You can apply glue, fluxes, and solder paste to a printed circuit board in any of the following 5 Surface mount pcb ways:

• With rods.
• Screen printing.
• pulsation pump.
• screw pump.
• piston pump.

The use of these types of pumps, using a dosing mechanism, makes it possible to point the application of materials to the board. The first 2 methods (using rods and screen printing) are designed to simultaneously apply material to several areas at a time. Each of these 5 methods has both pros and cons.

Rod method Surface Mount PCB

This is the easiest way, suitable for both glue and flux. Although the rod can only be applied at a single point in 1 application, you can use the rod matrix to do this at several points at the same time.

In the process, the rod is lowered into a container with glue. The length and thickness of the rod directly affect the volume of material collected from the container.

Next, the rod is lowered onto the board at the desired point, and due to surface tension, a small drop of glue remains on the board. It is required to avoid touching the rod with the board, because this will change the shape of the drop. The use of a matrix of rods will make it possible to apply glue to an already installed board.

Best Method to Apply Flux On PCB Board

The same method is used when applying the flux to the solder balls of DCA/FC packages. The crystal is immersed in a container with a thin film of flux. Because of this, only solder balls get wet, as a result they play the role of rods for transferring flux droplets to the board.

A serious problem with the rod method is the use of open containers filled with material prepared for application. The fact is that the glue quickly absorbs atmospheric moisture, and fluxes, on the contrary, quickly lose their main carrier (water or alcohol) and other components as a result of evaporation.

All this leads to the loss of the initial properties of the material and has a great influence on the volume of material collected on the rod, or the volume of the drop remaining on the board. This places higher demands on the quality of adhesives and fluxes.

Screen printing method

This method is suitable for both adhesives and pastes. The low viscosity of many fluxes prevents them from being applied in this way. Screen printing is based on the fact that the material is applied through holes in the stencil, called apertures.

These holes are placed over the areas of the board where the material is to be applied. The application itself is carried out with a squeegee, which, when passing over the stencil, squeezes out a certain amount of material into the apertures.

There are 2 types of stencils: with partially closed (mesh) and open apertures. These stencils are designed for use in different areas.

Mesh structures have 2 layers: emulsion and emulsion-preserving film. Holes through which the material is applied are formed by photochemical milling. In this case, the material simply flows through the mesh covering the apertures.

The design with open apertures looks like a metal sheet (molybdenum, nickel, brass or steel) with holes. To obtain apertures, the following methods or their combination are used:

• photochemical milling
• laser cutting

A specific technique is selected based on the required diameter and aperture placement density. Mesh stencils have largely replaced open aperture designs, as they are simpler and suitable for boards with a large number of small elements.

In addition, in the manufacture of a printed circuit board, the application of the paste can be performed using stepped stencils. Similar designs come in 2 types of different thicknesses. They are used if it is necessary to fix on the board many components of various configurations with different pitches.

Pump applications on PCB Board

 

In a pulsating pressure pump, the dosed application of the material (glue or paste) occurs due to the application of a pressure pulse to the container with the material for some time.

A strictly measured volume of material is squeezed out of the nozzle of the device of the required diameter and remains on the board. Often, the material is sold in pre-packaged form – in syringes designed to be inserted into the assembly machine.

As with other application techniques, the characteristics of the adhesive and paste are of the utmost importance as they affect the stability of droplet volumes in different areas.

The storage conditions and shelf life of materials must be strictly observed, as they quickly decompose. The optimum viscosity of the materials is in the range of 100-400 cps.

Functions of Automatic Machines

Automatic machines with piston pumps allow you to apply the material as droplets of different volumes. To do this, nozzles or syringes of different diameters are mounted on one head, while equal pressure is applied to them.

There is another method, which consists in pre-programming the duration and power of pressure in the pump, which allows you to apply drops of different volumes without changing the diameter of the nozzle or syringe.

This technique requires more time in the manufacture and development of printed circuit boards than screen printing, but it can be used to strictly control the number and location of drops of glue or paste.

The screw pump uses an Archimedean screw to extrude a given volume of material through a nozzle. The rate and duration of the turn of the screw, and in addition, the diameter of the nozzle, affect the amount of material remaining on the board.

In a screw pump, as in a piston pump, you can use different heads and programming to change the speed of the rotating screw or the duration of its rotation, thereby changing the size of the droplets of material without changing the diameter of the nozzle.

Output printed circuit boards

The main task of each of the considered methods is to apply droplets of material of a precisely defined and constant size to the entire surface of the printed circuit board. A very small drop of glue in the manufacture of printed circuit boards will not allow you to fix the part on the board, and too much glue will spread and lead to poor soldering.