Effects of metallic nanoparticle doped flux on interfacial intermetallic compounds between Sn-3.0Ag-0.5Cu and copper substrate

Intermetallic compounds (IMCs) formed between solder and substrate play a vital role in determining the long term reliability of microelectronic packages. Various attempts have been made by the researchers to control the morphology and thickness of IMC layers. The aim of this study is to investigate the effects of nanoparticle dopants into flux on the morphology and thickness of interfacial intermetallic compounds layers. Different types of nano-sized metallic particles were studied to understand their effects on the wetting characteristics and interfacial microstructural evaluations after first reflow by adding nanoparticles to flux at various percentages. Nanoparticles were dispersed manually with a water soluble flux to prepare a nanoparticles doped flux which was placed on the copper substrate. Lead-free Sn-3.0Ag-0.5Cu (SAC 305) solder balls of diameter 0.45mm were then placed on top of the flux and were reflowed in a reflow oven at a peak temperature of 240°C for 45s. Wetting area, contact angle and interfacial microstructure were investigated by optical microscopy, scanning electron microscopy (SEM), field emission scanning electron microscopy (FESEM) and energy-dispersive x-ray spectroscopy (EDX). It was found that doping of cobalt (Co) and nickel (Ni) nanoparticles with flux was successful in incorporating Co and Ni into the solder joint. Microstructural observations showed that both Co and Ni nanoparticles changed the interfacial morphology from a scallop to a planer type. This was suggested to be caused by alloying effect of these elements. In case of Co, this morphological change was evident down to 0.25 wt% Co addition to flux. For Ni, this effect was notable even at 0.1 wt% Ni addition to flux. Therefore, Nano doping of flux can be successfully used to cause in situ targeted alloying at the solder/substrate interface.