Design for Manufacturing: The 10 Commandments of Design for Assembly

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You might think that most PCB designers would know how to avoid the most obvious blunders that can gum up board assembly. But you would be wrong. I can tell you from daily experience that we’re just not there yet. So allow me to propose The 10 Commandments of DFA.

1st Commandment

Be sure all indicators are present. The leading show-stopper in assembly is missing pin 1 indicators or component polarity/orientation indicators on the silkscreen. Nearly 75% of the assembly orders my facility receives fail to identify the location of pin 1 for each IC, or they misrepresent or neglect to indicate the polarity of some capacitors, diodes, or LEDs.

Obey the convention for marking the polarity of diodes, including LEDs: Put a K on the silkscreen layer at the cathode end. Alternatively, use the electrical symbol for diodes in the correct orientation to guide assembly. Never indicate diode polarity based on the anode pad. Use a K to designate the cathode or line up the diode symbol in the correct position. Don’t substitute any other marks or your contractor will misinterpret what you intend.

To orient tantalum capacitors, tag the positive side with a plus symbol on the silkscreen. Remember, tantalum capacitors can ignite if mounted with swapped polarity. Short of reverse-engineering a schematic (and who bundles schematics with assembly files?) there’s no way for a contractor to figure out part polarity unless it’s clearly displayed.

Silkscreens must not interfere with pads, and symbols should not be printed beneath the body of any component.Read the full column here.Editor's Note: This column originally appeared in the April 2013 issue of The PCB Design Magazine.