For those of you at my PCB West session on the Sept. 27, thanks for attending. Here’s the final presentation as delivered:
http://screamingcircuits.typepad.com/files/pcbwest2012-duanebenson-reliablemanf.pdf
Duane Benson
Category Archives: Screaming Circuits Blog
Via in BGA Pads
The answer to the question “Is it ever okay to put open vias in BGA pads?” is simply “no.”
It’s no, no, no, no, not ever!!! That makes it easy. No technique to worry about. No tolerances. Nothing. Just don’t put an exposed via in a BGA pad. The only option is between the pads, with a complete solder mask dam between the pad and via, or have the vias filled and plated over at the board house. Nothing but metal is allowed on the BGA pad.
Now, other components give you more flexibility and thus require some choices and guidelines. Andy B. asked about large components, such as voltage regulators where the manufacturer has recommended vias to connect the thermal pad to the ground plane, or to additional thermal area on the back side of the PCB.
The easy answer is to just treat it like a QFN and read our various suggestions surrounding that form factor. Here’s some. Having the extra room does allow for additional flexibility, but if the vias are open, they still run the risk of sucking solder to the other side of your PCB. You can sometimes get away with really tiny vias, as in here. But it’s not best practice.
It’s really a matter of tradeoffs. I have seen opinions stating that you should never fill or cap the via because doing so might impede the thermal transfer. Well, power chip manufacturers, you shouldn’t rely on unbuildable design to meet product specs. You can fill the vias with thermally conductive material. You can cap the via with solder mask, as in the link I just gave you. Just make the via cap as small as possible — 100 to 125 microns larger than the via.
Finally, segment your paste stencil layer. If you put solder paste on top of an open via or even on top of a masked via, you can be asking for trouble. In this image, the six vias (which will be capped) are put between the openings of the stencil.
Duane Benson
Tesla says what?
Via in Pad X 8
Here’s an interesting via in pad case. On the one hand, the footprint is very symmetrical and clean looking. On the other hand, it has open vias in the pads.
At first glance, I thought this was a DIP footprint with extra long pads, but it’s not. It’s for an SMT part. Personally, I would have put mask between the pads. Looking at the rest of the board (not shown), the spacing between pads and mask is pretty wide, so there may be a good reason. I’m not sure though.
Definitely, though, I would not put the vias in the pads like that. Those open vias will cause solder to flow down to the other side of the board, make a mess there and leave the chips without sufficient solder.
Duane Benson
Sucking solder through a straw – or via
Passively Annoying
Passive components can be kind of offensive sometimes. I can understand them in analog circuits or charge pumps. But the fact that we need to put them all over our digital logic is just rude. Technically, I understand why they have to be there, but philosophically, they violate my basic principles of life.
Back in the early days of personal computers, there allegedly was a company that had its engineers remove bypass caps one by one until the motherboard stopped working. Then they’d add the last one back in and smile about the short-term cost savings. Well, that was a bad idea. The reality is that we need them.
I’ve written about some of the problems that can show up because of passives (or other small two lead parts like LEDs and other diodes). Like here, here and here. That last example has popped up recently and I have some more thoughts on it. Essentially, I’m talking about multiple two-lead components that have one lead tied together. That’s a pretty common scenario with bypass caps or LEDs (or the LED current limit resistor).
There are a couple of ways to do this. Some error prone and some not. First, the general rule of thumb for two lead passives is, if at all possible, to have the same amount of copper going into both sides. That means that if you have one 8 mil trace going to one pad, have one 8 mil trace going to the other. Also make sure that you have solder mask stopping the solder from going off pad.
Passively annoying bad way here is bad. It might just barely meet IPC standards, but it still is really not manufacturable. First, there are no thermals. That makes the solder melt much slower on the right side which can lead to unreliable solder joints or tombstoning.
Second, even though the theoretical solder mask openings don’t touch and the keep-out (it’s not shown but is just a hair narrower than the mask area) areas don’t touch, they are close enough that you might not have any mask between the parts on the thermal pad. That can lead to components shorting.
is also bad. You have your thermals in there so that’s good, but the parts are still so close together that you might not get any mask between them, leaving a path of bare copper between the parts that can cause them to drift around and mess things up.
Method B1, on the right here has the same issue. Likely no solder mask between the parts and a bare copper path between the parts.
Method C here is fine. The parts are still at risk of not having mask between them, but there isn’t bare copper running straight between them. There will be mask between the parts and the pad so there isn’t any way for solder to bridge or the parts to drift.
Method D here is also okay. You do need more room to spread the parts apart. That’s a bummer, but sometimes “bummer” is the cost of reliability. Here, there will be solder mask between the parts and there are thermals. Everything is happy.
Use method C if you have a little side to side room to play with or method D if you have a little top to bottom spare room.
Duane Benson
Prevent flanking maneuvers.
Don’t be like the Solders at Thermopylae
Robots on Mars!
And the Curiosity Rover has landed! People can be so cool when we set our minds to it. Touchdown at 10:14:39 Earth Pacific Daylight Time.
Duane
One small step for A robot. One giant leap for the rocket that got it there.
Cool Customer Application
It’s not all that often that we get to see or can talk about just what is done with the boards we build at Screaming Circuits. In most cases, it’s a proprietary product or some government thing. But, recently we built some boards for NTH SYNTH. They have a successful Kickstarter project to produce a music synthesizer. They describe it as: “It is fun to use, sonically-rich, and hackable by design.” Go check it out.
