Effect of solder volume on joint shape with variable chip-to-board contact pad ratio

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Jesudoss, Pio
Mathewson, Alan
Wright, William M. D.
O'Flynn, Brendan
Stam, Frank
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The objective of this paper is to investigate the effect of the pad size ratio between the chip and board end of a solder joint on the shape of that solder joint in combination with the solder volume available. The shape of the solder joint is correlated to its reliability and thus of importance. For low density chip bond pad applications Flip Chip (FC) manufacturing costs can be kept down by using larger size board pads suitable for solder application. By using “Surface Evolver” software package the solder joint shapes associated with different size/shape solder preforms and chip/board pad ratios are predicted. In this case a so called Flip-Chip Over Hole (FCOH) assembly format has been used. Assembly trials involved the deposition of lead-free 99.3Sn0.7Cu solder on the board side, followed by reflow, an underfill process and back die encapsulation. During the assembly work pad off-sets occurred that have been taken into account for the Surface Evolver solder joint shape prediction and accurately matched the real assembly. Overall, good correlation was found between the simulated solder joint shape and the actual fabricated solder joint shapes. Solder preforms were found to exhibit better control over the solder volume. Reflow simulation of commercially available solder preform volumes suggests that for a fixed stand-off height and chip-board pad ratio, the solder volume value and the surface tension determines the shape of the joint.
Flip Chip Over Hole
Jesudoss, P., Mathewson, A., Wright, W., O'Flynn, B., Stam, F., 2008. Effect of solder volume on joint shape with variable chip-to-board contact pad ratio. In: IMAPS (International Microelectronics And Packaging Society)-CPMT(Components, Packaging and Manufacturing Technology) 32nd International Microelectronics and Packaging IMAPS-CPMT Poland Conference. Warszawa-Pułtusk, Poland 21-24 September 2008.
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