Diffusion-controlled reactions in gold/lead-tin solder systems

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" Diffusion-controlled reactions in gold/lead-tin solder systems "
Hannech, El Bahi

Document Type	:	Latin Dissertation
Record Number	:	827780
Doc. No	:	TLets252957
Main Entry	:	Hannech, El Bahi
Title & Author	:	Diffusion-controlled reactions in gold/lead-tin solder systems\ Hannech, El Bahi
College	:	University of Warwick
Date	:	1989
student score	:	1989
Degree	:	Thesis (Ph.D.)
Abstract	:	Au/PbSn joints are widely used in the microelectronics industry. Intermetallic formation in Au/60Sn-40wt. %Pb and in Au/62Sn-36Pb-2wt. %Ag systems has been studied in the temperature range 80°C-160°C using diffusion couples. Interdiffusion between Au films and 60Sn-40wt. %Pb solder doped with 2 wt. % zinc has also been studied but at 125°C only, also using diffusion couples. The intermetallic phases were identified by Energy Dispersive X-ray Analysis (EDAX) in the Scanning Electron Microscope (SEM) and found to be gold-tin compounds. The predominant phases were found to be AuSn4 and AuSn2 but all the phases predicted in that range of temperatures by the equilibrium phase diagram of the Au-Sn system were present in the couples after long annealing times. The kinetics of the intermetallic layers were determined and an activation energy for the growth of the layer of AuSn4 of 0.84 ± 0.02 eV was found. The effect of both silver and zinc on the growth rates of the intermetallic layers were determined. It was found that both additives reduce the intermetallic layer growth rates. In addition to the Au-Sn intermetallics, a compound layer of composition Au0.3Sn0.3Zn0.4 forms between the AuSn4 layer and the solder in the Au/PbSnZn system. The tensile strengths of the joints, before and after aging, were also evaluated and the weak bonds within the joints identified. It was found that the weak bonds within a Au/PbSn joint, with or without silver in the solder, were the intermetallic layer interfaces. The joint tensile strength is not weakened by the increase of the intermetallic layer thicknesses with aging time. Joint weakening occurs after degradation of the gold film of the joint, but that happens a long aging time after the depletion of the gold film. The joint made with the zinc containing solder was found to lose strength rapidly after aging. The weak area of the joint is the solder/wire interface.
Subject	:	QC Physics
Added Entry	:	University of Warwick