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Publication Date: 07/1/2011
ARCHIVE >  July 2011 Issue >  Special Features: SMT and Production > 

Prototyping with Multi-Layer Boards
Engineers are shortening key phases of the PCB prototyping process by making use of multi-layer boards.

In spite of many areas where PTH designs can still do the job, electronic components continue to move relentlessly into SMT packages. Prototype manufacturing firms are increasingly seeing a shift in the fabrication and assembly work needed to implement engineering prototypes. The shift is subtle but unwavering: as new components come to market in surface-mount packages, QFPs, BGAs and the like, board design and assembly for prototypes must change to accommodate the surface mount components.

In some cases, a prototype design that might otherwise have used DIP and through-hole components now must make use of the SMT packages. And where that design might have once been assembled in-house by the design team from fabricated boards and standard parts kept on-hand in the design team's parts locker, now it needs the precise assembly work of a pick-and-place machine. This shift forces prototypes into a different manufacturing channel.

Furthermore, the smaller and smaller dimensions associated with these SMT devices and advanced packages put additional strain on the designers during the prototyping phase. Dimensions, spacing and tolerances are much tighter.

"It wasn't too terribly long ago that just about any design could still be built all through-hole parts," said Duane Benson, Marketing Manager at Screaming Circuits, a leading quick-turn assembly corporation. "Once the big CPU chips stopped showing up in PGA (pin grid array), thru-hole PC motherboard possibilities went out. Then, when Bluetooth® and Zigbee® came around, most if not all of those chips came out in BGA, LGA or QFN forms — no through-hole at all. Now, it's not too difficult to complete almost any design with all SMT parts."

Time-Saving Prototyping
At Sunstone Circuits, we've been tracking the increased use of a prototyping technique used by many of our customers. The increased complexity of SMT prototyping and the economic stresses of the current market mean that SMDs and advanced packages have begun to dominate new designs.

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. It's not that this technique is new, but the economic pivot point has clearly shifted, a product of technology, component packaging, and recession-style economics. Nevertheless, 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, but bear in mind that it will work only for certain types of designs.

Rather than struggle for a couple weeks to squeeze a prototype design onto a two-layer board, designers are adding an internal layer or two to the prototype board. With these additional layers available for routing, engineers can expand the spacing between traces "just a touch" and let the auto-router do the work. Where there might typically be a few days of effort to fit everything onto the two-layer proto board, now the effort is reduced to a few hours. This drastically shortens the layout time spent on the prototype and potentially shaves time from the overall prototyping process by allowing the designer larger tolerances away from the critical routing close in to the advanced packages.
The increased use of SMT components is shifting the PCB prototyping process.

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.

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:

  • 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 and assembly labor.

Well Established Technique
Large PCB design firms have been using this prototyping technique for a number of years. It works well for designs in which there isn't a lot of high-speed design, and where strict compliance is not a requirement. Obviously not all designs fit into this sweet spot, but when they do, you can use the technique to great advantage. So how does this all pencil out? 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:

  • One layout designer costs you $100/day. Whether employee or contractor, this number is probably conservatively low, once you factor in the costs of health care, and facilities costs.
  • The design in this example takes 10 days of schematic work.
  • The design in this example takes 12 work days to lay out as a two-layer, or four days to lay out as a multi-layer.
  • Two-layer PCB bare boards cost $450 to fabricate.
  • $1,500 in parts and assembly costs to populate your board.
  • A typical prototype process requires three spins.
    Let's also assume that the multi-layer technique results in a $600 per order cost for PCB fabrication, reduces layout design time from twelve days to four, and saves the design team one spin overall.

The overall effect to the design team is that the layout portion of the process takes much less time. The saved design team labor cost is the big savings here. The multi-layer board may cost a few extra dollars ($150) in this case, but the reduction in layout design time saves $800 per engineer in days saved alone, netting $650 per spin just in payroll. When we factor in the removal of one complete design spin — rework, revalidation, reorder, remanufacture, retest — the savings from the reduced spin just compounds on top of the per-spin savings.

In this example the project cost numbers roll up like this, resulting in a multi-layer prototype that's actually 61 percent of the cost of the 2-layer approach, and requires 54 percent of the overall design cycle.

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 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.
  • 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 your plan all along is 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."

Given the economic environment we've all be working under in recent months, and the inexorable march toward smaller, faster, cheaper, the trend is no longer just to be found in the features and dimensions of our product designs. It also can be found in the business practices of our design teams. By making full use of multi-layer, along with strategic use of assembly services, design teams can dramatically shorten key parts of the PCB prototyping design process, thereby validating their designs faster and easier. And, without a negative impact on the process of optimizing for production. In fact, the extra man weeks saved in prototyping are likely to deliver the extra time designers wish they'd had to get their production designs beyond merely good and into the realm of great.

Contact: Sunstone Circuits, 13626 S. Freeman Road, Mulino, OR 97042 800-228-8198 fax: 503-829-6657 E-mail: Web:

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