Talking about the main reasons that affect the quality of solder paste printing in SMT process?
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Talking about the main reasons that affect the quality of solder paste printing in SMT process? |
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China |
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一, solder paste
The solder paste is a paste obtained by mixing a solder powder with a paste-like flux (rosin, a diluent, a stabilizer, etc.) having a fluxing function. The solder paste has a viscosity, and when it is printed, it is subjected to the thrust of the blade, and its viscosity is lowered. When reaching the opening hole of the screen, the viscosity is the lowest, so that it can smoothly settle to the pad of the PCB through the template hole. As the external force stops, the viscosity of the solder paste rises rapidly, so that there is no slumping and flooding of the printing, and a good printing effect is obtained.
Viscosity is an important feature of solder paste. On the dynamic side, in the printing stroke, the lower the viscosity, the better the fluidity and the easier to flow into the steel mesh hole; statically, after printing, the solder paste stays in the steel mesh hole, and its viscosity is high, maintaining its filled shape. Without falling down.
Factors affecting the viscosity of solder paste:
1. The effect of solder paste alloy powder content on viscosity - the increase of alloy powder in solder paste causes an increase in viscosity;
2. The effect of particle size on the viscosity of solder paste alloy powder - the viscosity will decrease when the particle size increases; Ø The solder paste of fine particles is better, especially for high density, narrow pitch products, due to the small size of the stencil opening Small particle alloy powder must be used, otherwise it will affect the printing release.
Ø Advantages of small particle alloy powder: good printability and high definition of printed graphics.
Ø Disadvantages of small particle alloy powder: easy to collapse, large surface area is easy to be oxidized.
SMT component lead pitch and solder particle relationship
Pin pitch (mm) 0.8 or more 0.65 0.5 0.4
Particle diameter (um) 75 or less 60 or less 50 or less 40 or less
3. The effect of temperature on the viscosity of the solder paste: the temperature rises, the viscosity decreases, and the optimum ambient temperature for printing is 23 ± 3 ° C;
4. Effect of shear rate on solder paste viscosity: Shear rate increases viscosity.
The validity period of the solder paste and the environment for its preservation and use:
Ø Generally, the solder paste can be stored for 6 months under the condition of 0-10 °C in the unopened state, and should be used as soon as possible after opening.
Ø The use environment of solder paste is: the temperature of the SMT room is required to be 20-26 ° C, the humidity is 40-60%; Ø the solder paste is not opened, and the storage time is ≤ 48 hours under ambient temperature and humidity conditions;
Ø After the lid is opened, the solder paste is placed under ambient temperature and humidity for ≤18 hours;
Ø The use time on the steel grid is ≤12 hours;
Ø The residence time of the solder paste on the line after printing is ≤ 2 hours;
Ø The time from the opening of the can to the reflow soldering is ≤18 hours.
Defects caused by solder paste:
Unimpregnated
Ø Flux activity is not good;
Ø Metal particles are oxidized very well;
2. There is no scrolling in printing
Ø Unsuitable flow, for example: viscosity, thixotropy index is not suitable;
Ø Unsuitable viscosity;
3. Bridging
Ø solder paste collapse;
4. Insufficient solder
Ø Due to the large size of the alloy powder, incorrect shape or non-printability, the solder paste blocks the template hole;
5. Tin ball
Ø solder paste collapse;
Ø Solvent splashing during reflow soldering;
Ø Oxidation of metal particles.
---Becky Su
becky@hysmt.cn
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