How Many Spins?

The other day I wrote about my failure to follow my own advice. Obviously, advice is only for someone else. Just like the best standards are double. Right?

Hmmm. It got me to thinking about board spins. Years ago, I remember products produced by the company I worked for often coming out with double-digits worth of mod wires in production PC boards. I think with the ability to turn PCBs in a day or a few, that rarely happens anymore. But what about in the prototype stage?

Here at Screaming Circuits, surprisingly few repeats show up other than from people using us for small-lot production. We do see a lot of layout issues here, but likely we see a lot because we see a lot times a big multiplier of different designs here.

For my little dohickeys, I seem to need about one board spin due to design or layout problems for each five designs. Of course, mine are pretty simple. Most of my boards spins are due to me coming up with better ideas after using the thing for a while.

If my supposition is true that mods are required less often now, is it because designers are better now, tools are better now or components are better now? How many times do you typically re-spin a PCB due to design or layout problems?

Duane Benson

2 thoughts on “How Many Spins?

  1. I have found that the number of re-spins can vary depending on several factors, the Engineer doing the design, the Companies philosophy towards the design cycle, and the design complexity.
    Engineers can be split into Tortoises and Hares :). The Tortoises will mull over every aspect of the design from schematic to layout, and hover like a hawk, watching every track go down, often with numerous ‘helpful’ comments. They usually have very fixed views on design, quite often from a past century! When a Tortoises board finally… arrives it will work first time and will require one very minor mod, due I must add to the PCB guy laying out some circuitry when not being supervised.
    The Hares will throw a schematic down in about 30 minutes, lets the PCB guy get on with his job, and several days later a prototype board is being powered up. These boards will require several mods, again due to the layout guy; not realising a signal named $3029 was a 100MHz clock or similar lack of reading the engineers mind.
    Then there is the Company philosophy:
    We all work in a no blame culture, but there are different levels of no blame from the:
    NO BLAME, where management realise that people being mostly humans, make mistakes, and accept the fact. This tends to inspire a creative attitude to work, where re-spins are minimised, and the various departments work harmoniously together.
    no blame, where the slightest modification will cause a witch hunt that would make Matthew Hopkins proud. This is accompanied with the usual tirade of “why can’t you get it right first time”, and the belief that a prototype board should work perfectly and be ready for assembly runs into the millions. Here the departments build trenches and employ the “finger of blame” weapon. (This is a stiffly pointing finger aimed at the next department. Hardware engineers are best placed for using this weapon as they can target both the layout guys and the software guys.
    Finally on a more serious note, it is easier to mod a digital board or similar where links can be easily added. When it is something like a SMPS, a respin is often necessary due to high voltages or just the required complex switching current paths.
    So when you add all the combinations up, it’s surprising we ever get a board done, one thing for sure though, if it’s wrong, if its late, if it’s the wrong colour its gonna be the PCB designers fault.
    A cynical PCB Designer.

  2. Interesting characterization of engineers in two extremes.. almost like black and white…
    I am sure they come in many shades in between too, but a good read nonetheless.

    Over the years tools have become much better to aid in detecting variances from the design intent. There are two keys to success: First, you MUST capture the design intent as early in the process as possible. Second, the tool you use must warn you about deviation as you layout/implement the board, not as a post process at the end.

    In Allegro, for example, customers have the ability to specify constraints when creating schematics for EE/Design intent and then during PCB layout, there is feedback through heads-up display as well as through the Constraint Manager on adherence to constraints/designIntent.

    Another example of how tools have gotten better can be found in this interview – Pete Waddell of UPMedia interviews Robert Jardon of FreedomCAD on Design Planning. You can find the link to this interview on the UPMedia daily newsletter or on the PC University home page.


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