Board Negotiations: Design Rules and Tolerances

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In this column, I explore design rules and the constraints they put on the PCB fabricator. As you will see (and may have experienced), some work, but others do not.

1. Applications

Depending on the board application, you may need tighter tolerances. For example, with aerospace, automotive, and medical boards, the tolerances must be tighter. So, how do we get there?

2. Designing With Specific Tolerances

Here are several examples of how a PCB fabricator can deal with various tolerances. Let’s look at “press fit” applications for tool sizes. Typically, a given plated hole or slot is ±0.003” and a typical non-plated hole or slot is ±0.002”. So, what does the fabricator do when a plated hole is called out as ±0.002”?

The simple answer is to calculate how much plating there will be in the hole barrel, and then over-drill to accommodate the ±0.002 tolerance. Typically, this is done by labeling the ±0.002” hole as plus 0.004” minus zero in the CAM system. End-users on RF applications, such as antennas and phased arrays, have pre-calculated the copper in the hole barrel for many years. Often, end users will stipulate the drilled hole size (not the finished hole size as most designers call out) here so that they can pre-calculate how much plating will be in the hole barrel based on the PCB fabricators capabilities.

3. Don’t Be a Violator

For instance, we may receive an IPC callout for a 0.002” annular ring minimum. But what happens when the end user does not allow for that, creating gap violations? Or worse, what if they don’t understand that a PCB fabricator drills approximately 0.004–0.005” over the finished hole size as expressed on the drill drawing to meet the nominal hole size? Making the pads on a signal layer or the anti-pads (relief pads) 0.004” over the finished hole size without accounting for the additional plating to meet the IPC minimum of 0.002” per side annular ring does not work.

In a perfect world, the machine tolerances and true position are both ±0.000”, but that is just not the case in practice. It used to be said that if you added up all the tolerances needed to manufacture a given board, nothing would be possible. As an example, let’s add a ±0.003” true position tolerance to a ±0.003” machine tolerance. This would mean that +0.006”/-0.000” could occur at some point. I can assure you this would not work for a ±0.002” annular ring if the signal pads were only increased by 0.004” to meet an IPC ±0.002” minimum annular ring, and the over-drill for plating was not considered.

To read this entire column, which appeared in the May 2019 issue of Design007 Magazine, click here.



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