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Accomplishments in lead-free flip chip wafer bumping using stencil-printing technology

The present study intends to reveal the key process issues involved in stencil printing technology for very- fine- pitch wafer bumping up to 120µm pitch. In parallel, in face of the upcoming strict regulations for introduction of lead-free back-end processes, it provides insight into printability issues of newly developed lead-free pastes and attempts a comparison with their eutectic Sn63Pb37 counterparts. Type 6 Lead-free pastes with a powder size of 5-15µm were prepared with Sn4%Ag0.5%Cu and Sn3.5%Ag compositions. Their rheological behaviour and product stability were carefully examined using as guide well established type 6 eutectic Sn63Pb37 pastes.

Printing experiments have revealed the significance of stencil design parameters, print speed, print pressure, separation speed and shear thinning behaviour of the pastes to the success of the wafer printing up to 120µm pitches. Laser-cut stencils of 75µm were used for bumping of 300µm and 200µm pitch peripheral arrays. Furthermore, the study extended up to 120µm pitch peripheral and area arrays using an electroformed 30µm thick stencil. Bumping processes have yielded bump heights of 129 ± 2.6 µm and 110 ± 4.5µm for 300µm and 200µm pitch peripheral configurations, respectively. For very fine pitch structures, lead-free bumping has resulted in bump heights of 57.2±2.6µm and 41.6±2.8µm for 120µm pitch peripheral and area arrays, respectively. Cleaning of flux residues, at least for the flux used in these pastes, does not seem to differ from cleaning of the corresponding eutectic pastes. Shear tests have provided strength values of 4.26 g/ml² for the Sn4%Ag0.5%Cu bumps and 3.07 g/mil² for the Sn3.5%Ag bumps.

Reported by

Fraunhofer IZM/technical University Berlin
Gustav-meyer-Allee 25
13355 Berlin
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