What Do You Do If You Can’t Have Reference Designators?

The first answer to that question is probably going to be along the lines of, “Put them on the board.”

But, sometimes you can’t have reference designators on your board. Maybe it’s too densely populated and there isn’t room. Maybe, for aesthetic reasons, you’ve chosen to leave them off. With some products, like development boards, it’s sometimes necessary to use the space for instruction or functional identification and reference designators would confuse your customers. Figure 1

It’s always best to put reference designators as close to the part as possible, and on the same side as the part, but if that’s not possible, you can still create an assembly drawing. When laying out the board, put the reference designators in a different layer than the text you want in silk screen. Then, create a PDF that has all the component outlines in their place, with reference designators. Make one for the top and one for the bottom. Call this document “assembly drawing” and include it in the files sent in to be manufactured.

Figure 1 shows a good assembly drawing format. It has reference designators and polarity marks.

You might ask why reference designators are needed when all the surface-mount parts are machine-assembled. First, any through-hole parts are hand-assembled. Their locations and board side needs to be clear for the people stuffing them.

Second, CAD systems don’t always have 100% accurate information. If the center point of the footprint is off, surface mount machines (ours and anyone else’s) will center the part where file says to put it, which, in the case, would be the wrong spot.

The reference designators are also part of quality control. It’s another opportunity to remove ambiguity. Ambiguity bad. Certainty good.

Duane Benson
Car 54, where are you?

http://blog.screamingcircuits.com/

Speaking of Reference Designators …

In my prior post about BOMs, I gave a few examples of reference designator formats in the BOM. BOMs are a common item that have standards but no standards as are reference designators. There are actually a number of standards covering reference designators, but I still find people referring to documents published in the 1970s!

Some aspects are pretty obvious. They are a code letter followed by a sequential number. Each and every placement on the PCB has to have a unique reference designator. The code letters are somewhat standardized, in practice. Some vary based on the particular user. Pretty much everyone uses “R” for resistor and “C” for capacitor. The mostly standard designator for an integrated circuit chip is “U”, although I’ve seen “IC” used enough times. Crystals and oscillators are supposed to be “Y”, but I’ve also seen “X”, “Q” and “U” used. Check this page over at Mentor Graphics for its recommendations.

Things start to get sticky when people have more than 10 of a given type of component or when putting together a family panel (several different designs on the same PCB panel). Let’s say you have 15 resistors. You could designate them as R1, R2, R3 – R15. But maybe you’re a little OCD and you want them to all have two digits. In that case, you might have R01, R02, R03 – R15. To a human, “R1” and “R01” might very well be exactly the same thing. But to a surface mount robot, they are two different things. The robot would be happy with R01, R02, R3, R03, R4… but that could cause problems for a human reworking or maintaining the circuit later. It’s best to be consistent. Basically, the assembly systems see reference designators as text items, not numerics.

Let’s take the example of a family panel. One board has C1 and C2 are a 10uf, 24V tantalum cap. The other board has C1 as a .01uf, 50V ceramic and C2 as a 220uf 24V metal can electrolytic. If you were having them built separately, there wouldn’t be any problems, but the two of those on a surface mount machine in a family panel and you will have bad news.

First, you could avoid running your boards as family panels. That’s not always practical, though. Second, you could just start numbering the second design where the first one stops: design one: R1, R2, R3, R4. Design two: R5, R6, R7, R8. That makes a lot of sense for a family panel. Just treat it all like one big design. That can get confusing though if you later run them individually or need to do some rework. Some poor tech could go crazy looking for R1 on design two. Even worse would be: design on: R1, R3, R7. Design two: R2, R4, R5, R8. Again, fine as a family but darn confusing when separated.

Personally, I would probably go with something like: design one: R101, R102, R103. Design two: R201, R202, R203.

Duane Benson
You know the nearer your designator, the more you’re silk screening away

http://blog.screamingcircuits.com/

Reference Designators

A while back, I wrote about reference designators relative to family panels. Family panels can cause problems because often times, each individual circuit layout will have reference designators that start at the same place.

