The Impact of Solder Bump Voiding on Flip Chip Reliability

This paper investigates the reliability impact of copper doped eutectic tin-lead alloys, in association with solder voiding, upon flip chip bumps.

Reflow and differential scanning calorimetry analyses were conducted to determine feasible copper concentrations. Alloys with 62Sn-37Pb-XCu, where X is less than 4 % were then evaluated through wafer bumping, flip chip assembly, and board-level reliability tests.

Excellent results were obtained through each stage of the development. The reliability assessment tests included thermal cycle testing, high temperature storage, and high temperature operating life with test currents up to 350 milliamps. Failure modes of interest include solder bump fatigue, under-bump metallization consumption, and electro-migration.

Relatively large test die, 10mm x 10mm, with 250 and 200 micron pitches were used in the study. Flip chips were assembled to nickel-gold and OSP/copper finish laminate boards having a thickness of 0.8mm. Substrate cleaning and standard underfill protocol assembly was followed.

A significant incidence of solder voiding was observed and its effect upon reliability performance is also documented. X-ray analysis was used to statistically measure the incidence of solder voiding. The impact of solder voiding upon flip chip solder joint reliability has been a controversial topic. There is a concern that very large solder bump voids; 50 to 70 % of the bump diameter, either created by poor pad wetting or poor bump processing, can lead to current crowding and poor solder bump features susceptible to early solder fatigue.

Flip chips were prepared with an increased incidence of solder voiding during the bumping process and their reliability performance was compared to low-level voided solder bumps. The susceptibility of assembled flip chips to damage during handling was also evaluated by board-bending tests with low-level and high incidence solder bump voiding.

It is shown that increased solder voiding does not significantly impact the reliability of the solder joints nor make them more susceptible to damage from handling.