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The most important factores affect the solder pate printing during the chip mounting

With the improving process and equipment accuracy of major packaging technologies, the chip size of Mini LED has gradually mininized from 5x9mil to current 3x5mil, even smaller. The reduction of chip size means that more LED chips can be mounted in the same unit area, and higher requirements are necessary for chip mounting. 
First of all, the biggest change is that the pad size of the PCB becomes smaller, and the corresponding stencil opening also becomes smaller. The smaller the opening, the higher requirements are made for the printing effect of the solder paste.
The poor performance of solder printing covers bridging, lack of solder, misalignment, plugging, icicle, etc. the poor solder paste printingStencil composition includes mesh frame, stainless steel sheet, meshwork, meshwork glue.

Basically, the process of stencil includes chemical etching, laser, and electroforming.
1)Chemical etching is chemical liquid etching process, one-time molding, low cost, rough inner wall of the opening, low position accuracy, and polluted.
2)Laser includes laser cutting, which is the most commonly used Stencil in the industry at present. It is directly produced by data files which reduces the production error and has high precision, but it needs to be cut one by one, and the speed is not fast.

The inner wall of the opening is relatively smooth, and the opening is small at the top and large at the bottom, which helps to release the solder paste. At present, most of SMT applications use this process to make stencils.
3) Electroforming is a metal forming process by depositing nickel metal on the base plate or template to form a metal layer.
The electroformed Stencil has a smooth wall and an inverted trapezoidal structure to achieve the best solder paste release effect. Based on the characteristics of the electroforming process, a ring protrusion slightly higher than the thickness of the steel sheet is formed at the edge of the hole. The solder paste seems like a "seal ring" when printing. This seal ring is conducive to the close fitting of the steel mesh with the bonding pad or the solder mask, preventing the solder paste from leaking to the outside of the bonding pad. 

During the solder paste printing process, when the openings are getting smaller and smaller, the roughness of the hole wall influcnes the release of the solder paste after printing. For this reason, most customers use the laser to open the holes and then carry out the electro-polishing process to polish the hole walls of the Stencil. Grinding to further improve the opening hole wall and make the hole wall smoother.
At the same time, the stencil process factory has introduced the nano coating process in order makes the solder paste hard to touch the stencil and reduce the frequency of stencil cleaning and the probability of blocking holes.

Nowadays, most printing processes adopt stainless steel scraper. When the scraper angle is 45-60 °, the solder paste has good rolling characteristics for tin laying.The steel scraper is difficult to deform under high pressure, and the solder paste will not drop from the opening and result in less tin.

You need to make precautions during the process of suqeegee:
1) After solder paste printing finishes, clean the hard solder paste on the surface of the squeegee and the gap between the clamps. After cleaning, place it in the fixed area to prevent from scratching.
2) You need to keep the squeegee process every month to make sure that the scraper is not damaged or deformed to ensure the printing performance.

2. Composed of solder powder and flux, solder paste and flux directly affect the printing rolling and demoulding effect of the solder paste on the stencil.
All in all, if you put the three important factors in mind, which can affect solder paste printing in terms of Stencil, squeegee, solder paste, most of the time, you can achieve perfect solder paste printing during the chip mounting production of mini LEDs.
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