By Mark Thompson | Published on: November 18, 2013
In this post our focus is RF and high frequency designs. I will attempt to go over some common issues on artworks provided for RF/high frequency jobs.
First off, the very nature of RF is that the features must be tightly controlled. Typically, RF features are done as "Constructs” or "Islands” because using standard "drawn” features leave undesirable radii. This means a fabricator must be very careful when attempting an "etch compensation” for the known loss of trace or feature width at an etcher based on starting copper weight. All features must be selected on the constructs or they will not perform as expected. Net-lists are helpful, but can also cause delays on RF jobs. Let me give you an example: If the net-list definition assumes a later connection to a surface feature (for instance through a metal screw head or plated half hole) you can get false open nets that will require further communication with the customer or engineer resulting in lost time.
A good Fabricator sees an RF device for what it is and understands certain "rules” apply in CAM. For instance, the clipping back of metal running to a part edge (assuming no z-axis connection is to be made). Normally a shop may clip metal back anywhere from .004 to .010 to prevent burring at final rout. This is not the case with RF type jobs. Trimming the RF leads more than .005 may result in performance issues and is generally not done unless the customer is made aware and has approved the change.
Many RF features themselves can look very much like "stubs" or un-terminated traces and some CAM systems may flag them as such. Even on predominantly digital boards, RF features such as antenna's can look like stubs or un-terminated traces. A savvy CAM operator will typically know these are intentional; however, contacting the engineer is still required in most instances.
Here at Prototron, we know that unlike conventional designs, RF designs have characteristics that require an understanding of the intended performance of the product. Almost as important is communication with the customer when any modification is being considered. Even the slightest variation in the end product from the design can result in performance issues. Interestingly, clear solder mask seems to be a favorite of RF engineers who like to verify the feature geometries are as designed.
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