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We PCB designers are doing some truly great things with our layout tools. But we must remember that these tools are so powerful that they will sometimes allow us to design things that can’t be manufactured! We must collaborate with our fabricator and assembly brethren and embrace the best DFM practices, or face the consequences downstream.
Something as seemingly simple as copper-to-edge spacing provides us with plenty of examples of DFM techniques, potentially good and bad.
Providing a sufficient amount of copper-to-edge spacing allows for the least costly manufacturing processes at the PCB fabrication and assembly levels. Extremely tight manufacturing etching and routing tolerances enable the close registration of copper to the cut board edge. On very tight layouts, we see a router profile that is intended to come within .007” of a copper trace. You may have seen closer—and even cases where the copper is designed to extend beyond the the board edge effectively wrapping around the board edge.
Granted, sometimes we designers intend for the copper to exist in close proximity to a board edge profile. When close copper-to-edge distance is intended, we ought to always be sure that the copper will end up protected with a coating of resin (if still laminated within the PCB) or plated with a surface finish in order to prevent oxidation or other forms of contamination.
While copper print and etch factors are more accurate than ever, and your PCB layout tool will allow you place that copper trace very close to that board edge, stop and wait a minute. Think about how this PCB will be fabricated and assembled.
If you don’t know, set up a meeting with your PCB fabricator and assembler. Ask your manufacturing representatives about their capabilities and processes. Take note, though, with regard to the context of this conversation. If you ask how close you can design copper to the board edge, you will (and should) get an entirely different response than if you were to ask how far away should you keep your copper from the board edge. Here’s why:
A PCB board fabricator is in the business of creating very fine images out of copper that will be matched with a drill pattern and registered very accuratly onto a board outline. If this is accomplished as a one-up PCB intended for manual assembly, there are few problems if the PCB is designed such that the copper-to-edge spacing comes within .010” (0.25 mm) or greater. But this scenario can plant the seed of failure if the board design is destined for automated assembly.
After a prototype PCB layout is blessed by the engineering team, the determination is made to get on with production. This is when the lightning bolt of manufacturing reality is set to strike. While the PCB fabricator has done his best to accurately route the board edge very close to the copper conductors as designed, this awsome capability has tied the hands of the assembly provider who may be under contract to build thousands of these PCBs. You see, a prototype fabricator’s working panel can cut very close to the copper when building one-ups that will be shipped as single PCBs. But a PCB going to volume production must be designed to be included into an assembly array. There will be extra features which the assembly provider’s engineers will be adding to allow for ease of de-paneling or excising the boards from the array. These features all require varying amounts of space relative to the PCB edge.
To read this entire article, which appeared in the January 2017 issue of The PCB Design Magazine, click here.