Authors: Dr. Benlih Huang and Dr. Ning-Cheng Lee Company: Indium Corporation of America Date Published: 1/1/2002
Abstract: Several unique solder paste systems have been developed and tested for 63Sn/37Pb solder bumping for wafer, CSP, and BGA with the low cost print-detach-reflow process. The results indicate that the bump height achieved is very adequate and consistent for all three area array package systems. Microstructure of solder bumps appears normal. The yield is also very high for both before reflow and after reflow condition, and is dictated by printing performance. With the unique high slump resistance exhibited by those newly developed pastes, the paste transfer efficiency at printing stage becomes the most critical performance for this process. The transfer efficiency increases with increasing area ratio, increasing taper angle, decreasing pitch, decreasing stencil thickness, decreasing challenge, with adoption of square aperture design, and is not sensitive to aspect ratio of aperture to solder particle size. The paste systems appear to have more potential for depositing a larger amount of paste per unit pitch, as evidenced by the linear relation between expected paste volume and the deposited paste volume. Increasing metal content helps improving bumping performance. The bottleneck of increasing bumping performance for wafer applications appears to be developing a stencil manufacturing technology capable of providing an aperture pattern with spacing considerably smaller than the stencil thickness. Slow print speed is also essential for adequate printing. A non-shiny non-smooth stencil surface is considered beneficial for aiding paste rolling. The flux residue of those pastes is cleanable with solvents.
Key words: solder, bumping, solder paste, area array package, BGA, CSP, wafer, reflow, high speed, high throughput, low cost.