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Issues with using Indium for Wafer Bumping

Lets talk about some issues…

The first thing that I am worried about is the use of a small particle size of Indium-containing alloys. Indium is self-passivating, and will clump and cold weld to itself, even when stored as powder. For this reason we look at each individual case separately. Normally, we do not recommend the use of Indium alloys for solder powders that are smaller than Type 4 (20-38μm). For small aperture sizes, you would need a Type 5 (20-25μm). The smaller the powder size, the larger the surface area, so as the indium-containing powders get smaller, the more tendency to cold-weld in the packaging.
Which leads us to my second concern, which is the higher metal percentage in wafer pastes. Usually, in order to print through the smaller apertures (and lower area ratios) for wafer bumping, the solder paste has a higher metal percentage. For these wafer pastes, the metal percents are usually >92%. Which makes them very prone to cold-welding.
For example, the area ratio for an aperture opening of 140μm with a 90μm thick stencil is 0.39.  Area ratios that are below 0.50 are not recommended.
We can physically manufacture the paste, but whether it will be useable when you get it is the problems.

More information may be found at IKB: Indium Knowledge Base.