Circuits Assembly - September 2008 - (Page 48)

Pb-Free Rework Table 2. Component Descriptions Comp. Type CBGA PBGA PBGA PBGA CSP I/O 937 676 256 196 64 Pitch, mm 1 1 1.27 1 0.8 Ball Composition (wt. %) SnAg3.8%Cu0.5% SnAg3.8%Cu0.5% SnAg3.8%Cu0.5% SnAg3.8%Cu0.5% SnAg3.8%Cu0.5% Table 3. Rework Matrix Cell 6-1 6-2 6-3 6-4 Thickness 0.093” 0.093” 0.125” 0.125” Surface Finish ImAg ENIG ImAg ENIG Rework N2 N2 N2 N2 ATC Profile 0-100°C 0-100°C 0-100°C 0-100°C Environ. much below 217°C, the melting temperature of SAC solder balls. Reworked PBGA 196 solder joints finish melting at an even lower temperature (209°C), confirming SAC 385 solder balls are completely consumed, Figure 6b. Melting begins at 178°C. The final composition of CBGA joints after rework using Incontaining alloy A is 3.7-3.8 % Ag, 3.8-4.0 % In, and 0.4-1.0 % Cu, depending on the board surface finish. The microstructures of these joints are very similar to those of pure Sn-Ag-Cu alloys. CBGA reworked solder joints have Ag3Sn plates, Sn dendrites, and eutectic in inter-dendritic spaces. The number of Ag3Sn plates is small, and they are not large in size, (Figure 7a, online). The EDX analysis revealed indium in the Ag3Sn particles and the Sn matrix. These data are consistent with the literature on SnAgIn alloy microstructure formation.8 The ternary liquidus projection phase diagram9 (Figure 8, online) may be used to interpret the microstructures. It shows that for low In-content (0 to approximately 4 atom %), the compound phase is the Ag3Sn type, with some indium substituted for tin. The matrix phase is the tin type. PBGA 196 joints after rework contain 3.6-3.7 % Ag, 6.0-6.2 % In, and 0.4-1.0 % Cu. Tin dendrites and eutectic are present in the microstructure. Large primary intermetallic particles that have a specific flower shape, shown in Figure 7b, are detected in the PBGA 196 reworked solder joints. This compound contains a significant amount of indium and is more likely the Ag2.7 (In,Sn) type. It was found9 that for intermediate In content in alloys (approximately 4 to 6 at %), the compound phase has the approximate composition of Ag2.7 (In,Sn), where the indium and tin contents are approximately equal. It also may be the Ag2In type that forms in alloys with a high In-content (greater than approximately 6 at %). The occurrence and size of the particles depend on the rework parameters, and may be significantly reduced by optimizing the profile. The reworked CSP46 microstructure is similar to PBGA 196. There are no significant differences between 1X and 2X reworks on ImAg and ENIG finishes. The influence of rework of the adjacent PBGA 196 was negligible. The reaction intermetallic layer at the board side is (Cu,Ni)6Sn5 with 3-5 % Ni. The intermetallic on the component side is a ternary compound, SnCuNi, with a nickel content in the range of 48 Circuits Assembly SEPTEMBER 2008 Figure 6. DSC heating curve for reworked CBGA (a, top) and PBGA 196 (b) using In-containing alloy A. 10-15%. For the OSP and ImAg finished boards, the thickness of the intermetallic layer on the board side is about 5 µm in CBGA joints and 6 µm in PBGA 196 joints, in both cases after 1X and 2X rework. The intermetallic layers on the board side of the ENIG boards is much thinner, about 1.8 µm and 2.5 µm in CBGA and PBGA 196 joints, respectively. Both CBGA and PBGA 196 solder joint compositions after rework with In-containing alloy A provided an excellent balance of the desirable properties: strength, plasticity, fatigue life and superb fatigue resistance. The described microstructures are also favorable for high reliability of the reworked solder joints. There were no differences found between the microstructures of components reworked in air and nitrogen. Accelerated Thermal Cycling The ATC results reported below are based on 2500 cycles completed to date. The test is currently ongoing to a target of 6000 cycles. Weibull plots were generated for CBGA and PBGA 196 components reworked using the low melt In-containing alloy A, as well as for primary attached components and components reworked using conventional rework process (SAC 385 alloy). These are the only two components that have failed up to this cycle count. Continued on pg. 62 circuitsassembly.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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