AbstractsChemistry

The scope of this dissertation is to understand the reaction phenomena at the interfaces of solder joints since these reactions are very crucial from the manufacturing point-of-view. Lead-free ball-grid-array (BGA) solder balls, with a diameter of 0.76 mm, were placed on prefluxed bond pads of the substrates and then soldered at a particular temperature in an oven. Prolonged reflow, solid-state aging and multiple reflow cycles have been carried out systematically on the assembled packages. The flux used in this project was a commercial no-clean flux. Throughout the study, different types of lead-free solder alloy have been investigated. The chemical and microstructural analyses of the cross-sectioned samples have been conducted using a Philips XL 40 FEG scanning electron microscope (SEM) equipped with an energy dispersive x-ray spectrometer (EDX). The effects of reflow on the interfacial reactions of a Cu substrate with Pb-free BGA solder balls have been investigated. The highest Cu dissolution from the substrate pad was observed in the Sn-3.5%Ag (wt.%) solder. The Cu consumption in the Sn-Ag solder was so high that at 2500C, the entire 13μm thick Cu traces were fully dissolved within a minute. The In-containing Pb-free solder exhibited less dissolution of the Cu pad during long time reflow. Between the other two Cu-containing solders, the more Cu-containing Sn-0.7Cu solder showed a lower Cu consumption than the Sn-3.5Ag- 0.5Cu solder. The dissolution phenomenon of the Cu pad of the BGA substrate were also compared into the two different volumes of molten conventional eutectic Sn-Pb and a near eutectic Sn-Ag-Cu solders as a function of time and temperature. The Cu consumption was much higher for both alloy systems with a higher volume whereas the mean thickness of the intermetallic compounds was smaller. On the other hand, the mean thickness of the intermetallics for solder with a smaller volume was larger with less Cu consumption. There was a significant effect of the solder volume observed on the interfacial reaction flux, however little influence was observed on the ripening/coarsening flux of the soldering reaction among the two volumes for a particular alloy system. Between the two-alloy systems, it was observed that the ripening action in the Sn-Ag-Cu solder was much more pronounced than that in the Sn- Pb solder. A simplistic theoretical approach was carried out to find out the amount of Cu6Sn5 intermetallic compounds (IMCs) in the bulk of the solder by the measurement of the Cu consumption from the substrate and the thickness of the IMCs that form on the interface. The effects of reflow at four different temperatures on the dissolution of electroless Ni(P) surface finish in Sn-3.5%Ag, Sn-3.5%Ag-0.5%Cu and Sn-0.7%Cu (wt.%) BGA solder balls have been studied systematically. The Cu-containing Pb-free solders exhibit less dissolution of electroless Ni after a long time reflow. The diffusion of reacting species through the more Cu-containing ternary IMCs was restricted in the …