4 Things You Must Consider Before Prototyping a PCB

By Max Maxfield |

There's more to creating a PCB prototype than meets the eye. Do you have any PCB prototype successes or disasters that you would care to share with the rest of us?


Many of my friends are involved in designing silicon chips in the form of ASICs, ASSPs, and SoCs (see also ASIC, ASSP, SoC, FPGA – What's the Difference?), so if you say "prototyping" to them, it means something very different to someone who wishes to create a prototype of a design that will be realized on a printed circuit board (PCB). In this column, we will consider four of the things that you really should consider before prototyping your PCB.

(Source: pixabay.com)

Parts Availability (General)

This is a big one. I used to write a monthly column for an electronics and computing hobbyist magazine called EPE (Everyday Practical Electronics) in the UK. Readers would send incredibly interesting articles regarding home-grown circuits to be published in the magazine.

The problem was that, on many an occasion, the little rascals had constructed their prototypes using old and obscure components from their treasure chest of parts, and these parts were either hard to find or completely unavailable. 

I remember trying to build an Atmospheric Monitor project from the magazine -- it took me a couple of years to track down an appropriate ferrite rod. In fact, one of my chums, Alan Winstanley (who is now one of the EEWeb Experts in our forums), was in charge of tracking down substitute components. I'm sure Alan can tell a tale or two about this process -- maybe we can persuade him to write a column about this.

The point of all this is that it's not unheard of for professional engineers to do the same thing; that is, to use a component that happens to be lying around, or to re-use part of an earlier design, without first checking to ensure that of these components are still available.

As I wrote in a recent column (see What's New in Altium Designer 18), the point when you are poised to go to manufacturing is not a good time to discover that one or more of the components in your design are no longer available (either they've gone end-of-life or there's a world shortage or something).

Parts Availability (Multi-Package)

Let's assume that your final product is going to feature surface mount technology (SMT) components. It may be that you are happy to create a prototype PCB that uses these components. By comparison, I tend to use a lot of breadboards.

In this case, it's advantageous for me to use lead through-hole (LTH) components for my breadboard prototype, and to then swap over to their SMT counterparts when it comes to building the final product.

When it comes to microcontrollers (MCUs), for example, I've always appreciated the fact that the folks at Microchip Technology continue to provide their devices in both LTH and SMT packages. 

Manufacturability

Assuming you are going to build a full-up prototype of your PCB, then one of the key things to consider is manufacturability (just like you would do for the final PCB product). Oftentimes, the difference between "difficult and/or expensive-to-build" and "easy and less-expensive-to-build" comes down to some very small design decisions.

My EEWeb Expert chum, Duane Benson, works for a company called Screaming Circuits. These are the guys who specialize in "building your short-run, one-off, and prototype PCB assembly, with rapid turn times and instant online quoting and ordering." I asked Duane for a few pointers, and he replied as follows:

When it comes to manufacturing, small changes can have a big impact. The items below are the "big hitters" in terms of manufacturing things that need to be considered when designing a prototype:
  • Don't put open vias in pads. The solder will migrate to the back side of the board and you won't get a reliable electrical or mechanical connection. Either have the vias filled and plated at the board house, or move them off the pads and make sure there's some solder mask between the pad and the via.
  • Don't put small surface mount parts next to large components. The large component will act like a heat sink and slow the solder melt on one or more of the small component pads. This can lead to "tombstoning" (the part standing up vertical like a tombstone) or cold solder joints.
  • Make sure you have the same size of trace going to both leads of a small passive part, otherwise the larger trace will sink more heat, thereby slowing the solder melt on that side, which can -- again -- lead to "tombstoning."
  • Put fiducials on your board in a pattern that isn't symmetrical, because this results in a board that can't be inadvertently reversed.
  • Make sure there's no ambiguity in your polarity markings, especially with diodes. Plus '+' and/or minus '-' signs simply aren't good enough for a diode. Use the diode symbol, or use an 'A' for anode and/or a 'K' for cathode.
  • Don't put parts too close to the edge of the PCB. If certain parts, like some connectors, overhang the edge, make sure your board house knows to not put panel tabs in that area.
  • If you're using QFN or DFN parts with a large center pad, segment the solder paste layer. You want 50 to 75% paste coverage in the center pad. Otherwise, the part may "float up," thereby leaving some of the outside solder pads unconnected.

