What Is Solder Paste and What Is It Used For?

Jun 30, 2023

Solder paste is a very useful substance for soldering electronics and is fairly easy to use. Here's everything you need to know.

Solder paste is among the list of few essentials required to solder electronic components onto a printed circuit board. But what exactly is solder paste, what is it made up of, and how do you use it to solder components? Let's dive in to find out.

Solder paste is a mixture of powdered solder and solder flux. The solder is essentially a metal alloy made of two or more metals, whereas the solder flux is a cream-like material comprising different chemicals. For the longest time, companies made solders out of lead and tin. But due to its adverse impact on the environment and health, we're beginning to see more and more lead-free solder alloys nowadays.

Solder paste is primarily used in SMT (surface-mount technology) soldering. SMT soldering is the process of creating a printed circuit board out of SMD components—like resistors or capacitors, used for storing electrical energy—or mounting electronic components onto the pads on existing circuit boards.

Using solder paste in SMT soldering has several advantages:

We can classify solder pastes into different types based on the size of the solder powder and the type of flux used. By size, solder pastes can broadly be divided into eight types, ranging from Type 1 to Type 8. (The lower the type number, the larger the particle size within the paste.) For SMT mounting, Type 3, Type 4, and Type 5 remain the most widely used solder paste types.

Based on the type of flux used, solder pastes can be split into three categories:

Solder paste requires heat, just like regular solder. When you put solder paste onto a PCB and apply heat, the heat melts the paste into solder powder and flux.

While the solder powder creates a bond between metal surfaces, the flux does a few different things. First, it gives the solder powder flowing properties to make its application easy. Second, it acts as a temporary adhesive to hold the SMDs in place as you solder them, and third, it removes impurities and oxidation from electronic components for a better connection between the surfaces.

Using solder paste is pretty easy. But before starting, make sure the circuit board and the components you want to solder are clean. Likewise, make sure your solder paste is at room temperature. Assuming you've got the PCB and the components you want to install on it handy, here's a quick overview of how to use solder paste.

First, apply the solder paste to the area where you want to mount the components. You can do this using a syringe or stencil. In the case of the latter, you'll need to use a squeegee to even out the paste on the mounting area.

After this, place the components on the circuit board. Be gentle and extra careful when you do this, and ensure the components are aligned properly.

Finally, you need to apply heat to melt the solder powder in the paste, so it can create a bond between the components and the board. Ideally, this is done by passing the circuit board through an infrared reflow oven. But for most people, an economical way is to instead use a heat gun. Alternatively, you can use a soldering iron to heat the paste, although it's a little tricky to get it right at the first attempt.

If you're a budding engineer or hobbyist who wants to create PCBs or solder some components onto existing boards, knowing how to use solder paste can prove a major boon.

Now you understand how to apply solder paste, you can pick one according to your requirements and use it to solder components onto PCBs. If you're new to soldering, you can learn more about how to do it effectively.

Yash is a Staff Writer who covers Android, iOS, and Mac at MUO. He found his passion for writing while he used to develop apps for the web. In 2018, he decided to combine his experience in technology with his love for gadgets and venture into journalism. Since then, he has written hundreds of features, how-tos, and explainers across multiple online publications to help users get more out of their technology. You can also find his writings on TechPP, where he reviews consumer-tech products to help people make the right purchase decisions.