Solder paste printing and stencil design considerations for wafer bumping

Recently wafer bumping using solder paste with very fine solder powder has come into focus as more cost effective than conventional sputtered or plated methods. This additive method revolves around a stencil printing process similar to conventional SMT with the exception of extremely small pitch and desired deposit size. An optimal print process for a high aperture density design has been established. Print process variables such as squeegee type, print gap, and separation speed are tested and quantified as to their effect of deposit variability. This study was done on 200mm wafers and involved over 6 million bump deposits. Results of a second study on the effects of stencil aperture size and shape combined with several stencil thicknesses on bump height and diameter variability are presented. This second study was done with both water soluble and no clean formulations and involved a total of 135 variables and over 68,000 measured bumps. A more recent study designed to push paste performance and stencil design to their limits in order to determine the maximum bump height possible in full array designs down to 150 micron pitch is discussed with preliminary results presented. The combination of the findings of all three studies results in a print process and stencil design recipe for producing consistent wafer bumps of various sizes.