Design007 Magazine

PCBD-Jan2017

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18 The PCB Design Magazine • January 2017 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 reg- istered 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 auto- mated 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 as- sembly provider who may be under contract to build thousands of these PCBs. You see, a pro- totype 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 go- ing 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. Incorporating good DFM processes into a PCB means that the designer needs to have a familiarity of the processes that will remain available to the assembly provider after the PCB moves into production. One of the first things the assembly provider will be looking at is bare PCB material cost, and that means panel yield. How many PCB images can fit onto a panel array? The assembly supplier's manufacturing engineers look very closely at the the PCB's shape and thickness. Additionally, copper-to- edge spacing is measured in order to derive the most robust, stable and easy to excise PCB re- tention solution satisfying the many various as- sembly and even test process. The method of retention selected for the as- sembly array is based on quite a few factors, in- cluding: • Whether the board edge is straight or curved • The finished thickness of the PCB panel • The dimensional tolerance requirements of the PCB edge • Whether the PCB edge can tolerate a bare, frayed edge or must be smooth, sealed or plated If there is insufficient copper-to-edge spac- ing for a viable panel excising method, the assembly engineer may be faced with a stop order until the proper permissions are ob- tained to direct the fabricator to modify the copper-to-edge spacing to allow for a particu- lar excising method. It can take hours or days to obtain permission for this type of modifi- cation and is understadably awkward for all stakeholders in the manufacturing stream. The data for the modification is not included in Gerber and is usually added on a sub-set of fabrication documentation with a note and accompanying detail. It is hoped that the our designer audience Figure 1: Example of insufficient copper-to-edge clearance. TRUE DFM: TAKING CONTROL OF YOUR EDA TOOL

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