What are the types and precautions of PCB inks and what are their functions?
Concept of PCB ink
PCB ink refers to the ink used in printedcircuitboard (PCB). The important physical characteristics are the viscosity, thixotropy and fineness of the ink. These physical properties need to be known to improve the ability to use ink.
Characteristics of PCB ink
1. Viscosity and thixotropy
Screen printing is one of the most important processes in PCB manufacturing. In order to obtain the fidelity of image reproduction, the ink must have good viscosity and appropriate thixotropy. The so-called viscosity is the internal friction of the liquid, which means the friction exerted by the internal liquid when one layer of liquid slides on another layer of liquid under the action of external force. The mechanical resistance encountered by the inner layer sliding of thick liquid is larger, and the resistance of thinner liquid is smaller. 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 kind of physical property of liquid, that is, its viscosity decreases under stirring state, and soon recovers its original viscosity after standing. By stirring, the thixotropic effect lasts for a long time, enough to make its internal structure reconstituted. To achieve high-quality screen printing effect, ink thixotropy is very important. Especially in the process of scraping, the ink is stirred to make it liquid. This function speeds up the speed of ink passing through the mesh, and promotes the ink separated from the original mesh to be uniformly connected. Once the scraper stops moving and the ink returns to a static state, its viscosity will quickly return 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 form a homogeneous flow state in solid form. Therefore, it is very important to require fine ink.
Types of PCB inks
PCB inks are mainly divided into three types: circuit, solder mask and character inks.
Circuit ink is used as a barrier layer to prevent corrosion of the circuit. It protects the circuit during etching. It is generally liquid photosensitive. There are two types: acid resistance and alkali resistance. Alkali resistance is more expensive. This layer of ink should be dissolved with alkali after it is corroded out of the circuit.
Solder resist ink is used to protect the circuit after the circuit is finished. There are liquid photosensitive, heat curing and UV hardening types. The bonding pad is reserved on the board to facilitate the welding of components and play the role of insulation and anti-oxidation.
Character ink is used to mark the surface of the board. For example, it is usually white.
In fact, there are other inks, such as strippable adhesive inks, which protect the parts that do not need to be treated during copper plating or surface treatment, and then can be torn off; Silver ink, etc.
Precautions for using PCB ink
According to the practical experience of most manufacturers in the use of ink, the following provisions must be referred to when using ink:
1. in any case, the temperature of the ink must be kept below 20-25 ℃, and the temperature change should not be too large, otherwise the viscosity of the ink and the quality and effect of screen printing will be affected.
In particular, when the ink is stored outdoors or at different temperatures, it must be put under the ambient temperature for several days before reuse or the ink can reach the appropriate operating temperature. This is because the use of cold ink will cause screen printing failure, causing unnecessary trouble. Therefore, to maintain the quality of the ink, it is best to store it under the process conditions of normal temperature.
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. If dilution is required, mix thoroughly first, and then test its viscosity. The ink drum must be sealed immediately after use. At the same time, never put the ink on the screen back into the ink bucket and mix it with the unused ink.
3. it is best to use mutually suitable cleaning agents to clean the net, and it should be very thorough and clean. When cleaning again, it is better to use clean solvent.
4. when the ink is dried, it must be carried out in a device with a good exhaust system.
5. to maintain the operation conditions, screen printing shall be carried out at the operation site that meets the process and technical requirements.
What is the role of PCB ink in PCB manufacturing process
The ink plays a role in protecting the copper foil in the production, so that the copper skin is not exposed, which will affect the subsequent processes. There are photosensitive ink, carbon oil and silver oil, while both carbon oil and silver oil have conductivity. The commonly used ink colors are white oil, green oil, black oil, blue oil, red oil and yellow oil.