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Today more than ever, there is a high demand for low cost, powerful, reliable electronic products. Coupled with the global trend of product miniaturization, electronic circuitry must also be reduced in size and designed for higher density. Flip-chips are in theory the most efficient electronic package available today, supporting the highest I/O counts per area. However, there are a number of issues that currently limit the use of this technology in industry. The two most important, of course, are cost and reliability. One of the most expensive elements in the application of flip-chips is the bumping process. Bumping is the means of applying a solder alloy to the bonding pads of a silicon wafer that embodies all of the individual IC chips. This research explores a non-traditional method of bumping wafers by screening paste through stencil apertures. The utilization of the stencil printing machine on a standard surface mount assembly line offers a drastically reduced cost wafer bumping method. A variety of statistically valid experiments were conducted based on observation and measurement of the resultant reflowed solder bump sizes on the wafers. Performance of this bumping method was determined by mean bump height, paste transfer efficiency, standard deviation, and yield for two wafer test vehicles. The first experiments, conducted on the Test Vehicle A wafers, utilized two stencils. The first of two stencils that were used for bumping produced an array of reflowed bumps that were insufficiently tall. The second stencil used was modified with larger aperture openings which produced taller bumps, gave improved transfer efficiency, and reduced the overall scatter in the distribution of refowed bumps. Numerous statistical tests were performed on the measured bump height data to compare the influences that stencil design attributes and printing conditions have on these bump distributions. Some of the factors that were found to be significant include aperture area ratio, aperture orientation, stencil foil orientation, die location on the wafer, and pad location (with respect to position on the die). It was also discovered that using the Gerber designed aperture sizes to calculate aperture area ratios and corresponding transfer efficiencies can be an unsafe practice, resulting in inaccurate estimates of printing performance. In the second major investigation, the Test Vehicle B wafers were bumped by stencil printing using three stencil designs and two different stencil vendors. Stencils were produced with a variety of oblong, rectangular, circular, and square shape apertures in order to compare the differences in the resulting transfer efficiencies and corresponding bump distribution scatter. It was found that the aperture area ratio accounts for most of the influence on the resulting bump size rather than aperture shape. However, for equivalent area ratio apertures, the oblong gave a higher transfer efficiency than the rectangle and the square slightly exceeded the circle. In terms of bump scatter, there was no observed dependence on aperture shape. Nonetheless, there was a notable difference between the results between the bump distributions generated from the same apertures among the two stencil manufacturers. Another significant discovery was the impact of setting a delay period between the printing stroke and release process, leading to the reduction in bridging defects. Further statistical tests were conducted on the measured bump height data to compare the influences that stencil design attributes and printing conditions have on these bump distributions.


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