Electronics > Manufacturing & Assembly

Silkscreened Pads

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dangrie:
Hi all,

I was just cracking open a cheap USB current meter and found something I can't really explain. The main micro in a TQFP package has several pins covered in soldermask AND silkscreen (see attachment). Is there any reason why one would do that instead of just leaving out these pins on the solder stencil and/or just solder them to pads that aren't connected to any net?

I always solder n/c pins to unconnected pads just for the sake of the added mechanical stability. The only possible reason I can think of why they are not just left out from the solder paste stencil is to make sure they are also not connected when wave soldering or to make sure these pads don't "thieve" the solder from the surrounding pads, but this leaves the question of why not soldering them in the first place and the only explanation I can think of for this would be cost, but can this really be a reason? I think we're talking femto-cents/unit here :)

Also: Why silkscreening instead of just solder-masking the pads? Visual indication of which pads are unconnected?

Thanks for any insights
Daniel

Edit: I just noticed that there are actually traces going under / to these pads. In case they go under these pads it may be to make space to easily route these traces?

Pinkus:
As you can see, there are traces running under some of these pins.
Thus I suppose, they are using solder mask and silk screen as a kind of double isolation between these traces and the pins.


forrestc:

--- Quote from: dangrie on January 17, 2019, 10:06:52 am ---Edit: I just noticed that there are actually traces going under / to these pads. In case they go under these pads it may be to make space to easily route these traces?

--- End quote ---

I think that this is probably the reason.   There's a lot of this type of stuff done (although I haven't seen this particular method before) to make things fit on a too-crowded layout and still keep a double sided layout, since 4 layer boards can be rather expensive.   If the choice is between doing this and going to a 4 layer board or not fitting in the form factor, I can see why someone would choose this method..   I suspect the purpose of both the solder mask and silk screen is to give a double layer of insulation just in case.

I will say I wouldn't be likely to do this in a design though....  since I'm not sure I'd trust the silkscreen+solder mask to be good enough insulation.

d1wang:
The soldering is not symmetric. Won't the solder paste pull the component upward and cause shorts?

ar__systems:
Why guess, when you can look up the datasheets for the parts? Most likely some address or range or mode setting pins...

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