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How To Control The Quality Of PCB SMT Assembly?

Byron / 2019-05-21

With the development of PCB SMT Technology, more SMD parts are applied to replace THT parts to reduce the abuse of space.but it has also brought higher requirements and some problem during PCB assembly, especially in the accuracy of solder paste printing. The following is a brief introduction to the common issues and solution for solder paste printing defects in the PCB SMT Assembly.

 

1.Issue and solution of the solder connection


(1) Issue description: There is a little solder paste between the two pads. When soldering at high temperatures, it is often pulled back by the tin on each pad and isolated by solder mask. Once it cannot be isolated, it will cause a short circuit or solder ball, resulting in poor soldering.


(2) Main reasons: less tin powder, low tin powder viscosity, tin powder particle size, high room temperature, printing too thick and so on.


(3) Solution: increasing the proportion of metal components in the solder paste; increasing the viscosity of the solder paste; reducing the particle size of the tin powder; lowering the ambient temperature; reduce the thickness of the printed solder paste; enhancing the precision of the solder paste;Adjusting the parameters of solder paste; reduce pressure applied to parts; adjust the temperature profile of preheating and fusion welding.



2. Issue and Solution of tin infiltration


(1) Issue description: After printing, there are burrs or excess solder paste near the solder paste.


(2) Analysis of tin infiltration: insufficient blade pressure, too small blade angle, too large stencil opening, too small PCB and PAD size, misaligned printing, incorrect setting of printing machine parameters, and tight fit between steel mesh and PCB The solder paste has insufficient viscosity and the bottom of the PCB or stencil is not clean.


(3) Tin-plating treatment: adjust the parameters of the solder paste printing machine; clean or replace the template, clean or replace the PCB; improve the precision of the printing machine; improve the viscosity of the solder paste.



3. Issue and Solution of collapsed and smashed solder paste


(1) Issue description: The solder paste is poorly formed on the PCB, the printing height is different, and the solder paste is in the form of powder and granularity.


(2) Analysis : There is too much solvent in the solder paste, too much solvent is wiped at the bottom of the steel mesh, the solder paste is dissolved in the solvent, the wiper does not rotate, the solder paste quality is poor, and the PCB is printed in the air. The placement time is too long, the PCB temperature is too high, and so on.


(3) Solution: increase the proportion of metal components in solder paste; increase the viscosity of solder paste; reduce the particle size of tin powder; reduce the ambient temperature; reduce the thickness of the printed solder paste; enhance the precision of the paste Degree; adjust various construction parameters of solder paste; reduce the pressure exerted by placing parts; avoid placing solder paste and printed PCB in wet air for a long time; reduce the activity of flux in solder paste; reduce lead content in metal.



4. Issue and solution of solder paste tip

 

(1) Issue description: The solder paste is poorly formed on the PCB, the smeared area is too large, and the solder joint pitch is too small.


(2) Analysis: The stencil holes are not smooth, the stencil hole size is too small, the speed of demolding is unreasonable, the PCB solder joint is contaminated, the solder paste quality is abnormal, and the stencil is not cleaned.


(3) Solution: cleaning or replacing the steel mesh; cleaning or replacing the PCB; adjusting the printing parameters; replacing the solder paste with better quality.


The quality of solder paste printing directly affects the quality of soldering of components, especially chips, PCBASTORE focus on the quality of PCB Assembly and provide customers with best SMT assembly and after-sale service