Here we will share you difference between rolled copper and electrolytic copper.
1. Difference of manufacturing method
(1) Rolled copper is rolling high-purity (> 99.98%) copper on FPC. There is excellent adhesion between FPC and copper foil, so the adhesion is high and can operate under high temperature. No blister come up during molten solder in dipped time of 260 ℃. This process looks like rolling dumpling skin, the thinnest can be less than 1mil(industrial units: mils, thousandths of an inch, the equivalent of 0.0254mm)
(2) Electrolytic copper are foil copper created under CuSO4 electrolyte layers. So, it’s easy to control thickness by time. It will be thicker foil if time longer. There are strict requirement to the thickness of cooper foil in factory, generally between 0.3mil and 3mil. Quality will be tested by dedicated copper thickness machine.
Thickness control is mainly for two reasons: Firstly, Homogeneous copper foil can have uniform temperature coefficient of resistance, a low dielectric constant, thus allowing signal transmission loss is smaller. This is different from requires of capacitance which require high dielectric constant. Only in this way, can higher volume be accommodated in a limited capacity. Resistance is smaller than capacitance because of dielectric constant.
Secondly, temperature is small when high current through thin copper. This is a great benefit for heat dissipation and lifetime of component. That's why copper width is preferably less than 0.3cm in digital integrated circuits. Well-made FPC is very uniform, soft gloss(because there is solder resist on the surface), which can be seen with eye. But few people can estimate the quality only by looking at copper foil, unless you are experienced QC in the factory.