Electronics solder joint structure

In contrast to soldering, conductive adhesives are used for special electronic applications. Conductive adhesives simultaneously establish mechanical and electrical joints between PCBs and components by means of a particle-filled resin matrix. Whereas the polymer matrix is responsible for the mechanical interconnection, the fiUing particles (silver, palladium, or gold particles) provide the electrical contact between PCB and component. Therefore, in contrast to solder joints, conductive adhesive joints have a heterogeneous structure. 

The ability to tailor CTE is important in many applications. For example, titanium fittings are often used with carbon/epoxy (C/Ep) structures instead of aluminum, because the latter has a much larger CTE that can cause high thermal stresses under thermal cycling. Another application for which CTE is important is electronic packaging, because thermal stresses can cause failure of ceramic substrates, semiconductors, and solder joints. 

FIG. 2 Scanning electron microscope secondary electron emission photographs of the initial micro structure of Sn-Ag eutectic solder joints containing 20 vol.% (a) CueSns, (b)FeSn2, and (c)Ni3Sn4 reinforcement particles. (From Refs. 38 and 39.)... 

FIG. 7 Scanning electron microscope secondary electron images depicting the thermally aged micro structure of Sn-Ag eutectic solder joints with (a) Cu reinforcement, (b) Ag reinforcement, and (c) Ni reinforcement, which were mechanically added. The solder joints were aged at 150°C for 500 hr. 

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