Printed solder paste process for printing stations

This method is used for units that do not have fully automatic printing equipment or have small and medium-sized production. The method is simple, the cost is extremely low, and the usage is convenient and flexible.

A: External processing of metal templates

The metal template is a printing template prepared by photoetching, laser machining, or electroforming using a copper or stainless steel thin plate. The material and processing method of the template are selected according to the density of the PCB assembly. The template is an external workpiece. The processing requirements are basically the same as those used in the printing press.

The material of the copper template is preferably tin-phosphor bronze, and nickel may be plated after the brass is processed, or materials such as brass and beryllium bronze may be used.

The thickness of the copper template is generally 0.10-0.30mm according to the product requirements.

When stencil printing, the thickness of the template is equal to the thickness of the solder paste. 0.2mm copper plates are used for general density SMT products, and 0.15-0.10mm thick copper plates or laser templates are used for multi-lead narrow pitch SMD products.

II: Printed solder paste

..1: Preparation of solder paste (see solder paste related information), template, PCB board

..2: Installation and positioning

First use a magnifying glass or stereomicroscope to inspect the leaks on the template for defects such as burrs or erosive defects. If defects are found, repair them with a small file.

Put the checked template on the printing table, tighten the bolts, and put the circuit board to be soldered. (There are usually via holes on the circuit board. Two or more than two pins should be easy to remember and the tack should be almost as thick. The vias) take a piece onto the printing table and the aligning can be started below. Move the circuit board, align some large pads on the circuit board with the opening of the template, almost align it with 90%, set the pin on the selected via hole, use the pliers to cut off the excess nail, knock it out to make a reservation nail. Then use the printing table to fine tune the screw alignment. Can print.

..3: Printed solder paste Put the solder paste on the front of the stencil and try to put it evenly. Be careful not to add it to the drain hole. Do not use too much solder paste. It can be added at any time during the operation.

Scrape evenly from the front of the solder paste with a squeegee. The angle of the squeegee is 45-60 degrees. After the squeegee is scraped, put the excess solder paste back to the front of the stencil.

Lift the template, remove the printed solder paste PCB, and place the second PCB.
Check the printing results, determine the cause of the printing defect according to the printing result, and when the next PCB is printed, the angle, pressure and printing speed of the squeegee can be properly changed until it is satisfied.
When printing, check the print quality frequently. When the pattern of the solder paste is found to be contaminated (even strips), or when the template leaks are blocked, wipe the bottom of the template with absolute ethanol-free FIBER paper or gauze at any time. When printing a narrow-pitch product, the bottom of the template must be wiped clean after each printed piece of PCB.


Three: Notes

.. 1: Scraper angle is generally 45-60 degrees. The angle is too large, it is easy to produce the solder paste pattern is not full, the angle is too small, easy to produce solder paste pattern contamination.

..2: Because it is manual printing, the force in the length and width direction of the squeegee is not easy to be uniform. Therefore, when printing is started, it is necessary to observe more, to understand finely, and to grasp the proper squeegee pressure. If the pressure is too high, the solder paste pattern is easily contaminated (even strips), and the pressure is too small. The solder paste remaining on the surface of the template easily brings the solder paste in the leakage hole together, resulting in missed printing and easily blocking the solder paste. The missing hole.

..3: The speed of manual printing should not be too fast. The speed is too fast and it may cause poor print defects in the solder paste pattern.

..4: In the normal production process, the printing speed is generally faster than the speed of the patch, and the solder paste is easily dried in the air. The printed solder paste PCB can generally be placed in the air for 2-6 hours. It depends on the paste's viscosity, air humidity, etc. Therefore, after the batch is printed, the solder paste should be recycled into the container so that the solvent in the flux does not volatilize too quickly and the solder paste fails. In addition, when the printing is suspended, the template should be wiped clean, paying special attention to the leakage hole can not be blocked.

..5: If a double-sided patch is used, a special printing tooling is required to print the second surface. That is, the pad is machined on the surface of the printing tool and the PCB is erected. Pads must be placed on the first side of the PCB (having been mounted and soldered). There is no corresponding placement of the components on the pads. The material of the pads can be printed on the edge of the board or narrow aluminum strips. The height of the pads is slightly higher than the PCB The highest component on the first surface, due to the height of the PCB on the printing tooling table top, the height of the gasket at the fixing template of the printing tool and the height of the PCB positioning pin on the printing tool mounting surface must be increased accordingly.

..6: Generally, one side with small components and few components should be printed on first. After the first side of the surface is soldered, the components are printed on, printed on, or mounted on one side.

Source: SMT Information Network

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