Hirohisa Matsuki

TEM observation of interfaces in a solder joint in a semiconductor device

Abstract Microstructure of a joint between a Pb–Sn eutectic solder and an electroless Ni–8 mass% P has beenexamined using transmission electron microscopy. Four layers, i.e. Ni3Sn4, Ni48Sn52, Ni2SnP and Ni–20 mass% P, are formed between the solder and the electroless Ni–8 mass% P. Among them, Ni48Sn52 and Ni2SnP were found for the first time in a solder joint. Spherical voids are formed at the interface between Ni48Sn52 and Ni2SnP, and columnar voids are formed at the interface between Ni2SnP and Ni–20 maas% P. From the analysis of the migration of the respective interfaces observed during in situ heatingexperiments, it is concluded that these voids are Kirkendall voids formed due to the difference in diffusivity of Ni across the interfaces. Fracture takes place at either of those interfaces during a dropping test.

Fluxless bump reflow using carboxylic acid

We have established a new fluxless reflow technology and realized an organic solvent free wafer bump forming process. Some carboxylic acids have been used in order to reduce oxidized films on solder bumps of both eutectic and high lead tin types. We found that formic acid was most suitable for stripping of the oxidized films. To obtain good bump shape with this method, it is important to optimize the timing of supply of the acid to the vehicle, the maximum temperature, and the method used to reduce the formic acid on the vehicle. Bump shape, voids, shear strength, and the surface condition of the bumps have been researched with the vehicles obtained. The bumps which were formed by our new technology compare favourably to those formed by conventional reflow processing. The advantages of this technology are: (1) reduced material cost, e.g. flux and organic solvent; (2) environmentally friendly reflow process.