Assembly issues with Sn/Ag/Cu bumped flip chips

Abstract

Purpose

The objective of this research is to address issues that relate to the assembly of Sn/Ag/Cu bumped flip chips.

Design/methodology/approach

Flip chips bumped with Sn/Ag/Cu bumps were assembled onto different lead‐free surface finishes at lead‐free soldering temperatures. Sensitivity to fluxes, reflow profiles, pad finishes and pad designs were all investigated and the potential consequences for assembly yields were calculated numerically.

Findings

Soldering defects, such as incomplete wetting and collapse and poor self‐centring were observed in the assemblies. Defect levels were sensitive to contact pad metallurgy and flux type, but not to flux level and reflow profile within the ranges considered. Owing to a particularly robust substrate‐pad design, defects observed in this work were limited to incomplete wetting and collapse, as well as poor self‐centering.

Research limitations/implications

The scope of this work is limited to the lead‐free fluxes available at the time of research. A switch to lead‐free solder alloys in flip chip assemblies raises concerns with respect to the compatibilities of materials and the quality of soldering that is achievable. While this may be less of an issue in the case of larger area array components, such as ball grid arrays and chip scale packages, it is more of a concern for applications that use flip chips due to the smaller size of the solder spheres. Assembly yields tend to become more sensitive to the reduced collapse of the joints. More work is essential to investigate the potential benefits of more active lead‐free fluxes, both no‐clean tacky and liquid fluxes, in reducing or eliminating soldering defects.

Originality/value

The paper offers insights into assembly issues with Sn/Ag/Cu bumped flip chips.

Citation

Gopakumar, S., Borgesen, P. and Srihari, K. (2007), "Assembly issues with Sn/Ag/Cu bumped flip chips", Soldering & Surface Mount Technology, Vol. 19 No. 4, pp. 3-10. https://doi.org/10.1108/09540910710848491