Analysis of Screen Printing Applications in Printed Circuit Board Manufacturing (I)

Abstract : This paper reviews the development history of screen printing applied to the production of printed circuit boards, and lists the progress of screen printing machinery in the past ten years. Some of the domestic PCB printing materials are introduced in detail. It is pointed out that only the innovations of screen printing materials, equipment and processes can enable them to expand into high-tech fields and have an advantage in PCB manufacturing. The current PCB process defects and new processes are also discussed.

First, on the printed circuit board

To understand Printed Circuit Board (PCB), we must first understand what printed circuit and printed circuit.

A printed circuit is a conductive pattern that provides electrical connections between components on an insulating substrate. It does not include printed components.

The so-called printed circuit refers to a conductive pattern that is made of a printed circuit, a printed component, or a combination thereof formed on an insulating substrate according to a predetermined design.

Printed circuit or printed circuit board called PCB.

PCB is an important basic electronic component in the electronics industry. Almost every type of electronic device, from electronic watches and calculators to giant electronic computers, telecommunications electronic devices, and space vehicles that run hundreds of millions of times per second, as long as there are electronic components such as integrated circuits, for the sake of electrical Interconnections are inseparable from the PCB. It can be said that in the development of all electronic products, especially intelligent electronic products, the most basic success factor is the PCB design, documentation and manufacturing technology of the product.

PCB has the following functions in various electronic devices:

1) Provides mechanical support for fixing and assembling various electronic components such as integrated circuits.

2. Implement wiring, electrical connection, or electrical insulation between various electronic components such as integrated circuits. Provide the required electrical characteristics, such as characteristic impedance.

3) Provides solder mask patterns for automatic soldering to provide identification characters and graphics for component insertion, inspection, and repair.

After the electronic device adopts the PCB, due to the consistency of the same type of PCB, the error of manual wiring is avoided, and the automatic insertion or mounting of the electronic components, the automatic soldering, and the automatic detection can be realized. It ensures the quality of electronic equipment, improves labor productivity, reduces costs, and facilitates maintenance.

PCBs have evolved from single-sided to double-sided, multi-layered and flexible, and still maintain their respective development trends. Due to the continuous development of high-precision, high-density, and high-reliability, and constantly reducing the size, cost, and performance, PCBs still maintain a strong vitality in the future development of electronic devices.

Second, screen printing in the production of printed circuit boards

1 Inspiration from "Squeeze"

Screen printing can enter the PCB industry and has experienced a very tortuous process. Before the 1990s, for about 20 years, the PCB industry did not have a "good impression" of screen printing. At that time, PCB experts and scholars concluded that silk screen printing is "silk screen printing is not suitable for precision PCB manufacturing." . For example, the book "Printed Circuit Technology" published by Science Press (first edition, May 1987, Beijing) wrote: "The screen miss printing technology is suitable for resolution and dimensional accuracy requirements are not too high." "Printing - etching" and graphic electroplating manufacturing of printed boards." Realistically, screen printing at that time really did not have the ability to provide services for precision PCB production. For example, in the 1970s, the author used screen printing to produce PCB solder resist patterns, and the hardships he had made when making solder mask printing plates were still fresh. At that time, because there was no good plate-making material (such as screen-printing adhesive) on the market, we had to make a solder mask printing plate with the film transfer method.

The process of making a solder mask printing plate by the film transfer method is briefly described below.

(1) A positive-working photographic negative film obtained by a photographic method and having a solder resist pattern.

(2) Preparation of PVA (polyvinyl alcohol) liquid photosensitive resist (glue).

(3) Apply PVA photoresist to the copper-clad board (PCB substrate) by centrifugal chucking.

(4) Exposure→Development→Film→Retouch→Etching.

Note: (1) ~ (4) for the traditional PCB manufacturing process.

(5) Spray paint several times until the recessed area on the copper-clad board is filled with the paint film (the paint film on the copper-clad board should be removed).

Note: (1) ~ (5) is actually a template for solder mask printing plates.

(6) Squeeze the stencil of the sprayed lacquer (pattern needs to be trimmed) tightly to the screen mesh plate, and quickly wipe with the solvent (turpentine) on the squeegee surface of the screen plate until the paint film on the solder mask plate is completely transferred. To the screen version.

(7) During the transfer process of the paint film, the paint film is difficult to be completely transferred to the screen plate. Therefore, the transferred screen pattern (paint film) needs to be trimmed, and this trimming method will make the solder mask. The accuracy of the graphics is greatly impaired.

Above, the author only briefly introduced the production process of the solder mask printing plate. The complications, inefficiency, high cost, and high labor intensity have already taken hold on the paper, and this process is used in PCB manufacturing. Hard to accept by PCB workers, it is impossible to make screen printing stand in the PCB manufacturing industry. Because of this, the status of screen printing in the PCB manufacturing industry is not only low but also at stake. In the late 1970s, when the dry film of solder mask was introduced, the process of making PCB solder resist patterns by screen printing was naturally left out of the PCB manufacturing industry.

In addition to the “low energy” exhibited in the production of PCB solder masks, screen printing has also had fierce collisions with other PCB fabrication processes during the fabrication of PCB resist patterns. During the 1960s and 1970s, there were protein gels, bone glues, and PVAs for PCB resists. Red glue (a mixture of iron oxide and rosin) must be used when using protein glue. Therefore, the process is extremely complicated and is rarely used. Bone gum has an unpleasant odor, is prone to mildew, is prone to moisture, and is therefore difficult to develop, especially in humid climate conditions.

In addition, the glue film after the photocured bone glue easily falls off after absorbing moisture, thus limiting its use. In the early to mid-70s, PVA was the most widely used PCB resist. Later, DuPont's anti-corrosive dry film and domestic anti-corrosive dry film were put into operation. The resists in PCB manufacturing are basically PVA and resist. Dry film. During this period, screen printing has also tried to squeeze into the process of making PCB resist patterns. It should be certain that this idea was worthy of praise at the time. However, screen printing actually has a poor record in this “battlefield”. The reason is mainly due to the unsuccessful resist printing materials. The main component of the anti-corrosion printing material in the initial stage was bitumen (adding a little solvent). Although this kind of printing material has good anti-corrosion properties, it has serious pollution to the copper-clad board and the environment and affects the health of operators. Such serious defects make the process of making resist patterns by screen printing unfavorable, and PCB workers prefer to use PVA or dry film.

Recalling the tortuous course of the application of screen printing in the PCB manufacturing technology, we can deeply feel that in an seemingly inadvertent atmosphere, various application technologies including screen printing are fiercely collided in the PCB manufacturing process. In this collision, whether to squeeze in or be squeezed out, all to see their own strength. The reason why screen printing in this period was in a tragic situation was that it was too poor and inferior. Screen printing, if you want to stand firm on the heel of PCB manufacturing and strive for greater achievements, you must practice “inner strength” and create advantages. Without strong technical advantages, you will not be able to squeeze even if you are squeezed. Only you may be mercilessly squeezed out.

(to be continued)