Copper Thickness Measurement Unit

Theoretically, PCB (Printed Circuit Board) current carrying capacity is determined by cross-sectional area of trace and temperature rise. Furthermore, cross-sectional area of trace is directly proportional to trace width and copper thickness. Well, here comes a question: does this rule also apply to the relationship between current carrying capacity and trace cross-sectional area, that is, is trace carrying capacity directly proportional to its cross-sectional area? Under the same temperature rise that is 10°C, a 10mil-trace with 1oz copper weight is capable of withstanding current of 1amp at most and we rest assured that a 50mil-trace is capable of withstanding current larger than 1amp.

Then how exactly is the maximum current it is able to hold, 5amps based on a simple multiple calculation? It’s actually far more complicated than that. According to MIL-STD-275, we are told that the maximum current a 50mil-trace can stand is 2.6amps.

Nevertheless, the table above has been gradually replaced by IPC-2221 Generic Standard on Printed Board Design as a reference based on which a printed circuit board is accurately designed.

Copper Thickness Measurement Unit

Prior to genuine discussion, it’s necessary to wiki a unit applied for copper thickness, ounce (oz). It is generally accepted as a unit for weight measurement but in circuit board design, it has been employed for copper thickness measurement. When it comes to the conversion on copper thickness by ounce, some rules should be kept in mind. Because specifications of copper are measured by copper weight per square feet, 1oz that is usually mentioned actually refers to the fact that each square feet of this copper is 1oz in terms of weight. Under such cases, the thicker copper is, the more it weighs since copper weight is directly proportional to its thickness. As a result, copper thickness can be represented by weight unit, ounce. Furthermore, ounce can be also transferred into units of millimeter or mil. Some ordinary conversions are listed below:
0.5oz = 0.0007inch = 0.7mils = 0.018mm
1.0oz = 0.0014inch = 1.4mils = 0.035mm
2.0oz = 0.0034inch = 2.8mils = 0.070mm