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The Bare (Board) Truth: Tales From the Fab Shop
Wednesday, May 16, 2012 | Mark Thompson, CID, Prototron Circuits

Greetings all! It has been a while since I have been able to write with any consistency. Fortunately, I’ve had plenty of time to gather fodder for new columns, thanks to the many good and bad habits of designers and engineers. Now I can relate some of these examples back to you in an effort to help you avoid making similar mistakes.

We all want to be able to make assumptions about certain aspects of processes in a typical fab shop, but the truth is that there really are no more “typical” fab shops in the US. The large production fab houses have expanded their capabilities, while the smaller yet still-alive fabricators carefully weigh the consequences of newer technologies or processes.

Gone are the days of doing ROIs on eight pieces of equipment, plugging them all in simultaneously and crossing your fingers that everything will work as planned. A good fabricator carefully researches the new equipment or process and makes sound, educated purchases based on customer needs.

The salesperson comes in at the beginning of this process. Customers frequently ask outside sales folks about remedies to common industrywide issues. These folks should be prepared to offer those solutions. They don’t always have to involve a new process or a large technology shift. Most of the time, once the need is fully understood there are many options to be able to fulfill that need.

I am a huge believer in common sense in electronics. If a product does not have to contend with temperature excursions, available board space driving parts smaller, and the geometries of smaller features as well, it will be a more robust product in general.  

One thing often not considered when designing at smaller geometries: With the small features dictated by available board space, if you have impedances, the traditional 50 ohm and 60 ohm structures are very small. This is where tolerance plays a key role in whether or not your board will be a no bid. At .1mm (.0039”) there is little room for any process errors; just a half-mil deviation from trace width can result in a 5 ohm difference.

Sure, small lines and spaces can be dealt with better by starting on a lighter copper weight, but remember the analogy about digging a trench. If I dig a shallower trench, it will have to be wider to contain the same volume. The same is true for traces on a PCB; if you go lighter on the copper weight so you can impose less etch compensation, the traces get wider to accommodate the same impedance. In addition, the shallower trench now has different flow characteristics than the deeper, thinner version. Again, the same is true for the flow of current on a PCB trace. I cannot remember where I heard that analogy years ago, and it is a bit tongue-in-cheek, but you get the idea.

And another thing: Keep it real. When reviewing your drawing notes, make sure they actually pertain to the job! Sometimes when editing these drawing notes, new notes are added but old ones are not deleted. These notes, if in conflict, will mean a phone call from your fabricator.

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