(This image is from their website.) I wanted to take some photos for them of their PCBs being assembled on our SMT machines, but the boards ended up being built on one of our night shifts and I missed the chance.
Duane Benson
They’re the people out there turnin’ music into gold
But hopefully makin’ more than Jim Bass’ two-fifty for an hour
Electrolytic Ambiguity
I’ve written about ambiguity a few times before, like this post about fiducials. But I’m not talking about the PCB today. I’m talking parts. More specifically, I’m talking about silkscreen markings for your parts on the PCB.
Diodes have a lot of opportunity for ambiguity, as you can read here. There are many ways to mark parts, but fewer ways to clearly mark them. Take a typical electrolytic capacitor. It can be through-hole, SMT metal can, tantalum, or a few other form factors. The capacitor manufacturers aren’t doing any of us any favors insofar as “markation” is concerned.
Check out the image at the right. Yikes! In all cases shown here, I’ve oriented positive on the left, which, according to IPC is pin 1. This is also the zero degree rotation for the centroid value. But, isn’t it nice of those component manufacturers to put the identification bar on the positive side for tantalum capacitors and on the negative side for metal can electrolytics? Not!
So, how should you mark this in the silkscreen on your PCB? For an electrolytic capacitor, the best approach is to mark the positive sided with a plus (+) sign. If you mark pin 1 with the number 1, it can easily be mistaken for the minus sign. If you mark the negative side with a minus sign, it can easily be mistaken for pin 1.
For a metal can capacitor, it is also acceptable to put the notched outline in silkscreen. We still recommend that you place the plus (+) sign on there too.
Duane Benson
I’m just positive I put the negative right on the left
http://blog.screamingcircuits.com/
To Mod or Not to Mod? That is The Question
Many years ago, I was a product manager at a business-consumer electronics company developing some pretty leading-edge display equipment. Prototyping back then was a long and painful process. A PCB might take a month or two to arrive from fabrication. Parts had to be sourced by digging through massive catalogs and then hoping that what you needed would be on the companies approved vendor list. The whole process was a bear.
Well, the soldering up part wasn’t always so bad – unless you were the poor soul tasked with wire-wrapping or hand soldering the prototype.
Based on how difficult and expensive a board spin was back then, common practice was to just mod up the boards, even in production. Any given PCB might have a dozen or more cuts and mod wires. Those changes might not make it into the PCB for months. These days, though, you can get board fabbed overnight, your parts delivered over night, and when you have all of those parts and PCBs, you can get them assembled overnight. I suspect that increase in speed is the major reason mod wires seem to be nearing extinction these days. (note that Screaming Circuits didn’t build the board in this picture. It’s from my personal collection)
It may not seem cheap to pay to have someone respin a board so speedily; especially when set next to hand soldering. But when compared to the cost of idle engineers waiting for the next rev, the cost of adding mods, the reduced reliability from having mods and the additional manufacturing time caused by modding a board; today’s quickturn parts, fab and assembly options can end up saving gobs of time and money in the long run.
Duane Benson
There are more wires in heaven and earth, Horatio, than are soldered on your PC board.
How Not to Trick Your BGA Friends
Continuing with yesterday’s theme, I have a couple examples that should have been fine, but due to issues at the board house, improper storage or contamination, ended up very much not fine.
Behind door number one, we have an OSP finish that will make you very unhappy. That’s “Organic Solderability Preservatives” in long hand. I’ve also heard it called “Organic Surface Preservative”, but close enough. It is a nice flay surface which is good for BGAs. Years ago, it had a reputation for being poor quality, “cheap”, but newer formulas seem to work pretty good in both leaded and lead free. In this case, the darker pads were likely contaminated in some way – either at the board fab house or subsequently in handling.
Next is the worst example of surface degradation I’ve ever seen. Yes, it’s an extreme outlier case, but this is where a silver board can go if it wasn’t built with the best quality control, was stored too long, was exposed to polluted air or other contamination and had bad luck. This board probably has all of those issues, but any one alone can be problematic. Silver board especially should be stored in a cool dark place; preferably sealed in the original packaging.
Duane Benson
OSP can also mean Oregon State Patrol, but they don’t care about BGAs. Just safe driving.
Fiddling with Fiducials Again
I recently posted a note about fiducials but I didn’t have any images. Here’s a couple of examples:
This first example shows what IPC would like to see. If this is an individual board, this would be it. If it were part of a panel, you would follow the same pattern on the panel rails and also put it on each individual board in the panel.
As I wrote in the earlier post, we don’t require these, but it’s always a good idea. You’ll need them once you go into volume manufacturing anyway.
The next example won’t make IPC happy, but it will make Screaming Circuits happy:
It only uses two fiducial dots, but it isn’t reversible. Reversibility is okay for jackets, but not for circuit boards. Since one of the dots is offset, it can’t be placed on the machine and recognized as correct in any way except in the proper orientation.
The important aspect of both of these examples is that they remove ambiguity. There can’t be any uncertainty, which is good because uncertainty is your enemy. It’s a subtle enemy. It might not bother you 99.9 times out of a hundred, but then, when you’re not looking, it can strike. So, give a hoot and stomp out ambiguity.
Duane Benson
False data can act only as a distraction. Therefore, I shall refuse to perceive.