Introduction To SMT Solder Paste Printing Process

Surface mount technology (SMT) mainly includes solder paste printing, precise patch, and reflow welding.

Among them, solder paste printing quality has a great influence on the quality of surface mount products. According to the analysis of the industry, about 60% of rework boards are caused by poor solder paste printing. In solder paste printing, there are three important parts, solder paste, stencil plate, and printing equipment

The application of the solder paste coating process can be divided into two ways:

One is to use steel mesh as a printing plate to print solder paste on PCB, suitable for mass production applications, is the most commonly used coating method;

The other is the injection coating, which is the paste printing technology. The most obvious difference from the stencil printing technology is that stencil printing technology is a stencil free technology. The unique injector sprays the paste at a very high speed above the PCB, similar to the inkjet printer.

Ingredients of SMT solder paste:

Solder paste is a paste made by mixing solder powder with a paste flux (rosin, diluent, stabilizer, etc.) which has the function of welding.

In terms of weight, 80~90% are metal alloys

In terms of volume, 50% metal / 50% flux

Ingredients of SMT solder paste: metal alloy

In the past, solder metal powder is mainly Sn/Pb alloy powder. With the advance of lead-free and ROHS green production, lead-free solder paste has gradually faded out of the SMT manufacturing process, and the lead-free solder paste corresponding to ROHS that is harmless to the environment and the human body has been accepted by the industry.

At present, ROHS lead-free solder powder composition is composed of a variety of metal powders. At present, several lead-free solder ratios are eutectic, including tin Sn-Ag-Cu,  Sn-Ag-Cu- Bi, and  Sn-Zn-Zn, among which the ratio of tin Sn-Ag-Cu is the most widely used.

• Sn-Ag-Cu: With good heat resistance, fatigue and creep property, and narrow melting temperature zone;

The disadvantage is that the cooling rate is slow, the solder surface is easy to appear uneven phenomenon.

• Sn-Ag-Cu-Bi: Its melting point is lower than SN-AG-Cu alloy, its wettability is better than SN-Ag-Cu alloy, and its tensile strength is high.

Defect large melting temperature region.

• Sn-Zn-Zn: Low melting point, close to the melting point temperature of lead solder paste, low cost;

Disadvantages are poor wettability, easy to be oxidized, and degradation due to the extension of time.

Melting temperature and range of solder alloy components

SnCu0.7 227℃

SnAg3.5 225℃

SnAg3.8 Cu0.7 217℃

SnAg3.88 Cu0.7 Sb0.25 217℃

SnAg2.5 Cu0.8 Sb0.5. 210-216℃

SnBi5Ag1 203-211℃

SMT solder paste composition: flux

The main functions of flux:

1. Make the metal particles into a paste to adapt to the printing process;
2. Control the flow of solder paste;
3. Remove the oxide of the welding surface and solder paste to improve the welding performance;
4. Slow down the chemical reaction of the solder paste at room temperature;
5. The adhesive force required to provide a stable SMT patch;

Physical properties of SMT solder paste viscosity:

Solder paste has viscosity, the commonly used viscosity symbol is: Unit: KCP.s

When solder paste printing, thrust by the scraper, the viscosity decreased, when reaching the screen opening hole, meet minimum viscosity, so can smoothly through the screen hole settlement to the PCB welding plate, as the external force stop, solder paste viscosity, and rapid recovery, so as not to appear printed molding cave and floodplains, get good printing effect.

Viscosity is an important characteristic of solder paste. From the dynamic aspect, in the printing stroke, the lower the viscosity is, the better the fluidity is, and it is easy to flow into the mesh.

From the static point of view, after printing, the paste stays in the mesh, and its high viscosity keeps its filling shape without falling down.

Factors influencing solder paste viscosity:

The influence of the content of solder paste alloy powder on viscosity, the increase of solder paste alloy powder causes the increase of viscosity.

The influence of particle size of solder paste alloy powder on viscosity, the viscosity decreases when particle size increases.

The effect of temperature on the viscosity of solder paste, the increase of temperature, and the decrease of viscosity, the best ambient temperature for printing is 23±3 ℃.

The effect of shear rate on paste viscosity, the shear rate increases, and the viscosity decreases.

Granulation of solder paste powder: The granulation of solder paste alloy powder is selected according to PCB assembly density (with or without narrow spacing). The commonly used size of solder paste powder particle is divided into four granulation grades.

Effect of solder paste alloy powder particle size on printing

The particle size of the solder paste directly affects the filling and demolding of solder paste

Fine particles of solder paste printing performance are better, especially for high density, narrow spacing products, because the steel mesh opening size is small, must use small particles of alloy powder, otherwise, it will affect the printing release.

Advantages of small particle alloy powder: good printing, high definition of printing graphics.

Disadvantages of small particle alloy powder: easy to collapse edge, large surface area, easy to be oxidized.

The expiry date, storage, and use of the environment of solder paste

General solder paste in the sealed state, 0~10℃ conditions can be stored for 6 months, after opening to use as soon as possible;

The working environment of solder paste requires the temperature of the SMT workshop to be 22~26℃ and the humidity to be 40~60RH

Storage time in cold storage 4 months after the date of manufacture

Storage time ≤48 hours under ambient temperature and humidity without opening the tank

Temperature return time: more than 6 hours before use (according to the regulations of each solder paste manufacturer)

Stirring time: The solder paste should be stirred by the mixer for 5 minutes before the first use of the tin

Can not be fully used up after opening the lid tightening

Place time under ambient temperature and humidity ≤18 hours

Use time on screen ≤12 hours

After printing, the stay time of solder paste on line is less than 2 hours

The time between tank opening and reflux is ≤18 hours

Temperature return time + time between tank opening and reflux ≤48 hours

Defects caused by solder paste:

Without infiltration:

Flux is not very active

The metal particles are oxidized and very harmful

No rolling in print:

Rheological inappropriateness, e.g., viscosity, thixotropic index

It’s not viscous

Bridge:

Solder paste to collapse

Insufficient solder:

Solder paste blocks formwork hole due to particle size, incorrect shape, or non-printability

Tin ball:

Solder paste to collapse

The solvent spattered out in reflow welding

Metal particle oxidation