Types of solder pastes. Why do we need different sizes of balls? Which flux to choose in the paste?

What do we know about solder pastes?

• Solder paste alloys are different. Lead-free, lead-free, low temperature, high temperature, various exotic alloys.
• Solder pastes may have different flux types. According to their classification, there are 18 and, depending on the requirements of production, on the speed of production, from many other technical processes, the desired type of flux is selected. The types of fluxes are standardized, but they differ from manufacturer to manufacturer. Practice ultimately determines the choice.

• Solder pastes also have a characteristic such as viscosity. The most common values are 1600 and 2000. The higher the value, the higher the viscosity of the solder paste and it can retain its shape on the PCB longer, but the thinner solder paste penetrates better into the holes for the subsequent installation of DIP components.

Now I want to talk about the types of solder pastes. You can see the most common types below:

• 3 type with 25-45 micron diameter balls.
• 4 type with 20-40 micron diameter balls.
• 5 type with 15-25 micron diameter balls.
• 6 type with 5-15 microns diameter balls.

The smaller the size of the components to be soldered, the smaller the size of the balls in the solder paste should be. This is very important for the application of solder paste for 0402 and 0201 components and microcircuits with a pin spacing of 0.4mm or less.

The smaller the aperture (hole) in the stencil, the more difficult it is for large balls to get through such a stencil onto the printed circuit board. Therefore, you need to choose the right type of solder paste with the right ball size for your specific PCB. What's the easiest way to do this?

You can use the 5-ball rule. The bottom line is that you need to choose the smallest aperture on the stencil in width and 5 balls of solder paste should fit in its width. This rule must be monitored through a microscope, which is not always convenient.

If more than 5 balls enter the width of the aperture, then this is always good. If 5 or fewer balls enter the aperture, then the solder paste may start to get stuck in the aperture, the print of the solder paste will be bad.

Apertures for microcircuits with a fine pitch should be ~ 15% smaller than the contact area.

There is another simple approach - depending on the smallest aperture width on your PCB, we select the type of solder paste. You can clarify this moment with the developer or in the files of the printed circuit board.

• 3 type of solder paste - minimum aperture width 0.19 - 0.24 mm.
• 4 type of solder paste - minimum aperture width 0.14 - 0.19 mm.
• 5 type of solder paste - minimum aperture width 0.14 mm.
• 6 type of solder paste - minimum aperture width 0.1 mm.

Since the dimensions of the balls in the solder paste are not ideal, there is a tolerance.

This information will be useful if you ordered the correct polished stencil, which excludes solder paste sticking due to poor quality apertures and poor surface treatment. It is also assumed that the solder paste has not passed the expiration date, has been properly stored and properly prepared for work.

I hope this article was useful to you!

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