Squeegee bump technology

An innovative solder bumping technology, termed squeegee bumping, has been developed at Motorola´s Interconnect Systems Laboratory that uses baked photoresist as a mask for solder printing to deposit fine pitch solder bumps on wafers. This process provides much better alignment accuracy and is capable of bumping finer pitch devices than stencil printing technology. Solder paste printing uses a screen printer similar to that used for stencil printing. Greater versatility of solder materials can be obtained through solder paste than the electroplating. Cost modeling shows that the squeegee bump technology has a significant cost benefit over controlled collapse chip connection (C4) technology. This is because the C4 process has very low efficiency in labor and materials usage. Statistical process control data show an average bump height of 118 ± 3.5 μm, and a maximum-to-minimum bump height range of 17 μm over a 150 mm-diameter wafer and have been produced repeatedly on test wafers with 210 μm peripheral pitch. A 109.6 ± 1.3 μm bump height on orthogonal array with 250 μm pitch has been successfully demonstrated with greater than 90% die yield. Bump reliability has been studied using both multiple reflows and extended thermal/humidity storage procedures. No degradation of shear strength was observed after up to 10 × reflows and 1008 h of a thermal/humidity stress environment. Bump reliability was also evaluated by assembling squeegee bumped dice on a plastic chip scale package (CSP). Liquid-to-liquid thermal shock cycling at a temperature range of -55°C to +125°C had a characteristic life of 2764 cycles with a 1st failure at 1050 cycles. No failures were observed after 432 h of autoclave stress at 121°C, 100% RH, 15 psig test condition