Screaming Circuits: PCB Finishes

PCB Planarity, Not Polarity

Via-in-pad can ruin a manufacturer's whole day. Or, if properly done, can go completely unnoticed. There are a number of ways to properly put a via in a pad but the best is to have it filled and plated oCopper filled via bulgever at the board fab house.

Copper filled via droop If you do that, check with them on their planarity standards. If they don't hold tight, you can end up with a  dip or a bump where the via is. Neither of those are as big a  problem as an open via, but they can still lead to some difficulties.

Speaking of bumps, the old standby, HASL, generally leaves bumps on the pads too. And, across the span of a BGA, the bumps can vary in size and shape. That's not such a good thing either. If you're designing with a fine-pitch BGA, you might want to consider a flatter surface such as ENIG or Immersion Silver. BGA on HASL close

Duane Benson
Fight Uni

To Lead or not to Lead. That is the question

Back at the Embedded Systems Conference in September, I had a number of folks ask me about mixing leaded and lead-free components on a PCB. It's a difficult situation for some people - especially when using old and very new BGA form-factor components.

We generally tell people to follow the BGA. Since the BGA has those little solder balls on it, it's the most sensitive to temperature as far as soldering is concerned. Reflow a leaded BGA at no-lead temperatures and the flux may all burn off and the solder may sag down too far and bridge or dry and crack. Do the reverse and reflow a no-lead BGA at leaded temps and you won't get a good intermetalic mix and the solder joint will be prone to cracking and other bad stuff.

In most cases no-lead components, other than BGAs can be used on a leaded board. Going the other way isn't always so easy though because of the additional 20 degrees C in the no-lead process. Everything's more sensitive to moisture absorption so baking parts or keeping them sealed in moisture-free packaging is more important. Some components may melt, especially chip LEDs. And metal can capacitors can pop.

In a prototype world, where you just need to see if something works, you can sometimes get away with a lot more than you can in production, but it's still not an easy question to answer. Unfortunately if you're in the situation of one of the guys that asked about it and have one leaded BGA and one no-lead BGA, you may have to get one of the BGAs re-balled or you may just need to redesign on of them out. No easy answer there.

Duane Benson
My 24 hours is almost come
When I to sulphrous and tormenting flames
Must reflow up myself

Lots Of Things Can Be Fixed

I recently posted a bit of a bad OSP board with the conclusion that it was only worthy of scrap. Over on the OSP_bad_BGA_finish Circuits Assembly blog, Mike commented that if the board was expensive enough, he'd rework it and make it usable. He gave a pretty detailed description of the process he'd use too. And he's absolutely right. Here's a picture of the full BGA land pattern on the right. You can see just how many pads were messed up and how random the pattern seems to be.

While not everything can be fixed, a lot of things can. It all depends on the cost of remaking vs. reworking, the time to remake vs. the time to rework and what you need ultimately in reliability. In the prototype world, we do a lot of work to just make messy things work simply because of the time constraints.

Most of things would never be done in a production environment these days, but sometimes they are. Not that longCenter pad oops both ago, judicious use of mod wires was commonplace on shipping PCB assemblies.

 Here's an example of something yucky that we made work. We've also cleaned tarnish off of silver boards and done a bunch of other things to recover from difficult challenges. As always, when we do do something like this, you're really in test-pilot mode.

Duane Benson
Rolly polly fish heads
Eat them up, yum.

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