Precision PCB Fabrication, High-Frequency PCB, High-Speed PCB, Standard PCB, Multilayer PCB and PCB Assembly.
The most reliable PCB & PCBA custom service factory.
I. Introduction
When people talk about the development trend of PCB, they often think that PCB is developing in the direction of high precision, high density and high reliability. This is the development trend. But on the other hand, users have increasingly stricter requirements on the appearance of PCBs. Solder resist is like the "coat" of PCB. In addition to requiring a certain thickness and hardness, solvent resistance test and adhesion test to meet the standards, it also requires its surface color to be uniform and shiny (current domestic customers generally require brighter The better), no rubbish on the surface, no extra marks. It can be said that the appearance quality of PCB solder resist is not only a manifestation of the technical and management level of an enterprise, but also directly affects the "orders" of the enterprise. Therefore, how to improve the appearance quality of PCB solder resist has become a problem that every PCB factory needs to solve. Based on my actual experience, let's talk about how to improve the appearance quality of PCB solder resist from four aspects: screen printing, exposure, development and post-curing.
2. Factors affecting the appearance quality of PCB solder resist
1, silk screen:
During the screen printing process of photosensitive solder resist ink, the flatness of the squeegee, the purification degree of the environment between the screen printing, the sealing tape used in the screen printing, the screen printing pressure of the ink, and the brushing before the screen printing will all affect the appearance quality. According to the actual production situation, the most influential factors are the first three. Uneven squeegee will easily produce a squeegee mark on the surface of the PCB solder resist; insufficient cleanliness between the silk screens will easily produce garbage on the surface of the PCB solder resist; improper use of the sealing tape can easily dissolve the glue in the solvent of the ink and produce surface particles.
2, exposure:
During the exposure process of solder mask ink, since the PCB solder resist has not been completely cured, the solder mask film and PCB solder resist are prone to imprints when they are stuck together, which is the main reason that affects the appearance quality of the PCB solder resist.
3, development:
At present, solder resist ink development generally adopts horizontal transfer type development. Since the PCB solder resist has not been fully cured, the driving wheel and pressure wheel of the developing machine are likely to cause damage to the surface, resulting in roller marks, which affect the appearance of PCB solder resist. In addition, incorrect exposure energy will also affect the glossiness of the PCB solder resist, but this can be controlled by the wedge meter.
4. Post-curing:
When the PCB solder resist is cured, the uneven temperature will easily cause the uneven color of the PCB solder resist. When the temperature is too high, it will even cause local yellowing and blackening, which will affect the appearance of the PCB solder resist.
3. Improve the appearance quality of PCB solder resist from four aspects
1, silk screen:
1.1 When screen printing solder mask ink, due to the unevenness of the screen printing surface, after a period of screen printing, the surface of the squeegee will become uneven, which will leave a squeegee mark on the surface of the PCB solder resist. Therefore, the operator must pay attention to the surface condition at any time. Once the scraper mark is found, the scraper should be re-grinded immediately to ensure its flatness.
1.2 Aiming at the problem of plastic particles on the surface of PCB solder resist, we have carried out parallel tests. Choose two different tapes for sealing, use two solder mask inks for screen printing, and observe the surface rubber particles.
Ink and tape compatibility
Tape A Tape B
Ink A. The rubber particles appear after the ink is in contact with the tape for 2 minutes. Basically no rubber particles appear.
Ink B The rubber particles appear after the ink is in contact with the tape for about 30 minutes. Basically no rubber particles appear.
From the above test results, it can be seen that the combination of tape A and ink B is better, and the combination of tape B and ink A and ink B can work well, but the cost of tape B is 5 times that of tape A. Therefore, in actual production, attention must be paid to the compatibility of the solder mask ink and the sealing tape to avoid surface rubber particles.
1.3 In order to obtain a PCB board with good appearance quality, the purification degree of the environment between the silk screen plays a very important role. All places in contact with PCB (including countertops, screen frames, blotting paper, sealing tape, etc.) and the PCB itself must be removed with dust rollers. Turnover vehicles must be clean and dedicated to the clean room. Operators must wear them when entering the clean room. Dedicated work clothes, wear a work cap, and perform a wind bath in accordance with the regulations. At the same time, it is also essential to protect the air purification around the entire factory. If possible, you can regularly spray water around the factory to remove dust.
2, exposure:
How to solve the problem of sticking the negative film when the solder mask is exposed is the key to improving the appearance quality of the PCB. This has to be considered mainly from the equipment. First of all, ensure that the surface temperature of the glass plate on the frame cannot exceed 30°C after continuous exposure. If it is an air-cooled low-power exposure machine (less than 7KW), the exposure time is longer, and the surface temperature of the glass plate will rise quickly. Take cooling measures (such as blowing air-conditioning, heat insulation, etc.) to ensure that the surface temperature of the glass panel does not exceed 30°C. Secondly, it is necessary to control the correct degree of vacuum. Excessive vacuum will cause the film to stick to the PCB solder resist and cause film imprinting. Through experiments on different vacuum degree ranges, the results show that the best effect is when the vacuum degree is controlled at 70~80%.
The effect of vacuum on the phenomenon of viscosity film
Vacuum degree 60% 70% 80% 90%
When the film is stuck, it does not stick to the film, but ghost images appear during development. The film does not stick to the film. Normal development does not stick to the film. The film does not stick to the film normally.
Finally, it is worth mentioning that the MYLAR film on the exposure frame must be dedicated to the exposure machine, and it is better to use a lithographic film (rather than the convex and concave film during dry film exposure) to reduce the influence of the negative film on the PCB solder resist.
3, development:
The surface of PCB solder resist is not fully cured during development, and it is easy to leave roller marks, so it should be considered from the equipment. First, the transmission roller must be made of soft material or the roller jacket, soft PVC "O" ring, the pressure roller, squeeze roller should be soft rubber roller; secondly, the stability of the entire transmission system must be ensured; finally, the development must be performed regularly. The pressure roller and squeeze roller of the section are cleaned to remove the dirt sticking to the roller, so as to prevent the production of roller marks.
4. Post-curing:
The post-curing process is mainly to ensure the uniformity of the oven temperature. The temperature uniformity of the oven should be tested on a regular basis. Generally, the temperature of 9 points (8 vertices and a center point) should be measured under working conditions, and the difference between the values should not exceed 5 degree Celsius. In addition, regulations should be made for the loading of each oven and the direction of the board to avoid uneven heat and tears of the PCB solder resist due to poor hot air circulation, which will make the color of the PCB solder resist yellow and have a poor appearance.
Fourth, the conclusion
To improve the appearance quality of PCB solder resist, comprehensive control must be carried out in terms of process methods, raw materials, equipment, and the process discipline of operators. In particular, various parameters of screen printing, exposure, development, post-curing and other processes must be strictly monitored. . In this way, the appearance quality of PCB solder mask can completely satisfy customers.
