Are you a fan of soldering electronic components on your circuit board? Then you probably know how to use a soldering iron, a flux, and solder wire.
But did you know that you can make the entire process even simpler by using a solder paste?
The paste is the main component in the reflow soldering process. Unlike the typical hand soldering, in the reflow process, the components are placed in the printed circuit board and then heated in a special oven.
Alternatively, you can attach the components by using the solder paste and an iron. By using the paste, you can eliminate the use of a solder wire entirely. This is because the paste has solder suspended in the flux itself in the form of tiny spheres.
In this article, we will discuss how to use solder paste with soldering iron. By using this technique, you won’t have to use a separate solder wire in your circuit soldering projects!
What Is A Solder Paste?
Solder paste is a substance that is used to solder surface mount electronics to the printed circuit boards. It is a viscous semi-solid that has a paste-like texture, hence the name.
The grey-colored paste is a rich mixture of flux and metal solder powder. These are the elements that are required to bond separate metal pieces successfully. The paste may consist of different alloy types such as gold or silver.
The temperature at which the solder melts can be different. They can vary in hardness and are designated as ‘hard’, ‘medium’, and ‘easy’ solder pastes. It is used in most surface-mount technology assembly, as it is a great medium for soldering the component to the PCB.
Solder Paste Vs Solder Flux- A Comparison
Solder flux is a paste that is applied on metal surfaces prior to soldering. It helps prevent the metal surfaces from oxidizing, which may occur due to high temperatures during the soldering process.
The problem with metal surfaces is that it is prone to oxidation. This affects elements such as aluminum, copper, silver, and almost every element that is used in electronics.
Due to the formation of oxides on metal surfaces, soldering becomes difficult and in some cases, impossible. This is because they prevent the metallurgical bond between the metal surface and the substance itself.
The rate of oxidation increases with higher temperatures. A flux effectively cleans the metal surface by removing any oxidation layer and leaves the surface primed. This allows for good bonding between the metal and the solder. There are multiple types of flux, each having a use of its own.
The paste is basically a mixture of tiny solder spheres and a suitable flux. It is used and designed for reflow operations. It is an all in one solution where you don’t have to use a separate flux to bond components with the PCB.
It is a soldering process where solder paste is used in order to stick thousands of small electrical components on the PCB contact pads. The entire board along with the components is then subjected to heat.
The paste in its molten form permanently attaches the components. The assembly is usually heated in a reflow oven where it firms the joints. A flux, on the other hand, requires separate solder in the form of a wire. Unlike the paste, a flux is meant for hand-soldering purposes using a soldering iron.
How To Use Solder Paste With Soldering Iron- The Steps
The most common way of attaching tiny electrical components on PCB is by using an oven instead of a soldering iron in a process known as reflow soldering. However, you can use a simple iron with a good quality paste to achieve the same results!
Here’s how you can do it:
1. Gather The Tools
The two essential tools for the job are soldering iron and paste. An iron with a built-in temperature controller is preferable. Also, make sure that it is easy to grip. Since the components are extremely small, handle them with a tweezer. This will help you to accurately position each component.
2. Apply The Solder Paste
After you have your tools ready, the next step is to apply the solder paste. Put some of the paste on the tip of the iron, and apply it on the pads of the circuit board. Then pick up the component using a tweezer and place it on top of the applied paste. You can also apply flux on the board prior to applying the solder paste.
3. Heat The Solder
Hold the part on top of the pads using the tweezer and heat the pad using the soldering iron. Apply the heat on the joints in a circular motion. The iron will instantly melt the paste. The flux in the paste will burn off eventually. As the joints are heated, you will notice that the substance will start to ball up.
You have to make sure that the heat completely melts the solder and the component contacts. This will cause it to fuse together.
4. Allow Some Cooling Time
After the component contact and the substance has been melted, allow them to cool off. The solder will solidify and will form a firm bond. After that, clean off the circuit board and remove any bits of residual solder paste or flux. You can get rid of the residue by using a small brush and a bit of isopropyl alcohol.
