Circuits Assembly - November 2008 - (Page 39)

Nothing to Hide Shrink holes in Pb-free boards are effects, not defects. ne problem we have begun to see more of with rough surface appearance regardless of the process Pb-free soldering is the phenomenon known settings. Therefore, optimization of the appearance as shrink holes, or hot tears on Pb-free solder might not work for all joints in the same assembly. joints. We have little understanding about why shrink Boards were subjected to thermal cycling to deterholes form or how they affect field reliability. Recently, mine how the shrink holes evolve and if they will a study was performed on joints formed in a wave affect reliability. SEM analysis of the surfaces, as well soldering process, and was used to as cross-section analysis, was percharacterize shrink holes and meaformed after 600 and 1200 cycles. sure their effect on Pb-free solder Surface differences were observed; joint reliability. the joints formed at a slower conIt is believed the dull, rough veyor speed also showed the presappearance of SAC solder joints is ence of tin dendrites and shrink an effect of the solidification behavholes, and generally had a rough ior of the alloy, at which residual appearance. eutectic liquid contracts its volume Cross-section analysis after upon solidification. The result is 1200 cycles showed minor changes shrink holes, located between the in the surface and no evidence of tin dendrites that solidify first. any shrink hole evolving into a For this study, boards with crack. through-hole components were Figure 2 shows shrink hole forassembled with SnAg3.9Cu0.6 mation inside the joints. This type alloy and a commercial VOC-free of shrink hole might jeopardize flux. The effects of PCB surface the electrical connection if many finishes (Cu OSP, ENIG, immervoids are formed around them by sion Ag and immersion Sn) and increasing the path for the cracks. conveyor speeds (50 and 137 cm/ Figure 1. SEM images of the surfaces of However, in this analysis, there was min.) on shrink-hole formation two solder joints. Top) Conveyor speed no evidence of cracks that proand joint surface roughness were 137 cm/min. (dull) and bottom) conduce a discontinuity in the electrievaluated. Assembly reliability was veyor speed 50 cm/min. (shiny). cal interconnection of any solder characterized using thermal cycling joint, even if the shrink holes were (-50° to 80°C with a dwell time of located near voids. IPC-A-610D states shrink holes 15 min.). or hot tears are acceptable for Class Both wave profiles had similar 1, 2 and 3 and for connections cooling rates (~27°C/s) and simimade with Pb-free alloys, if the lar topside preheat temperatures bottom of the tear is visible and the (~150°C). The main difference was shrink hole does not contact the the dwell time (3 and 7 sec.) and lead, land or barrel wall.1 Therepeak temperature (240° and 260°C). Three repetitions were assembled Figure 2. Shrink hole inside a solder fore, shrink holes are here to stay for each condition. One board per joint. and end-users should treat them as combination was used for time an effect, not a defect. zero analysis; the other two were We may conclude the appearsubjected to thermal cycling (600 and 1200 cycles). ance of SAC solder joints cannot be controlled for the Visual inspection showed the largest components entire assembly. However, it is important to know no (pad diameter of 5.4 mm) had shiny, smooth solder evidence indicates shrink holes adversely affect Pb-free joint surfaces when using the slower conveyor speed, assembly reliability. n and rough surfaces when using the fast conveyor speed (Figure 1). This is true for all surface finishes, with References the exception of ENIG. The reason for this behavior is unknown. On the other hand, smaller pad diameters 1. IPC-A-610D, Acceptability of Electronic Assemblies, Febru(less than 2.9 mm) showed, in most cases, dull and ary 2005. circuitsassembly.com Circuits Assembly NOVEMBER 2008 Wave Soldering O Ursula Marquez de Tino is a process and research engineer at Vitronics Soltec, based in the Unovis SMT Lab (vitronics-soltec.com); umarquez@vsww.com. 39 http://www.vitronics-soltec.com http://www.circuitsassembly.com

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Circuits Assembly - November 2008 Contents Caveat Lector Industry News Market Watch Talking Heads Focus on Business On the Forefront Screen Printing Better Manufacturing RoHS Conversion for Medical Devices Supporting Full-Service Customer Requirements at the Regional EMS Level Speed Thrills Tech Tips Wave Soldering Process Doctor Pb-Free Lessons Learned The Defects Database Getting Lean Materials World Product Spotlight Ad Index Assembly Insider Technical Abstracts

Circuits Assembly - November 2008

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