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Papers about volatility

• Voiding Mechanism in BGA Assembly

  by Wanda B. O’Hara, Dr. Ning-Cheng Lee

  Voiding in BGA assembly using Sn63 solder bumps is primarily introduced at board-level assembly stage. On the pretinned PCBs, voiding of BGA joints increases with increasing solvent volatility, increasing metal content, and increasing reflow temperature, and with decreasing powder size. This can be explained by a viscosity dictated flux-exclusion-rate model. In this model, a higher viscosity in fluxing medium at reflow temperature could hinder the exclusion of flux from the interior of molten solder, hence increase the chance of outgassing due to the increasing amount of entrapped flux, and consequently result in a higher voiding in BGA assembly. Flux activity and reflow atmosphere appear to have negligible effect on voiding when the solderability of the immobile metallization is not a concern. Increase in void content is accompanied by an increase in fraction of large voids. This suggests that, similar to voiding phenomena in SMT process, factors causing voiding in BGA will have an even greater impact on the joint reliability than what shown by the total-void-volume analysis results.

  void, BGA, viscosity, volatility, flux-exclusion-rate, soldering, pb-free, lead-free

  [Permanent Link to this Paper]

  Posted on 1 Jan 2009

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