If you are not happy with the way the joints are done, you can simply re-clean the board and repeat the process.
Tips And Caution When Using Solder Paste
1. Store The Paste Properly
Solder paste requires specific conditions to be stored properly. It needs to be kept in a refrigeration unit at temperatures around 0-10 degrees Celsius. It should never be stored at room temperature for prolonged periods of time.
As the paste has a low shelf life, exposure to temperatures exceeding 29 degrees Celsius can decrease the performance of the paste. This will shorten its shelf life. Also if you are thinking of storing the paste for a long time, close the lid properly and put it in a zip lock bag. This way, air will not get in and dry out the paste.
2. Prepare The Paste Before Applying
To get the best performance from the paste, keep the paste at room temperature for 3 or 4 hours prior to the application. Mix the paste properly using a small spatula for about 30 to 60 seconds. This will ensure the paste homogeneity.
3. Apply The Right Amount
You need to be careful when applying the paste. For a good bond, the right amount of the paste needs to be applied. Too much of the paste on the contact pads can cause short-circuit, whereas less amount of the substance may result in poor electrical conductivity.
4. Clean The Surface Before Applying The Paste
Make sure to clean the metal surface from any dirt or greasy fingerprints. An unclean surface might interfere with the soldering process. This can make the process difficult and ineffective. A clean surface will allow the solder to flow properly through the joints.
5. Unclog Blocked Solder Paste Syringes Using Warm Water
If you have a blocked solder paste syringe, you can easily unclog it with warm water. Simply soak the syringe in a bowl of warm water and loosen the dry paste. If that doesn’t work, remove the dry paste using a thin wire or a needle.
Classification Of Solder Pastes
Solder pastes can be classified according to the type of flux. The three main flux types are:
1. Rosin Based Pastes
As the name suggests, rosin-based pastes contain rosin in it. It is an extract that occurs naturally in pine trees. The fluxes in the rosin-based paste can be easily cleaned with a solvent when the soldering has been completed.
2. Water Soluble Flux Pastes
These fluxes are manufactured using organic materials along with glycol bases. Water-soluble fluxes usually have a range of cleaning agents. They have excellent reflow characteristics and provides superior wetting.
3. No Clean Pastes
This variant of solder paste is efficient. The residue formed after the reflow process doesn’t need to be washed off from the electrical board. This makes the assembly process simple and also enhances productivity.
What Type Of Solder Paste Should I Buy?
If you want to do some surface mount soldering or SMD, then you need to know what type of solder paste you need to buy. In general, there are three types of pastes. They are:
The Type 3 variant is the most commonly used solder paste. It is also the cheapest among the three. The solder ball size of type 3 paste ranges from 25-45 micrometers. It is considered to be an industry standard for most printing applications.
The sphere size of the type 4 variant is between 20-38 micrometers. This variant has 20% more surface area and higher transfer efficiency than Type 3 pastes. In terms of performance, the variant performs slightly better than type 3 pastes.
It has a better rate of reaction between the solder powder and the flux. Due to this reason, Type 4 pastes have a significantly lower shelf life compared to type 3 pastes.
Type 5 solder paste has a sphere size ranging 15-25 micrometers. It is the smallest of the three and has 75% more surface area than its Type 3 variant. Type 5 is better for finer pitched components. It is also much easier to work with compared to the other variants as it gives better printability.
One of the problems associated with using a solder wire is that it can release toxic fumes when melted. This can be hazardous. But if you know how to use solder paste with soldering iron, this problem can be eliminated.
Using a paste instead of melting a wire is much easier. Most pastes come in syringes that allows you to accurately apply the substance on the contact pads. This gives you much more control over the amount of the substance that will be applied.
You don’t have to own a specialized oven to attach the electrical components on the PCB. Using an iron will help you achieve the same results!
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