Originally Published MDDI April 2003

PRODUCT DEVELOPMENT INSIGHT

Printed circuit board (PCB) fabrication successfully crossed the threshold to paperless fabrication about 10 years ago. Now virtually all PCBs are fabricated directly from a digital file, with no accompanying drawing. For parts designed using solid modeling, however, the transition has been more difficult. A comparison of the two helps explain why. 

The first reason that PCB fabrication has been able to go to an all-digital format is that PCB design rules are highly standardized worldwide. Important design characteristics such as tolerances and relationships between features are the same, whether the PCB is in a child's toy or an ECG machine. PCB design software enforces these standards through a rigorous set of internal design rules. The resulting PCB design output from the application usually meets these international design standards. Consequently, both engineer and supplier are confident that there will be few, if any, problems producing the PCB.

For the typical part designed with 3-D CAD, however, the tolerances and relationships with other parts are not standardized. Critical dimensions and tolerances are different in every part designed. This is the major weakness of solid modeling: The virtual model gives equal emphasis to all dimensions, although some dimensions on a part are much more important than others, and the virtual model ignores tolerances completely. Consequently, understanding critical relationships between features and identifying critical tolerances is difficult for the vendor.

This is important because it would be prohibitively expensive and almost impossible to design a mold to make a part that would exactly match the virtual solid model. Creating an injection molding tool or a sheet-metal die is a painstaking process done by real people who make large and small mistakes, which take time and money to correct. The toolmaker is also faced with the reality that the plastic may shrink or the metal might bend in unpredictable ways. The key to successful mold design is to identify those dimensions and tolerances that are critical on the part drawing, and then control them. Mark Vaughan, director of engineering, at Bayer Healthcare's Diagnostics Division, notes a typical situation. “One problem we have had when working with vendors from solid models during fast-paced, concurrent development,” he says, “is that there have been generic dimensions that turned out to be important, but were not called out as critical on the initial revisions of the drawing.”

The second reason that PCB fabrication has gone all-digital is that the file formats used by PCB layout software have themselves become highly standardized. The photoplot and drill files created by the PCB layout software are well-defined and created specifically to be loaded into typical sets of PCB fabrication machines. 

The fabrication of parts from solid models, on the other hand, has just begun to get to that point, because of the variety of solid-model packages, and the equally large variety of often-incompatible fabrication software used by the vendors. 

According to Charles Deschenes, mechanical engineering manager at Abbott Laboratories, MediSense Products, the key to saving time with solid models is compatibility of file systems. “It has taken a lot of years, and it still takes a lot of hand holding [with the vendor] before you cut steel.” Norm Desmarais, a mechanical engineer at Bayer's Diagnostics Division, went into more detail on this problem. “When vendors use different software systems, the construction of the CAD model can become important. Certain methods of constructing the 3-D model geometry in the original CAD system do not always translate cleanly when imported to the vendor's CAD/CAM system. Time and effort are needed to sort out the resulting confusion when it happens.”

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