Circuits Assembly - December 2007 - (Page 27)

Solder Materials desired thickness range. X-ray intermetallic mapping. Both alloys were subjected to x-ray mapping to show copper diffusion at the intermetallic layer level. Copper diffusion was limited strictly to the intermetallic layer for the cobalt enhanced alloy, while the SAC 305 exhibited more copper diffusion into the bulk solder (Figure 6). The greater the copper diffusion beyond the intermetallic layer, the more brittle the solder joint can become. Pull test. The SAC 305 alloy slightly outperformed the cobalt-enhanced alloy on the three devices pulled with 352.5N for the SAC alloy and 316.5N for the cobaltenhanced alloy (Table 2). However, the SAC alloy exhibited a larger deviation in the results (41.6N for the SAC alloy and 11.2N for the cobalt-enhanced alloy), which suggests the cobalt-enhanced alloy shows a greater ability to maintain consistency in the soldering process. Since no figures exist for pass/fail criteria, and the test is to failure, it is not known if the forces are indicative of real-world use. Temperature/humidity. After the parameters discussed above were completed, the cobalt-enhanced solder attachments were examined for defects precipitated by the test. No structural defects on the board were observed at 30x magnification. No tin whiskers were observed (Figure 7). Conclusions Although much industry testing has yet to be accomplished and evaluated, independent testing has shown an enhanced SnCu Pb-free alloy can provide benefits equal to, and in some cases better than, those of SAC alloys. In addition, the cobalt-enhanced alloy provides a much brighter and shinier solder joint with a tighter grain structure when compared to SAC 305 joints; in fact, the solder joints are virtually indistinguishable from Sn63 joints. When used in throughhole applications, a binary alloy (such as cobalt-enhanced) is much easier to maintain operating specifications when compared to a tertiary alloy (such as SAC 305); thus, less solder pot maintenance becomes necessary. ■ Figure 7. Magnified solder joint showing no tin whiskers. Test Results Thermal cycling. Both the SAC 305 and cobalt alloys exhibited no deterioration in solder attachment integrity after 1,000 thermal cycles (Figure 1). Minor voiding was observed in the solder attachments and TSOP devices for the SAC 305 as well as the cobalt alloy. Voiding was estimated at 2 to 12%, well within the range of IPC-A-610D (25% max.) and much below the IPC SPVC’s recommendation. Thermal shock. For the SAC 305, minor voiding was observed on tested J-leads and no voiding of TSOP devices (Figure 2). For the cobalt alloy, the TSOP devices exhibited minor voiding, while no voiding was observed on the J-leads. The cobalt-enhanced alloy exhibits a much finer and more uniform grain structure than that of the SAC 305. As indicated on the left in Figure 2, the dark areas are silverenriched pockets of the SAC alloy. Vibration. For the SAC 305 alloy, minor voiding and minor cracks were observed on both the J-leads and TSOP devices (Figure 3). The cobalt-enhanced alloy performed the same as SAC 305. The cobalt-enhanced alloy’s grain structure is much finer and tighter than that of SAC 305, providing a significantly brighter solder joint. Shear. Table 1 shows the cobalt-enhanced alloy outperformed SAC 305 on resistors while SAC 305 outperformed the cobalt-enhanced alloy on capacitors. Averaging all results, the cobalt-enhanced alloy slightly outperformed SAC 305. The smaller deviation of the cobalt-enhanced alloy suggests a greater ability to maintain consistency in all soldering processes. Intermetallic examination. The cobalt-enhanced alloy showed an average intermetallic layer of 2.18 µm for the unstressed sample, while the average thickness after 1,000 thermal cycles was 2.32 µm (Figure 4). This suggests little or no growth of the intermetallic layer, indicating long term solder joint reliability. Both unstressed and stressed samples were well within the 1 to 5 µm desired thickness. Per Figure 5, the SAC 305 alloy showed an average intermetallic layer of 2.00 µm for the unstressed sample, while the average thickness after 1,000 thermal cycles was 3.43 µm, still within the References 1. Thomas Siewert, Stephen Liu, David R. Smith, Juan Carlos Madeni, “Properties of Lead-Free Solders Release 4.0,” p. 21, Feb. 11, 2002. 2. IPC, IPC-A-610D, "Acceptability of Electronic Assemblies,” p. 8-83, February 2005. 3. IPC Solder Products Value Council, “The Effect of Voiding in Solder Interconnections formed from Lead Free Solder Pastes with Alloys of Tin, Silver and Copper,” p. 11. 4. iNEMI, “iNEMI Recommendations on Lead-Free Finishes for Components Used in High-Reliability Products,” version 4, pp. 3-4, Dec. 1, 2006. Howard Stevens is vice president sales and marketing at Metallic Resources Inc. (metallicresources.com); hstevens@metallicresources.com. circuitsassembly.com Circuits Assembly DECEMBER 2007 27 http://metallicresources.com http://circuitsassembly.com

Table of Contents Feed for the Digital Edition of Circuits Assembly - December 2007

Circuits Assembly - December 2007 Contents Caveat Lector Industry News Market Watch Talking Heads Global Sourcing Screen Printing Better Manufacturing Metalization Options for COB Assembly A Test Comparison of SAC and Non-SAC Pb-Free Solders An A-to-Z Guide to X-Ray Inspection BEST: A ‘Funky Chicken’ with an EMS Niche Tech Tips Wave Soldering Process Doctor Pb-Free Lessons Learned Getting Lean Eastern Approaches Product Spotlight Ad Index Assembly Insider Technical Abstracts

Circuits Assembly - December 2007

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