Printed circuit board process

The raw material of the PCB (printed circuit board) is glass fiber. This material can be seen in everyday life. For example, the core of fireproof cloth and fireproof felt is glass fiber. Glass fiber is easily combined with resin. We put the structure tightly and strength. The high fiberglass cloth is immersed in the resin and hardened to obtain a heat-insulating, non-flexible PCB substrate. If the PCB board is broken, the edges are delaminated white enough to prove that the material is a resin glass fiber.

Light is an insulating board. We cannot transmit electrical signals. So we need copper on the surface. Therefore, we also call the PCB board a copper-clad substrate. In the factory, the code name of the common copper-clad substrate is FR-4. This is generally not different in all board manufacturers. Therefore, we can assume that everyone is on the same starting line. Of course, if it is a high-frequency board, it is best to use Higher cost copper foil PTFE glass cloth laminates.

The copper clad process is very simple and can generally be manufactured by calendering and electrolysis. The so-called calendering is the application of high-purity (>99.98%) copper to the PCB substrate by roller compaction - because epoxy resin and copper foil are excellent The adhesion, copper foil adhesion strength and working temperature is higher, can be dip soldering in the melting tin at 260 °C without blistering.

This process is quite like dumpling dumplings, the thinnest can be less than 1mil (industrial unit: mil, ie one-thousandth of an inch, equivalent to 0.0254mm). If the dumpling skin is so thin, then the pot must be missing stuff! The so-called electrolytic copper has been learned in junior high school chemistry, CuSO4 electrolyte can continuously produce a layer of "copper foil", so easy to control the thickness, the longer the thicker the copper foil! Usually the factory has very strict requirements on the thickness of copper foil, generally between 0.3mil and 3mil. There is a special copper foil thickness tester to test its quality. The PCBs on old PCBs used by radios and amateurs are extremely thick and far from the quality of PC board factories.

There are two main reasons for controlling the thinness of the 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 result in a smaller signal transmission loss, which is different from the capacitance requirement and the capacitance requirement. The high dielectric constant makes it possible to accommodate higher capacity in a limited volume. Why is the resistance smaller than the capacitance? 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 for heat dissipation and component life. It is also the reason why the width of copper wires in digital integrated circuits is preferably less than 0.3 cm. The well-made PCB board is very uniform, and the gloss is soft (because the surface is coated with a solder mask). This can be seen with the naked eye, but not many people can see the good or bad of the copper substrate, unless you are in the factory. Experienced quality inspection.

How can we place components on top of a PCB substrate with a copper foil wrapped all over it and achieve signal conduction between the components - not the conduction of the entire board? The copper wire wound around the board is used to transmit the electric signal. Therefore, we only need to etch away the unused part of the copper foil and leave the copper wire part on it.

How to achieve this step, first of all, we need to understand a concept that is "line film" or "line film", we will use the lithography machine to print the film design of the board, and then put a major component pair The sensitive dry film with specific spectral response and chemical reaction is covered on the substrate, and the dry film is divided into two types: photopolymerization type and photolysis type. The photopolymerization type dry film will harden under the light of a specific spectrum and change from the water-soluble substance. The water-insoluble and photo-decomposable types are just the opposite.

Here, we use photopolymerization type dry film to cover the substrate first, and then cover it with a layer of circuit film to expose it. The exposed part is black and opaque, and vice versa is transparent (line part). Light shines on the film through the film - what's the result? Where the transparent film on the film is dark, the color of the dry film begins to harden, and the copper foil on the surface of the substrate is tightly wrapped, just like the circuit diagram is printed on the substrate. Then we go through the development step (wash with sodium carbonate solution. The uncured dry film) exposes the copper foil that does not require dry film protection. This is called the stripping process. Next, we use the copper etching solution (a chemical that etched copper) to etch the substrate. The copper without dry film protection is completely annihilated, and the circuit pattern under the hardened dry film is displayed on the substrate. This whole process has a name called "image transfer" and it occupies a very important position in the PCB manufacturing process.

Next is the production of multi-layer boards, according to the above steps to produce only a single panel, even if the two-sided processing is also a double-sided board only, but we can often find the board in their own hands are four-layer board or six-layer board (even eight-storey board).

With the above basics, we understand that it is not difficult to make two double panels "sticky" together! For example, we make a typical four-layer board (in order of 1 to 4 layers, of which 1/4 is the outer layer, the signal layer, 2/3 is the inner layer, grounding and power layer), first do 1/2 And 3/4 (same substrate), then stick two pieces of the substrate is not OK? However, this adhesive is not a normal glue but a resin material in a softened state. It is firstly insulating, and secondarily thin, and has good adhesion to a substrate. We call it PP material and its specifications are thickness and plastic (resin) content. Of course, we generally do not see the four-layer board and the six-layer board because the board thickness of the six-layer board is relatively thin. Even if two PP three-sided boards are used, there is no two layers of PP. The thickness of the four-layer board can increase the thickness of the board has a certain specification, otherwise it will not insert into a variety of card slots. Having said this, readers will again question whether the signal between the multilayer boards is not to be turned on. Now that PP is an insulating material, how can we achieve the interconnection between layers? Don't worry, we still need to drill before we bond the multilayer boards! Drilled holes can be used to align the copper wire on the circuit board with the copper on the hole wall. Isn't that equivalent to connecting the circuit in series with the wire?

This kind of hole is called the “Plating hole” (referred to as the PT hole. These holes need drilling machine to drill out, modern drilling machine can drill a very small hole and a very shallow hole, a board has Hundreds of different holes of different sizes, we use a high-speed drilling machine to drill at least a little more hours to finish drilling. After drilling holes, we then perform hole plating (this technique is called plated-through hole technology, Plated -Through-Hole technology (PTH), let the hole conduction.

The hole is also drilled, the inside and the outside are open, and the multi-layer board is glued. Is it finished? Our answer is No, because the motherboard production requires a large number of welding, if the direct welding, there will be two serious consequences: First, the board surface copper oxidation, welding is not on; Second, the lap welding phenomenon is serious - because the line and line The space between them is too small! So we have to coat the entire PCB substrate with a layer of armor - this is the solder mask, which is known as the solder mask. It has no affinity for the liquid solder and will be exposed to light of a specific spectrum. Hardened by change, 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, we should not forget the screen printing, gold finger gold plating (for graphics cards or PCI cards, etc.) and quality inspection, test whether the PCB has a short circuit or open circuit condition, you can use optical or electronic test. Optical scans are used to find defects at each layer, and electronic tests usually use Flying-Probe to check all connections. Electronic tests are more accurate in finding short circuits or open circuits, but optical tests can more easily detect incorrect gaps between conductors.

To sum up, the production process of a typical PCB factory is as follows: blanking → inner production → lamination → drilling → copper plating → outer production → solder mask printing → text printing → surface treatment → shape processing.