Selective Soldering: Design, Process Challenges and Practical Solutions

Estimated reading time: 6 minutes

He described three different options for mini-wave selective soldering: wettable nozzle, non-wettable nozzle and non-wettable nozzle with the work angled from the horizontal. Wettable nozzles were useful for soldering close to other components and, because solder flowed in all directions, no rotation was required. Solder bridging was also reduced, although lead protrusion length was a critical parameter. Non-wettable nozzles, with solder flowing in a single direction, could give more consistent results, but there was the consequent need for board or solder pot rotation. Bridging was avoided by a directed nitrogen stream heated to above the alloy melting point, and lead protrusion length was then less critical. Best results on pitches as fine as 1 mm were achieved by keeping the protrusion length as small as possible and tilting the work at an angle of 7–10 degrees.

For high volume work, short cycle times could be achieved by dip-soldering using a multiple-wave system, with dedicated tooling plates and laser-cut screens to avoid bridging. Soldering of fine-pitch connectors was feasible, but required higher pump speeds and some form of hold-down unit to prevent components from lifting. Care was needed if using a multiple-dip process, to avoid hairline bridging due to oxide buildup.

This SMART Group workshop very positively achieved its objectives: an excellent learning opportunity with a good balance between technical explanation and practical involvement, which generated plenty of interactive discussion and sharing of experiences from delegates and presenters. Nigel Burtt wrapped up the proceedings with thanks to delegates for their attention, to Vitronics Soltec, Kester, MBDA and Renishaw for providing expert speakers, and to APP Electronics for generously offering the use of their conference and demonstration facilities.

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