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SMT Webcast Explores Lead-free Rework
December 31, 1969 |Estimated reading time: 2 minutes
NASHUA, N.H. With boards becoming more complex and lead-free pastes melting at higher temperatures, rework-and-repair operations have become a necessity. Rework often involves hand soldering tools, rework stations with movable X/Y stages for PCB mounting and support, hot air/gas nozzles, and other tools for maneuvering within the tight parameters of lead-free. The basic steps include: identify failed components and causes of failures; remove failed components; do PCB site preparation; then replace and reflow the failed component. Final clean-up and inspection are important to ensure reliable rework.
Peter Biocca, senior market development engineer at Kester, describes the materials-selection process for lead-free hand soldering and rework. To perform lead-free rework, Biocca suggests using a solder wire and compatible flux with high and sustainable activity, adding that the materials should tolerate high temperatures and work with a variety of board finishes. In a list of popular solder choices for wave, hand, and reflow soldering, tin/silver/copper (SAC) was a predominant alloy of choice. Biocca covered the impacts made by board finishes and rework temperatures, and found that higher activity fluxes provide the most suitable soldering environment. Variables included the melting temperature and wettability of the solder alloy, and necessary nitrogen levels. Flux variables such as volatility, cleanability, and sputter also factored. He noted that using the proper solder-iron tip for a given application was key, and gave several examples of factors causing tip failure. Andy Mitchell, product manager at American Hakko, presented advice for hand-soldering in a lead-free environment, starting with the basics of precise technique and temperature control, including pre-heat. Preheating a board to between 80 and 100°C increases spreadability and improves rework, especially with high-mass applications. One big mistake people make with lead-free, according to Mitchell, is that they turn the iron up too high. Tip shape will affect thermal performance when reworking a board; lead-free solder will erode tip plating due to the intermetallic relationship of tin and iron. Mitchell suggests proper tip care and stringent cleaning of the iron and the workspace.
Stan Kench, general manager of Precision Manufacturing Tools, gave a brief tutorial on modern technology for reworking complex components. Advanced BGA rework technology will improve rework-and-repair in the lead-free era, according to Kench. If bottom heaters energize the board properly, top heaters are more effective; automatic profiling reduces risks of heat damage to the board. Convective heaters closely replicate the reflow-oven environment, says Kench, as long as the power is coupled with repeatability and control. He demonstrates the method of "teaching" a solder profile on software, which could yield safe, effective rework.
Michael D. Jones, vice president of Micro Care Corp., focused on benchtop cleaning for lead-free rework opening with an explanation of the pros and cons of cleaning in a lead-free environment. Overall, he says, cleaning lead-free assemblies is more difficult, but creates higher quality and lower costs. He detailed solvent options and factors to consider in selection, giving positive and negative attributes for popular chemistries. Vapor degreasing, a "dry" process, was also examined. Jones concluded that cleaning must be considered in a cost-and-quality evaluation, and proper selections will minimize cost-per-board.
The Webcast, "Improving Rework and Repair," is available on our Website, archived for on-demand viewing.