Unit Conversion in PCB and Hardware Design
Hardware engineers move constantly between unit systems. Mechanical drawings may use millimeters, older PCB rules may use mils, fabrication notes may describe copper in ounces, and component data sheets may list dimensions in microns. The math is not difficult, but conversion mistakes can create real board errors. A trace width, clearance, drill size, or copper thickness entered in the wrong unit can change manufacturability and electrical behavior.
A mil is one thousandth of an inch, not one millimeter. One mil equals 0.0254 mm, or 25.4 microns. This unit is common in PCB layout because many historical design rules were specified in thousandths of an inch. Modern CAD tools often support both metric and imperial grids, so engineers need to convert accurately when reading older design notes or communicating with fabricators.
The risk is not only arithmetic; it is also communication. A mechanical engineer may discuss enclosure clearance in millimeters while a PCB designer discusses trace spacing in mils. A board house may list solder mask clearance in microns and minimum drill sizes in millimeters. A shared conversion reference reduces ambiguity during design reviews and helps catch assumptions before files are released for fabrication.
Copper Weight
PCB copper thickness is often specified by weight in ounces per square foot. One ounce copper corresponds to roughly 34.8 microns, or about 1.378 mils, of finished copper thickness before fabrication details are considered. Two ounce copper is approximately double that thickness. This matters for current carrying capacity, impedance, etching limits, and thermal spreading. A high-current board may require heavier copper or wider pours than a low-power digital board.
Why Small Errors Matter
A difference of a few mils can be significant for fine-pitch components, controlled-impedance routing, and manufacturing clearances. A via annular ring, solder mask web, or differential-pair gap may be close to the fabricator's minimum capability. Converting units explicitly helps avoid ambiguous instructions such as "make it ten wide" when one person means ten mils and another thinks in tenths of a millimeter.
Workflow Use
Unit conversion is useful during schematic review, footprint creation, stackup planning, mechanical fit checks, and layout constraints. It also helps when comparing online calculators, IPC formulas, and vendor capability tables that use different conventions. Keeping the converted values visible reduces friction and prevents the quiet arithmetic mistakes that tend to appear late in a board bring-up.
Engineering Judgment
Converted values should be treated as design inputs, not manufacturing guarantees. Finished copper thickness, plating, etch compensation, solder mask, and fabrication tolerance depend on the board house and process. For critical impedance, high current, or high voltage, engineers should use the fabricator's stackup and capability data rather than generic conversions alone. This tool is a fast reference for early design and review.
Unit conversions are also useful when reading application notes from different eras or regions. RF layout guidance might describe a 50 mil spacing, while a modern connector drawing gives pad geometry in millimeters. Translating those values into a common mental scale makes it easier to compare recommendations and identify whether a rule is about manufacturing, electrical performance, or simple mechanical fit.
Rounding should match the manufacturing context. Reporting six decimal places can imply precision that the PCB process cannot hold, while rounding too aggressively can violate a minimum clearance. For drawings and design rules, keep enough significant figures to preserve the requirement and then apply the fabricator's stated tolerances. Conversion is exact; manufacturability is not.
Copper weight conversions are nominal. Plating, etch-back, surface finish, and process variation can change the finished conductor thickness. Controlled-impedance and high-current designs should use the fabricator actual stackup data rather than assuming ideal one-ounce or two-ounce copper thickness.
Mechanical constraints should be checked after conversion. A converted pad size may be electrically acceptable but still conflict with enclosure ribs, connector tolerances, or assembly clearance. Unit conversion keeps dimensions consistent, but the final board still needs cross-discipline review against mechanical drawings and fabrication notes.