Circuits Assembly - September 2008 - (Page 62)

Pb-Free Rework Continued from pg. 48 CBGA. As can be seen in Figure 9 (online), cell 6-1, the thermal fatigue reliability of the solder joints formed after rework using low melt In-containing alloy A is significantly better than that of the primary attached joints where SAC 385 alloy was used. A similar trend was observed for the other cells. Figure 10 (online) compares the Weibull plots of reworked solder joints formed on thick (0.125") boards with different surface finishes. This illustrates CBGAs reworked using the low melt In-containing alloy performed slightly better on boards with ENIG than on ImAg. Figure 11 shows the performance of low melt reworked CBGAs on boards with different thicknesses. No difference was observed between cells 6-1 and 6-3, which had board thicknesses of 0.093" and 0.125", respectively. Weibull plots for all four low melt reworked cells are shown in Figure 12 (online). The thermal fatigue reliability of CBGA components reworked using low melt In-containing alloy is very similar for all cells. Figure 13 (online) compares thermal fatigue reliability of low melt reworked CBGAs on thick boards (0.125") with ImAg and ENIG finishes with that of the conventionally reworked components on 0.125"-thick boards with OSP finish. The conventional rework was part of the original matrix and was performed on the same test vehicle using the optimized rework process parameters. The surface finish on the conventionally reworked cell was OSP. These plots illustrate the performance of low melt reworked components in both cells 6-3 and 6-4 is comparable to that of the conventionally reworked parts. In general, CBGAs reworked using the Incontaining alloy (with a final resultant joint composition of 3.7-3.8 % Ag, 3.8-4.0 % In, 0.4-1.0 % copper, and remainder tin) outperformed non-reworked pure SAC 385 joints. Figure 14 (online) shows the 1% failures for reworked and primary cells 6-1 and 6-2. The 1% failure for the low melt reworked components is better than that of the primary attach. Reworked solder joints outperform as- assembled joints formed on ImAg and ENIG by a factor of 2 and 6, respectively. Conclusions The alloy of this low melt rework process contains indium and has a melting temperature close to that of SnPb eutectic. When the low melt solder paste melts, dissolution of the SAC balls occurs. When the rework process parameters such as time and temperature are properly controlled, the result is full mixing of the solder ball with the solder paste, forming a uniform joint microstructure. The reduced temperature of the process prevents component overheating, reduces risk of board warpage and pad cratering, and prevents neighboring and mirror-imaged components from thermal damage. In general, thermal fatigue reliability of low melt reworked components is comparable to or slightly better than that of the as-assembled and conventionally reworked components. There is not a statistically significant difference between the thermal fatigue reliability of low melt reworked components on different surface finishes and on boards with different thicknesses. Reworking BGAs using the low melting alloy provides an excellent combination of processability and reliability. In-containing alloy A, in combination with modified process parameters such as solder paste volume and reflow profile, may be recommended for manufacturing rework and field failure rework. Use of this alloy results in high-reliability solder joints without damage to boards, packages and adjacent components. n Instant Online Quotes, Day or Night SPECIAL LIMITED-TIME PROMOTION Delivered, all testing and shipping included: Single/Double-Sided $79.99 1st piece $29.99 up to 5th piece allow 7 working days Even One Piece, We Care!! 4-Layer $129.99 1st piece $79.99 up to 5th piece allow 8 working days Acknowledgments The authors would like to thank Joel Trudell of Celestica for rework profiling and Russell Brush of Celestica for ATC testing and data analysis. Ed.: This paper was originally presented at SMTA Toronto 2008 and is published with permission. Simin Bagheri, Polina Snugovsky, Zohreh Bagheri, Craig Hamilton and Heather McCormick are with Celestica (celestica.com); sbagheri@celestica.com. 6-Layer $149.99 1st piece $99.99 up to 5th piece allow 9 working days Based on standard speciﬁcations. See the HOME page and Calculate for other options. Register to receive other special promotions. www.jetpcb.com/eng VISIT WWW.JETPCB.COM/ENG See us at PCB West, Booth 15, and the FOR OTHER PROMOTIONAL Electronics Assembly Show, Booth 2430 LOW-COST PCB PROTOTYPES circuitsassembly.com 62 Circuits Assembly SEPTEMBER 2008 http://www.bareboard.com http://www.bareboard.com http://www.jetpcb.com/eng http://celestica.com http://celestica.com 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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