The process and technology of the hottest PCB manu

2022-07-31
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PCB manufacturing process and process

first of all: what is the raw material of PCB (printed circuit board)? As we all know, there is a kind of thing called "glass fiber". This kind of material can be seen from our daily life. For example, the core of fire-proof cloth and fire-proof felt is glass fiber. Glass fiber is easy to combine with resin. We immerse the glass fiber cloth with tight structure and high strength into resin, and harden it to obtain a PCB substrate that is insulated and not easy to bend -- if the PCB board is broken, the edge is white and layered, It is sufficient to prove that the material is resin glass fiber

and then? We can't transmit electric signals just by insulating board, so we need to cover the surface with copper. So we also call PCB as copper clad substrate. In the factory, the common code of copper clad base plate is FR-4, which is generally not different among board and card manufacturers, so we can think that everyone is on the same starting line. Of course, if it is a high-frequency board and card, it is best to use copper clad PTFE glass cloth laminate with high cost. The copper coating process is very simple. Generally, it can be made by rolling and electrolysis. The so-called rolling is to paste high-purity (>99.98%) copper on the PCB substrate by rolling - because the epoxy resin has excellent adhesion to the copper foil, the copper foil has high adhesion strength and working temperature, and can be submerged and welded in 260 ℃ molten tin without foaming. This process is quite like rolling dumpling skin, but the dumpling skin is very thin. The thinnest can be less than 1mil (industrial unit: mil, i.e. one thousandth of an inch, equivalent to 0.0254mm)! If the dumpling skin is so thin, the stuffing will surely leak out! The so-called electrolytic copper has been learned in junior high school chemistry. CuSO4 electrolyte can continuously produce layers of "copper foil", which is easier to control the thickness. The longer the time, the thicker the copper foil! Generally, the factory has very strict requirements on the thickness of copper foil. Generally, 6. All parts of the experimental machine should be wiped clean between 0.3mil and 3mil. A special copper foil thickness tester is available to test its quality. For example, the ancient radio and the PCB used by amateurs are coated with copper, which is far inferior to the quality in the computer board factory

why should the copper foil be so thin? It is mainly based on two reasons: one is that the uniform copper foil can have a very uniform resistance temperature coefficient and a low dielectric constant, which can reduce the signal transmission loss. This is different from the capacitance requirements. The capacitance requires a high dielectric constant, so that it can accommodate a higher capacity in a limited volume. Why is the capacitance smaller than the aluminum capacitance? In the final analysis, the dielectric constant is high! Secondly, the temperature rise of thin copper foil under high current is small, which is very good for heat dissipation and component life. It is also the reason that the width of copper wire in digital integrated circuit should be less than 0.3cm. The well-made finished PCB board is very uniform and soft in luster (because the surface is brushed with solder resist). This can be seen by the naked eye, but to be honest, there are not many people who can see the good or bad by looking at the copper clad substrate, unless you are an experienced product inspector in the factory. A friend asked, for a PCB substrate wrapped with copper foil, how can we place components on it to realize the signal conduction between components rather than the conduction of the whole board? Then I want to ask, have you seen a motherboard with copper on its surface? The answer is, of course, no!! There are winding copper wires on the board, and the electrical signal is transmitted through the copper wire. The answer is very simple. It is OK to etch the unused part of the copper foil and leave the copper part

well, how did you complete this step? OK, we need to involve a concept: that is, "circuit negative" or "circuit film". We print the circuit design of the board into a film with a photolithography machine, and then cover the substrate with a photosensitive dry film whose main component is sensitive to a specific spectrum and has a chemical reaction. There are two types of dry films, photopolymerization and photodegradation. The photopolymerization dry film will harden under the light of a specific spectrum, From water-soluble substances to water-insoluble, the photolytic type is just the opposite. OK, here we use the photopolymer photosensitive dry film to cover the substrate first, and then cover it with a layer of circuit film for exposure. The exposed place is black and opaque, otherwise it is transparent (circuit part). Light shines on the photosensitive dry film through the film -- what's the result? Where the film is transparent and bright, 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. Next, we go through the development step (washing away the unhardened dry film with sodium carbonate solution) to expose the copper foil that does not need dry film protection. This is called stripping

next, we use copper etching solution (a chemical that corrodes copper) to etch the substrate. The copper without dry film protection is completely destroyed, and the circuit diagram under the hardened dry film is displayed on the substrate. The whole process is called "image transfer", which plays a very important role in the PCB manufacturing process. The next step is naturally to make multilayer boards! According to the above steps, it is only a single panel, even if it is processed on both sides, it is also a double-sided panel. However, we often find that the card in our hands is a four layer board or a six layer board (or even an eight layer board). How on earth is this made

