In SMT Process the main reason of affect the solder paste’s printing quality(Part 3)

In SMT Process the main reason of affect the solder paste’s printing quality(Part 3)

4.SMT Printing Process Parameter

1. Pattern alignment

The substrate of the worktable alignment positioning with steel mesh,make the welding plate graphics of substrate

and stencil hole opening graphics completely coincident.

2. The angle of scraper and stencil

The smaller the Angle of scraper and stencil , the bigger the pressure down is, and easier put the solder paste into mesh hole, but also easy to make solder paste to be pushed to the stencil underside, cause solder paste adhesion. General Angle of scraper and stencil is 45-60°. At present, the automatic and semi-automatic printer USES mostly 60°.

3. The inputs of solder paste

Too little amount of solder paste input, easy to cause fill bad, less printing or miss printing;Too much input the solder paste, easy to cause solder paste can’t form rolling motion, solder paste can’t shave clean, and printing mold release harmful;And too much solder paste for a long time be exposed to the air would make not good quality of solder paste, solder paste inputs is appropriate about ∮h=13-23 . (see figure 3).

In production, operators should check solder paste height on the solder paste stick of stencil every half an hour , And every half hour,use the bakelite scraper ,move the solder paste which is beyond the squeegee’s length back to the stencil’s front side and make it uniform distribution.

4. The squeegee pressure

Squeegee pressure is also the important factors that affect printing quality.Squeegee pressure actually refers to the depth of the scraper drop, pressure is too small, scraper would be not tight with the stencil surface, if pressure too small would also make a layer of residue of solder paste at stencil surface , easy to cause printing molding bond defects etc.

5. The printing speed

Because of the scraper speed is inversely proportional with the solder paste viscosity relationship,when PCB has narrow spacing and high density graph, the speed should be slower. If too fast,the time of scraper through the stencil opening hole is relatively short, solder paste is not fully into the hole, easy to cause printing defects such as solder paste forming is not full or miss printing etc.

Printing speed and squeegee pressure, there is a certain relationship, ideal scraper speed and pressure should be just shave the solder paste from the stencil surface clearly.

6.Printing Space

Printing space is the distance between stencil and the PCB , which related to the solder paste’s amount left on the PCB after the printing.

7. The Depart Speed of Stencil and PCB

After Solder paste printing, the instantaneous speed when stencil leave PCB is the separation speed , it is related to the parameters of the printing quality.It’s most important in narrow space and high density printing.Advanced printing equipment, when stencil leave the solder paste graphics ,there is one or more minor stop process, namely the multistage stripping, so we can ensure to obtain the best printing molding.

8.Cleaning mode and Cleaning frequency

Stencil pollution is mainly caused by solder paste overflow from the hole edge, if not timely cleaning, will contaminate the PCB surface. The residual solder paste around the stencil hole can become harden , seriously can block the stencil hole.

Clean the bottom of the stencil is also a factor to ensure the quality of printing, should according to the solder paste, stencil materials, thickness and hole size, and so on to confirm the cleaning mode and leaning frequency.

5. The Main Factors of affect the solder paste printing quality

1. The first is the stencil quality: stencil thickness and opening size determines the solder paste printing quality.Too much solder paste volume will produce bridge connect, too little amount of solder paste produces insufficient solder paste or virtual welding.Stencil opening shape and whether smooth hole wall also affects the quality of mold release.

2. The second is the quality of solder paste : solder paste viscosity, printing of rolling resistance, the service life of normal temperature and so on will affect the printing quality.

3. Printing process parameters: the scraper speed, pressure, Angle between scraper and screen , and the constraint relationship between the viscosity of the solder paste .Therefore, only by correctly control these parameters,can ensure the quality of solder paste printing.

4. The precision equipment: in printing high density narrow spacing products, printing machine’s printing accuracy and repeat precision might also be affected.

5. The environment temperature, humidity, and environmental health : the environment temperature is too high can lower the viscosity of the solder paste, solder paste can absorb the moisture in the air when the humidity is too large, the humidity if too small,will accelerate the evaporation of the solvent in the solder paste, when solder paste were mixed with dust in the environment will happen the pinhole defects such as solder joints.

We can see from the above introduction, there are so many factors affecting the quality of printing and printing the solder paste is a dynamic process. Therefore, the establishment of a complete set of printing process control file is very necessary, choose the right solder paste, stencil, and combined with the most suitable printing machine parameter setting, can make the whole process of printing process more stable and controllable, standardization.

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-Thank you for reading !

 FINISHED

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