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 had wanted to take some photos for them of their PC boards being assembled on our SMT machines, but the boards ended up being built on one of our night shifts and I missed the chance.
They're the people out there turnin' music into gold
But hopefully makin' more than Jim Bass' two-fifty for an hour
I've written about ambiguity a few times before, like this post about fiducials. But I'm not talking about the PC board today. I'm talking parts. More specifically, I'm talking about silk screen 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 thru-hole, smt metal can, tantalum, or a few other form factors. The capacitor manufacturers aren't doing any of us any favors where markation is concerned.
Check out this image. 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 silk screen 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 silk screen. We still recommend that you place the (+) plus sign on there too.
I'm just positive I put the negative right on the left
As usual, we are closed on Independance day, July 4th. This means that the day won't be counted toward your turn times. For example, if you have asked for a 48 hour turn time and we received your kit on the afternoon (past the receiving cut off) of the 3rd, your 48 hour clock will start on Thursday, the 5th.
We apologize for any inconvenience and wish you a happy holiday.
The other day, I needed just a few things at the grocery store which, given the small town I'm in, should have been a quick no-stress fifteen minutes. But, some kids were sitting in the middle of the first intersection I came to and didn't seem to want to move out of the way of my five thousand pounds of rolling danger. Traffic at another intersection was backed up due to a train. On the next block, I had to follow someone, likely looking for an address, at about ten miles per hour. Then there were pedestrians crossing the street far slower then human body mechanics are designed for. In the store, it seemed like every isle I tried to go down was blocked by carts or people. The "short line" at check out turned out to be short because a customer and checker were having payment issues. The drive home was much like the drive in. In short, there was nothing short about the trip. Nor was there anything low-stress about it.
But this is a blog about electronics stuff. It's not a shopping blog or a driving blog. The point is, that trip reminded me of projects I've been involved in years ago. Someone changes a spec after that part of the design is complete. The only version of a key component on the approved vendor list has a 12 week lead time. It's Friday, at 4:00pm, the board files have to be shipped off by five, but there's still several hours of double checking left to do. While placing the prototype parts order, you keep getting distracted by loud talking in the background.
Ugh. Not only is such a thing blood pressure raising, but it also can lead directly to problems any of us would never dream of letting out the door. Like these here:
Too little time can cause problems. So can too much stress and distraction. There's not always a good solution, but anything to reduce stress and agravation while doing final checks is probably a good thing.
Is there "lab rage" like there's "road rage"?
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 PC board 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 re-spin 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 quick-turn parts, fab and assembly options can end up saving gobs of time and money in the long run.
There are more wires in heaven and earth, Horatio, Than are soldered on your pc board.
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 flat 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.
OSP can also mean Oregon State Patrol, but they don't care about BGAs. Just safe driving.
Over the years, most of what we see are good PC boards. But some standout in the other direction as examples of what not to do. Some didn't make it through the board house alive. Some were unknowingly rendered useless in layout and some were just held on to too long or not stored properly.
In this first image, we see a guaranteed not to work example. Open vias in BGA pads will ruin your whole day. And you can't just cap them with solder mask either. For BGAs, the only two via solutions are to have them filled and plated over at the board house, or not be in the pads at all. Having a via in a BGA pad is like trying to cook scrambled eggs over a camp fire without a skillet. The eggs will in fact cook, but they'll be all mixed in with the fire and coals and stuff and you won't be able to eat them.
This next guaranteed not to work example shows a valiant attempt at keeping the vias out of the pads. But, as we used to say on the playground: "close only counts in horseshoes and hand grenades - and sometimes atom bombs." Here on the right, first, the mask registration is way off. That's not good but doesn't necessarily spell BGA death on its own. What will kill this assembly is the clear metal path between some of the pads and the vias. You need to have some soldermask blocking the metal path between the pad and the via. If you don't, it's almost as bad as putting the via in the pad. This board has a few places where there is a thin solder mask dam between the via and the pad. But, in the cases where there is no mask, the solder and solder ball will most likely migrate over to and down through the via.
Close might also count with badgers.
Let's say you have two options: First, you could send in your PC boards for assembly as individuals. Second, you could send them in a panel. That's all fine and dandy. For a few, send individuals. For a bunch, panels might make more sense. But, when you do go to quote and order, how do you count the parts?
Let's take this example. As a single, this board has 32 line items on it's bill of materials. That's 32 unique parts. Counting all of the individual part placements, there are 56 total parts: 42 smt and 14 thru-hole. So, naturally, if you quoted the assembly of 20 of this board at Screaming Circuits, you would enter your desired board quantity as 20, 32 total unique parts, 42 smt and 14 thru-hole.
But what do you do if you send it in panel form? How do you count? It's actually not as difficult as it seems. In this example, it's in a panel of four. There are still only 32 BOM line items, but there are four times as many placements. That means that if you quoted this, as a panel, you would enter 32 total unique parts, 168 smt and 56 thru-hole parts. If you still need 20 of the final boards assembled, you would enter 5 as your desired board quantity.
In the end, you will have 20 assembled boards. In case you are wondering about the cost, there won't be a difference. As long as the final number of boards (after the panel is broken apart) are the same, your cost will be exactly the same for panel vs. one up. You don't save any money by sending in singles. However, if your board is panelized and all of your parts on on reels, full or partial, you can save money by ordering Short-Run production.
50 Years ago today
Robert Rushworth flew the X-15 to Mach 5.03 at 100,400 feet altitude
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.
False data can act only as a distraction. Therefore, I shall refuse to perceive.