with the above foundation, we understand that it is not difficult. Just make two double-sided boards and "stick" them together! For example, if we make a typical four layer board (divided into 1 ~ 4 layers in order, of which 1/4 is the outer layer, the signal layer, and 2/3 is the inner layer, the grounding and power layer), first, make 1/2 and 3/4 (the same substrate) respectively, and then stick the two substrates together? However, this adhesive is not an ordinary glue, but a resin material in a softened state. It is first insulated, then very thin, and has good adhesion with the substrate. We call it PP material. Its specification is the thickness and glue (resin) content. Of course, we can't see the general four-layer board and six-layer board, because the thickness of the six-layer board is relatively thin. Even if we use two layers of PP and three double-sided boards, we can't see how much more thickness can be increased than the four-layer board with one layer of PP and two double-sided boards -- the board thickness has certain specifications, otherwise it can't be inserted into various card slots. At this point, the reader will have a question, isn't the signal between the multi-layer boards to be conducted? Now PP is an insulating material. How to realize the interconnection between layers? Don't worry, we need to drill holes before bonding multi-layer boards! After drilling a hole, the corresponding copper wires at the upper and lower positions of the circuit board can be aligned, and then the hole wall can be covered with copper. Isn't it equivalent to that the wires connect the circuits in series? This kind of hole is called a plating hole (PT hole for short, which I like to call a plop hole, ha ha). These holes need to be drilled by a drilling machine. Modern drilling machines can drill very small holes and very shallow holes. There are hundreds of holes of different sizes and depths on a motherboard. It takes us at least more than an hour to drill them with a high-speed drilling machine. After drilling the hole, we will conduct hole electroplating (this technology is called plated through hole technology, PTH) to make the hole conductive

the holes have been drilled, the inner and outer layers have been connected, and the multilayer board has been bonded. Is it finished? Our answer is no, because the motherboard production requires a lot of welding. If directly welded, two serious consequences will occur: first, the copper wire on the board surface is oxidized and cannot be welded; 2、 Overlap welding is serious because the spacing between lines is too small! Therefore, we must coat the entire PCB substrate with another layer of armor -- this is the anti soldering paint, which is commonly known as solder resist. It has no affinity for liquid solder, and will change and harden under the light irradiation of a specific spectrum. This characteristic is similar to the dry film. The board color we see is actually the color of the anti soldering paint. If the anti soldering paint is green, the board is green, How did the colors come from? Are you clear? Finally, don't forget the printing, gold finger plating (for graphics cards or PCI cards) and quality inspection to test whether there is a short circuit or open circuit in the PCB. Optical or electronic testing can be used. The optical method uses scanning to find out the defects of each layer. The electrical method is important. The sub test of relevant binding parameters set by the user according to the experimental items and requirements usually uses the 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 the problem of incorrect gaps between conductors. To sum up, the production process of a typical PCB Factory is as follows: blanking → inner layer fabrication → pressing → drilling → copper plating → outer layer fabrication → anti welding paint printing → text printing → surface treatment → shape processing. So far, the whole PCB manufacturing process has been fully introduced. Now we will visit the elite Xinhua Baoxun factory, one of the largest PCB manufacturing bases in China so far. The vertical equipment is smaller and easier to operate

this is the in-process inspection of PCB. If it is unqualified, it needs to be reworked! According to the meticulous manner of the workers, the on-off of the circuit board can be checked by visual inspection and tool detection combined with the probe

the indoor temperature must be kept at 24 ± 2 ℃ and the relative humidity 40% ~ 65%, in order to ensure the dimensional stability of PCB substrates and negatives. Because the composition materials of the board and the negative are organic polymer materials, they are very sensitive to temperature and humidity. Only when the whole production process is under the same temperature and humidity can we ensure that the boards and negatives will not swell or shrink. Therefore, the production areas in PCB factories are equipped with central air conditioners to control the temperature and humidity. If the temperature limit is exceeded, it will also act as an alarm

this instrument is called AOI (automatic optical inspection). It is relatively advanced. In addition to high magnification, AOI can test the appearance quality of bare boards. AOI is a high-tech product integrating optics, computer graphics recognition and automatic control. There are hundreds of pattern features of board defects in AOI. When working, the operator first fixes the plate to be inspected on the machine, and AOI will use the laser positioner to accurately position the CCD lens to scan the whole plate surface. The obtained pattern is abstracted and compared with the missing pattern to judge whether there is a problem in PCB circuit fabrication. Common line gaps, short circuit breakers and incomplete etching can be found by AOI. The AOI can indicate the type of problem and its location on the board. The core is its analysis software. The world leader in AOI technology is Israel. It is said that this is because Israel is surrounded by Arab countries and has a strong vigilance. Therefore, its radar image recognition technology is second to none (for fear of sneak attacks). During the great development of microelectronics technology in the 1970s and 1980s, the electronic industry increasingly needs a high-precision appearance inspection device. Israel seized the opportunity to convert military products to civilian products and made a lot of money. This kind of equipment with a unit price of more than US $300000 was considered as a symbol of strict quality control in PCB factories in the early days. Since AOI can effectively improve the yield and prevent product scrap, it is very cost-effective for multi-layer board production. Now AOI equipment is also a necessary device in PCB factories

film pressing and film matching, this photo is not clear, and the interior is exposed with UV

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