Development and reliability studies of stencil printing solder-bumped flip chip technology

Flip chip offers many advantages including more I/O count at a limited chip size, enhanced electrical and thermal performance, self-alignment capabil-ity, SMT equipment and process compatibility, low bumping cost, low profile and light weight. The development of a low-cost, reliable solder-bumped flip chip technology is essential to the application of flip chip technology. A low-cost stencil printing process with electroless Ni/Au under bump met-allurgy( UBM) was developed. The zincation is an indispensable step to deposit electroless Ni on aluminium pads. The immersion potential of aluminium sub-strates during zincation was measured. The effect of aluminium substrate on the zincation morphologies was studied. The zinc films produced by two zincation solutions were compared. Single and double zincation pretreatments were con-ducted. The mechanical properties of Ni bumps grown on aluminium pads by single and double zincation pretreatments were compared. The thermal aging testing and the temperature-humidity testing with various test time were per-formed for the samples with bumps. The solder bumps at different aging time were cross-sectioned and Intermetallic compound (IMC) growth was examined. The influence of UBM surface roughness and immersed gold amount on the IMC growth was discussed. The shear tests were performed after thermal aging test. The relation between shear strength and IMC growth was established. Fine pitch stencil printing was achieved by optimizing the stencil design and printing parameters. The bridging was the major defects for fine pitch printing and could be avoid by selecting proper stencil design. 8 test runs designed by Taguchi method were carried out. The major printing process parameters includ-ing printing force, printing speed, separation speed and snap-off were studied. The analysis of mean and analysis of variance were conducted to determine the effect of each factor. An optimal combination of printing parameters was pro-posed to achieve high transfer efficiency and meet the requirements of defects rate at the same time