Smooth copper isn't always what it is purported to be.

I always trusted datasheets. Why shouldn't I? Every time I picked one up, I understood they were harbingers of truth about what something did and how to interact with it. From chips to components to widgets, manufacturers have a vested interest in ensuring users know what and how to use their products.

My trust in datasheets was eviscerated the very week I entered the printed circuit board industry. I was attending a weeklong training session on signal integrity taught by Eric Bogatin when he surveyed the attendees about datasheets and their accuracy. While my memory of the day and exact content isn't perfect, I do recall that as the very moment I learned the harsh reality: Datasheets are sometimes not as accurate as one would hope! (As an aside, Eric Bogatin's books on signal integrity are excellent resources because he has that special gift of teaching and explaining complex topics in approachable and understandable ways. The concept of "being the signal" and walking down a transmission line is just fantastic.)

Don't mix your units.

When I first started in the PCB fabrication industry, I was fresh from college and ready to jump in and make an impact. The first hurdle I hit was likely the same as many first-time designers: units of measure for PCB design. Remember back in school, your physics professors or science teachers likely pointed out missing units in tables, graphs or homework assignments? That problem.

Figure 1. The three units of measure commonly used to describe the relative thickness of a PCB layer.