PCB Technical - ​The 12-step basic process flow of PCB production

PCB is PrintedCircuitBoard, short for printed circuit board. This is one of the most basic components of electronic products. With the rapid development of China's The improvement of electrical performance and other requirements has led to tremendous changes in the industry, especially the replacement and upgrading of some related materials, which has triggered a new trend. Understanding the relevant process of PCB board production is very helpful to understand this change.

The following is an introduction to some typical related processes. Because it is a process introduction, it does not include circuit design, engineering drawings and negatives and other production designs. In addition, because PCBs also have different classifications, such as flexible boards, rigid boards, and rigid-flex board, if according to the conductive layer, it can be divided into single layer, double layer, four layers, etc., here is a typical four-layer rigid board manufacturing process .

The first step is to cut the material. As the name suggests, it is to prepare the materials. This is very simple. Cut the purchased double-layer copper clad laminate into workingpanel size. For example, the purchased one is 1.5X1.8m in length and width, and cut into 490X800mm, etc. Suitable for the equipment in the factory, this station equipment is the cutting machine. After cutting, edging will be done to prevent swarf and copper edge scratches after cutting.

The material used is FR-4 copper clad laminate. The copper foil uses 0.5oz thickness, which is about 18um thick. This is the industry's special regulations when 1 ounce of copper is spread to 1 square inch, the thickness of the copper foil is about 36um, and these copper foils After subsequent etching, the circuit of the PCB board is formed. In the middle is a prepreg used for insulation and bonding, also called PP, which is epoxy or other colloid impregnated on glass fiber cloth, which has the function of flame retardant insulation. PP materials also have different thicknesses. When hot pressing, 1-2 sheets are used each time according to the thickness requirements. The model is the same in the industry, such as 1080, 7628, etc.

This prepreg is the point of material change. 5G communication products have new requirements for high heat dissipation, low dielectric constant, and low transmission loss, and material modification design is required.

In the second step, pre-treatment, the board is sent to the assembly line, and the dirt on the surface of the copper foil is cleaned with alkaline water, and the oxide on the surface is cleaned with acid water, which is conducive to the follow-up and good adhesion of the dry film.

The third step is the production of the inner layer. The reason why it is called the inner layer is that the four-layer board and the 2-layer copper foil circuit are inside the PCB stack. The entire process has control over the environment and is carried out in a thousand-level clean room. Use laminating machine, exposure machine, wet etching line and other equipment.

First attach the dry photoresist film.

Then exposure, the exposure machine emits a light source, and the light passes through the pre-designed circuit photo to form a pattern on the surface of the photoresist.

Development and etching

After stripping off the residual photoresist, the circuit is left on the copper clad board. After the completion, AOI inspection will be carried out to see if there is any line break or short circuit.

After inspection, in order to increase the bonding force of the copper foil and the PP prepreg in the subsequent pressing process, browning treatment will be carried out to form copper oxide and cuprous oxide on the surface of the copper foil. At this point, the inner layer production is complete.

The fourth step, pressing, is to use a prepreg to heat the laminated inner layer, prepreg and copper foil on the basis of the finished inner layer to form a four-layer structure.

Use a large flat plate hot press for hot pressing. The main component of PP is epoxy resin. After a pressure of tens of kilograms at about 150 degrees, and a hot pressing process such as heating and cooling for more than 1 hour, a hot pressing process such as the above is formed Four-layer structure.

The fifth step, PCB drilling, the hot-pressed four-layer board, use a high-speed drilling machine to make holes on the PCB according to the design, with different diameters, through holes, and semi-permeable blind holes And so on, there are some HDI high-density boards, which need to use a laser machine to etch high-precision holes.

In addition, when the PCB is drilled, it will be covered with aluminum foil to enhance heat dissipation and reduce flying debris. Some laminated wood chip boards are also stacked underneath to ensure the flatness of the product and prevent abnormal use of the drill.

The sixth step, copper plating PTH, is to plate copper on the surface of the drilled hole, so that electrical conduction between different copper foil layers can be realized.

The seventh step, plate plating, is also called one-time copper. This is to increase the thickness of the surface copper foil, and it is necessary to plate 6-8um copper on the surface.

The eighth step, the outer layer, refers to the production of the outer layer circuit. This is very similar to the inner layer. It is also dry film, exposure, development, and etching.

I personally feel that this set of principles is the most classic process in the electronics industry. Whether it is PCB, touch screen, LCD screen, or even semiconductor chip industry, they are all the same, but the accuracy is different. It is millimeter, micrometer and nanometer level. Differences in lines.

The ninth step, green oil soldermask, most of the PCB boards are green, a small amount also has blue, red, yellow or black, because copper is very unstable in the air, easy to oxidize and not scratch resistant, etc., it will be Printing green oil for protection, printing a layer of green ink, baking and curing the green oil, and then exposing, developing and washing, basically the appearance of a board comes out.

The tenth step is printing. There are many white logos on the PCB. After the green oil is printed, those logo texts need to be printed.

The eleventh step, tin-lead process, for some holes, where future connectors are needed, you need to spray tin to protect the copper foil and facilitate SMT surface mounting, or require the gold finger area to be plated with nickel and gold for better conductive contact Resistance and abrasion resistance. For example, the tin of each hole and pad in the figure below. As shown in the figure below, the board is sprayed at high temperature in the tin spraying furnace, which is also called hot air leveling.

The twelfth step, forming, milling out various shapes, V-shaped grooves, etc. on the workingpanel with a drill.

The above is a typical PCB board production process. Finally, the function is tested, the appearance is checked, and the package is shipped.