A high-mix, high-rel EMS finds selective tops masking and taping.

Many assemblers of high-reliability circuits have turned to selective soldering as an alternative to wave-soldering selected underside areas or PTH components using pallets. There are a number of reasons, but often it is because wave soldering was not originally designed for this. Defect levels can be high, and preparation times – such as masking – become prohibitively long. Selective soldering is a good way to avoid hiring a platoon of hand soldering technicians as orders (and throughput) increase.

APT Electronics, a contract assembler of high-reliability military and avionics, faced this problem. Initially, APT was processing high-rel products on pallets in a wave soldering machine, complemented by hand soldering, but the complexity and high-mass nature of the products – thick boards with massive ground planes – made hand soldering difficult or impossible for many applications. Wave soldering was balky and time-consuming, and wasn’t delivering hole-fill requirements. Also, the large number of different products – it’s a high-mix environment – had varying requirements and process recipes.

“Right now, I’m running 38 different products through this machine,” says manufacturing engineer Joe Garcia, “using only three ‘universal’ fixtures that cost perhaps $500 each. Prior to that, we used expensive custom fixtures for every part in a wave machine and still weren’t getting the results we needed, especially in terms of hole filling.” A big problem was getting sufficient solder into holes for connector attachment on big backplanes.

“A lot of time was eaten masking the entire assembly bottom with water-soluble or peelable mask, or taping with Kapton tape, which is very expensive. We needed a unit that could be programmed to selectively solder only what we wanted,” Garcia says.

“For example, I was hand-soldering four connectors on a 0.090" board. With hand soldering, it would take 8 min. to hand-solder four connectors. Now I can do it in 4:37, start to finish. For a two-up array, to mask and wave solder 18 components took 25 min. Now, we’re doing it with the selective machine in 7 min. No masking, no pallets. We found that we would need seven to 10 pallets at $300 to $500 per pallet just to be efficient with the wave machine in a high-mix environment. That’s not to say that we don’t do any taping. If I have to use a little bit of Kapton tape in a few places to mask something, it’s well worth it, because of what I’m saving overall in taping, touchup, masking and the rest. [But] prior to this, we would mask the entire bottom of the PCB, in part because the customer did not want to spend up to $3000 for pallets for 25 to 100 assemblies.”

A preprogrammed selective soldering machine’s fountain can get solder into the holes despite the assembly’s high mass, but the process is more automated (Figure 1). “Once you get it set up properly, programmed, dialed in, you just plug it in and start running,” Garcia says. “It has cut our process time 50 to 70%, sometimes 80%. We’re not masking or taping product comprehensively prior to processing, and we don’t have the volume of rework or touchup that we had, either.”


Rework has also been cut 30 to 40%, Garcia says, because the solder joints fill all the way to the top, even on heavy 0.120" backplanes with heavy ground planes and gold antenna connectors.

Ursula Marquez de Tino is a process and research engineer for Vitronics Soltec (vitronics.com); umarquez@vitronics-soltec.com.