can soldermask + paste keep a package from shorting to vias?

Other Hardware General Electronics
1

I received a board back from OSHPARK.com that did not have tented vias under an IC package that has a exposed pad on the bottom of it. ENIG plating is visible on the edges of the holes, instead of complete solder mask up to the drill. I'm wondering if the thickness of the solder mask around the hole, in combination with the solder paste holding the chip down keeps the package just high enough off the board so that the Exposed pad does not touch the vias? I've done some quick and dirty continuity checks between a ground pad and the vias on the bottom under the chip, which shows no short, but that may be because the EP is wired differently or something.

Should I be fairly safe in assuming nothing is shorted if the continuity test shows negative?

2

Ooh, poor mix of design and/or parts selection there.
Still, if the signals are not showing as being shorted to Gnd (or whatever signal the pad is - read a datasheet today that had the chip pad at 3.3V) you might be okay.

3

When I first saw pictures of the DUE layout, I wondered whether it didn't have a
potential pad-to-via shorting problem, too. ????

4

2560 TQFP does not have exposed pad on the bottom tho.

The leadless M62 package does, but that is not what's used in your links.

5

The exposed pad is there for a reason. If you fail to use it you risk the component going out of specification and breaking.

Either select a different component that doesn't require the exposed pad, or re-design your board to use the exposed pad.

6

Yeah I was hoping to save a redesign. I neglected to check the ATMega328 datasheet for a QFN package to see if there was indeed a pad, as the part I used (sparkfun library) did NOT have an EP in the layout. Once I got the boards back, and then the chip came in...i realized the issue. I went ahead and reflowed the board to see if it would work with just the existing ground pins (I'm not running anything heavy duty in terms of code, so I may be ok, but yeah...was a pretty big oversight on my part. Should've triple checked :frowning:

since my multimeter doesn't show a short between ground and the vias on the backside, I'm going to power it up to see if it works, but I'll most likely have to either redesign the layout to move the vias out from the middle of the part, or use the TQFP version. Fail.

7

If any large voltages (greater than logic level) were involved then I'd be wary - 0.05mm of air can
act as a nice insulator at a few volts, but crank than up to a few tens of volts and it could breakdown.

0.05mm of soldermask on the other hand probably handles 100V without strain.