Printing Solder Paste in Dry Film - A Low Cost Fine-Pitch Bumping Technique

There is a steady trend to decrease bump pitch and the total cost for the bumping process. Several techniques for bumping are available such as screen printing of solder paste through metal stencils, electroplating of bumps, ball placement or C4NP. Initially there appears to be a number of choices for bumping, however after looking at the details the choices are significantly reduced. The use of ball placement or screen printing is limited to larger sized bumps and coarser pitches. Bumping by electroplating which is popular for fine-pitch bumping is restricted to single or double component systems. Electroplating of triple component systems like SnAgCu which are interesting for lead free bumping requires a very complex process monitoring. Additionally, electroplating of ternary metal systems is not yet suitable for manufacturing. In this paper the basic process steps for a different bumping technology are discussed in detail. This technology uses a solder paste which is printed into photo-structured dry film. Test wafers with Ni + flash-Au final metallisation were provided. A dry film photo resist was laminated onto the wafers, exposed and developed. Next, solder paste was printed into the structured resist. The wafers were then passed through a Pb-free reflow profile. After the reflow, the dry film was successfully stripped. A variation of pad size and resist openings were investigated. Correlation between design and results are discussed.