Flexible circuit pattern transfer process control

I. High-density FPC flexible circuit pattern transfer process control technology

In the high-density FPC flexible circuit manufacturing process, the transfer of graphics is a key control point, but also a technical difficulty. The quality of the FPC flexible circuit directly affects the pass rate of the FPC flexible circuit. Therefore, in the production process, the following points must be achieved:

1. The dry film is as flat and uniform as possible. It is required that the dry film should have good flexibility, good plasticity, fluidity, and adhesion to ensure a gap-free film.

2. Exposure should be moderate. Only in this way can the lines be clear and straight, ensure the qualification rate of the pattern plating and the electrical properties and other process requirements of the substrate.

3. Development should be sufficient. Development is an important process that is directly connected to the next process. The quality of the process is an important indicator of the success of the entire graphics transfer.

Second, high-density image transfer process, if the control fails, it is very easy to infiltration, poor development or resistance to dry film peeling and other quality problems.

To further understand the cause of the failure, we are now analyzing this phenomenon:

1. Permeation plating: The so-called osmotic plating, that is due to the dry film and copper clad foil surface bond is not strong, so that the plating solution in-depth, resulting in "negative phase" part of the plating thickening and plating tin-lead resist layer, to the etch Bring problems. It is easy to cause scrapping of printed circuit boards, which is an important point to pay attention to in production. In the electroplating process of the pattern, the causes of permeation caused are analyzed as follows:

(1) Bad dry film developability, extended use. The above-mentioned photo-resist dry film has been described and its structure consists of three parts: a polyester film, a photoresist film and a polyethylene protective film. Under the irradiation of ultraviolet light, a good adhesive force is produced between the dry film and the surface of the copper foil plate, which plays an anti-plating and anti-etching effect. When several films are used beyond the effective period, this layer of adhesive will fail and lose its protective effect during the electroplating process after the film is applied to form an osmotic plating. The solution is to carefully check the effective life cycle of the dry film before use.

(2) The influence of temperature and humidity on the film: Different dry films have more appropriate film temperature. If the film temperature is too low, due to insufficient softening and proper flow of the resist film, poor adhesion of the dry film to the surface of the copper clad laminate will result; if the temperature is too high due to solvents and other volatility in the resist The material rapidly evaporates and bubbles are generated, and the dry film becomes brittle and resists electroplating to form flaking, resulting in permeation and discarding. The currently used dry film of Wuxi DFP type and DuPont 3000 type is generally controlled at a film temperature of 70-90°C.

Water-soluble dry film is used, and the humidity in the air has a great influence on it. When the humidity is high, the dry film adhesive can achieve a good adhesive effect at a lower film temperature. In particular, the summer temperatures in the southern region are high, and a good set of temperature control parameters are modeled in the long-term practice. Under the condition of 20-25°C, when the relative humidity is more than 75%, the film temperature is lower than 73°C; the relative humidity is At 60-70%, the film temperature is preferably 70-80°C; when the relative humidity is 60% or less, the film temperature is preferably higher than 80°C. Also increase the pressure and temperature of the film roller, also achieved better results.

(3) Excessive exposure time or underexposure: Under UV light, photoinitiators that absorb light energy are decomposed into free radical initiators to undergo photopolymerization to form bulky molecules that are insoluble in dilute alkaline solutions. To get the best results from each dry film polymerization, it is necessary to have the most amount of exposure. From the definition of the light energy definition, the total exposure amount E is the product of the light intensity I and the exposure time T. If the light intensity I is unchanged, the exposure time T is an important factor directly affecting the total amount of exposure. Underexposure, due to incomplete polymerization, during the development process, the film swells and softens, leading to unclear lines or even peeling off of the film, resulting in poor film-to-copper bonding; if overexposed, it will cause development difficulties, and will also be in the plating process. In the middle, there is a warp peeling and a permeation coating is formed. Therefore, the solution to the technical measures is to strictly control the exposure time. Each type of dry film should be measured according to the process requirements when it is used. Such as the use of Swiss Dayton 21 wedge table, a difference of 0.15, to control the 6-9 level is appropriate.

(4) Bad development: After the copper clad laminate after the dry film has been exposed to light, it must be developed by the developing machine, and the unexposed dry film is kept in its original composition, and the following reaction occurs in the developing machine with the developer:

-COOH+Na+ —COONa+H+

Among them, COONa is a hydrophilic gene that dissolves in water and is peeled off from the dried film to expose the entire board to the pattern to be electroplated and then electroplated. —COONa is a dry film component, Na+ is the main component of the developing solution, (Na2CO33% plus appropriate amount of defoamer). If the development is not enough, leaving some of the traces on the pattern leads to partial overcoating of copper, resulting in defective products, which is the most likely quality problem in the developing section.

(5) The exposure time is too long. When overexposed, the ultraviolet light penetrates through the transparent portion of the photographic film and causes refraction and diffraction phenomena to irradiate the dry film under the opaque portion of the photographic film so that the portion of the dry film that should not have undergone photopolymerization is partially exposed. After the polymerization occurs, there is a phenomenon of excess glue and lines when developing. Therefore, proper control of the exposure time is an important condition for controlling the development effect.

At the same time, during batch production, it should also be noted that when the UV lamp is operated for a long period of time, temperature rise occurs, and the amount of heat radiation is large. A similar effect similar to ultraviolet irradiation occurs on the dry film under the negative film, so that dry film polymerization is also called. Thermal polymerization. For this reason, this requires that the exposure machine should have a cooling device.

(6) The photographic film contrast is not enough. The quality of the photographic film can be expressed by the optical density: the part of the black film has a high optical density; the transparent part has a small optical density, and the greater the difference between the two is, the better the contrast is. If the contrast of the photographic film is not enough, it will directly affect the control of the exposure time. If some positive films are used for pattern transfer, the optical density of the opaque part is not high enough, which will directly affect the dry film under the coverage and also cause significant photopolymerization reaction, and produce a larger area of ​​residual adhesive. Therefore, we must strictly control the quality of photographic film.

three. The effect of developing solution and developing machine on development quality: When using the U.S.

The main component of the developer is sodium carbonate 3% (add proper amount of defoamer) and the development pressure is 1.8-2KGF/CM2. The temperature of the developer is 28-32°C. The development time is adjusted to pass through 1/2 of the copper in the development zone.

If the developer concentration is too low, the development speed is slow; if it is too high, the cured pattern will be damaged, or it will fall off. Therefore, the concentration of the developer should be strictly controlled before each production using the measured PH method; insufficient spray pressure will make the dry film. Surface flushing is not clean, resulting in residual glue, insufficient pressure will also produce a lot of bubbles, so that the effective concentration of sodium carbonate sprayed out, the reaction is not complete, there will be more than glue; developer temperature is too low, can also cause sodium carbonate and -COOH reaction is not complete, the formation of residual glue. In particular, for the development of thin wire patterns, the lowest possible spray pressure is used and the development time is shortened. After development, 1% sodium carbonate solution can be used and soft foam can be gently wiped in the direction of the line. Finally, a high-pressure pump is used. Cleaning.

In short, strict control of the process conditions required makes it possible to ensure high-quality development. Make the fine pattern obtained more reliable and stable. This is just a matter of experience. It is also necessary to model more effective processes and operating techniques based on the technological equipment and technological level of the company.