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Title: A study on interfacial interaction behavior between lead-free solders and electroless Nickel metallization for advanced electronic packaging
Other Titles: Xian jin dian zi feng zhuang zhong wu qian han liao yu hua xue du nie ceng zhi jian de jie mian fan ying xing wei yan jiu
Authors: Sharif, Ahmed
Department: Dept. of Electronic Engineering
Degree: Doctor of Philosophy
Issue Date: 2005
Publisher: City University of Hong Kong
Subjects: Electroless plating
Electronic packaging
Lead-free electronics manufacturing processes
Solder and soldering
Notes: CityU Call Number: TK7870.15.S53 2005
Includes bibliographical references (leaves 162-174)
Thesis (Ph.D.)--City University of Hong Kong, 2005
xxiii, 177 leaves : ill. ; 30 cm.
Type: Thesis
Abstract: 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 extended period of the molten reaction. The Sn-0.7%Cu solder demonstrated the formation of thick IMC layers at the interface in the initial stage. It was also seen that the formation of Ni3P on the electroless Ni(P) with Pb-free solders was diffusion controlled. The effect of high temperature storage on the joint reliability and microstructure of the Pb-free solder on the electroless Ni has also been investigated. The Sn-3.5%Ag solders showed relatively better ball shear strength than the 0.5% Cu added Sn-3.5%Ag solders during aging. A relatively high consumption of Ni was observed in the case of Sn-Ag solder alloys. The IMC growth rate in the interface of the Sn-Ag solder was higher than that of the Sn-Ag-Cu solder. The redistribution of Au as Au-Ni-Sn was detected at the interface of the Sn-Ag solders. A larger amount of Au was incorporated in the Cu-containing interfacial IMCs of Sn-Ag-Cu solder through aging. A very high consumption of Ni(P) was observed in the case of the Sn-3.5%Ag solder alloy after multiple reflow cycles. A relatively smaller amount of original Ni(P) plating was consumed in the Sn-3.5%Ag-0.5% Cu solder. A white layer of P-rich Ni- Sn compound was detected above the dark Ni3P layer in Sn-3.5%Ag solders after 10 reflows. It is believed that the diffusivity of the reacting atoms through this white layer was much higher than the Ni3P layer. The addition of 0.5% Cu in the Sn-3.5%Ag solder prevents the formation of such a white layer at the interface of the solder joint. Overall, the Sn-3.5%Ag-0.5%Cu solder alloy on electroless Ni layer showed a better performance regarding the original Ni(P) consumption and IMC formation under a large number of reflow cycles. The effects of Ag and In additions on the interfacial reactions after extended reflow, multiple reflow cycles and high temperature solid-state storage of the Sn-Ag- Cu solder have also been investigated. Both the low Ag-containing solders and the In- containing solders exhibit less interfacial reaction with the electroless Ni(P) layer during long time reflow. Electroless Ni consumption was higher in the high Ag- containing solder than in the other two solder systems during multiple reflow cycles. Ni-Sn-P compounds were found on the P-rich Ni layer at the interface of the high Ag- containing solder. By the addition of 9% indium, the interfacial reaction was slightly increased with a solid-state high temperature storage of the solder system. A comparison of the interfacial reaction between the electroless Ni(P) and the electrolytic Ni with the In added Sn-Ag-Cu solder has been studied. The electroless Ni(P) layer showed less dissolution than the electrolytic Ni layer after long time reflow. With the straightforward reactions, the interfacial IMC formation on the electrolytic Ni layer was larger and more uniform throughout the interface
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