Surface Mount International Conference Proceedings


Author: Chingchen S. Chiu
Company: Iridium Corporation of America
Date Published: 4/28/1997   Conference: Surface Mount International

Abstract: The solder bumping process for BGA is investigated by using solder paste alone, solder spheres with solder paste, and solder spheres with fluxes. Also explored is the use of integrated preform together with either flux or solder paste. For bumping process involving Sn62 or Sn63 spheres, use of paste for sphere attachment produces excellent alignment results. In the case of using fluxes for Sn62 or Sn63 sphere attachment, the defect rate increases with decreasing flux viscosity, decreasing solvent volatility, decreasing pitch dimension, increasing flux deposition thickness, increasing flux activity, and increasing pad diameter. For overall better yield, a solder paste with long stencil life, good printability, and good solder ball performance should be the most promising eutectic sphere attachment material. For systems using pastes for Sn10 sphere attachment, no missing is observed, and the alignment improves with decreasing paste deposition thickness, decreasing solvent volatility, increasing sphere solderability, increasing flux activity, increasing pad dimension, increasing metal load, increasing pad solderability. Paste viscosity, pitch, and reflow profile has negligible effect on the Sn10 bumping yield using Sn63 solder paste. An easily releasable solder paste is crucial for area-array BGA if a regular print-release process is desired for bumping with solder paste alone. Bumping with integrated preform is promising. Reducing the thickness and width of the solder link is considered essential for improving the bumping success rate. Other potential bumping processes may include (1) dispense paste/reflow, (2) print paste/reflow/release, (3) apply solder mask/print paste/release /reflow/strip solder mask, (4) solder jet/reflow, and (5) sphere welding, and are briefly introduced and commented on. Key words: solder, balling, bump, bumping, BGA, solder sphere, solder paste, integrated preform, flux, defect rate

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