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Development of adaptive solder technology for reliability and environment compatibility of electronic assemblies


Electroless coating process has been optimised in order to coat NiTi spherical micronic powders with metal. Coating metals are nickel, gold and copper. In our experiments, the electroless process is essentially based on the dipping of the NiTi particles in four different and successive baths. Control of the dipping time in the metal bath (last bath) will allow the control of the metal thickness around each individual NiTi particles. Metal thickness ranging from 0,1µm to 5µm can be obtained.
Cu coating: An intermetallic layer is built between the copper coating and the matrix and there is a copper layer left on most of the particles, even after three re-flows. Particles with a copper layer of 2 microns have copper left after soldering. The copper layer does not decrease much between the first and the third soldering. Consequently, a copper layer of 2 microns is sufficient to undergo as much as three re-flow soldering processes. With a thick Cu coating a large intermetallic layer is formed resulting in severe agglomeration, especially when the particles are small. Ni/Au coating: A Ni/Au coating of 2µm gives a thin well-adhered intermetallic layer around the particles. The interface between the coating/intermetallic and matrix remains intact during fatigue.
In order to allow the incorporation of NiTi particles inside the SnAgCu liquid matrix, Cu thin film has to be deposited onto the particles. De-wetting of the NiTi particles can occur after total dissolution of the Cu sacrificial interface zone. In order to optimise the copper thickness, liquid state diffusion, solid state diffusion and the Cu-Sn inter-metallics formation have been analysed. The speed formation of the interface reaction zone (Cu/ Cu3Sn/ Cu6Sn5/ Sn), has been followed for an increasing contact time (0 to 300s) of solid copper with liquid SnAgCu (240°C). Three regimes can be observed: - 1. regime A (0-10 s): corresponds to an interfacial reaction zone speed of 0,3 mm/s characteristic to a diffusion regime of tin at the Cu6Sn5 inter-metallic grain boundaries. - 2. Regime B (10-50s): corresponds to an interfacial reaction zone speed of 0,1µm/s characteristic to a diffusion process of tin through the Cu6Sn5 intermetallic grains. - 3. Regime C (50-00s and more): corresponds to an interfacial reaction zone speed of 5.10-3µm/s characteristic to a diffusion of tin through the Cu3Sn and Cu6Sn5 inter-metallic grain. The solid diffusion has been found to be negligible in respect with the liquid one.
Microstructure evolutions of lead free solder paste (SnAgCu) have been analysed with the annealing process and the cooling spread (high (100°C/min) and low (1°C/min)). This evolution has tentatively been correlated with the evolution of the mechanical behaviour of the solder paste. For non-annealed samples and high cooling rates the SnAgCu microstructure can be defined as a ternary eutectic phase embedded by primary tin dendrites. The ternary eutectic phase is composed of Sn, Ag and Cu which are homogeneously dispersed. For all annealed samples (125°C, time: 1 hour to 600 hours) the primary tin dendrites are stable when an evolution of the ternary eutectic phase can be observed;. Starting from an annealing time of 10 hours, silver inside the eutectic phase start to form spherical Ag3Sn compounds where Cu forms Cu6Sn5 ones. From long annealing time the size of Ag3Sn and Cu6Sn5 spherical precipitates can reach a few micrometers. It has to be mentioned that low cooling speed SnAgCu microstructure is similar than the microstructure observed for long annealing times. The only change is the evolution of the precipitate shape: spherical for long annealing time, rod-like for low cooling rate.
The following information and knowledge was made from metallographic analysis after fatigue: - Proportion eutectic/dendrites: the proportion of eutectic compared to Sn-rich dendrites is higher after mechanical cycling than before. This is probably due to precipitation of Ag3Sn and Cu6Sn5 in the dendrites, which transform into an apparent eutectic structure. - Re-crystallisation of the tin dendrites is observed in all the tested samples. The degree of re-crystallisation and the size of re-crystallised grains are very different in different samples. They may only be very locally re-crystallised in the most stressed zones or almost totally re-crystallised in the whole sample. In all the samples, tin grains are re-crystallised along the cracks. - Coarsening of the eutectic structure is observed in the most stressed regions. This coarsening possibly enables re-crystallisation in theses regions to take place. - Cracks propagate in the matrix and not at the interface with intermetallics. Thus, the intermetallic layer does not influence the crack propagation in these tests