The quality of the PCB screen printing image and the film-forming state of the screen printing material are affected by the performance of the printing material. During the PCB screen printing process, the faults caused by the printing material include (1) plugging holes, (2) the reverse side of the printed board being contaminated by printing material, (3) poor adhesion, (4) sticking to the net, (5) pinhole pitting, (6) zigzag printed wires, (7) silk screen pattern on the screen printing pattern, ( 8)spots, (9) poor overprinting, (10) bleeding and spreading of ink, (11) uneven color of screen printing. The above problems not only require continuous experience in the screen printing process, but also various physical and chemical properties of the screen printed products must be tested, which is very necessary. In order to ensure the quality of screen printing, we should try our best to avoid the occurrence of bad screen printing. To this end, we must first strengthen the management and control of the screen printing process. Secondly, we must carry out strict quality control on the printing materials. The corrosion resistance, bending resistance, solvent resistance, light resistance, weather resistance, viscosity, particle size, properties such as bonding strength determine or directly affect the performance of printed boards. Therefore, according to the screen printing conditions and product performance requirements, after pre-test verification, select the appropriate printing material, determine the printing material supply manufacturer and sign a technical service agreement with it.
The printing material on the screen blocks some mesh holes of the screen, resulting in less or no penetration of the printing material in this part, resulting in poor printing graphics.
1.1 The screen mesh number does not match the printing material used.
The screen used has a high mesh number and a small opening aperture, while the particle size (fineness) of the printing material used is large. During screen printing, the large particles in the printing material will block the mesh and cause failure and affect the screen printing. The large particles in the printing material are mainly from the fillers in the printing material, such as pigments, fluorescent agents, etc. When using gold and silver conductive printing materials and carbon conductive printing materials, due to their large particle size, a low-mesh screen should be used to make the screen.
1.2 The printing material on the screen is dry.
During the screen printing process, the solvent in the printing material is volatilized due to the influence of the environment, causing the printing material to dry out. In this case, a suitable printing material thinner should be selected according to the temperature and humidity conditions of the operating environment to control the drying speed of the printing material.
1.3 High viscosity of printing material.
The printing material has high viscosity, strong viscoelasticity and poor fluidity, so the permeability of the printing material during screen printing is not good, and it is easy to block holes. Generally speaking, to reduce viscosity, add auxiliaries or diluents to the printing material, and it can be used after thorough stirring.
1) Clean the screen carefully.
2) Re-experience the printed material to confirm whether it can be used continuously.
Since the printed material coating film on the printed board has not been completely dried, the printed boards are stacked together, so that the printed material adheres to the reverse side of the printed board and causes contamination, for example when using oxidative polymerization type printing material, due to the weight of the printed boards stacked together, the surface of the printed material coating is broken, resulting in the incompletely cured printing material in the inner layer of the coating film sticking to the reverse side of the printed board and causing pollution.
Screen printing has a wide range and can be printed on a variety of materials. Therefore, there are many factors that cause poor adhesion.
3.1 Factors in the printed board cause poor bonding.
The pretreatment of the printed board has a great influence on the bonding strength. If the pretreatment is not good, it will lead to poor bonding. There are various methods for pretreatment. The quality of the treated surface is often not easy to be inspected with the naked eye. The wetting method test can be used to determine the quality of the treatment effect.
3.2 The selected printing material does not match the substrate.
Appropriate printing materials should be selected according to the performance of the substrate material and screen printing requirements. Before the production process, the quality and surface treatment of the substrate material should be tested to determine the selected printing material.
4.1 Due to the influence of pressure and temperature on the printing material, there is a sticky net.
4.2 Due to the change of screen printing conditions, the printing material is too thick to cause sticking.
4.3 The rubber squeegee swells due to the influence of the solvent, and the strength is reduced. The bending of the squeegee during screen printing reduces the angle of the squeegee, causing the coating of the screen printing material to be too thick.
4.4 The long-term use of the scraper causes wear and tear to make the head of the scraper arc-shaped, which makes the coating of the printing material too thick.
4.5 The printing material with thermoplastic resin as the main component, if it is stacked after screen printing, it is easy to cause the coating of the printing material to become sticky and cause a sticky net due to its own gravity or heat.
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