Screaming Circuits: PCB Finishes

And What Is Wrong Is...

What is wrong isQuick. If you haven't had your chance to answer the question, go back and read it first.

Done? Okay.

#1. This is an OSP (Organic Surface preservative) finished PCB. That in itself isn't a problem. However, take notice of the two different colors of pads. The darker pads are oxidized or otherwise contaminated - not a well preserved surface. And that means that this board isn't going to work.

#2. Oops. I already answered #2 in with my answer to #1. It's got some oxidized/contaminated pads. Perhaps the PCBs weren't stored properly, were kept too long or the OSP finish wasn't well applied at the fab house.

#3. Planet ten

#4. Real Soon.

And, no it can't be used as is. The solder most likely just wont take on those bad pads.

A couple of other notes: The mask registration isn't bad. Not perfect, but not bad. Big pitch BGAs like this do tend to work best with NSMD (non solder mask defined) pads like this. That allows the BGA ball to sag down a bit and grasp the side edges of the copper pad for improved adhesion. (Note that for extra fine pitch BGAs, like 0.4mm pitch, you need soldermask defined pads) And, note that there is a good web of soldermask between the pad and the via. This will keep the solder from being sucked down the via.

Duane Benson
I've been ionized, but I'm okay now

What's Wrong With This Picture?

Questions for the day:

  1. What are we looking at here? (Come on, you can be more specific than that)
  2. What's wrong with it?
  3. Where are we going?
  4. When?
What is wrong

And... will it work? Or is it just scrap? Stay tuned. Answers will appear on the Screaming Circuits blog either in the comments or in a new post.

Duane Benson
It flies like a truck.

DB25 - Survey Says...

DB25 with 
issuesI've gotten a number of answers to my question: "What's wrong with this picture?" All good responses.

  • Via between the connector pins
  • Soldermask covering the pads
  • Tight zig zag on some traces
  • Traces real close to some of the pads
  • Uncovered/partially covered via in between the pads
  • No clear pin one markation
  • Tear dropped pads might be helpful

The biggest problem that I see in terms of buildability is the solder mask on the pads. Yes, we can make it work. In a prototype world, we make a lot of stuff work that by all rights should not, but once into production, or for best reliability, that mask is a problem.

The tight spacing between some of the traces and the pads is not inherently a problem, but based on the mask issues and the poor drill alignment in the via, I'm lead to question the workmanship on this PCB to the point that I would be concerned about anything too close to anything else on this board.

DB25 with issues del The third concern that I noted, I'm no longer quite sure of. I just have the photo in hand, not the actual PCB. When I first looked at this photo, I interpreted the lighter area around some of the pads as delamination. We see that sometimes and it's a bad problem. With a second look though, I'm not so sure that I'm not just seeing an inner copper layer with openings around most of the pads. Hard to say just by the photo.

I'm not terribly concerned by the lack of pin one designator. This is the solder-side of the board and as long as this connector is on the correct side of the board, it's not reversible. The via doesn't bother me much either, as long as the connector doesn't have any exposed metal over that side. Most of these would have plastic down there now.

Duane Benson

DB25 With Issues

DB25 with issues

Take a look at this footprint. We're looking at the solder-side of a DB-25 connector footprint. Given all of the smt and HDI (High Density Interconnect) we see these days, this looks pretty primitive; like it could be straight out of 1979 or something. But it is a recent PCB design. Some of those good old-standard parts still have a place in today's world.

However, old or new, issues can still pop up. So, just what do you see wrong with this one? There are at least two pretty obvious, and a third issue that may be a little open to debate.

Duane Benson
What's wrong with this picture?

Oxidized PCB Finish

I've written a few times about potential and actual problems with different PCB finishes. Sometimes, the results are tragic and sometimes, just annoying. I'm not sure where this one falls on the continuum between the two extremes, or maybe even outside. I suppose it depends on who's paying the bill and how close these need to be to a final production product.

Oxidized pcb surface 

The surface on the pads had been exposed to the elements a little too long such that, even with good flux, the solder balled up instead of adhering as it was supposed to.

PCB finishes are more robust then they were even a few years ago, (although, a few years before that when we were mostly using leaded HASL, things weren't so bad) but still, your raw boards should be treated more like food then like shoes. Avoid fingerprints and other contaminants. Try to use them promptly. Store them properly. If your air quality is poor, be even more cautious in your handling and storage.

Duane Benson
Let not the sands of time get in your lunch

Hmmm. Black Pad Does Happen On Other Parts Too.

