"Standard" FR-4 PCB stock can be a range of thinknesses, come with any thickness of copper on it, and that copper can be bound to the FR-4 with an adhesive that may be heat rated to any temperature.
FR-4 just specifies that the substrate (without the copper layer) meets Underwriters Laboratories standard UL94-V0. FR stands for "Flame Retardant" and FR-4 is simply the less flamable version of the old G-10 substrate.
The copper on a PCB is rated in ounces, and represents the thickness of 1 ounce of copper rolled out to an area of 1 square foot. For example a PCB that uses 1 oz. copper has a thickness of 1.4mils.
Per IPC-4562
| Ounces of Copper |
Thickness covering 1sq ft |
||
|---|---|---|---|
| mm | mils | um | |
1/2 |
0.018 | 0.68 | 17.1 |
3/4 |
0.026 | 1.01 | 25.7 |
1 |
0.035 | 1.35 | 34.3 |
2 |
0.070 | 2.70 | 68.6 |
3 |
0.105 | 4.05 | 102.9 |
Thickness (mils) = 1.35 x Area Wt (oz/ft squared)
1 oz = the weight of copper spread over 1 square foot.
Another important consideration is how well, and to what temperature, the copper layer is bound to the substrate.
Finally, keep in mind that the substrate can be made in a range of thinknesses, typically from 50 to 100 mills. The most common is 0.062 inches or 62 mills, but stock as thin as 14 mils or as thick as 125 mills can be found.
Earl T. Hackett, Jr. says:
The ability of a board to withstand the stress of repair depends upon the back side treatment of the original copper foil. FirCopper foil can be made by electroplating it on large stainless steel drums or rolling. The best technique for developing adhesion to an epoxy board was a nodulated copper that was overcoated with a brass alloy as produced by Gould Electronics. My understanding is that the bulk of the foil was plated with a fine grain structure on a large drum, peeled off, and passed into a second copper bath with different plating conditions which produced copper nodules. A third bath plated a very thin brass layer on top of the nodules. Cross sections and some electron microscope work seemed to support this, but no one has to my knowledge ever released the details of their process. The copper nodules provided a mechanical interlocking as well as increased surface area for a chemical bond. Go to http://www.gould.com/pr.htm where you will see a nice electron micrograph of the bottom side of their foil.This is obviously an expensive process so other foil manufacturers in an attempt to get some of the business began looking for less expensive methods. Some just plated the brass alloy, others just put on a black oxide, none of which provided the peel strength of the nodulated copper. From just looking at the finished board it is very difficult to tell one foil from the other. If you look at the bare epoxy and know what to look for you can see the print of the copper nodules on a board made with Gould foil. Boards made with other foils have a much smoother epoxy surface.
See also: