Investigations of void forming and shear strength of Sn42Bi58 solder joints for low cost applications

Abstract

There are many investigations and publications in the current discussion for lead free applications. It can be seen, that a nearly compatible lead free solder alloy to the current used SnPb, like SnAgCu was investigated and preferred in many projects [l, 21. The higher melting point of this alloys and the given temperature limitations for the electronic components decrease considerably the reflow soldering process windows. For this reasons the peak temperature in the reflow profiles are limited to 250T - 235OC. It is also published in some paper that the SnAgCu alloys needs a longer wetting time to reach equivalent soldering results like tin lead alloy [3. 41. Therefore the flux chemistry and the soldering profiles have to adjust to the new wetting behaviour. In contrast to that it can be possible to use the Sn42Bi58 alloy with a low melting point of 138T for low cost applications lie toys or consumer electronic. This alloy can be used in many lead free applications with a small temperature range in the real fatigue life, if we can prevent the BiPb intermetallic problem. The soldering temperature is uncritical and the use of bismuth decreases the surface tension of the alloy, so that the wetting behaviour is not problematical. The decreasing sizes of components and packaging lead to a reduced solder joint in the electronic assembly. For that reason the iniluence of voids in solder joints will become more significant for the lietime of the interconnections. Against the background of these statements we’ve investigated in our work the influence of different reflow profiles for void forming in lead free solder joints of Sn42Bi58 alloy. The influence of void forming and different reflow profiles were investigated and evaluated with shear tests for the chip resistors CR0805 and CR0603 assembled on the 3 different PCB fmishes. The testhoard f~shes were varied with Cu+OPS, electroless tin and - if bonding processes are necessary - as third electroless nickel over plated with immersion gold (ENIG) surface. The chip resistors have had an electroless tin metal fmish. The two investigated Sn42Bi58 solder pastes were from different manufacturers, so that it was possible to compare the influence of flux chemistry and shape of reflow profiles on the void forming of solder joints.