Nick Koop

If the clearance is not at least 10 mils, yields may drop.

One of the often-overlooked aspects of a board design is the moat. Perhaps this conjures up images of Monty Python’s Holy Grail, but moat does not refer to the ring around a castle. Instead, this is the clearance between pads and a surrounding copper plane, sometimes also referred to as embedded clearance.

These clearances often are 0.004" to 0.005" wide. This may seem like plenty of room, but Pareto analysis tells us this can lower overall manufacturing yield. These clearances often lead to unexpected yield loss, depending on certain design and processing factors. Believe it or not, etching these moats or clearances is difficult, due to the closed-ended, circular nature of the clearances. They do not image or etch well and are prone to shorting.

One reason is that driving the energy into the resist can result in bleeding and create an imaging short. But etching is also more difficult, as the etchant flow is trapped in a dead-end donut. These can conspire to create unintended image/etch shorts.

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