The Effects Of The Laser Soldering Parameters On The Interfacial Microstructure In Soldering

Lead-free solders have been the choice for the package industry for the traditional solder Sn-Pb has many bad efforts on the environment and the human. As a new type of heating resources, laser-soldering has received more and more attentions. Laser-soldering has the advantages such as high heat energy density, fast cooling speed, low heat stress when local heating, low soldering time, easy and accurate control the output power, etc. The morphology and the growth behavior of the IMCs which formed between the solders and substrate have big effects on the reliability of the joint. It is very important to study the effects of laser-soldering parameters on the interfacial reaction.In this study, fiber laser was adopted as the heating resources with different combinations values of the laser soldering output power and the laser scanning speed to solder the Sn, Sn-0.7Cu, Sn-3.5Ag. Sn-3.0Ag-0.5Cu and polycrystalline Cu substrate. The combinations values of the laser soldering output power and speed which can acquired good shape joint can be obtained. The SEM was used to observe the top-view and the cross-section view of the samples after soldering. The growth behavior and mechanism of IMCs formed between lead-free solders and Cu substrate at the condition of laser soldering were studied. These conclusions can be drawn:(1) Under certain laser scanning speed, the morphology of the Cu6Sns will change as the increased laser soldering output power as follows:when the output power was very low, the morphology of the Cu6Sns was the scallop type with smooth top on it; the scallop type grains will change to prism-type with the increased; when the energy of the laser soldering is more, Cu6Sn5grains which were bottom closely arranged; to further increase the output power, plate-like Cu6Sn5can be found.(2)Under certain laser soldering output power with the increased laser scanning speed, the morphology of the Cu6Sn5grains will verify as follow:when the laser scanning speed was very fast, the Gu6Sn5grains will be very fine grains with scallop type; when the speed is lower, the Cu6Sn5became faceted; when the speed was reduced, the morphology will change from the scallop type to the prism type which size is bigger as the speed is lower; again to slow down the laser scanning speed, the Cu6Sn5grains closely arranged at the bottom were instead of prism type grains; as the laser energy was increased, the plate-like Cu6Sn5were observed. (3) The prism type Cu6Sn5formed at the interface between different solders and Cu was different. The prism type Cu6Sn5formed between Sn, Sn-0.7Cu and polycrystalline Cu was in two planes with a certain angle. When the solders contained Ag reacted with Cu, the morphology of Cu6Sn5tended to be hexagonal prism.(5) To measure the top-view of the Cu6Sn5SEM photos and then analyz the data, the following conclusion can be obtained:at a certain laser soldering output power with the increased laser scanning speed, the number of the grains in1unit square(1μm2) showed an increasing trend. This value of solders which contained Ag element was bigger than that of Sn-0.7Cu.(6) When soldering at a certain soldering output power, there were a fastest scanning speed and a lowest scanning speed:the joint can be ruined if the laser scanning speed lower than the lowest soldering scanning speed while the joint cannot be in good shape or the joint strength was very low if the laser scanning speed was faster than the fastest laser scanning speed.(7)When soldering a certain solder, the fastest laser scanning speed was increased as the laser output power increased.(8) In the condition of this experiment, at a certain laser output power, the range of laser scanning speed which can get a good shape joint has the following rules: Sn-3.0Ag-0.5Cu<Sn-3.5Ag-Sn-0.7C<Sn.