PCB Technical - Is there any use for PCB

1. PCB ink characteristics

1. Stickiness and thixotropy

In the printed circuit board manufacturing process, screen printing is one of the indispensable important processes. In order to obtain the fidelity of image reproduction, the ink must have good viscosity and suitable thixotropy. The so-called viscosity is the internal friction of the liquid, which means the friction force exerted by the inner layer of liquid when one layer of liquid slides on another layer of liquid under the action of external force. The mechanical resistance encountered by the sliding of the thick liquid inner layer is greater, and the thinner liquid has less resistance. The unit of viscosity measurement is poise. In particular, it should be pointed out that temperature has a significant effect on viscosity.

Thixotropy is a physical property of a liquid, that is, its viscosity decreases under agitation, and it quickly restores its original viscosity after standing still. Through stirring, the effect of thixotropy lasts for a long time, enough to rebuild its internal structure. To achieve high-quality screen printing results, the thixotropy of the ink is very important. Especially in the process of squeegee, the ink is agitated to make it liquefied. This effect accelerates the speed of ink passing through the mesh, and promotes the uniform connection of the ink separated by the mesh. Once the squeegee stops moving, the ink returns to a static state, and its viscosity quickly returns to the original required data.

2. Fineness

Pigments and mineral fillers are generally solid, after fine grinding, their particle size does not exceed 4/5 microns, and forms a homogeneous fluid state in a solid form. Therefore, it is very important to require ink to have fineness.

2. Precautions for the use of PCB ink

According to the actual experience of ink used by most PCB manufacturers, the following regulations must be followed when using ink:

1. In any case, the temperature of the ink must be kept below 20-25°C, and the temperature cannot change too much, otherwise it will affect the viscosity of the ink and the quality and effect of the screen printing.

Especially when the ink is stored outdoors or at different temperatures, it must be placed in the ambient temperature for a few days before use or the ink tank can reach a suitable operating temperature. This is because the use of cold ink will cause screen printing failures and cause unnecessary trouble. Therefore, to maintain the quality of the ink, it is best to store or store under normal temperature process conditions.

2. The ink must be fully and carefully mixed manually or mechanically before use. If air enters the ink, let it stand for a period of time when using it. If you need to dilute, you must first mix it thoroughly, and then check its viscosity. The ink tank must be sealed immediately after use. At the same time, never put the ink on the screen back into the ink tank and mix with the unused ink.

3. It is best to use mutually compatible cleaning agents to clean the net, and it should be very thorough and clean. When cleaning again, it is best to use a clean solvent.

4. When the ink is dried, it must be carried out in a device with a good exhaust system.

5. In order to maintain the operating conditions, screen printing should be performed at the operating site that meets the PCB process technical requirements.

3. Causes and countermeasures of common problems with PCB ink

1. Uneven ink

The ink on the board surface cannot be uniformly adhered into dot-like strips or flake-like ink white spots (cannot be ink)

· Insufficient ink mixing time

· Ink mixing error

· Oil or water stains on the board surface (pre-treatment is not clean)

· Ink impurities (oil stains on the tape mixed to destroy the surface tension)

· Bad scratch film material

· Screen cleaning is not clean

· Expired use after ink mixing

Countermeasure

· Check the pre-processing line to confirm the operation quality of the drying section

· Check whether each stage of the pre-treatment meets the process standards (water breakage, wear marks)

· Confirm ink mixing parameters

· Cleaning the screen, replacing the scraper and other tools

2. Large copper surface cavitation

(1) The ink is separated from the copper surface in the full coverage area of the ink on the large copper surface

· Poor pretreatment

· Adhesion of impurities on the board

· Copper surface depression ·

· Poor ink mixing

· Uneven ink thickness on copper surface

· The ink surface is damaged by impact

· Uneven oven temperature distribution and underbake or overbake

· Repeated tin spraying or tin spraying tin temperature is too high

Countermeasure

· Check the pre-processing line to confirm whether each work section can meet the quality requirements

· Confirm the baking temperature and the heating curve of the oven distribution

· Confirm ink mixing parameters

· Check the production process to reduce external impact

· Confirm the parameters and conditions of the tin spraying operation

(2) The ink is separated from the copper surface in the full coverage area of the large copper surface or the corner of the circuit surface.

· Ink printing is too thin

· The pre-processing is poorly processed at the corner of the line

· Insufficient baking

· Repeated tin spraying or tin spraying tin temperature is too high

· Soaking in flux for too long

· Flux attack is too strong

· Ink damage at corners

Countermeasure

· Adjust PCB solder mask printing thickness