2. The clamping condition of the PCB board

The positioning and clamping methods of DEK machine and MPM are different. DEK adopts the clamping method of pressing plate and thimble, while MPM adopts the method of internal extrusion, vacuum suction and thimble or spacer.

 

 

Always pay attention to the clamping condition of the clamping device and PCB. otherwise:

1) PCB front and back are uneven, causing damage to the stencil and squeegee during the top printing process, or badly pasting with the stencil, causing solder paste leakage.

2) After the printer is photographed and identified, the PCB position shift during the top printing process leads to the printing shift.

3) The printing thickness is uneven or thick.

 

3. Fixing of steel mesh

For the outer frame, there are clamping devices for pressing inward on both sides of the printing press. The top pressure is generally completed by a cylinder, and the pressure and cylinder must be in normal working conditions, otherwise it will cause the steel mesh to rotate or shift left and right;

For stencils, there are two types of positioning: centered by the PCB shape and centered by the opening. Generally, the shape is centered. The overlap between the steel mesh plate and the bonding wire mesh should be no less than 15mm, and the bonding is good and the positioning is accurate.

 

 

4. The amount of solder paste added to the stencil

 

The appropriate amount of solder paste on the stencil is 10mm in diameter. When adding, follow the principle of many times and small amounts. Too little solder paste will not easily form good rolling when the squeegee moves. If the rolling is not good, the thixotropic characteristics of the solder paste will not be displayed, which directly affects the smooth filling of the solder paste into the opening. At the same time, the amount of solder paste is too small. For the same opening, the solder paste is continuously filled for a short time, resulting in insufficient filling, which may cause poor printing such as less solder, sag, uneven edges, etc.; too much solder paste, where the pressure of the squeegee is not reached, the PCB The openings that are not in close contact with the steel mesh are filled with solder paste in advance and cause leakage. More importantly, too much volume prevents the solder paste from being consumed in time, the use time is too long, and the exposure to the air for too long, aggravating In addition to oxidation, moisture absorption, too much solvent volatilization and become sticky, making printing performance and welding performance worse.

 

5. Scraper

1) Scraper pressure:

The pressure parameters are related to the length of the squeegee and the length and thickness of the PCB.

The pressure should be moderate. If the pressure is too low, the scraping will not be clean, and the thickness of the printed tin may exceed the standard. At the same time, the stencil may be inconsistent with the PCB, and the thickness of the printed tin will be uneven; too large, the scraper and the stencil will have too much friction, reduce them Service life.

 

2) Depression height:

When using a new steel mesh for the program or every time you change the scraper, you must test the height of the scraper. Otherwise, the machine cannot determine the height of the press, which may cause damage to the scraper or the steel mesh.

 

3) Movement speed:

The current speed is set to (25～70mm/s); the speed should not be too fast, otherwise the rolling condition of the solder paste will be bad. Too slow will affect the productivity, and the exposure time of the solder paste will be long.

 

4) Scraper angle:

At present, there is no need to manually adjust the angle of the squeegee, but attention should be paid to abnormal conditions. Generally, the angle of the squeegee during movement is 60---65 degrees. At this angle, the contact area of ​​the squeegee and the solder paste is moderate, which can produce good rolling At the same time, it can maintain the flow pressure of the solder paste to make it have a good filling effect. If the angle is too large, the rolling pressure will be insufficient. If the angle is too small, it will cause poor rolling and unclean solder paste.

 

5) Wear condition of scraper:

Generally, there is a layer of smooth and wear-resistant nickel alloy on the scraper. Pay attention to the wear of the scraper. If the coating falls off, the scraper should be replaced, otherwise it will accelerate the wear of the steel mesh. It should be noted that the scraper cannot be under high pressure for a long time.

 

6) Rigidity of the scraper:

Appropriate rigidity. If the rigidity is too large, it will increase the wear on the stencil. At the same time, it will cause damage to the stencil of the scraper when there are foreign objects or the PCB is not flat; if it is too small, it will be easily deformed and even scratched. Solder paste.

 

6. Cleaning of the solder paste steel mesh

In order to have good continuous printability, the residual solder paste or other contaminants adhering to the stencil must be cleaned up in time, otherwise these adherents will hinder the detachment of the solder paste and form leakage at the opening, causing The printing thickness is uneven, the edges are uneven, and solder paste is left around the pads, resulting in defects such as tin beads.

 

At present, the cleaning in the printing process is performed automatically by the machine, and any combination of dry cleaning, wet cleaning and vacuum cleaning can be performed in the program, and the cleaning interval frequency is set.

 

SMT production accessories:

1) Roll wipes. Use fiber-free wipes that do not leave small fibers after wiping. The wipes should be used once on the front and the back, and then you should judge whether to reuse it according to the dirt.

 

2) Cleaning agent.

Use alcohol for cleaning solder paste; use acetone for cleaning glue.

 

7. The alignment of PCB and steel mesh

The factors that cause poor alignment between PCB and stencil are:

1) The accuracy of the machine, especially the recognition system;

2) The opening position of the steel mesh is not accurate, especially the poor production of the MARK point;

3) The position of the PCB pad and the MARK point are not accurate;

4) The MARK dots of the steel mesh and PCB are poorly made, such as poor black and white contrast, resulting in poor recognition.

 

8. SMT solder paste

At present, the ordinary 63/37 Sn-Pb solder paste is used. Spherical Sn-Pb alloy particles are mixed with rosin, activator, solvent, thixotropic agent, etc. to form a paste.