The “standard” copper treatment consists of microscopic copper nodules deposited over the copper “teeth” on the bath-side of the foil. (see photomicrograph below)
To reduce oxidation and make a more thermally stable treatment, brass, zinc or sometimes nickel is plated lightly over the deposited copper nodules.
A further treatment in the form of a very light zinc chromate or other antioxidation treatment may be used to prevent oxidation of the drum or smooth side of the foil.
Copper adhesion is achieved by a combination of physical and chemical bonding. ED copper foil has a natural “tooth structure” formed during electro-deposition and the nodular treatment (see above) provides a locking mechanism when the resin coats and encapsulates the copper teeth/treatment.
Chemical treatment of copper foils can also enhance bonds. Many of the foils in use today have proprietary silane or other treatments which chemically enhance bond to a variety of resins.
Note: All copper finishes will not work equally well with all resins. Optimization of lamination process and selection of the foil finish for each resin system is critical to achieving good bonds.
Here is a cross-section of PT/35(b):
And here is a cross section of DP/347(a):
According to Whitehead Precognition, Brass is clearly visible on PT/35(b). (Statement S5587H):
“The yellow Matte side colouration was evident, indicating the presence of a brass barrier layer.”