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(Note: This question is in particular for low-volume self/manual assembly, i.e., without automatic pick and place machine).

Generally, stencils are used for well-aligned application of solder paste to a PCB.

Suppose you were to follow that stage with a second, similar stencil but this one for part placement, where the cutouts on this custom-made "part-stencil" are made to match the dimensions of your components.

Then one could place this part-stencil over the PCB, then use your vacuum-pickup or tweezers to quickly drop the components into these cutouts/slots/windows on the part-stencil thus making aligned placement easier and faster. Then you could lift the part-stencil, and move the PCB to reflow.

Could one make such a part-stencil approach work, or does it have any critical issues? Is it or some similar variant used?

For example, I see some issues such as the components being nudged when this part-stencil is removed, but if you play with the tolerances on the cutout slots, and set up this part-stencil slightly offset above the PCB, it might work (?)

(Above idea is inspired by a comment made by Scott under this blog post)

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    \$\begingroup\$ How are you going to keep the parts locator stencil from mushing the solder paste put down by the previous operation? Sounds like a mess. \$\endgroup\$ – Olin Lathrop Nov 2 '12 at 13:31
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There's generally no need to do this, because when the stuffed board is run through reflow, and the solder melts, the surface tension of the solder pulls the parts into a centered position over their pads.

If the centering action of the solder surface tension doesn't give accurate enough centering for whatever it is that is motivating this idea, you have a problem. Because if you make your part-locator "stencil" with tighter tolerance than the solder will achieve on its own, then in reflow the solder will pull some of the parts against the locator and you will need to be very careful removing the locator to avoid damaging the parts.

Even with the surface tension working in your favor, some problems can still occur, like tombstoning, but this wouldn't be solved by your idea.

Edit

Re-reading your question, I see you might have another idea in mind--the stencil is used as guide for hand-placing the parts, and then removed before reflow.

In that case, why not just design your silkscreen to give adequate cues for parts placement? Then you don't have to design (and pay for) a whole extra fabricated part that will have to be built as an expensive one-off.

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  • \$\begingroup\$ Noted. What if you were to remove the part-stencil before reflow, instead of after? Part-stencil being mainly for rough positioning. (Although if I understand you correctly, you are also implying that the tolerance allowed on the part-stencil probably needs to be large enough that it won't accomplish any more accuracy than the natural alignment that reflow itself does.) \$\endgroup\$ – OrCa Nov 2 '12 at 5:02
  • \$\begingroup\$ In response to your edited-in portion: I guess my reasoning is that a visual indicator on the silkscreen wouldn't be as helpful/fast as a mechanical slot will, for a human to insert something right in place? With the visual method, I have mainly had trouble with fine-pitch ICs, QFPs and QFNs. Hence I was hoping these kinds of windowed cutouts could help, hmm... \$\endgroup\$ – OrCa Nov 2 '12 at 5:04
  • \$\begingroup\$ Consider a fine pitch TQFP. The pins stick out around the body of the part, and the solder pads stick out farther than the pins (to allow the solder to form a fillet on the end of the pin). How big do you make the stencil. It can't be sized to the body of the part, or you couldn't fit the whole part through the hole. If its just big enough for the pins, its likely to contact the solder paste and mess it up. So it's got to be bigger than the pads --- and then it doesn't do any good for actually fixing the location of the part. \$\endgroup\$ – The Photon Nov 2 '12 at 5:15
  • \$\begingroup\$ Indeed. But for this: "If its just big enough for the pins, its likely to contact the solder paste and mess it up."... What if the part-stencil were placed a bit raised, i.e., with a slight vertical clearance from the solder paste? \$\endgroup\$ – OrCa Nov 2 '12 at 5:25
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    \$\begingroup\$ If you're having trouble placing components, working under a magnifier may be a big help. Your eye/brain will compensate for the magnification by making much smaller motions. \$\endgroup\$ – lyndon Nov 3 '12 at 1:19
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Not all production lines are automated. There are many PCB's that are stuffed by hand, soldered by hand, and you would never know it. I'm not talking about the hobbyists, but the professionals that work on assembly lines that specialize in making small runs of PCBs (where it is more economical to do the work by hand rather than machine-assembling it).

These people have to work quickly, accurately, and produce good results-- and they deliver. They don't need special guides to put the parts down accurately and quickly. And as I mentioned before, their work looks as good as the machine-placed parts.

I would posit that if you need a stencil or other guide to place parts accurately then you are doing something wrong. If the professional part-stuffers can do it with little more than a standard soldering iron and maybe an hot-air rework station, and can do it quickly enough to make a successful business out of it, then you can place the parts without a stencil.

I should also mention that these businesses are located in the U.S., not China.

I have assembled many of my own PCB's and my craftsmanship is similar to what the professional assemblers can do. This includes TQFPs, TSSOPs, 0402s, etc. (but not QFN and BGAs).

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  • \$\begingroup\$ This is certainly motivating; thank you for this note. Perhaps just a matter of more frequent practice instead of tricks. \$\endgroup\$ – OrCa Nov 13 '12 at 7:27
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I think the idea has an implementation problem, too: How do you remove the "part stencil" after placing the parts, without it disturbing the already placed parts and/or solder paste? If you add enough slop on the sides of the part stencil to solve this problem, then there's enough slop to allow mis-placed parts.

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