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First Pass Success with Hi-Rel Circuits
6745 selective soldering system.
By Michael L. Martel, Contributing Editor
Many electronics assemblers have turned to selective soldering as a replacement for wavesoldering of selected underside areas or through-hole components using pallets. There are a number of reasons for this: custom pallets are expensive, the wavesoldering process was not originally designed for this type of use, the incidence of rework due to defects is high, and preparation times — such as masking — can become prohibitively long.
The situation is worse for hi-rel and complex assemblies where acceptability criteria are stringent; inadequate hole filling, especially on backplanes and high-density boards, can be a serious and constant problem.
Selective soldering is designed for such applications, and there are any number of systems to choose from, varying from low-end benchtop systems to high-end units incorporating robotic material handling and sophisticated process controls. In the end, the right machine is the one that gets the job done, and the more problematic the assembly — such as military or avionics hardware — the more capability is needed. This notably includes enough heat to deliver solder to the top of through-holes even on high-mass assemblies. Bringing in a selective soldering machine is a good way to avoid having to hire a platoon of hand soldering technicians as orders and throughput increase. It has been effectively shown that the introduction of selective soldering brings greater consistency of results and repeatability to the process. Even with the small degree of automation brought to the table by selective soldering, it doesn't put employees out of work. Some hand solderers are still needed; others are reassigned to different critical areas of the manufacturing line; and generally, one or two people are required to operate the machine, performing such tasks as programming, maintainence, loading/unloading and servicing.
Significant Time Savings
But by far the biggest benefit resulting from putting selective soldering technology to work on the production floor is time savings. Time savings are cumulative, and after even a brief amount of time — several weeks or months — they can make a huge impact on manufacturing. This is because the time savings involve a lot more than speed of production over hand soldering. Time is saved in masking and individual board preparation; there is far less rework; there are far fewer custom pallets to purchase, make, store, and use. Time is, after all, money, and increased production with fewer defects and associated overhead has an immediate and direct effect on an assembler's bottom line. Improved quality of product has a longer-term and arguably greater effect on the assembler's overall business.
A dramatic example is APT Electronics in Anaheim, California, a contract assembler of high-reliability military and avionics hardware. Initially, APT was soldering these products in pallets in a wave soldering machine, complemented by hand soldering, but the complexity and high-mass nature of the products, many thick boards with massive ground planes, made hand soldering difficult or impossible for many of the applications, and wave soldering was balky and time-consuming, and still wasn't delivering the required results in terms of hole filling. Also, there were quite a number of different products — it's a high-mix environment — and all have varying requirements and process recipes.
Joe Garcia, manufacturing engineer at APT (firstname.lastname@example.org), explains that the decision to bring a selective soldering machine, in this case the Vitronics Soltec Model 6745, was necessitated by the need to get better results and save time. The 6745 is at the low end of Vitronics Soltec's selective soldering machines in terms of initial investment, but it proved to be a marked improvement over wavesoldering in pallets. The company has had the machine online for six months as we go to press. "Right now, I'm running 38 different products through this machine," Garcia says, "using only three `universal' fixtures that each cost me perhaps $500 each. Prior to that, we were using expensive custom fixtures for every part in a wave machine and still weren't getting the results, especially in terms of hole filling, that we needed." They had tried a low-end selective soldering machine that simply couldn't do the job; it wasn't beefy enough to get solder up into the holes for connector attachment for some of the big backplanes that APT processes.
No Masking Needed
"A lot of time was being eaten up masking the entire bottom of the assembly with a water-soluble or peelable mask, or even taping it up with Kapton tape, which is very expensive. All that effort made no sense; we needed a unit that could be programmed to selectively solder only what we wanted," Joe says.
"I was hand-soldering four connectors on a 90-thousandths board. With hand soldering, it would take 8 minutes to hand solder 4 connectors, now I can do it in 4 minutes 37 seconds, start to finish. We had a 2-up array, mask and wave solder 18 components; it took 25 minutes, now we're doing it with the selective machine in 7 minutes. No masking, no pallets. We found that we would need 7 to 10 pallets at $300 to $500 per pallet just to be efficient with the wave machine in a high-mix environment. That cost adds up, and a lot of customers don't want to pay it, or they want the contract manufacturer to find a solution. Well, we did. That's not to say that we don't do any taping — sometimes we do a little here and there. 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, touch up, masking, and all the rest. Prior to this, we would mask the entire bottom of the PC board, in part because the customer did not want to spend sometimes up to three thousand dollars for pallets for 25 to 100 assemblies, so we ended up with the tedious masking process instead."
Tiny "wave" of solder from focused fountain hits just those parts of the board needing soldering.
The 6745 is equipped with "Easyteach", a software tool and interface that makes selective soldering set-up fast and simple. With it, an operator uses a simple graphic image of the assembly — a flat scan and covert to a Bitmap Image — to program the soldering sequence for any assembly in a matter of minutes. Because APT's products are quite complex, more time is needed to "debug" some parts and get the sequence right. But, as Joe says, "One of the definite advantages of this machine is once you get it set up properly, programmed, dialed in, you just plug it in and start running. What it has really done is cut down our process time from 50-70 percent, sometimes 80 percent. Time savings are in multiple areas. We're not masking or comprehensively taping up the product prior to processing, and we don't have the volume of rework or touch-up that we had, either. Using this machine cuts down on my setup time dramatically, and I can pump them through at 3 up, 2 up, 4 up, as many as the area will hold. The amount of time that I save varies, up to 19 minutes, up to an 80 percent reduction in masking and taping."
Rework has been cut down because we're getting good solder joints, filling all the way to the top, even on heavy backplanes that we're running, under 120 thousandths, heavy ground planes, gold antenna connectors. We're meeting target conditions, with solder all the way to the top when even only 50 percent is required. I cannot achieve that even with a dip solder machine! Now, I can get enough heat up there, and at the same time I am not exposing the whole board to the entire wave soldering thermal cycle."
The 6745 has several special features that have contributed to APT's process success, including angle and rotation adjustment capability. These features allow the customer to angle and rotate the board, thus opening up the process window for the user. In addition, the machine uses non-wettable nozzles, eliminating wear and the need for maintenance. Garcia says that these features have "further reduced the time on previously programmed boards by about 20 to 30 seconds." The SDC nozzle (Solder Drainage Conditioner) does, at times, reduce the solder bridging especially on sub-miniature connectors. "We also use the SDC nozzle to keep an area being soldered up to temperature to get a topside fillet on heavily-grounded boards."
Rework has been cut down from 30 to 40 percent, at least, Joe says. "Every once in a while I'll get a rare icicle, and have to tweak the process a little, but the main thing is that I am getting a full solder fillet in the hole. A lot of our customers are Class 3, J-standard. If I can't see the fillet, I've got to stick it into the x-ray machine. It's better to see the fillet, and when the solder comes to the top, that's easy."
This is especially significant, because these military products are RoHS-exempt, using lead-bearing solders, whose wettability is generally better than lead-free solders — the next challenge that Joe and APT may face in the near future.
For more information contact: Vitronics Soltec North America, 2 Marin Way, Stratham, NH 03885
603-772-7778 fax: 603-772-9340 Web:
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