Selecting a Selective Soldering System, Part 5

Estimated reading time: 3 minutes

For the conclusion of our selective soldering discussion, I thought it would be useful to do a quick wrap-up to remind potential buyers about some important considerations that affect the purchase decisions they make. So, here goes:

Buy the Right Machine for the Intended Purpose

Selective soldering is often touted as being the “latest and greatest,” but it’s not the best technology for all boards. The fact is, selective is a much slower process than wave because it has to process each lead or closely spaced set of components one at a time. Selective is the only way to solder through-hole components in a production environment on a mixed technology board or if there are obstructions on the underside of the board. But wave soldering is by far the fastest way to preheat, flux and solder a board with only through-hole components.

Wave soldering dates back to a time even before integrated semiconductors were used, so it’s not a new technology. However, there have been many advances to the equipment as the industry has evolved, and it remains a very effective and cost-efficient manufacturing/assembly method to this day. No technology can match it for speed and no other method can improve on its joint strength.

Selective soldering is becoming much more common today, for very good reasons. It can process through-hole components on boards with mixed technologies, multiple shapes, patterns and devices; it is significantly faster and more reliable than hand soldering, so it’s a natural step up from a prototype setting to a production routine; once programmed, recipes can be archived to perform the same operation in exactly the same way in the future, ensuring reliable repeatability.

Hybrid boards exist for a number of reasons. In spite of the proliferation of surface mount components, which are perfect for high density functions on smaller and smaller footprints, through-hole devices are still used for high power applications and those with connectors which require a very strong, stable joint. And this is where selective soldering shines.

Understand the Process That’s Best for Your Assembly

While selective soldering does not necessarily replace a wave machine, depending on the nozzle type used it can replicate the wave technique in a compact way. For instance, a fairly large row of connectors/leads not in close proximity to SMD components could use a wide nozzle to swipe (or wave) the entire row at once. On the other hand, a small area situated near an SMD would require a very small nozzle to avoid disturbing the surface mount device.

There are myriads of standard nozzle designs for performing dips and drags, and they are limited only by your imagination or the complexity of the board. Selective gives you many ways to tackle a soldering routine, so you want to think about the sequence, the nozzle configurations you use, and the best way to save processing time. It may make sense to buy a custom nozzle if you have a high volume board with a unique profile. After all, nozzles aren’t very expensive and they last quite a long time.

Jet nozzles attack the board at about the same angle (7°) as a wave machine does and can deliver a high volume of solder using a tapered tip which guides solder roll-off in one direction, returning unused solder back to the pot from its trailing edge.

Wettable nozzles provide finer accuracy than jet type which makes them better suited to connections in close proximity to SMDs. It also produces less oxidation because there’s less contact with air. For this reason, wettable nozzles are also best suited for lead-free solder, which tends to be more vulnerable to oxidation.

To read this entire article, which appeared in the August 2016 issue of SMT Magazine, click here.