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.

0.4mm Pitch BGAs - Staggered?

I wrote not long ago and not far, far away about the type of pad recommended for 0.4mm pitch BGAs, as in non-soldermask defined vs. soldermask defined (NSMD vs SMD). The wisdom being that with the pads being so close on a 0.4mm pitch land, the BGA balls may be too close together and may bridge if the pads are NSMD.

I got a comment from an engineer stating that not all 0.4mm pitch BGAs are Staggerd .4mm NSMD pads created equal, that some have staggered lands which would still require NSMD pads. I searched and couldn't find a picture of such a part or corresponding land - too many millions to go through - so I simulated what that might look like in this image.

IPC calls this an "Interspersed array" (IPC-7095B, March 2008, Figure 6-13) or a "Staggered Matrix" (IPC-7351A, February 2007, Figure 14-8)

The thought being that, when staggered like this, the center to center distance between the pads is 0.57mm even though the column center to center distance is 0.4mm and that would put it back in the realm of getting better results with NSMD pads because 0.5's and above generally want NSMD pads.

Hmmm. Makes sense. Thanks Mr. Pythagoras for the extra 0.1656854mm of space.

Duane Benson
Use the reflow, Luke. Use the reflow

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.

How Fast Is Fast? I Mean, Really?

We talk a lot about speed here at Screaming Circuits. Back in 2003, one of the main reasons our parent company started us up was because their customers were telling them that they needed prototypes faster.

So, I know that getting everything built is faster these days - we can ship fully built boards as fast as 24 hours after we receive a kit, and you can get the raw fabbed PCBs in a day too. Certainly, everyone knows that you can place an order with Digi-Key and have the parts on your desk (or in our shop) the next morning. But has the rest of the process gotten faster too? [See if you can find the shameless plug in there. Sorry]

Whether you're using Sunstone PCB123, Ultiboard, Eagle, Pads, Altium, Allegro, or any of the common Bb input pwr sect CAD packaged, you'll probably spend most of your prototyping time in the software. It's also probably the least predictable segment of the process.

What takes longer? The schematic capture or the layout? Or are both completely variable and totally unpredictable? If your boss came running into your office and said:

"Bob! Quick! We just spilled something. We need an underwater temperature sensor with a video camera that can send a real-time temperature data stream and live video feed a mile back a cable to a host computer. And we need it NOW!"

How long would "NOW" take for you?

Duane Benson
Changing your reaction to the duration of time since 2003...

It's Not Just Planes and Passives

Tombstoning uneven route Planes can cause problems with passives, but it's not just planes. Uneven traces - a big one on one side and a little one on the other side - can cause just about as many problems too.

The scenario in the image on the right can do a lot of damage to manufacturerability and reliability.The problems get worse as the parts get smaller. You can end up with a cold solder joint on the right side or tombstoning. And cold solder joints are worse than cold oatmeal.

Duane Benson
Planes can hurt, but what about Euclids?

Reminders and Stress

Yesterday, I wrote about how much faster we can get things done in the prototype world than we could back when many of us were starting our careers. I mentioned that the decrease in time from weeks to days needed to re-spin a board and get a new prototype run completed is a double edged sword, but I kind of only mentioned the benefits - time savings, cost savings, quicker time to revenue generation. I didn't mention the down-side.

I also didn't really elaborate on another benefit - higher quality product. Instead of shipping product with PCBs riddled with mod wire, it's fast and cheap to get new boards and new assembled prototypes built and tested. These additional test cycles allow for much more reliable product. It can even improve the quality of firmware - shorter time to get a working prototype built means more time to write and test software/firmware. It's all good.

So, what is that second sword edge that I refer to?

RCA12ax7_sq_arms_smoke Stress. Yes. Stress. Back in the days before email and electronic projectors and speedy prototypes, we could relax more. A color presentation generally had to be sent out to have color transparencies or 35mm slides made up. If the boss wanted changes, they had to be made a week or so ahead of time. Now, with electronic projectors, changes can (and are too often) made at 2:00am the morning before the big presentation.

Since I can build you a prototype in a day or so, now those marketing geeks can, and do, throw changes in just a few days before release to production. Everything has to get done faster and faster. I want it now! We enable faster turns so the expectations increase and then we enable even faster turns and the expectation increase that much more. It never stops.

Duane Benson
Sorry. Sort of...

Reminders are Good Sometimes

I was just recently reading an article on another website that caused me to reflect a bit on where we've been and how far we've come in this industry. The article covered a design engineer's experience with modding a board back in the 80's and being required to ship the board with the mods instead of getting new ones made properly.

Back in the late 80's and early 90's, I worked for a company that designed and built business-oriented displays. One of the products was particularly troublesome to get going and the first production versions shipped with something like 24 different mods. If the company had re-spun the boards, we would have added at least a month to the schedule and payed somewhere in the range of $20,000 to $40,000 dollars. If I recall correctly, one of the biggest problem areas was the PLL (phase-locked loop). We were over-driving the parts a bit and that made all of the support passives and the layout that much more critical. Not smart, but I guess that came from one of those "cost-benefit" analysis type things.

Contrast that today where you can get a new set of boards from a PCB fab company like Sunstone.com in a few days for a few hundred dollars, get the parts from Digi-Key overnight and have us (Screaming Circuits) assemble them in a day or two.

Of course, it's a bit of a double edged sword. Like when faxes and later email came along. Written communications cycles that used to be measured in days became measured in hours and minutes. The expectations changed. Can you imagine writing a letter to a company and waiting a couple of weeks to get a response?!! That's the way it used to be.

In the same vein, us here at Screaming Circuits (and some other people too) have changed the prototype cycle expectations. Can you even imagine finishing your layout and having to wait four to six weeks for assembled boards to come back? Yikes! But that's what it used to be like. We're all making things go faster and faster. It keeps getting faster and it won't slow down. But that's good because time = money so less time building = less money spent and more time selling = more money earned. Right?

Duane Benson
I... Just... Need... More... Coffee... NOW!!!!

Passive Problems

Here's a common scenario: You have an array of small components. Maybe some SOT23 transistors or a set Common ground 0402s schof LEDs. On one side, you have wires and chips and stuff hooked up all over the place. On the other side, you have a ground plane.

The easCommon ground 0402s 
lay1y route would just plop the grounded pad of the part right on the ground plane.  You would get better heat sinking if needed. You's get a much more direct path to ground. It would be quicker to lay out.

But - and there's almost always a "but" to such questions - you could get tombstoning. Especially if the parts are 0402s or smaller. You would also likely have soldering problems because the plane will act like a heat sink and may keep the solder paste from melting.

If you really need to, You could do the pad directly on plane thing, but you'd probably have to hand retouch each connection on the big pad and maybe rework tombstoned or crooked parts.

Common ground 0402s lay2 Much better would be to do something like the image on the right. You could also use thermal pads in the plane. With really small parts though, you might still be opening yourself up to soldering problems because of the heatsinking of the plane. The thermal pads would typically have three connections to the plane in a setup like this and that could still be an unequal amount of copper connecting on one side vs the other. You generally want to have the same amount of copper on both sides of the small parts.

You could also just run the eight traces straight to the plane. How would you approach this seemingly simple but surprisingly error-prone layout?

Duane Benson
You'll take the left road and I'll take the right road
And I'll be in reflow before you

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