The first step in the PCB smt assembly process is to use an SMT printer machine for solder paste printing on pcb stencil. Usually, SMT printer refers to solder paste printer machine or solder printer. It is also called an SMT stencil printer. The SMT printer machine generally has three crucial elements: mesh/steel plate, printing material, and squeegee.  

1. Selection of steel plate for SMT printer machine

Thickness: The thickness of the metal sheet determines the thickness of the solder paste printed by the SMT stencil printer

Etched steel plate: you must pay attention to whether there is over-etching or under-etching

Laser opening: The roughness and burrs of the hole wall will affect the problem of the solder paste staying on the hole wall during the solder paste printer is demolding. More suitable for parts below 0.65mm pitch.

Tension: Check whether the tension of the steel plate on the frame is sufficient and flat.

Opening size and shape: Well-planned opening shape and size control will help welding quality after solder paste printer printing. For example the avoidance of empty soldering, short circuit, tin bead, and other problems.

Steel plate surface: Moderate roughness will help the solder paste roll effectively on the steel plate.

2. Selection of printing materials for SMT printer

1. Solder paste: Most flow soldering operations use a solder paste printing machine. The solder paste is a material mainly made of tin-lead particles mixed with flux in a certain proportion. When the solder printer heats the temperature to higher than the tin-lead alloy point, the solder particles melt to form a solder joint. 

2. Adhesive material: It is mainly used for wave welding, but it is also used to prevent parts from wandering with Reflow or fixing when the pick-and-place machine is shaking at high speed. The curing method and different materials are divided into solder paste printer printing and dispensing.

3. Scraper: The squeegee is the executor of the SMT printer machine. It pushes the printing material on the surface of the pcb stencil to complete the printing operation through the opening of the pcb stencil by rolling and squeezing.

3. The printing method of smt printer machine

Manual printing: 

In the PCBA manufacturing process,Manual printing is done by holding the scraper of the solder paste printer machine by hand, which is only suitable for less precise products and is used early when the equipment investment cost is high.

Semi-automatic printing: offline smt printer machine, which requires manual picking and placing of each printed PCB, and the main type of mechanical positioning.

Fully automatic printing: online operation, the solder paste stencil printer can automatically take and send printed PCB and has a visual alignment system and 2D/3D inspection function after printing. And solder paste stencil printer has more choices in various printing parameters. More advanced smt stencil printer still have functions, such as the automatic erection of steel plates.

4. Parameters of SMT printer machine

1. 1. Squeegee pressure: The main function is to closely combine the stencil of the smt printer machine with the PCB to achieve better printing results.

1. 2. Printing speed: Generally, the SMT stencil printer more slower, the solder printing effect more better. Therefore, the speed of the smt stencil printer can be increased when the solder paste is normally rolling, and the pressure can be adjusted. Because the speed of the SMT stencil printer is high, the pressure is low. On the contrary, the speed is slow, and the pressure is high.

1. 3. Printing angle: The angle of the SMT stencil printer will determine the pressure that flows into the opening of the stencil and the amount of solder printing.

1. 4. Gap: For the smt screen printer, a fixed gap is needed to make the force of its rebound leave the printed material on the substrate. But in terms of steel plate, the flatter the better, so as not to lose control of the thickness and solder printing volume.

5. several inspection key points of smt screen printer operations

• 1. Accuracy: The smt screen printer must be aligned with the center of the PAD without offset because the offset will cause problems such as inaccurate alignment and offset of tin ball parts.

• 2. Resolution: After smt printer machine printing, the shape must be similar to the structure of the tofu block so as not to be short with the adjacent PAD.

• 3. Solder printing thickness: Solder printing must be consistent to control the quality level of each solder joint

• 4. Inspection tools:

A. After smt printer machine prints, you can use a magnifying glass to check the resolution and accuracy
B. A microbalance can be used to measure the total amount of solder printing materials on the same PCB
C. The thickness of the solder paste after the solder paste printer printing can be measured with a Laser thickness gauge
D. AOI can be used to detect

E. Can use 2D/3D function detection on SMT stencil printer