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After you finish your design, it winds up in the hands of people like Mark Thompson, the man who runs the CAM department at Prototron Circuits in Redmond, Washington. He sees CAD data firsthand, and often has to address errors and inconsistencies in PCB designs. For this issue, we asked Mark to discuss the today’s tight tolerances, some of the problems they can cause PCB designers, and what designers can do when dealing with shrinking features.
Andy Shaughnessy: What are the tightest tolerances you are currently building?
Mark Thompson: First off, that is a great question. We have long said in fabrication if you added up all the accumulative tolerances a fab shop has to deal with the part would be physically impossible to build. Having said that, unusual process tolerances CAN be achieved, such as plated holes with a +/-.002” tolerance for press-fit devices. One fab shop may say that the best they can do for plated holes and slots/cutouts would be +/-.003” but often we can do +/-.002”. How is that possible, you ask? For one thing, we can tell the CAM system to select a tool that is +.004-.000”. This selects a tool that works best for a +/-.002” final tolerance.
Another typical tolerance issue is with controlled impedance. Many fabricators ask for +/-15% tolerance for traces thinner than .0035”. This is not uncommon considering that just 10% = less than half a mil of total accumulated deviation throughout the fabrication process. The good news is that most fabricators use a field solver for the impedances, which means they can adjust for process variables like plate, etch, mask thickness, etc. And ultimately this means that even in situations where a fabricator may ask for +/-15%, they may incur as little as 5% deviation if they have good process control. I guess I would end by saying if the customer has some unusual tolerances they need to achieve. I recommend speak with your chosen fabricator to make sure they can be met.
Shaughnessy: What are the most challenging issues fabricators face regarding fine spaces, traces, and pitch?
Thompson: Another great question. There are many. First and foremost is the chosen copper weight vs. trace and space. Many times this comes down to what a fabricator has to do for compensations for the process. In this case, we are talking about etch compensations.
Let’s say you have a .1 mm trace and space design and you desire 3 oz. finish. The general rule of thumb is that for every half ounce of starting copper, we do a half-mil etch compensation. For three ounces we would need a .003” etch comp, and if the space is .00393” (.1 mm) we would be left with a .00093”, space which is way outside of most folks’ capabilities.
To read this entire article, which appeared in the June 2016 issue of The PCB Design Magazine, click here.