For example, circuit A, down in the lower right corner of the panel, will have resistors R1, R2, R3… Looking at the other three circuits on this hypothetical family panel, all of them also start their reference designators with R1, R2, R3… That’s bad. It can lead to confusion and wrong parts on the board in the wrong spots. If we see this here at Screaming Circuits, we may spend some extra time and sort through it manually or we may ask you to fix it first. Fixing it here is a labor intensive and risky process. It’s bad news.

Anyway, to the point of this post: In the original post, I listed one wrong way and three right ways. There are two other wrong ways not in the original post, which I’ll list here.

Wrong way number one: R1-1, R1-2, R1-3. Bad. Most assembly software will interpret a dash as meaning a range. It will see “R1-3” as equalling “R1, R2, R3”. That can be bad.

Wrong way number two: Leading zeros. Don’t do “R1, R01, R001”. The leading zeros are stripped and that can cause all of those the be seen as “R1”. Just don’t put leading zeros in your reference designators.

Duane Benson
Corrigan says Long Beach is actually in Ireland

Family Reference

I’ve written a bit about reference designators here and there. There are a few more factors that we run into now and then. Take the family panel. In case you aren’t familiar with the term, it means that you have several different designs laid out into in one panel, as opposed to multiple copies of the same design in one panel.

Using a family panel can be a convenient way to deal with a multi-board design and can sometimes save a bit of money. Just a caution, though. Make sure to check with your fab house first. Some don’t like family panels and some won’t separate them for you. If you do have them separated prior to assembly, either at the fab house or by you, then you don’t have any reference designator worries.

If you leave them in the panel and wish to have them machine assembled, it can get a bit more complex though. “Why?”, you say. I’ll tell you why. Generally, most people start at “1” for each new design. i.e. “D1, D2, D3… R1, R2, R3…” If the boards go into the machine independently, that’s no problem. However, if you send the panel into a smt assembly robot, it may very well see that as your board having multiple D1’s, R1’s, etc. That would be rejected as an error in most cases.

If you are using the family panel approach, don’t restart your numbering when you move to another one of the designs that will be in the panel. Either continue on from the last number in the prior design, add in a hundred’s, with each design getting a different hundred’s number or add a unique suffix on each board.

  1. Wrong way: PCB1: “R1, R2, R3, R4, C1, C2”. PCB2: “R1, R2, R3, R4, C1, C2”.
  2. Right way: PCB1: “R1, R2, R3, R4, C1, C2”. PCB2: “R5, R6, R7, R8, C3, C4”.
  3. Right way: PCB1: “R101, R102, R103, R104, C101, C102”. PCB2: “R201, R202, R203, R204, C201, C202”.
  4. Right way: PCB1: “R1A, R2A, R3A, R4A, C1A, C2A”. PCB2: “R1B, R2B, R3B, R4B, C1B, C2B”.

There are a lot of ways to do this. Just make sure that no reference designators are repeated from one board design to the next. I prefer method #3 myself.

Duane Benson
Is it immediate or extended? Does it matter?

blog.screamingcircuits.com

Need a Reference for the Reference

Not long ago, I wrote a short post about non-standard use of reference designators. After doing that, I’ve been looking at some of my own microcontroller and motor driver boards with an eye for how close to standards I am.

All of the R’s, C’s, D’s and U’s are okay, but there are some differences. For example, the Eagle library I’ve been using calls crystals “X” instead of the more standard “Y.” I have seen crystals designated as “X”, “Y” and “Q.” LEDs seem to go by “LED” instead of “D” as indicated in the Wikipedia list. Headers go by “J”, “JP”, or “H.” Wikipedia says “J” is for a female jack connector, “JP” is for jumper, and it doesn’t list a “H.” My board has break away two-row male headers and keyed single-row male headers. Wikipedia does note that its list is a set of commonly used designators. Not necessarily standard.

We probably do have the specific standards document laying around here someplace, and if I were doing real work on a professional basis, I’d hunt it down and make sure I followed the actual standards. But I’m not doing real work with my controllers and drivers, so I just do the best I can. I wonder how often that happens everywhere. The standards books are “somewhere” but no one really knows where.

Duane Benson
Somwhere over the reflow…

http://blog.screamingcircuits.com/