Well, lookie here. Just the other day, I wrote about black pad happening on gull-wing partsQFP fillet bp. I also wrote a while back about the footprint on gull-wing parts - the fillet under the part is even more important to mechanical strength then is the visible fillet on the outside of the part. This morning, I was browsing back through some of my old parts images and I accidentally gave this one a good bump. And with a spot of ironic annoyance, a couple of the leads popped loose due to what appears to be black-pad.

I thought I was being careful with my nickle when I made this illustration earlier this month, but I must have let some contamination through. Maybe I was eating cookies at my desk between the nickle step and the gold step. How rude.

Duane Benson
If they were chocolate chip, then it was worth it

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

Gold Fever

We call it Gold Fever. It's when you've got a hankerin to have a nice flat surface to put that BGA on and the lust for that precious metallic surface turns into an obsession. It's all you can think of. You forsake your family, your job and even your level 68 Night Elf druid for but a glimpse of that resident of period 6, group 11. Ahhh, yes (or Auuu, yes), the all desired number 79...

Gold fever Okay, maybe we don't call it "gold fever", but it certainly can cause you a fever in your time line and budget if your gold board comes back with spots like this one did. In addition to the obvious four spots with visibly degraded Au layer, this whole set of PCBs probably has black-pad written all over it. If you have other boards that came from the same batch as one like this, you should give them a very close examination. At least make sure you've got a good healer class in your guild.

In case you haven't been caught by the scourge of black pad, just know that the Argent Dawn can't help you here. Black pad is caused (and I'm generalizing, not going into exact technical details) when there's a little contamination in the nickle layer of an ENIG (Electroless Nickle Immersion Gold) pcb during fab.

When the component is then soldered on, the solder mixes with the gold but not the underlying nickel layer and the part can later pop off or at least not conduct your signals or complete your quests. It's most commonly associated with BGAs, but can occur with other types of components too.

Duane Benson
It's safe back here in Goldshire, but all I can find is copper.

And Another Reason...

Another reason to inspect your PCBs before sending them on to the assembly house.

Missing barrel 

At first glance, these boards looked fine. But with a little closer inspection, you can see that the middle barrel isn't plated through. Bummer.

There are a number of possible causes for this.

  1. It could simply have been a goof at the board house. Sometimes a process will slip or someone in CAM will introduce an error.
  2. It could have been caused by improperly creating a library part. Maybe the symbol was built up by hand and ended up with a non-plated hole in that spot.
  3. It could have been built without a library part. Sometimes designers will just create a place for a thru-hole part using vias and traces instead of creating the library part. If that's the case, the center could have ended up with a non-plated hole instead of a plated via.

Myself, I'm betting this was a board fab problem. In any case - another good reason to check out those boards before sending them on for assembly.

And even better then just looking at them - also have them electrical tested at the fab house. Our fab partners at Sunstone Circuits can do that for you.

Duane Benson
Roll out the barrel...
and have a barrel of open solder joints

Oxidized Silver? What do you do???

I've written before about the shelf life of immersion silver PCBs. In that post, I alluded to the possibility of carefully cleaning an oxidized immersion silver pcb but I didn't say how to do it. Sometimes an oxidized board can be cleaned. Sometimes it can't.

Their are really only two industry recommended practices:

  1. Send the boards back to the fab house to have them re-plated
  2. Get new boards if re-plating is too expensive

That being said, you can sometimes use a large rubber eraser to clean them. It's easy to mess the boards up though so it won't always work. I would not do that in a production environment or with an expensive board. And it won't do anything for plated through holes for thru-hole parts. For that, you'll need to go back to your board house.

Here's what one of our engineers said about the process:

"The problem with immersion finishes, silver in this case, is that the oxidation or tarnish is the remaining oxide/sulfide/sulfate/chloride compound (depending on what's in the area to react with) conversion of original silver content. The immersion coating is so thin to begin with, cleaning away the tarnish would basically leave you with little to no silver and the undercoat (usually nickel) exposed. Also, the type of tarnish/contamination is a big factor. The sulfur compounds (sulfide/sulfate) are worse - i.e. cause more solderability problems, then the oxide/chloride compounds. I'm not a chemist, so don't ask why.... :) Of course, the only way to find out what particular type of contamination is most prevalent on a give board is to have it analyzed.... Probably cheaper to get new boards!"

I've heard about some studies at Sandia National Labs regarding the life and solderability of tarnished silver boards. I'll try to look that up and if I can find it, I'll post some notes about it.

Duane Benson
sulfide, sulfate
sulfide, sulfate
Swiftly fly the years
One season following another
Laden with happiness and tears