/2 The interface IMC is mainly composed of Ni3Sn4 after soldering and agingindicating that the electroplated Ni layer can effectively hinder the inter-diffusion between Sn-3.5Ag solder and Cu substrate. The activation energy of the Ni3Sn4 IMCs is 132.404KJ/mol which is larger than that of Cu6Sn5 (58.95KJ/mol). It means that the growth rate of the IMCs is much slower than that of Cu6Sn5 under low temperature aging while a bit faster under high temperature aging.(3) A continuous planar IMC layer is formed at the interface of Sn-9Zn-3Bi/Cu. The growthkinetics of the IMC layer during soldering versus t1/6 also shows a linear dependence. The Cu-Zn IMC layer formed during soldering is instable during aging. A continuous single layer of Cu5Zn8 intermetallic compound appears when aging within 200h. However, there are three compound layers at the interface after aging for 500h and 1000h: (from Cu substrate, they are Cu-Sn, Cu-Zn, Sn-Cu IMC layer respectively). The instability of Cu-Zn IMC layer has detrimental effects on the joints reliability during aging.