July, 2011
www.us-tech.com
Page 53 Prototyping with Multi-Layer Boards By Nolan Johnson, CAD/EDA Manager, Sunstone Circuits, Mulino, OR
move relentlessly into SMT pack- ages. Prototype manufacturing firms are increasingly seeing a shift in the fabrication and assembly work need- ed to implement engineering proto- types. The shift is subtle but unwa- vering: 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
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design that might otherwise have used DIP and through-hole compo- nents now must make use of the SMT packages. And where that design might have once been assem- bled 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 pro- totypes into a different manufactur- ing channel.
The trend toward totally SMT PCBs is creating increasingly difficult engineering design decisions.
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 tol- erances 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 possibili- ties 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 com- plexity of SMT prototyping and the economic stresses of the current mar- ket mean that SMDs and advanced packages have begun to dominate new designs. It’s an interesting development
that also shows up in our conversa- tions with PCB engineers at trade shows, customer sites, or whenever we find ourselves discussing proto- typing 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 tech- nology, component packaging, and recession-style economics. Nevertheless, designers are
n spite of many areas where PTH designs can still do the job, elec- tronic components continue to
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 cou-
ple 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 avail- able for routing, engineers can expand the spacing between traces “just a touch” and let the auto-router do the work. Where there might typ- ically 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 allow- ing the designer larger tolerances
away from the critical routing close in to the advanced packages. 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 tak- ing a week or so off the prototype lay- out process are much greater than the incremental fab costs. These designers tend to enter the production optimization phase
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