Surface Mount International Conference Proceedings


Author: Tony Huang
Company: Adaptec
Date Published: 8/23/1998   Conference: Surface Mount International

Abstract: The eutectic solder balls of PBGA (Plastic Ball Grid Array) packages tend to collapse at about 0.15 mm (6 mils) to 0.20 mm (8 mils) during the reflow soldering process’ . In fact, the actual collapses depend on the solder paste volume, solder ball coplanarity, package weight, PCB pad size, reflow temperature profile, etc. Due to the controlled collapse characteristics, the BGAs, which use the eutectic solder balls, require less compliant coplanarity specifications compared with “high-temp- solder-ball” BGAs such as CBGAs. Indeed, the solder ball coplanarity of BGA packages is a critical factor impacting not only the yield of assembly processes but also long term solder joint integrity2, especially for the larger packages. JEDEC has recently changed the coplanarity tolerance specifications on “eutectic-solder-ball” BGAs with 1.27 mm and 1.5 nm ball pitches from 6 mils to 8 mils. Change of the specification by JEDEC has brought up some concerns in the industry. Numerous reports have documented how the PBGA solder joint formation is impacted by solder ball collapse behaviors and how solder joint integrity impacts solder joint life fatigue. Therefore, a study was conducted to examine the sensitivity of solder joint integrity for PBGA assembly and rework process yields to the change of the solder ball coplanarity from 6 mils to 8 mils as well as to increase the overall comfort level with respect to the specification change. The study considered the following experiment variables:  Solder ball coplanarity  Stencil thickness and aperture design  Wave solder requirement  lRework method - dispensing paste or flux Electrical continuity tests were performed after PBGA assembly and rework. Accelerated thermal cycling and thermal shock were done to measure the solder joint reliability. Additionally, standoff heights of PBGA solder joints were measured through cross sections. Statistical comparisons of mean standoff height between different PBGA coplanarity groups, where different experiment variables were applied, allowed for determination of the variation of BGA solder joint integrity. Key Words: Plastic Ball Grid Array (PBGA), Coplanarity, Reliability of Solder Ball Joints, and Cross Sectioning Analysis.

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