SMT fine pitch pins bridging

[nikeplato]

Hello experts,

Issue: bridging between 3 adjacent pins at SMT
Given: an IC 36 pins with very fine pitch, 3 adjacent pins are conected together by a single wider copper trace by design.
Question: These 3 pins are getting bridged and Manufacturing Dept brought this up as an issue since it fails the optical inspection. I asked them to ignore this failure since I'm not bothered by the bridging (those pins needed to be tied together anyways)
But now they are saying there is another potential problem that 2 of the pins will "steal" solder from the third and this one would remain unsoldered. that would be because the higher mass of solder acumulated on the 2 pins will easily "attract" some solder from the third, in the extreme condition leaving it unsoldered.

I can't believe this could happen (I asked them to show me an example of such a bord and they didn't have one), since the capilarity between this third pin and the copper pad will strongly keep the solder there.

Please help me argue that. Is there any book/document clearly stating this?

Thank you,

nike

 

[Verba]

I can hardly believe that. As far as i know, it will not remain unsoldered because of the cappilary effect.
b.t.w., is there any solder mask between the pins on that copper pad ? It can prevent "stealing" of the solder from the third pin.
Unfortunately I havn't found any document stating that

 

[nikeplato]

Thank you Verba,

Yes there is some very thin solder mask in between the pins and we tried to widen it but the PCB manufacturer couldn't do it.
Meanwhile I found some documents talking something about "solder robbing" and found some guy on the IPC site blog, mentioning same kind of issue, and he basically didn't give a solution but he said he will try reduce the quantity of solder (i.e.: reduce the stencil aperture).

Thanks anyways,

nike

 

[danthrax]

our design guidlines are to route an "M" shaped track at the ends of the pads to join them. This helps thermally isolate the joints (less chance of bridging) and makes it obvious to manual inspection that the bridge is ok.
For super fine pitch we also use as narrow tracks as possible.

 

[abc123]

yes, use the m shaped track as suggested by danthrax, you can route m shaped track under the chip if you don't want inspection people bother you. soldering bridge and solder stealing is a problem when your volume is large and re-work costs a lot. and special instructions costs money too(in terms of time). you can change the PCB lauout for the next batch and ask the manufacture people to accept current PCB