In PCB fabrication drawings, the nominal value gets the spotlight, but the tolerances decide what actually ships.
When it comes to coloring in the fabrication drawing, the way we provide the data creates the space the fabricator must fill. For each datapoint, there is a least material condition (LMC), a maximum material condition (MMC) and a nominal. Process variation is permitted between the two extremes and rejected when it is outside the envelope.
The nominal value is usually the midpoint between those two extremes. A typical feature will be specified by a nominal value and an implied tolerance. The “implied” tolerance is usually stated in the title block or fabrication notes. The number of digits on the right of the dimension can be used to assign tolerance. For instance, 10.0mm would give 10 +/-0.4 while 10.00mm would permit a feature size of 10 +/-0.13.
Variations from this process would be specified along with the nominal value. There is another method called limit dimensions, where the minimum and maximum are given, but no nominal value is noted. The fabricator is likely to seek the middle ground between min and max to accommodate normal process distribution. It’s also possible to use different tolerances for plus and minus. A dimension like 25 +0.10/-0.00 would likely be interpreted as 25.05 +/-0.05, which is why I do not recommend “unilateral tolerances.”
Datum Dimensions: Quick, Compact and Easy
The simplest way to complete the fabrication drawing’s dimensional requirements is to assign a 0.0 location to a feature and base all the following dimensions on that single datum. Co-locating the system origin and the dimensions will simplify the task. From there, the features' coordinates will match the numbers in the dimensions.
Figure 1. Using the board origin as the datum for the dimensions is good when you start at a hole that is part of a pattern of mounting/tooling holes. Dimensional values correspond with the X-Y position of the hole or other feature. The board edge can be more of a tolerance issue since V-score and mouse-bite processes are less precise than drilling holes. (All images courtesy of the author)
While a board edge makes sense in most cases, I would use a mounting hole or a tooling hole as the system's datum and the datum dimensions. Often, aligning the bolt pattern is more important than a tight tolerance on the edges of the PCB. I’ve heard the argument that the processes are all automated and that drilling and milling machines use nominal values for their tool paths anyway.
ANSI Standard Dimensioning (ASME Y14.5)
I still believe some dimensions require more precision than others. The stated purpose of a fabrication drawing is as an inspection document. The traditional dimensions are more flexible in that regard. Using the method above, we can control the pitch between two mounting holes as a function of each hole’s distance from the datum. That’s two separate measurements, each with its own degree of freedom. A direct measurement allows only the implied or stated tolerance, not the accumulation of tolerances.
Figure 2. This is ANSI-style dimensions. The tolerance stackup is almost the same as Figure 1, except for two holes chain-dimensioned in order to control their interrelationship. The overall board dimensions set above and to the right of the outline are handy for quotation purposes.
Geometric Dimensioning and Tolerancing
Up until this point, we have been using X and Y coordinates to establish a tolerance defined by a square zone. The size and shape of the zone are determined by the tolerance allowance. We can tighten this by using a positional tolerance for hole pattern geometry. Instead of the square that permits equal freedom going up, down and side to side from the nominal, we can define the tolerance as a circle within that square. Cutting off the corners of the tolerance zone permits the same displacement but only from the nominal center outward.
Figure 3. The feature control box at the bottom of this view can display a variety of figures, in addition to the cross-hair symbol for positional tolerance. Other geometric tolerances that are common on PCBs include flatness, runout, parallelism, and profile. Each one creates a zone of acceptable outcomes.
Geometric dimensioning and tolerancing also allows us to control PCB flatness and the kinds of edges that are not easily dimensioned, such as mid-mounted cooling fans. These flat fans use Bernoulli’s principle to move air. The result is a fan shaped somewhat like a sectioned nautilus shell, with a complex, changing radius. To control this outline, designers follow a profile specification (shaped like a D on its side) that instructs the manufacturer to follow the arbitrary line or curve with an allowable deviation from the design.
The idea of following the data is built into our process from initial library generation to those emails and phone calls that follow tape-out day. One of the fab notes will (or should) cover layer-to-layer misregistration allowance. It would be rare to actually dimension any circuits themselves. You know how those antenna people are.
Product enablement means taking the image on the screen and turning it into hardware. We want to ensure the product resembles the artwork as closely as possible without over-constraining it. Good judgment demands that we consider the chain linking components, boards, and other subassemblies to products and their end use.
Figure 4. The USB connector pins are the springy type that zig-zag through the board. It’s not quite press-fit, but it does require some force to insert. The way we yank on those cords is reason enough for a solid build.
While the nominal values are important, it’s the tolerances that set the bar for manufacturing. Whether it’s metal thickness in the via barrels or components along the edge, there is almost always a call for a little more wiggle room here and there. We only want to tighten a spec if the process is already failing.
Process control limits are there for a reason. The drama plays out between what the fabricator can do and what the assembler can’t. A lot of that churn is down in these details. I may not have needed specific dimensions on the USB connector, but the component vendor provided the package details and suggested footprint for a reason. Carrying that over to the fab drawing leaves one less thing to chance.
In the end, this layout didn’t require anything beyond the first effort. All information on the fabrication drawing is subject to verification on the actual PCB. Each style uses similar degrees of tolerance but applies them in more specific ways than the one before. Your choice will depend on the use case, but the tolerance values generally define product values.
John Burkhert, Jr. is a principle PCB designer in retirement. For the past several years, he has been sharing what he has learned for the sake of helping fresh and ambitious PCB designers. The knowledge is passed along through stories and lessons learned from three decades of design, including the most basic one-layer board up to the high-reliability rigid-flex HDI designs for aerospace and military applications. His well-earned free time is spent on a bike, or with a mic doing a karaoke jam.