In Summary

Well, I think it's clear that there's more to creating a PCB prototype than meets the eye. For example, we haven’t even considered visibility and testability; similarly, we haven’t considered creating test firmware. What else have I forgotten? And do you have any PCB prototype successes or disasters that you would care to share with the rest of us?

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  • by  Elizabeth Simon
    Another thing to consider when creating a PCB prototype is the size and shape of the enclosure. While considering that, also consider where to place the LEDs, connectors, buttons and switches. It would be a shame to have something that didn't quite fit and have to create another prototype just for that. That being said, there is a place for making a "breadboard like" prototype with extra space and maybe a prototyping area, especially if you have to use components that are NOT available in through-hole packages.
  • by  Aubrey Kagan

    Max


    For example, we haven’t even considered visibility and testability; similarly, we haven’t considered creating test firmware. What else have I forgotten? And do you have any PCB prototype successes or disasters that you would care to share with the rest of us?

     I hope you remember the blogs that I have written on related topics, that may be relevant to the questions that you pose.

    Think Outside the Board – Think of the Panel: 3 Hints

    Strange Brews: Designs That Shouldn't Involve a PCB, but Do!

    The Art of Test, Part 1: Boards, Subassemblies, &Products

    The Art of Test, Part 2: Controlling the Test

    The Art of Test, Part 3: Lessons Learned

    PCB Test Jigs & My China Connection

    Testing Your Test Equipment

    • by  Aubrey Kagan

      Oops- forgot this one

      Do You Check Your PCBs?

    • by  Max Maxfield

      @Aubrey: "...I hope you remember the blogs that I have written on related topics..."

      How could I forget? LOL

      I'm constantly amazed by the number of articles you've written on almost every conceivable subject -- when do you find the time to do real work? :-)

  • by  Aubrey Kagan

    Oops- forgot this one

    Do You Check Your PCBs?

  • by  Max Maxfield

    @Aubrey: "...I hope you remember the blogs that I have written on related topics..."

    How could I forget? LOL

    I'm constantly amazed by the number of articles you've written on almost every conceivable subject -- when do you find the time to do real work? :-)

  • by  Elizabeth Simon

    Don't forget to check that the pin out and pad spacing on your PCB layout matches what is in the data sheets for your parts.

    Pay particular attention to three pin through-hole packages which, for some reason, are often shown in bottom view on the data sheets.

    And while you are checking things, make sure that the power pins on all your parts are all connected to the right power sources.

  • by  Aubrey Kagan

    Elizabeth


    Following your train of thought:

    1. Add test points on every signal (it may be no more than a via) so that a wire may be soldered on for a scope probe attachment.

    2. On a micro bring out every unused pin to a test point to help with debugging and timing measurement

    • by  Max Maxfield (edited)

      @Elizabeth: "...On a micro bring out every unused pin to a test point to help with debugging and timing measurement..."

      Yes!!!

  • by  Max Maxfield (edited)

    @Elizabeth: "...On a micro bring out every unused pin to a test point to help with debugging and timing measurement..."

    Yes!!!

  • by  Max Maxfield
    One thing I forgot to mention is "Include some LEDs" -- these are so useful for indicating simple things like "power is on" -- all the way to debugging firmware and indicating error conditions.
  • by  Peter White
    And don't forget to add two machine pins for a Logic Probe.
    • by  Max Maxfield

      @Peter: "...And don't forget to add two machine pins for a Logic Probe..."

      Are you talking about ground pins? If so, why only two?

  • by  Max Maxfield

    @Peter: "...And don't forget to add two machine pins for a Logic Probe..."

    Are you talking about ground pins? If so, why only two?

  • by  Peter White
    I am talking about a simple HIGH-PULSE-LOW logic probe requiring just 5v and 0v connections.
    Many times a board does not even have a pin or pins where you can connect a simple logic probe.
    By using machine pins you can fit pins into them or use alligator clips.

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