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.

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Despite the tackiness offered by ball attach fluxes, they have failed to prevent ball movement on Solder on Pad (SoP) finishes when the solder protrudes above the solder mask. Dippable solder paste provides the tackiness needed for these adverse situations as well as small solder particles that inhibit ball rolling due to mechanical or convective forces downstream. Also, other pad finishes like OSP has issues with oxidation with partial pad wetting as a common defect. The use of dippable paste has also demonstrated relief with these wetting related issues. The most current application of dippable pastes is in the assembly of stacked BGA's also known as Package on Package (PoP). Although water clean formulations are typical in BGA assembly, PoP assembly typically utilizes no clean formulations. Although tacky flux has been successful in many PoP applications, issues such as coplanarity and package tolerances that require increasing the sphere size during assembly have found benefit from dippable solder paste.

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