Reading a datasheet that mixes mil and mm

A mil is 1/1000 inch

A mil is 1/1000 inch, so 1mil = 0.0254mm, and the reverse is 1mm = 39.37mil. It has a long history in US PCB design and survives in CAD default settings (Eagle, Altium, and others), in older footprints, and in datasheets from US-based manufacturers. Some regions call it a "thou" instead.

The values you actually meet in PCB design cluster around a handful of numbers. 4mil ≈ 0.1mm, 6mil ≈ 0.15mm, 8mil ≈ 0.2mm, 10mil = 0.254mm, 20mil ≈ 0.5mm, 50mil = 1.27mm, 100mil = 2.54mm. Keep this table in your head and you can skip converting every time a datasheet opens.

"Mil" and "mil-li-meter" get confused often enough in speech that a 40x error can slip through unnoticed. Get in the habit of confirming which one someone means when a drawing discussion says just "mil." Micrometers (um) get mixed in too: since 1mil = 25.4um, a 35um copper foil is about 1.4mil.

Is it fine to round "4mil = 0.1mm"?

4mil is exactly 0.1016mm. If a fab's minimum trace width is "0.1mm" and the design is drawn at 4mil, then 0.1016mm ≥ 0.1mm and the rule is met. Go the other way — the fab says "4mil" but the design is drawn at 0.1mm — and 0.1mm < 0.1016mm gives you a marginal violation.

In practice a fab will never flag a 0.0016mm (1.6um) difference. But a DRC (design rule check) judges mechanically. A CAD rule set to 0.1mm passes a trace drawn at 4mil; a CAD rule set to 4mil flags a trace drawn at 0.1mm as an error.

The fix is simple: enter the fab's rule into CAD in whichever unit the fab gave it in. mm as mm, mil as mil. The moment you convert and round it yourself, you drift off in one direction or the other.

5mil is 0.127mm — not 0.125mm and not 0.13mm. If a fab's table reads "0.127mm" you know the rule is mil-based underneath; if it reads "0.125mm" you know it is mm-based.

Which unit system does a component's pitch belong to

An SOIC's 1.27mm pitch is 50mil, and a DIP's 2.54mm is 100mil — these are inch-family parts. QFP pitches of 0.5mm, 0.65mm, 0.8mm and BGA pitches of 0.8mm, 1.0mm, on the other hand, are metric, and convert to awkward fractions like 19.69mil or 25.59mil.

The trouble shows up when you convert a pitch, round it, and apply that to a part with many pins. Build a 100-pin QFP treating 0.5mm as 20mil (0.508mm) and the outermost pins are off by 100 x 0.008 = 0.8mm, or 0.4mm with 50 pins to a side. Against a 0.25mm pin width, a 0.4mm offset is enough to put the land completely off the pin.

Build a footprint in the same unit system as the component drawing. Rather than placing each pin at an absolute coordinate (position of pin n = n x pitch), specify the exact pitch through your CAD's pin-array feature. When reusing an existing library part, check the accumulated dimension from the origin as well.

• Grid. Place inch-family parts on a 0.635mm (25mil) grid and metric parts on a 0.5mm grid, and the two will never land on a shared grid. Use a fine routing grid (0.05mm or 0.025mm) and let each part sit on its own native pitch.
• Hole size. Drills come in 0.05mm mm-based steps (0.3, 0.35, 0.4...) from some fabs, or in inch-based number drills (#60 = 1.016mm, etc.) from others. Specify an awkward drill size and the fab will substitute the nearest one it stocks.
• Board thickness. 1.6mm is 0.063inch (63mil), 0.8mm is 31mil, 1.0mm is 39mil. US card-edge connectors are built for 0.062inch, which 1.6mm matches closely enough.

oz expresses thickness as a weight

Copper weight in oz is the weight of copper per square foot. Spread 1oz (28.35g) of copper over one square foot (929cm²) and you get a thickness of 34.8um, so 1oz ≈ 35um ≈ 1.4mil. 2oz is 70um, 0.5oz is 18um.

The catch: outer-layer copper gets thicker from plating. Start with 1oz foil and through-hole plating adds 20–25um, landing at a finished 50–60um. Use the fab's "finished copper thickness" for current-capacity and trace-width calculations. Inner layers see no plating, so 1oz there stays 35um.

Japanese fab spec sheets tend to state copper weight in um ("18um / 35um / 70um"), while overseas fabs write "0.5oz / 1oz / 2oz." Both describe the same thing.

Practical rules for avoiding the mix-up

• Set the CAD display unit to mm, but enter each rule in the unit it arrived in. Most CAD tools let display and input units be set independently.
• When writing a converted value on a drawing, keep the original alongside it. Something like "1.27 (50mil)" or "0.254 (10mil)" lets whoever reads it next see where the rounding came from.
• Build footprints in the component drawing's own unit system. This is what avoids accumulated pitch error.
• Specify to the fab in mm, and stay off values sitting right at the rule limit. Against a fab with a 0.1mm minimum, drawing at 0.12mm or 0.15mm instead of exactly 0.1mm passes regardless of whether the rule gets read in mil or mm.
• Ask for copper thickness as a "finished" figure. An oz value is the starting foil weight, not the thickness after plating.