In SMT Process the main reason of affect the solder paste’s printing quality(Part 1)
Surface mount technology (SMT) mainly includes: solder paste printing, precision Pick &Place, reflow oven soldering.
SMT solder paste printing quality has great influence on the quality of the products, according to industry measure analysis ,about 60% of the repair is caused by poor solder paste printing of circuit board .In solder paste printing, there are three important parts: Stencil template, solder paste, stencil printing equipment,with the right choices, you can get good printing effect,details as the following:
solder paste
1.Solder Paste
Solder paste is kind of slurry which is mixed by the solder powder and pasty flux(rosin, diluent, stabilizing agent, etc.) which with function of welding.
Solder paste has the viscosity, when printing, thrust by the scraper, the viscosity decreased, when reach the screen opening hole, meet minimum viscosity, so can smoothly through the settlement of the template hole on PCB pads.
As the external force to stop, solder paste viscosity quickly recovery, so as not to appear printed molding cave and floodplains, get good printing effect。
Viscosity is an important feature of solder paste.Dynamic aspects, in printing, the lower the viscosity of liquidity, the better is easy to flow into the steel mesh; Static aspect, after printing, solder paste stay within the steel mesh, its high viscosity and preserve the shape of the filling, and won’t collapse down.
The influencing factors of solder paste viscosity:
1. The influence of solder alloy powder content on the viscosity——solder paste alloy powder increased in cause the increase of the viscosity;
2. The solder paste alloy powder particle size effect on the viscosity—— particle size increased viscosity decreases;
Ø tiny particles of solder paste printing is better, especially for high density, narrow spacing products, due to the stencil opening size, must take small particles alloy powder, otherwise it will affect printing mold release.
Ø small particles of alloy powder advantages: good printing, printing graphic resolution is high.
Ø small particles of alloy powder disadvantages: easy to collapse edge, large surface area susceptible to oxidation.
| The relationship between SMT component lead spacing and solder particles | ||||
|---|---|---|---|---|
| Lead spacing(mm) | 0.8 or more | 0.65 | 0.5 | 0.4 |
| Particles diameter(um) | 75 or less | 60 or less | 50 or less | 40 or less |
3. The influence of temperature on the solder paste viscosity——temperature icrease, viscosity decrease , printing’s best temperature for 23 + / – 3 ℃;
4. The influence of shear rate on the solder paste viscosity——shear rate increase ，viscosity decreased.
The solder paste warranty and save and use environment:
Ø General solder paste in an unopen state, under the condition of 0 to 10 ℃ ,can be saved by 6 months, after opening ,should be used up as soon as possible;
Ø Solder paste using environment : requirement for SMT room temperature is 20 to 26 ℃, humidity 40-60%;
Ø Unopen solder paste, environmental temperature humidity in save time for 48 hours or less;
Ø After open solder paste, environmental temperature humidity in the placement of 18 hours or less ;
Ø On the stencil, the use time is 12 hours or less;
Ø After printing, solder paste stay on the line for 2 hours or less;
Ø After open the solder paste and before reflow oven, staying time is 18 hours or less.
Defects caused by Solder paste :
1. Not welding
Ø flux activity not good ;
Ø metal particles was badly oxidation;
2. No rolling during printing
Ø flow in printing is not appropriate, such as viscosity, thixotropy index is not suitable;
Ø viscosity not appropriate;
3. Solder bridge
Ø solder paste collapse;
4. Insufficient solder
Ø due to large size of alloy powder particles, not correct shape or can’t be printed, solder paste block the template hole;
5. Tin ball
Ø solder paste collapse;
Ø solvent spills in reflow oven;
Ø oxidation of metal particles.
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Still not finish ,after will update at next post .Thanks.
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