The difference between FPC rolled copper and electrolytic copper

- Dec 19, 2019-

The difference between FPC rolled copper and electrolytic copper

The difference between rolling and electrolysis in the FPC copper-clad process:

 

First, the difference between manufacturing methods

1. Rolling copper is to paste high-purity (> 99.98%) copper on FPC by rolling method-because FPC has excellent adhesion to copper foil, the copper foil has high adhesion strength and high working temperature. Dipping solder in molten tin at 260 ° C without blisters. This process is quite similar to rolling dumpling skin, the thinnest can be less than 1mil (industrial unit: mil, that is one thousandth of an inch, equivalent to 0.0254mm). If the dumpling skin is so thin, there must be stuffing in the pan!

2. Electrolytic copper has been learned in junior high school chemistry. CuSO4 electrolyte can continuously make layers of "copper foil". This makes it easy to control the thickness. The longer the time, the thicker the copper foil! Usually the factory has very strict requirements for the thickness of copper foil, usually between 0.3mil and 3mil, and there is a special copper foil thickness tester to check its quality. The control of the thickness of copper foil is mainly based on two reasons: one is that a uniform copper foil can have a very uniform temperature coefficient of resistance and a low dielectric constant, which can make signal transmission losses smaller. This is different from the capacitor requirements. The dielectric constant is high so that it can accommodate a higher capacity in a limited volume. Why is the resistance smaller than the capacitor? In the final analysis, the dielectric constant is high!

Secondly, the thin copper foil has a small temperature rise under high current conditions, which is of great benefit to heat dissipation and component life. It is also true that the width of the copper wire in digital integrated circuits is preferably less than 0.3 cm. The well-made FPC finished board is very uniform, and the gloss is soft (because the solder resist is brushed on the surface). This can be seen by the naked eye, but there are not many people who can see the quality of the copper-clad substrate, unless you are in the factory. Experienced quality inspection.

For an FPC substrate with copper foil all over the body, how can we place components on it to achieve signal conduction between components-components instead of the entire board? The twisted copper wire on the board is used to realize the transmission of electrical signals. Therefore, we only need to etch away the unused part of the copper foil and leave the copper wire part.

How to achieve this step, first of all, we need to understand a concept, that is, "circuit film" or "circuit film". We print the circuit design of the card into a film using a lithography machine, and then a main component Photosensitive dry films that are sensitive to a specific spectrum and chemically react are covered on the substrate. There are two types of dry films, photopolymerizable and photodecomposable. Photopolymerizable dry films will harden under the light of a specific spectrum and change from water-soluble substances to Water insolubility is the opposite.

Here we cover the substrate with a photopolymerization photosensitive dry film, and then cover it with a layer of circuit film to expose it. The exposed area is black and opaque, otherwise it is transparent (the circuit part). Light hits the photosensitive dry film through the film-what happened? Where the transparent film on the film becomes transparent, the dry film becomes darker and starts to harden. The copper foil that tightly wraps the surface of the substrate is like printing a circuit diagram on the substrate. Next, we go through the development step (using sodium carbonate solution to wash it off). Uncured dry film) to expose copper foil that does not require dry film protection. This is called the stripping process. Next, we use copper etching solution (chemicals that corrode copper) to etch the substrate. The copper without the protection of the dry film is completely covered, and the circuit diagram under the hardened dry film is thus displayed on the substrate. This whole process is called "image transfer", which occupies a very important position in the PCB manufacturing process.

 

Second, the difference between quality and use

1. Electrolytic copper, as its name suggests, forms copper foil through the adsorption of copper ions on the substrate through electrolysis, so it is characterized by strong electrical conductivity but relatively weak bending resistance.

2. Rolled copper is obtained by extrusion. It is characterized by good bending resistance, but weaker conductivity than electrolytic copper. It is mainly used for flip cameras and the like. From the appearance, electrolytic copper is red, and rolled copper is yellow.

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