Does anyone have any experience with placing a silkscreen line parallel between the pads of small-pitch SMD ICs? For example I intend to hand-solder (or reflow) some 0.5 mm pitch packages, and I was thinking of adding some thickness between the pads to aid in placement and solder flow / bridge prevention, somewhat like an extra layer of soldermask. Will it have any use or shouldn’t I bother with it in my footprints?
No experience but the first question is can your board house do silkscreen that fine AND that accurate?
OSHPark can do a minimum ‘feature size’ of 5 mil / 0.0127 mm silkscreen (which is the line width of the other two silkscreen lines). My between-pad lines are 0.2 mm width.
For logo’s they mention the following: The maximum DPI we can correctly process is 500 DPI, corresponding to pixels of 2 mil or larger. Images at a higher DPI may be be printed on the board incorrectly, or omitted entirely. Due to the complexities of bitmaps through fabrication, we advise keeping images to a DPI of under 400 and a corresponding pixel height of 2.5 mil.
So for 400 DPI = 15.7 DPmm * (2.5 mil = 0.0635 mm) = 1 mm. Not sure what this figure says though, they can print 400 2.5 mil dots?
I could just order a bunch of breakout boards and try it out or contact them before. I think I may have gotten the idea from some kind of special protoboard with groove-like pads for the ICs - but in my case adding height instead of a groove.
If you are only doing this to make soldering .5mm pitch ICs easier, I would not bother. The gap between pins of such ICs is usually .2mm, you have to account for silkscreen alignment accuracy for something so thin. I don’t think most fabs can guarantee (at least for their cheap-ish offerings) enough precision to put silkscreen exactly where it needs to go even if they can print, say, .1mm thin lines.
On the other hand if they properly print soldermask, which is totally doable even on cheapest chinese fabs, you should have no trouble soldering .5mm IC with proper application of flux (the most important part) and right technique and solder tip combination (second most important part). Just watch some videos on youtube by EEVblog Dave or other known engineers (not amateurs like GreatScott who despite their popularity show about the worst ways of how to solder). This channel has amazing quality tutorials on proper soldering technique.
You must also take into account their placement tolerance. To avoid printing on pads they will have their own clearance envelope I think. But you are right. Ask them directly for the ‘most correct’ answer.
I’d advise against it. As the others mentioned, placement tolerance for silkscreen is likely not that great (because nobody cares), and I’d expect the paint to be slightly worse acting as a solder resist than the green stuff that is designed to work as solder resist.
The surface tension of the solder should take care of most issues. If you have issues, you can go the extra mile and do solder paste wedges, but without direct support in the program, this is a lot of effort that is probably only worth it when your prototype run shows that there is a real problem.
This is engineering. Optimize to the point where you get diminishing returns, then stop.
I have never used the “wedges” tool, but it looks like a clever idea worth trying. I have had great results with inexpensive plastic stencils (Kapton? Don’t recall the exact material) from “OSH Stencils”, in conjunction with solder paste (Kester EP256 from “CML Supply”). For rework, or initial assembly of a dozen parts or less, I touch the paste with a very fine soldering iron tip, and the solder melts and flows like magic. For larger assemblies I stencil the paste into place, populate the board with parts, and cook it in a table-top electric skillet. This has almost entirely eliminated solder bridges and solder splashes, compared to soldering with a soldering iron and wire solder.
Oh, but my 28-digit calculator shows we can improve things by half a part-per-billion if we just . . . . 
Dale
What model number? I think I need one!
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