Effects of Cu nanoparticles doped flux on the microstructure of IMCs between Sn solder and Cu substrate

Recently, developing thick interfacial intermetallic compound layers with rough morphology has been detrimental to the long term solder joint reliability for lead free solders currently. A novel method has been developed to control the morphology and growth of intermetallic compound layers between pure Sn solder ball and Cu substrate by doping flux with amount of 2wt.% metallic Cu nanoparticles. And lead-free Sn solder balls of diameter 1.4 mm were placed on top of the flux and solder at temperature of 250°C for 10s, 30s and 60s, respectively. Microstructural characterization and elemental compositions of solder joints were examined by field emission scanning electron microscopy (FESEM) equipped with energy dispersive X-ray spectroscopy (OXFORD, EDX). Although at first 10 s of soldering time, both doped and undoped solder joints were characterized with scalloped Cu
₆
Sn
₅
IMC, it was observed that with the extension of soldering time, the IMC of undoped specimen underwent faster transition to sharp edged, faceted grains or prismatic crystal shaped morphology. And that Cu nanoparticles doped into flux has significantly influenced the morphology of IMC at the interface of solder joint. Moreover, Cu nanoparticles may block the channels between the IMC scallops and are preferentially absorbed at the growth front of IMC scallops, and the Cu nanoparticles can have a reaction with the Sn in solder and feed the interfacial reaction, which lead to the coarsening of IMC grains. Thus, doping of flux with appropriate metallic nanoparticles can be successfully used to control the morphology and growth of intermetallic compound layers at the solder/substrate interface which is expected to lead to better reliability of electronic devices.