Screaming Circuits: October 2009

Black Pad, One More Time

The thing that wouldn't leave...

Sometimes black pad seems like that. It should be pretty well understood by now, but it still pops up (or should I say "pops off") now and then. I just got a call from an engineer asking for some help in talking about a black pad problem with his board fab house.

He's having issues with ENIG boards from one specific board house and gull-wing parts. We most commonly associate black pad with BGAs and they do seem to be more susceptible, but any part can suffer from black pad. In this case, the fab house suggested that the specified gold layer was too think and the house said that the thick gold layer was causing the black pad.

I double checked with one of my engineers and he responded that a thicker gold layer may hide the black pad problem for longer then a thin gold layer, but it won't cause it. The industry accepted understanding is that it is a problem with the nickle layer during fab that causes it. As Phil Zarrow states (Circuits Assembly April 2008, pg 20), IPC Class II calls for a minimum of 1.97 um of gold, but sometimes that may not be quite enough because of normal surface variations. His failure analysis guru friend suggests 3, minimum and nominal 4 micro meter thickness.

There may be some issues with too thick a gold layer, but Phil and everyone else I've read from or spoken to is pretty clear that it's the nickle layer processing or too thin a gold layer that leads to black pad.

Duane Benson
Pop goes the SSOP

Unequal Pad Sizes, Take 2

I wrote a post not long ago about via-in-pad and unequal pad sizes as a cause of tombstoning and I received a question from Aaron about unequal pad sizes:

"In my designs, I have ground pours that directly connect to pads. So as an example, a cap that goes from vcc to ground would end up with one pad attached to the ground pour. Because of the solder mask expansion, the ground side pad will be noticeable larger."

This is a pretty common practice. I do it myself sometimes. The risk of tombstoning or other solderability issues relates to a couple of things. First, the smaller the part, the more critical all of this is and the more likely it will be for problems to crawl up. I'm assuming we're talking about SMT parts. Thru-hole parts can have some issues with copper pours, but not near as many as can SMT.

Pour-with thermal Pour-no thermalThe first issue is that the copper pour on one side will act as a big heat sink and may lead to tombstoning, or at least a poor solder joint on the pour side. If high current isn't needed, then use thermal pads. That will help. I would guess that with a cap like you're describing, high current isn't a requirement.

The other issue Aaron mentioned was the soldermask expanding on the copper pour and making the aperture size smaller then on the other pad. You can try to make soldermask defined pads on both sides. Then, in theory, the mask expansion should be close to the same on both sides. You can also make a larger mask opening on the side that goes on the pour so that after expansion, it will approximate the other pad size. The problem with this approach is that if you change board houses, you may not have the same amount of expansion. The best option might be a call to your PCB fab house for a chat about control of the mask expansion.

Also, if you do use thermal pads, as in the illustration, that might just solve the expansion problem right there. Most CAD packages have either a global setting to make all pads in pours thermal pads or a properties setting for each part that would make thermal pads as illustrated on the right above.

Duane Benson
Pour poor pitiful me into a picture of a pitcher

QFP/SOIC/SSOP Mechanical Security

I've written a lot about QFN footprint issues, but the venerable old leaded SMT configuration is still around. It gets its share of disrespect and questionable footprints too. When you give it a post-solder visual QFP fillet inspection, mostly what you see is the fillet "B" in my illustration. But, here's a question: which fillet is more important, A or B? The hidden fillet, A actually gives more of the mechanical strength then does B.

Most CAD library parts will have the proper footprint for these parts, but not all. Sometimes you have something close, but not exact. If that's the case, make sure the pad leaves room for a good fillet on both sides of the lead. Doing so will ensure that you have the best mechanical connection. Don't use red solder though. It might actually be Play-dough and Play-dough isn't a good conductor.

Duane Benson
Silly Putty won't work either

SMTA Conference


Hey there -

Next week, if you happen to be down in San Diego, I'll be talking at the SMTA conference. I speak on October 7, at 3:30. I'm in session EMS2, in Royal Palms 1-2. My talk is called "Strengthening Your Downsized Design Teams Through a Strong Prototype Partner"

Duane Benson
It's a long, long way to San Jose
And even further to San Diego

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