General > General Technical Chat
Is partial SMT board assembly a practical option?
Pete_UK:
Don't know whether this is an unusual suggestion and whether it's too problematic, but I've been wondering if its possible to get PCBs partially-stuffed, say with all of the basic Rs and Cs etc, plus the other jelly bean parts, leaving the more expensive and obscure parts to be hand fitted later? The main problem as I see it is that the you'd probably want to avoid solder paste on pads destined for later hand-soldered parts (especially if using fine pitch QFPs etc.), which would require the pick'n'place solder paste stencil to have these parts removed. It all seems quite feasible, but in practice I guess this could be problematic to organise with the manufacturer/assembler? Has anybody done something similar, especially with a Chinese manufacturer such as JLC for instance?
KL27x:
Yeah, I do that. Two very important things to remember. 1. Remove the cream layer from the pads/footprints of the omitted parts. That one is obvious.
2. DON'T FORGET the finish! Immersion GOLD FINISH! If you don't spec this gold finish, then when the boards get partially assembled and baked, the unused pads will form a hard oxide layer in the oven. This makes putting on the remaining parts almost completely impractical. In a pinch, I have sat down and removed this layer using a rotary tool and felt polishing tips with compound. Is no fun, at all. In less dire circumstances, I have thrown out and redone entire batches of boards for this very mistake. For some parts with large spacing you can sorta get by with just turning your iron to super-hot and using an aggressive flux, but the soldering still crawls and takes way longer than it should; the residue you leave will be chock full of salt, and your iron will require non-stop attention/cleaning in the short time before the tip erodes into a blob.
Pete_UK:
Thanks, that's super helpful! Can I ask how you edit your paste file - do you do this in the PCB software package or with a separate Gerber editor? I'll have a play around with my PCB software tomorrow to see if I can figure this out. Thanks again!
Someone:
As above, if you're needed planarity for the later added parts then gold is the surface finish of choice. For less critical things like SOP8 or large surface mount passives, leaving the paste as the finish is ok if its washed well.
My assembly house does all that work in the gerbers when they order the stencils.
KL27x:
I edit it in the CAD software. I can't remember for sure if there's an easy box to uncheck for the paste layer. Or if I actually edited the footprints in the device layer. But it's the same thing if you want to make ICSP pads, so it's something a lot of people might have to do for other than this reason. So I think there's a box to uncheck, somewheres.
Yeah, I think it's a check box IN the device layer/footprint of the library part. In Eagle, anyway. Then update library and spin a new gerber.
^Good to know that HASL of solder paste isn't so bad, Someone. My nightmare mistakes were with OSP, w/e that is.
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