This project was organized to assess the process parameters for assembling Pb-free SnAgCu BGAs under the temperature constraints of a conventional tin-lead (SnPb) assembly process. The objective was to understand the reliability of the resulting mixed-alloy solder joints. For companies taking the RoHS exemption and continuing to manufacture SnPb products, there is a growing issue with the lack of availability of SnPb components. Many companies may be compelled to use Pb-free BGAs in an SnPb process, for which the process and reliability have not yet been characterized. The first phase of the project characterized the peak temperature effects on Pb-free BGAs in SnPb paste. The second phase studied the reliability of Pb-free BGAs processed within the temperature constraints of SnPb assembly conditions.
A careful experimental investigation evaluated the reliability of solder joints of SAC BGA components formed using SnPb solder paste. This evaluation specifically looked at the impact of package size, solder ball volume, printed circuit board (PCB) surface finish, time above liquidus and peak temperature on reliability. Four different BGA package sizes (ranging from 8 to 45 mm2) were selected with ball-to-ball pitch size ranging from 0.5mm to 1.27mm. Two different PCB finishes were used: electroless nickel immersion gold (ENIG) and organic solderability preservative (OSP) on copper.
Four different profiles were developed with the maximum peak temperatures of 210°C and 215°C and time above liquidus ranging from 60 to 120 seconds using Sn-Pb paste. One profile was generated for a lead-free control. A total of 60 boards were assembled. Some of the boards were subjected to an as assembled analysis while others were subjected to an accelerated thermal cycling (ATC) test in the temperature range of -40°C to 125°C for a maximum of 3500 cycles in accordance with the IPC-9701A standard. Weibull plots were created and failure analysis performed. Analysis of as-assembled solder joints revealed that for a time above liquidus of 120 seconds and below, the degree of mixing between the BGA SAC ball alloy and the SnPb solder paste was less than 100 percent for packages with a ball pitch of 0.8mm or greater. Depending on package size, the peak reflow temperature was observed to have a significant impact on the solder joint microstructural homogeneity. The influence of reflow process parameters on solder joint reliability was clearly manifested in the Weibull plots.
Paper & Presentations
Solder Joint Reliability of Pb-free Sn-Ag-Cu Ball Grid Array (BGA) Components in SnPb Assembly Process, Robert Kinyanjui, Ph.D. (Sanmina-SCI Corporation, Quyen Chu (Jabil), Polina Snugovsky, Ph.D. (Celestica Inc.), Richard Coyle, Ph.D. (Alcatel-Lucent), presented April 2, 2008, IPC Expo, APEX and the Designers Summit, Session S15 — Backward Compatibility — Where Leaded and Lead Free Collide (Las Vegas, Nevada, USA). This paper provides a discussion of the impact of various profiles' characteristics on the extent of mixing between SAC and SnPb solder alloys and the associated thermal cyclic fatigue performance.
Pb-Free BGAs in SnPb Assembly Project, Robert Kinyanjui (Sanmina-SCI), Quyen Chu (Jabil), iNEMI Lead-Free Forum, April 1, 2008, IPC Expo, APEX and the Designers Summit (Las Vegas).
PROJECT WEBINAR (members only): Pb-Free BGAs in SnPb Assemblies Project, Phase II (October 4, 2007)
Presentation: iNEMI Council of Members meeting held at SMTA International, October 11, 2007 (Kissimmee, Florida)
Pb-Free BGAs in SnPb Assembly Project, Robert Kinyanjui (Sanmina-SCI) and Quyen Chu (Jabil), iNEMI Forum: Optimizing Lead-Free Processes, February 20, 2007, IPC Printed Circuits Expo, APEX and the Designers Summit (Los Angeles, California).