If your engineering team could deliver just the netlist for your fully modeled and simulated circuit, along with the system’s operating-software code, your design life would be much easier.Reality, however, is that you need to build a working prototype for debugging,
for testing your software in the real world, and for working out the packaging issues, such as EMI/RFI, battery and connector placement, usability, and overall look and feel. Building a prototype was not as difficult for designers even a few years ago when each aspect of the design process—the product itself, its components, and even your team’s development
schedule— had more room and flexibility. To aggravate your situation, many products now
have custom-designed enclosures that try to meet the many conflicting constraints of small size, user convenience, unique batteries, and special user interfaces while looking distinctive to the market and meeting demanding RF requirements. The basic rectangular, standard-sized plastic or metal enclosure that you can get from a convenient electronics-supply catalog can’t meet your stringent needs.

Fortunately, the same CAD and EDA technologies that drive today’s increases in system functionality, up-front modeling and simulation, and circuit density also provide you with the tools to meet these design objectives.With PC-driven electromechanical systems, you can make detailed, ready-to-load circuit boards in a few hours, which you can then fit into the board’s unique enclosure, along with the ancillary components and connectors, for a meaningful test fitting.

THE BOARD COMES FIRST

Layout demands on your pc board for component placement, connectors, and high-frequency performance—plus your need to debug software on a “real” system—
mean that you normally want to confirm your pc-board design before you commit to an enclosure design. In the “Middle Ages”of the solid-state era when circuits had many larger discrete components and DIPs, you could make the board in-house using basic photographic, chemical etching, and a drill press (see sidebar “You still want to be in pictures?’’). For today’s high-density designs, these relatively crude techniques are insufficient.

But an attractive alternative offers you the benefits of short cycles and layout flexibility and gives you that near-total control of pc-board fabrication that most design teams doing prototype work really need: benchtop milling machines. Such benchtop board fabrication is the prototyping analogy to desktop publishing as far as the direct contact and control
you have over the final product.

LPKF Laser and Electronics (www. lpkfcadcam.com) and T-Tech Inc (www. t-tech.com) offer these units; systems from these vendors are roughly comparable in features and performance. For a starting cost of around $8000 to $10,000 (but usually somewhat more, depending on the options you choose), you can make those prototype boards as you need
them, with minimal mess and frustration. Your payback—measured only in dollars—is typically a year or two compared with the cost of going outside; this payback doesn’t include your productivity and time-to-market improvements because you can now do your board right away. Thus, you can proceed to the next step in the design and debugging cycle.

These systems mill selected areas of the conducting copper layer from a nonconducting
base layer, such as a standard glass-epoxy circuit board; drill any holes for component leads, mounting, or support hardware; and route the final board shape or any large interior open areas. They perform these functions with precision commensurate with today’s highdensity designs. The machines use X-Y control of an 8000- to 60,000-rpm, (depending
on model), spindle coupled with Z-axis depth control. The motion of the spindle is based on commands from software on your PC to transform 2-D PC laminate or other material—for example, FR3, FR4, G10, PTFE, Teflon, or Duroid—into a 3-D pc board.When you
see the combination of sophisticated CAD software and precision motion control in action, it’s easy to admire what such a combination can do.