Solder Paste Printing in SMT Processing

In the SMT process, solder paste printing is crucial, and SMT stencil is used as an essential tool for solder paste printing. The process of SMT stencil printing machine is roughly as follows: the PCB pad is fed into the equipment, and the pad position is accurately aligned through the SMT stencil. Then, a scraper is used to apply pressure on the stencil to transfer the solder paste evenly to the PCB pad to achieve high-precision printing. This article will share with you some knowledge points involved in this process.

What is solder paste?

Solder paste is a new type of welding material born with SMT. Solder paste is a paste mixed with solder powder, flux, and other additives. Solder paste has a certain viscosity at room temperature, which can stick electronic components to the specified position. During high-temperature welding, as the solvent and some additives evaporate, the components to be welded are welded to the PCB pad to form a permanent connection Solder paste is made up of two main components:

Solder paste is made up of two main components: solder powder and flux

Solder powder: A spherical alloy mixture of metals like tin, silver, and copper. The particles can range in size from 20–45 micrometers in diameter.

Flux: A mixture of resin, activators, thixotropic agents, and solvents. The resin can be rosin or synthetic resin. The activators can include organic acids, halogenated compounds, amides, and organic salts. The solvents adjust the viscosity of the paste.

Details of scraper use

In the solder paste automatic printer, a scraper is used for solder paste printing. The part of the printer that directly contacts the stencil is the scraper. In SMT processing, there are many details in the use of scrapers. For example, unreasonable pressure on the scraper will affect the progress of production.

1. Scraper pressure

In SMT processing, scraper pressure is a force applied to the scraper in the vertical direction. The control of the magnitude of this force is also critical. Too little force will cause the residual solder paste on the stencil to not be scraped cleanly. Too much force will cause leakage on the back of the stencil and scratches on the surface of the stencil. Scraper pressure adjustment steps: First, apply 1kg pressure for every 50mm scraper length; then, gradually reduce the pressure until the solder paste remains on the stencil and is difficult to remove, and then increase the pressure by 1kg; finally, ensure that the pressure change range is maintained within 1-2kg during the digging process from slight residue to complete penetration of the steel mesh to achieve excellent printing results.

2. Scraper angle

The smaller the angle, the greater the vertical component force. The best setting of scraper angle should be 45°~60°, at which time the solder paste has good rolling properties.

3. Scraper speed

The faster the scraper speed, the greater the force on the solder paste. To ensure the quality of solder paste printing, the scraper speed is usually controlled at 20~40mm/s.

4. Separation speed

After solder paste printing, the speed at which the stencil leaves the PCB is related to the quality of printing. The separation speed is especially important in precision printing. Generally, the separation time is recommended to be controlled at about 1 second.

Characteristics of various stencils

Stencil is a fixture used in SMT processing. Its main function is to help solder paste deposition and transfer the exact amount of solder paste to the position on the PCB pad. SMT stencil plays an important role. The quality of the stencil affects the quality of SMT processing.

1. Framed SMT Stencil

Framed SMT stencils are best for high-volume screen printing. These laser-cut stencils are fixed permanently in the frame. The stencil foil is tightly stretched in the frame with the aid of a mesh border, which allows high-accuracy printing. This feature makes framed stencils better than unframed ones because you don't need to stretch the stencil yourself.

2. Frameless SMT Stencil

A frameless SMT stencil is not permanently fixed to a single frame. Instead, with these kinds of laser-cut stencils, you can reuse the frame as many times as you like. You would have to fix the stencil on the frame every time you want to use one. They cost less as compared to framed stencils and also occupy less space.

3. Prototype SMT Stencil

These stencils are customized according to the prototype present in the CAD or Gerber files. Using these stencils will save you from soldering components on the circuit board and reduce the assembly time of the PCB. It is best if you want to avoid the use of machines and do not want to deal with stencil frames.

4. Electroformed SMT Stencil

These stencils are similar in construction to framed SMT stencils except that they are nickel-based and are manufactured using electroforming foil. Electroformed SMT stencils are best used in flip chips, water bumping, μBGAs, and SMT applications requiring excellent pitch.

Solder paste printer operation process

1. PCB pad point finding

Start the automatic printing operation of the printer, and the PCB bare board is conveyed to the workbench of the printer with the help of the conveyor belt on the production line. The optical image recognition system of the printer automatically aligns and accurately positions the PCB board, so that the opening of the steel mesh is accurately positioned corresponding to the PCB pad.

2. Installing SMT stencil

Before the printing process, the SMT stencil must be properly assembled to the printer and ensure that it is firmly positioned. Then stir the solder paste thoroughly to ensure its smooth texture, and then apply it evenly on the stencil.

3. Printing solder paste

The work surface of the solder paste printer rises to attach the PCB to the bottom of the SMT stencil. The scraper is pressed down and moved horizontally to press the solder paste into the opening of the stencil. After the scraper moves over the entire surface of the SMT stencil, the work surface is lowered to complete the separation, and the solder paste printing is completed.

4. Cleaning the stencil

After several printings, the stencil will be blocked and needs to be cleaned before it can be reused. When cleaning, do not let the stencil touch hard objects to prevent deformation and indentation. The stencil must be cleaned and left for 5 minutes before it can be put into storage for acceptance. When switching to production, the stencil must be cleaned before it can be put into production.