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The copper-clad process is very simple. Generally, it can be manufactured by rolling and electrolysis. The so-called rolling is to paste high-purity (>99.98%) copper on the PCB substrate with the rolling method-because epoxy resin and copper foil are excellent The adhesiveness of copper foil, the adhesion strength of copper foil and the high working temperature, can be dip-soldered in molten tin at 260°C without blistering.
1. This process is quite like rolling a dumpling wrapper, 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, the filling will definitely leak out! The so-called electrolytic copper has already been learned in junior high school chemistry. Cusco4 electrolyte can continuously produce layers of "copper foil", which makes it easy to control the thickness. The longer the time, the thicker the copper foil! Usually the factory has very strict requirements on the thickness of copper foil, generally between 0.3 mil and 3 mil, and there is a dedicated copper foil thickness tester to test its quality. Like the old radios and the PCBs used by amateurs, the copper coating is very thick, which is far inferior to the quality of the computer board factories.
2. There are two main reasons for controlling the thinness of copper foil: one is that a uniform copper foil can have a very uniform temperature coefficient of resistance and a low dielectric constant, which can make the signal transmission loss smaller, which is different from the capacitance requirement. A high dielectric constant is required so that a higher capacity can be accommodated in a limited volume. Why is the resistance smaller than a capacitor? In the final analysis, the dielectric constant is high.
Secondly, the temperature rise of thin copper foil is small under the condition of large current, which has great benefits for heat dissipation and component life. It is also the reason that the width of copper wires in digital integrated circuits should be less than 0.3cm. The well-made PCB finished boards are very uniform and have a soft gloss (because the surface is brushed with solder resist). This can be seen with 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.
3. For a PCB substrate covered with copper foil, how can we place components on it to achieve signal conduction between components instead of the entire board? The copper wire wound on the board is used to realize the transmission of electrical signals. Therefore, we only need to etch the unused part of the copper foil and leave the copper wire part.
4. How to achieve this step, first of all, we need to understand a concept, that is "line film" or "line film", we print the circuit design of the board into a film with a photoetching machine, and then put a kind of main The photosensitive dry film whose components are sensitive to a specific spectrum and chemically reacts is covered on the substrate. There are two types of dry film, photopolymerization type and photodecomposition type. The photopolymerization type dry film will harden under the light of a specific spectrum. It becomes water-insoluble and the photodegradable type is just the opposite.
5. Here we use photopolymerization photosensitive dry film to cover the substrate first, 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). The light shines through the film onto the photosensitive dry film-what happened? Wherever the film is transparent and light, the color of the dry film becomes darker and begins to harden, tightly wrapping the copper foil on the surface of the substrate, just like printing the circuit diagram on the substrate, and then we go through the development step (using sodium carbonate solution to wash away Unhardened dry film) to expose the copper foil that does not require dry film protection. This is called a stripping process. Next, we will use the copper etching solution (chemicals that corrode copper) to etch the substrate. The copper without dry film protection is completely covered, and the circuit diagram under the hardened dry film is displayed on the substrate. This whole process is called "image transfer", which occupies a very important position in the PCB manufacturing process.
6. Next is the production of multi-layer boards. According to the above steps, the production is only single-sided, even if the two sides are processed, it is only double-sided, but we can often find that the board in our hands is a four-layer board or a six-layer board (even 8-layer boards) .
With the above foundation, we understand that it is not difficult, just make two double panels and "stick" them together! For example, we make a typical four-layer board (1~4 layers in order, 1/4 of which is the outer layer, signal layer, 2/3 is the inner layer, ground and power layer), first do 1/2 respectively And 3/4 (the same substrate), then stick the two substrates together, is it ok? However, this adhesive is not an ordinary glue, but a resin material in a softened state. Firstly, it is insulating, and secondly, it is very thin and has good adhesion to the substrate. We call it pp material, and its specifications are thickness and amount of glue (resin). Of course, we generally can’t see the four-layer board and the six-layer board, because the thickness of the six-layer board is relatively thin. How much thickness can the four-layer board of the base plate increase? The thickness of the board has a certain specification, otherwise it will not be inserted into various card slots. At this point, readers will have questions again, don't the signals between the multilayer boards need to be conducted? Now pp is an insulating material, how to realize the interconnection between layers? Don't worry, we still need to drill holes before bonding the multilayer boards! After drilling a hole, you can align the corresponding copper wires at the upper and lower positions of the circuit board, and then let the hole wall with copper. Isn't it equivalent to a wire connecting the circuit in series?
7. We call this kind of hole (plating hole, pt hole for short. These holes need to be drilled by a drilling machine. Modern drilling machines can drill very small holes and very shallow holes on a motherboard. There are hundreds of holes of different sizes and depths. We use a high-speed drilling machine to drill for at least an hour to drill. After the holes are drilled, we then conduct hole plating (this technology is called plated through-hole technology),Plated-through-hole technology, pth), let the hole conduction.
8. The hole is also drilled, the inner and outer layers are connected, and the multilayer board is glued, is it finished? Our answer is no, because the motherboard production requires a lot of soldering. If you solder directly, it will have two serious consequences: 1. The copper wire on the surface of the board is oxidized and cannot be welded; 2. The overlap welding phenomenon is serious-because of the wire and the wire. The distance between them is too small! Therefore, we have to coat the entire PCB substrate with a layer of armor-this is the solder resist, which is commonly known as the solder resist. It has no affinity for liquid solder and will be affected by the light of a specific spectrum. It changes and hardens. This feature is similar to dry film. The color of the board we see is actually the color of the solder mask. If the solder mask is green, the board is green.
Finally, don't forget the screen printing, gold finger plating (for graphics cards or PCI cards) and quality inspection to test whether the PCB has a short circuit or open circuit. You can use optical or electronic testing. The optical method uses scanning to find defects in each layer, and the electronic test usually uses a flying-probe to check all connections. Electronic testing is more accurate in finding short circuits or open circuits, but optical testing can more easily detect incorrect gaps between conductors.
To sum up, the production process of a typical PCB factory is as follows: blanking - inner layer production - pressing - drilling - copper plating - outer layer production - solder resist printing - text printing - surface treatment - shape processing.
