| Summary: | 博士 === 國立臺灣大學 === 材料科學與工程學研究所 === 88 === Soldering has been widely used on electronics assemblies and PbSn alloys are very popular solder alloys due to their superior properties as good soldering properties, fatigue resistance, adequate thermal and mechanical properties and low cost. However, growing concerns about environmental pollution and lead toxicity have increased the regulations and legislation on lead usage. The development of lead-free solders has been prompted and intensified for the past years. In soldering, the reaction between solder and substrate will result in intermetallic compound growth on the interface, which is crucial to the quality of solder joint. The morphology and growth kinetics of intermetallic compound formed during the soldering reaction of lead-free In/Au, In49Sn/Au and In10Ag/Ag systems and solid-state diffusion couple of Ag2In/In have been investigated in this study. Also, the wettability of these solder systems are determined by contact angle measurements in sessile drop method.
The results show that AuIn2 phase, in the shape of wavy layer, grows as a major phase at the reaction interface in both In/Au and In49Sn/Au reactions. The growth of AuIn2 layer in In/Au reaction is a diffusion-controlled process with an activation energy 39kJ/mol, while the AuIn2 growth in In49Sn/Au reaction is interfacial reaction controlled with an activation energy 51kJ/mol. As reaction time and temperature increase, two minor phases AuIn and Au7In3 phases grow at the Au/AuIn2 interface. The growth of Au7In3 phase induces cracks at the interface and impairs the mechanical integrity of the solder joint. Accompanying the interfacial intermetallic compound growth, the Au atoms dissolving into solder in soldering reaction precipitate as a cluster of AuIn2 islands in the solder matrix during cooling.
In the In10Ag/Ag reaction, a continuous layer of scallop-shaped or dendrite-shaped Ag2In compounds enveloped in thin AgIn2 shells appears at the In10Ag/Ag interface. The growth of the Ag2In phase is diffusion-controlled and the activation energy is 45kJ/mol. The Ag atoms precipitate as a cluster of spherical or dendrite-shaped Ag2In islands in the interior of solder matrix. Solid-state reaction happens on both interfacial and precipitated intermetallic compounds when the soldered specimens are stored at room temperature that the AgIn2 shells grow and consume the former Ag2In phases. The study of Ag2In/In diffusion couple shows that the growth of AgIn2 phase is diffusion-controlled and the activation energy is 41kJ/mol which is similar to the AgIn2 growth energy in thin film In/Ag reaction.
The sessile drop method is used to evaluate the wettability of the former soldering reactions. The result shows that there are three stages on the contact angle variation of In/Au and In10Ag/Ag reactions. The contact angles decline rapidly when the solders begin melting, the first stage, and then the curves remain as a plateau, the second stage, for a period of time before they finally collapse to a very low value which is considered as the last stage. A mechanism involving the interfacial intermetallic compound growth and precursor halo formation has been propounded for this wetting behavior. This mechanism is confirmed through SEM observation of the cross-sections of specimens after wetting test. The contact angle variation in In49Sn/Au reaction includes three stages too. However, the curves in the second stage is not a plateau but a gradual descent. Through comparison with In/Au and Sn/Au reactions, it was brought to light that the contact angle variation in In49Sn/Au reaction is resulted from the effects of In/Au and Sn/Au reactions.
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