Wetting, nucleation and growth in soldering

Abstract : In this work we have studied some fundamental aspects of (i) wetting, (ii) nucleation at reactive interfaces as well as inside the solder bulk and (iii) growth kinetics of intermetallic layers during a soldering process in electronic industry (Cu/liquid Sn system).Wetting of metallic substrates Cu, Ag and their intermetallics with Sn (Cu3Sn, Cu6Sn5 and Ag3Sn) by liquid Sn-Cu solders was studied by the dispensed drop technique in a high-vacuum furnace and using a rapid camera for recording the spreading process. In temperature range 300-600C, the first stage of wetting of Cu by liquid alloy occurs in less than 10ms and the spreading rate is about 0.25 m.s-1. During this stage (non reactive spreading) the spreading kinetics is practically temperature independent and similar to that of the non-reactive liquid Pb on Cu. The equilibrium contact angle of liquid Sn on a non-reacted and clean surface of Cu is lower than 300. The non-reactive contact angles on Cu3Sn, attained in less than 10 ms, decrease from 23 to 100 when T increases from 300 to 500C. During wetting of Cu6Sn5 at 390C, low contact angle of about 20 are attained in less than 10 ms. Results of wetting experiments of Ag and Ag3Sn substrates allowed to propose a mechanism of the spreading kinetics in two stages: a very rapid non reactive wetting and a slow reactive wetting stage.By implementing a fast dipping experimental set-up and SEM-FEG and TEM techniques, we succeed to study for the first time the sequence of formation of intermetallics at the Cu/liquid alloy interface at the very beginning of reaction (1 ms to 1 s) at 250C. These experiments give, for the first time, the answer to one of the most challenging questions in soldering: the first phase that appears at Cu/liquid Sn interface is the Cu6Sn5 phase. Afterwards, we developed a theoretical approach on the suppression criteria of the second phase formation (Cu3Sn) by assuming that Cu6Sn5 is the first phase that grows at the interface in form of a continuous layer. Moreover, a theoretical modeling of even earlier stages of the isolated nucleus of Cu3Sn phase nucleation is developed.Crystallization of eutectic Sn-Cu alloy was studied by performing specific DSC experiment by applying thermal cycles of complete and partial melting of the solder. Different categories of the undercooling degrees are obtained. In particular, low undercooling degrees (1-7K) are detected for the first time in the case of partial melting. A theoretical approach on heterogeneous nucleation is developed and the responsible sites of nucleation in the case of low undercooling degrees are proposed.Direct comparison of Cu6Sn5 growth kinetics in standard Cu/liquid Sn and in incremental Cu3Sn/liquid Sn couple gave a link with the operating mechanism of the Cu6Sn5 phase growth at solid Cu/liquid Sn interface. The obtained results are also consistent with the FDR model.Specific experiments dealing with interfacial reactions between Cu substrate and metastable liquid Sn-0.7wt.%Cu alloy at 222°C for reaction times as long as 32 h were performed for the first time. This is achieved by performing specific DSC experiments in order to monitor and control the physical state of the alloy as well as to set the accurate reaction temperature. Direct comparison of Cu/liquid and Cu/solid solder growth reaction kinetics and layer morphology at the same temperature lead to the conclusion that the large difference in the growth kinetics between the two couples can be explained if the growth occurs by the liquid state diffusion via the liquid channels of nanometric width formed between grain boundaries of Cu6Sn5 phase and/or by cylindrical channels of radius of some tens of nanometers formed at the triple line junctions of Cu6Sn5 scallops. A theoretical evaluation of the liquid channel width is performed for the first time. Evaluation is consistent with the FDR model.
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Oleksii Liashenko. Wetting, nucleation and growth in soldering. Other. Université Grenoble Alpes, 2015. English. ⟨NNT : 2015GREAI076⟩. ⟨tel-01266355⟩

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