Fluxing for flip chip assembly - effect of bump damage

Abstract

Solder bumps on flip chips could arrive at the manufacturing floor with prior damage. While bumps with insufficient solder volume may not solder well even if adequately fluxed, the bumps that are flattened during damage are also of concern for fluxing. Good assembly yields will depend on whether these bumps are fluxed adequately and whether the collapse offered by the substrate design is sufficient to bring the damaged bumps in contact with their target pads. The former depends on the flux application method and the latter depends on the pad design and the solder alloy. When assembled on the same substrates, leadfiee solder joints have been shown to exhibit less collapse than eutectic tin-lead solder. Therefore, a damaged lead-free solder joint may not solder well even if it is adequately fluxed. The sensitivity of four flux application methods to bump damage is described in this paper. Solder bumps on flip chips were systematically damaged by flattening them to an extent of 40-45 pn. These damaged chips were then assembled using four different fluxing techniques: dip fluxing, stencil printing, flux jetting and no-flow encapsulation. Soldering of three alloys, eutectic tin-lead, Sn-Ag-Cu (LF2) and Sn-Ag-Cu-In (LFl) was studied. Assembly was followed by X-ray inspection, micro-sectioning and electrical testing. As expected, dip fluxing proved to be sensitive to bump damage. Defects were observed with both tin-lead and LF2 chips with the least thickness of flux. Damaged LF 1 bumps soldered very well even with a limited amount of flux. Both stencil printing and flux jetting were insensitive to bump damage and resulted in good soldering for all three alloys. The no-flow encapsulation process>gave good soldering with both tin-lead and LFl chips when used with the appropriate reflow profile. Since the reflow encapsulant material is designed for use with tin-lead, the encapsulant gelled before soldering of the LF2 bumps occurred.