Circuits Assembly - September 2008 - (Page 46)

Pb-Free Rework Solder Joint Reliability of Different BGAs Reworked Using Low Melting Point Pb-Free Alloys By Simin Bagheri, Polina Snugovsky, Zohreh Bagheri, Craig Hamilton and Heather McCormick Under testing, uniform joint microstructures and reliable joints were observed. Ed.: For the complete article, please see circuitsassembly.com/ cms/content/view/7054. T he initial low melt rework investigation was performed in four sequential phases on RIA2 test vehicles. The first phase was an investigation of joint microstructures resulting after rework using three different low melt alloys. Three commercially available alloys were selected (Table 1). Alloys A and C were alloys containing indium, and Alloy B was a Bi-containing alloy. In this phase, all BGAs on a limited number of RIA2 boards with OSP finish were reworked in air. Hot gas rework profiles were created and optimized for each solder paste. The criteria for rework parameter optimization were proper shape of solder joint after rework, minimized voiding, uniform microstructure and absence of or reduced low-melt phases in the solder joint. In the second phase, optimized rework profiles were used. This phase was performed on a limited number of RIA2 boards with an ENIG finish. Only alloys A and B were included for this part of the investigation. Alloy C was dropped off the matrix due to the unknown properties of the high In-content solder joints.5 The third phase compared 1X and 2X rework processes in air and nitrogen environments. In this phase, boards with immersion Ag, OSP and ENIG finishes were reworked using alloy A. The influence of rework on adjacent components, PBGA 196 and CSP 46 (Figure 1) was also investigated at this stage. Two scenarios were analyzed. First, both PBGA 196 and CSP 46 were removed at the same time. Then the CSP 46 component was replaced using the low melt In-containing alloy A. After that, the PBGA 196 was placed using the same low melt alloy. The second scenario was more thermally challenging. The CSP 46 component was removed and a new component was placed using low melt In-containing alloy A. Then the PBGA 196, which was originally assembled with SAC 385 solder paste, was removed using high heat to melt the SAC alloy. Finally, the new PBGA 196 was placed using low melt In-containing alloy A. In the fourth phase, the influence on thermal fatigue life of each of 1X and 2X reworked components using optimized profiles was studied on limited RIA2 boards using alloy A and alloy B. On all boards, two identical BGAs were assembled. During this phase, one of each component was reworked using low melting alloys and one remained untouched (primary attach). After low melt rework, the boards were tested using accelerated thermal cycling (ATC). Results indicated solder joints reworked using In-containing alloy A performed better or comparable to primary attach joints. Joints reworked with Bi-containing circuitsassembly.com 46 Circuits Assembly SEPTEMBER 2008 http://circuitsassembly.com/cms/content/view/7054 http://circuitsassembly.com/cms/content/view/7054 http://circuitsassembly.com

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Circuits Assembly - September 2008 Contents Caveat Lector Industry News Market Watch Talking Heads Focus on Business Global Sourcing On the Forefront Screen Printing Better Manufacturing Reflow Soldering with a SnCu Eutectic Pb-Free Alloy Improving OEE in High Mix Facilities Effectively Managing RF Design in Utility Metering Applications Solder Joint Reliability of Different BGAs Reworked Using Low Melting Point Pb-Free Alloys Tech Tips Wave Soldering Pb-Free Lessons Learned Materials World Process Doctor Equipment Advances Product Spotlight Ad Index Assembly Insider Technical Abstracts

Circuits Assembly - September 2008

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