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I understand there are a number of ways of reflowing individual SMT boards. I've seen small temperature profile ovens, hotplates, skillets, airbaths, and hot air guns used. I'm trying to determine which method is most appropriate for low-volume (<100 per year) commercial production of SMT PCBs.

I've observed something being done by another company, though at smaller volumes. Their reflow method was to preheat the board with an air bath, then spot-heat the board with an air gun to complete the reflow. They claimed a long and effectively problem-free history with this method.

Is this a viable method of SMT reflow for the volumes I'm talking about? Are there particular flaws or problems to watch out for?

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  • \$\begingroup\$ Are you saying <100 PCBs per product per year, or <100 total per year? \$\endgroup\$
    – Matt Young
    Jun 9, 2014 at 14:57
  • \$\begingroup\$ I'd be interested in answers for both scenarios. \$\endgroup\$ Jun 9, 2014 at 15:00

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Depends on who's paying the guy who's doing it, what he'd be doing if he weren't hand-assembling, and what the boards look like. As a number for comparison, my US domestic assembly experiences seem to come in at around 6.5 cents per pad (maybe a bit more at low volume), plus NRE's.

Also depends on what your goals are for the assembly. Do you need it done, or do you need it done inexpensively? At less than 100/year, I'd assume the price of the PCB has not much to do with your business model, but I could be wrong. If this is the case, I think the assembly method you describe, when carried out by an experienced assembler, and followed by good QC testing, is an OK approach.

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that method is used quite frequently for high value one off construction. I believe nasa published a load of high reliability design guidance that used a similar method. Its much more time consuming then paste+reflow oven, and you can get a pretty decent bench reflow oven for a few hundred usd, so even for small prototype runs I do reflow now.

The rules of thumb I use for different batch sizes(which may be different for your circumstances):

one-offs to 10x - hand soldering (usually prototypes, usually have some corrections / testing to do)

10x-50x - solder paste stencil + hand placement. I have a cheap reflow oven, so tackle the easy jobs myself, but sometimes I outsource to a local assembler, depends on time/cost requirements.

100x+ - board assembly house, ideally a turnkey service, either hand placement or machine placement depending on the board

pcb assembly in china is cheap as chips, and if you can justify just placing one big order a year, its not that expensive. I'm guessing board costs isn't a big part of your business costs if you only make 100 units a year.

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