Circuits Assembly - September 2008 - (Page 47)

Pb-Free Rework alloy (alloy B) failed before the nonreworked component; therefore, alloy B was excluded from further analysis. Experiment Based on results obtained in the initial investigation, In-containing alloy A was chosen for further analysis in Stage 2. In Stage 2, a comprehensive investigation of the thermal fatigue reliability of the low melt reworked solder joints was performed per IPC-9701.1 BGAs on RIA3 boards were reworked using In-containing alloy A. The reworked solder joint performance was then compared to that of the primary attach joints, as well as to components conventionally reworked using SAC 385 paste. The RIA3 test vehicle was 8" x 10" with 12 copper layers and available in 0.093" and 0.125" thicknesses. This TV was designed to represent a mid-range complexity product with greater assembly process challenges, and was made from a Pb-free compatible laminate material capable of withstanding the higher temperature requirements of Pb-free processing. Surface finishes used on the RIA3 test vehicles for this investigation were immersion silver (ImAg) and electrolytic nickel immersion gold (ENIG). This test vehicle is daisy-chained and permits four wire in-situ monitoring of the components during ATC (Figure 2, online). The RIA3 TV covers a range of component technologies (Table 2). In the TV design, two PBGA 196 components were added at the bottom side for the rework cells, which were a mirror image of the two PBGA 196s populated on the topside. Primary SMT assembly was performed with no-clean SAC 385 paste using a standard 10-zone reflow oven, in air. One-time hot gas rework was performed on all BGAs on RIA3 TVs with different thicknesses. Site re-dressing was performed following component removal. Low melt solder (In-containing alloy A) was then applied to the re-dressed sites for all components. The rework process was performed under nitrogen for all boards. Figure 3 (online) shows locations of the reworked components. The matrix for this investigation is in Table 3. circuitsassembly.com Cell 6-1 and 6-2 were designed to investigate the thermal fatigue reliability of solder joints formed on all components reworked using low melt In-containing alloy A on 0.093" boards with ImmAg and ENIG finishes. Cell 6-3 and 6-4 were included to assess similar properties, but on 0.125"-thick boards. These low melt rework cells 6-1 to 6-4 were incorporated in a much larger experimental matrix in Celestica’s RIA3 Lead-Free project. This project included conventional rework as well. A comparison between low melting and conventional rework was also performed. Microstructure of Reworked Solder Joints As-assembled and reworked solder joints were examined before and after thermal cycling using optical and scanning electron microscopy and x-ray analysis. Differential scanning calorimetry (DSC) analysis was also performed to study the solder joint solidification after mixing of the low melt solder with SAC 385 balls during rework. The DSC results showed, if full mixing was achieved, the low melt liquid was completely consumed and the resulting composition crystallized at a reasonably high temperature range. In general, in this study, the low melt solder paste was melted at temperatures below the SAC 385 solder ball melting point. The solid solder balls dissolved in the molten solder. In general, the dissolution process depends on the rework parameters of temperature and time, and may cause full or partial mixing. In addition to temperature and time, the ratio between solder ball and solder paste volume is especially important for complete mixing, and causes different microstructures for different components. Snugovsky, et al described the theory of solid solder ball dissolution in molten solder.7 Metallurgical analysis was performed on all solder joints after rework, and their microstructure was characterized. No time-zero defects or open joints were found using a BGA scope, optical microscopy or SEM. Solder joints were properly formed, collapsed normally and wellshaped (Figure 5). It was also observed that for all reworked joints, the micro- Table 1. Melting Points of Low Melt Alloys Solder Alloy Alloy A Alloy B Alloy C Melting Temperature, °C 181-187 138 118 Figure 5. Microstructure of reworked solder joints, In-containing alloy A: (a, top) CBGA, (b) PBGA 196. structure was uniform and fully mixed. There was no portion of initial solder ball visible in the cross-sections of reworked solder joints. This finding confirms the component solder balls were fully dissolved in the liquefied solder paste during reflow, and the resulting liquid solidified during cooling. Transmissive x-ray was used to inspect the assemblies for defects and to assess the level of voiding. No assembly defects were noted, but all cells showed some voiding. In all cases, voiding was within acceptable levels per IPC-A-610-D.9 The DSC heating curves for reworked CBGA and PBGA196 solder joints using In-containing alloy A are illustrated in Figure 6. These curves confirm the metallurgical observations reported above. As shown in Figure 6a, for reworked CBGAs, the melting of a joint starts at 208°C, 17°C higher than the In-containing solder paste melting point. It melts in a relatively narrow temperature range, and stops melting at 212°C. This range is 47 Circuits Assembly SEPTEMBER 2008 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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