Circuits Assembly - July 2008 - (Page 71)

Preventing Pb-Free Solder Pot Contamination Brittle joints are one common result of poor maintenance. onitoring the composition of the alloy in the solder pot is critical to maintain a stable wave or selective soldering process. Deviation from the original composition of specified alloy results in physical property modification, such as melting behavior and intermetallics formation. In the typical wave or selective soldering process, the main elements of concern for a Pb-free solder pot are copper, iron and lead. The copper percentage increases based on board and component types used. Copper levels above 1% potentially impact the stability of the process by affecting the melting point and encouraging Cu6Sn5 intermetallics formation. The risk of these intermetallics traveling in the flow of the wave and ending in the joint increases when the copper content is not kept below 1%. The potential result: brittle joints. To reduce the copper in a SnAgCu bath, feed the solder pot with pure bars of SnAg or remove the alloy and replace with an appropriate amount of a corrective alloy. Controlling the amount of lead and iron is also important. Sources of lead contamination are board or component surface finishes. Over time, lead dissolves into the solder pot. A maximum of 0.1% is permitted to form solder joints that will comply with RoHS. In a case study, a differential scanning calorimetry (DSC) was used to analyze the effect of lead on SnBi alloy. The original composition of the alloy was SnBi42 with a eutectic melting point of 139˚C. The addition of 30% lead lowered the melting point to 119˚-132˚C, from eutectic to a pasty range, and formed a second alloy: SnBiPb with a melting point at 97.5˚C (Figure 1). Once the solder pot is contaminated with lead, the entire bath must be discarded and the pot cleaned. Iron contamination is a major concern. The only source of iron is the soldering equipment itself. A large body of research is dedicated to understanding the behavior of various Pb-free alloys, as well as compatible materials to resist the Wave Soldering M corrosive properties of these alloys. This work found all alloys have some corrosive effect on incompatible materials. The rate of corrosion varies significantly, ranging from insoluble to very soluble. The effect of alloys that are extremely aggressive to selected metals or materials is to react with the iron of the solder pot and its internal parts to form FeSn2 needles (Figure 2). These needles have a melting point of 510˚C; once formed, they neither melt nor dissolve. The needles grow over time and deposit in the areas where the solder pot has a low or stagnant flow. In some instances, they can flow into the wave, forming brittle joints. Iron content of more than 0.02% can indicate a non-compatible wave/selective soldering machine. It has been observed that pot materials are more vulnerable where solder flow is at its highest, such as in wave formers and pumps. Iron dissolution is a safety hazard that impacts equipment integrity and solder joint reliability. It is important that the solder pot and internal parts are compatible with Pb-free processing. They should be protected by materials such as titanium, cast iron or stainless steel with a chromium carbide finish. Once the pot is contaminated with iron, the bath should be discarded and solder pot cleaned, just as with lead contamination. Users should schedule monitoring of a solder pot’s elemental composition. In the beginning, they should monitor the pot on a regular basis until a clear equilibrium is achieved. Solder pot composition can be monitored by x-ray fluorescence (XRF), atomic absorption spectroscopy, inductive couple plasma, scanning electron microscope (SEM), and gas chromatography mass spectrometry. Many solder suppliers offer these types of services at no cost to customers. n 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. Figure 1. Lead contamination of SnBi alloy. The lead lowers the alloy melting point and can lead to out-of-spec boards. circuitsassembly.com Figure 2. FeSn2 needles are caused by the iron solder pot and can lead to brittle joints. Circuits Assembly JULY 2008 71 http://vitronics-soltec.com http://circuitsassembly.com

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Circuits Assembly - July 2008 Contents Caveat Lector Industry News Market Watch Talking Heads Focus on Business Global Sourcing On the Forefront Screen Printing Tech Tips Will Electronics Follow the Sun? ‘Warm’ Manufacturing Heats Up Reputation Trumps Recession, China ‘Web Circuits’ A Novel Non-VOC Conformal Coating Krypton: Benchmarking Customer Satisfaction 2008-09 CIRCUITS ASSEMBLY Buyers Guide Wave Soldering Pb-Free Lessons Learned Process Doctor Materials World SMTA Int'l Product Preview Ad Index Assembly Insider Technical Abstracts

Circuits Assembly - July 2008

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