SMTA China Conference Proceedings


RELIABILITY IMPACT OF COPPER-DOPED EUTECTIC TIN-LEAD BUMP AND ITS VOIDING UPON FLIP CHIP ASSEMBLIES

Authors: David Ihms and Shing Yeh
Company: Delphi Electronics & Safety
Date Published: 4/8/2008   Conference: SMTA China


Abstract: This paper investigates the reliability impact of copper doped eutectic tin-lead (Sn-37Pb) alloys, in association with solder voiding, upon flip chip bumps. Previous work demonstrated that the addition of 1% copper in eutectic SnPb bump has significantly improve flip chip reliability against diffusion related failure mechanisms [1]. It is believed that additional copper doping should further improve flip chip reliability. Reflow experiments and differential scanning calorimetry analyses were conducted to determine feasible copper concentrations. Alloys with Sn-36Pb-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 (HTS), and high temperature operating life (HTOL) with test currents up to 600 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 (NiAu) 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 in the SnPbCu bumps 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 large solder bump voids; > 50% 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 typical voided solder bumps from current production environment. The susceptibility of assembled flip chips to damage during handling was also evaluated by board-bending tests with typical-level and high incidence solder bump voiding. It is shown that the increased solder voiding does not significantly impact the reliability of the solder joints nor make them susceptible to damage from handling. However, the addition of Cu in the Sn-37Pb solder has significantly increased flip chip reliability, particularly against diffusion related failure mechanisms, regardless of the increasing level of bump voiding.

Key words: SnPbCu, flipchip, reliability, solder voiding



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