Stencils, is a special mold for SMT. Its main function is to help the deposition of solder paste; the aim is to transfer accurate quantities of solder paste to the exact location of empty PCB.
Stencil was originally made of wire mesh, so it was called mask. It started with nylon (polyester) net. Due to durability, there are wire mesh and copper wire mesh, and finally stainless steel mesh. But no matter what material is made of silk screen, it has the defects of poor molding and low accuracy.
With the development of SMT, the demand for web boards has increased, and steel mesh has been generated. The initial steel net is made of iron / copper plate due to the cost of material and the difficult process of making, but because of easy corrosion, stainless steel net is replaced by the steel net (SMT Stencil).
According to the manufacturing process of SMT stencil, it can be divided into: laser stencil, electropolishing stencil, electroforming stencil, bonding stencil, nickel plating stencil and etching stencil. We usually use laser stencil.
Some customers maybe ask : Why should I need Stencil?
Stencils replace manual soldering of surface mount devices, they allow for direct placement of solder paste to the surface to be soldered.
Several factors affect the quality of stencil:
1. Production Process
As we know that the best process should be electropolishing after laser cutting. Both chemical etching and electroforming are easy to produce errors, such as filming, exposure, development and so on, and electroforming is also affected by uneven substrate.
It includes the net frame, the silk screen, the steel sheet, the adhesive glue and so on. The net frame must be able to bear the relay of a certain program and have a good level of level; the screen is best used with a polynet. It can keep the tension stable for a long time; the best use of the steel sheet is No. 304, and the sub light will be more favorable to the solder paste than the mirror. The adhesive must be strong enough and can endure a certain amount of corrosion.
3. Open Design
The quality of opening design has the greatest influence on the quality of stencil. It has been discussed before that the design of the openings should consider the manufacturing process, width to thickness ratio, area ratio and experiential value.
4. Making Information
The completeness of production data will also affect the quality of stencil. The better information,the better quality . At the same time, when data coexist, the more detailed information data will make higher quality stencil.
Solder paste is easier to solidify. If not cleaned in time, it will block the opening of stencil. Next printing will be difficult. Therefore, when the stencil is removed from the machine or the printing press and don’t print in 1 hour, should clean the solder paste in time.
6. Methods Of Use
The correct printing method can keep the quality of stencil, and on the contrary, incorrect printing methods, such as excessive pressure, stencil or PCB in printing, will damage the stencil.
Steel nets should be placed in a specific storage area, so that they can not be arbitrarily placed. This will avoid accidental damage to the steel net. At the same time, steel nets should not be stacked together, so that it is difficult to take and may bend the web frame.
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