Screaming Circuits: Prototyping with Multi Layer Boards


Prototyping with Multi Layer Boards

a guest post by Nolan Johnson of Sunstone Circuits (PCBexpress)

Over here at Sunstone Circuits, we’ve been tracking the increased use of a technique with which many of our PCB123 users have been prototyping. It’s an interesting development that also shows up in our conversations with PCB engineers at trade shows, customer sites, or whenever we find ourselves discussing prototyping techniques with designers using a wide variety of PCB layout tools.

What’s unique about this new technique is that designers are increasingly using multi-layer board designs throughout the prototype phase to shorten the initial design times and save project dollars in the long run.

Here’s how this approach works for many engineers:

  1. Rather than struggle for a couple weeks to squeeze their prototype design onto a two-layer board, designers are adding an internal layer or two to the prototype board. Expand the spacing between traces a just touch and let the auto-router do the work for you. This will drastically shorten the layout time spent on the prototype and potentially shaves a spin off the overall prototyping process by allowing you larger tolerances.
  2. The designers then get the boards assembled and validate the prototype’s core functionality. While the purchase price for the prototype’s bare board will be a bit higher, the saved labor costs that result by taking a week or so off the prototype layout process are much greater than the incremental fab costs.
  3. These designers tend to enter the production optimization phase ahead of schedule and under budget, leaving them valuable breathing room with which to optimize down to a production-ready two-layer board.

The benefits to the design team are as follows:

  • Shorter design time – less labor cost
  • Better DFM tolerances – reduced risk of shorts or design mistakes
  • Fewer prototype spins – saved budget dollars on boards and, more importantly, components & assembly labor

Large PCB design firms have been using this prototyping technique for a number of years now. It works well for designs in which there isn’t a lot of high-speed design, and where strict compliance is not a requirement.

So how does this all pencil? Let’s look at an example using some hypothetical numbers. As always, your specifics will be different than the ones in this example. Work the numbers with your specific data to estimate the savings you’ll see in your environment.

For the meantime, though, let’s assume:

  • layout designer costs you $100/day,
  • design takes 12 work days to lay out,
  • PCB bare boards cost $350 to fabricate,
  • $1,500 in parts and assembly costs to populate your board, and
  • a typical prototype process requires three spins.

Let’s also assume that the multi-layer technique results in a $500/order cost for PCB fabrication, reduces design time to four days, and cuts your spins down by one. The numbers roll up like this, resulting in a multi-layer prototype that’s actually 52% of the cost of the 2-layer approach, and requires 35% of the design days:

Sunstone_guest_table_10907_2

Now, for this technique to work well there are a few prerequisites. If only some of these conditions apply, your payoff may not be as noticeable. If none apply, you’re probably not a good candidate for this approach:

  • Your company accounts for staff labor as a part of the project cost. If you’re a hobbyist or an individual working on your own (no-cost) time, then this technique still works but is of more limited value.
  • Your prototype board will be optimized before production. If your prototype is likely to be used unchanged for production, your product will carry an ongoing incremental cost increase as a result of the multi-layer board. If you’re plan all along to optimize down to a two-layer configuration, then the 33 days you saved ought to give you plenty of time to get the optimization ‘just right’.
  • Your board will fit within the restrictions of a two-layer format once optimized. The first step is to assure yourself that you’ll be able to get your circuit to fit on your target production format before you even begin.

We’d love to hear from you on how this technique has worked for you on your designs. Drop a comment here, or email Sunstone at: njohnson@sunstone.com.

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