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PCBD-June2016

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20 The PCB Design Magazine • June 2016 Shaughnessy: What are the most challenging is- sues fabricators face regarding fine spaces, traces, and pitch? Thompson: Another great question. There are many. First and foremost is the chosen cop- per 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 compen- sation. 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. As far as pitch is concerned, there are a cou- ple of things to consider. First, if you are look- ing at a .4 or .5 mm BGA pitch, you won't have the ability to run two sets of traces between the pads, so you would look to keep any differen- tial pairs on the inside. Additionally, with the smaller pitch constraints it is difficult to main- tain a minimum web of mask material to pre- vent wicking at surface finish. In order to main- tain a .004" web of mask material, for instance, sometimes a fabricator will reduce the clearanc- es down to as little as half or one mil per side to be able to maintain the web and not create other issues for the end-user at assembly. Shaughnessy: What can designers do to alleviate some of that pain downstream when they're de- signing boards with such fine features? Thompson: So much of this just comes down to understanding the manufacturing process. Let's use the example of the mask clearances vs. web. For instance, if the designer knows that there will be SMTs less than .004" apart from each other, they may choose to create a mask-defined clear- ance where the clearance is smaller than the un- derlying SMT to be able to maintain that critical web of mask between mounts. In other cases, if the surface finish is a very thin deposition and does not tend to "wick" between mounts, they may choose to "gang relieve" the mask area so there are no physical webs between mounts. The same holds true with egressing from tight pitch BGA parts with traces. This is where you will see "neck down" areas where the traces get thinner to be able to get in and out of the BGAs. Shaughnessy: What are some of the more common mistakes designers make with fine spaces, traces and pitch? Thompson: First off, space value on the de- sign vs. copper weight is a big issue. So many times these days, we see a tight-pitch part that dictates the use of true .003" trace and space. This can only be done on either one quarter or three eighths ounce starting foils. Attempting to start a 3/3 design on half-ounce would mean .035"/.0025" after etch compensation, which is too small for most folks. Second, select a mask color that will allow for the tightest mask webs between surface mounts. Likewise, select a sur- face finish that does not tend to "wick" or short the mounts at assembly. Shaughnessy: I hear a lot of horror stories about BGA fanout issues. Do you have any advice for de- signers dealing with ultra-fine-pitch BGAs? Thompson: Yes, as much as possible, keep break- out traces on the inside layers. Externally, try to FROM THE CAM SHOP: TIGHT TOLERANCE DESIGN TIPS Mark Thompson

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