A basic guide to PCB soldering methods

Soldering   |   7 min read

Soldering methods vary, but they all refer to the same result: connecting 2 metals using a different element, the solder. While there are different types of solder, it is usually a metal alloy such as lead or tin combined with brass or silver, with a low melting point. As the solder metal is heated with a soldering iron, it melts. Think of it as a “glue” that attaches different parts together.

When soldering takes place on a printed circuit board (PCB), it creates a printed circuit board assemby (PCBA). You probably have seen a PCBA within an electronic product – it’s made up of a board with copper wires and plates with electronic components attached to it.

PCBAs are an integral part of the electronics we use in our daily lives. They can be found in many consumer products, including electrical products, like aircons, refrigerators, and microwave ovens, and computing and communication devices such as computers, smartphones, and routers.

So, if your business manufactures any of the above and you are wondering what the PCB soldering method requires, here’s a layperson’s guide to help you understand more.

Types of PCB soldering methods

PCB soldering involves joining two or more different electrical components together on the surface of a circuit board. Soldering is integral to the printed circuit board design process – the only way to get your circuits to attach to your board is through soldering them.

Let’s look at the different PCB soldering methods in more detail:

• Hand soldering
• Reflow soldering
• Wave soldering

Hand soldering is done manually, whereas reflow and wave soldering are performed by SMT (surface mount technology) machines.

Hand soldering

Hand soldering is a process where humans or robots manually apply pressure using the pre-tinned soldering tip of a soldering iron, heating the parts and melting the solder.

In the industry, you might hear people refer to this technique as soft soldering. That’s because the temperature required for hand soldering is below 400 degrees. The solder alloys mainly contain lead with a melting point below 350 degrees. When applying heat to the fusible components, we try to use the lowest temperatures possible so that the metals will not melt. The end product is a ‘soft’ joint.

Hand soldering PCBs, that can be done using Thermaltronics soldering and rework tools, is usually the last step in the prototype assembly process and is used to finish details for through-hole and surface-mounted components. Through-hole technology refers to components inserted into printed circuit boards and soldered to pads on the opposite side.

Surface-mount technology (SMT) is when electrical components are mounted directly onto the surface of a PCB. The electrical part installed in this manner is known as a surface-mount device (SMD).

While hand soldering requires knowledge and practice to master, many engineers will tell you that this is the simplest method. However, it does have its pros and cons:

Pros

• You can develop a reliable electrical joint connection.
• For smaller components, hand soldering can reach them easily.

Cons

• Since we cannot use hand soldering for high temperatures, it cannot make strong joints.
• You should not use it in high load-bearing applications.

Reflow soldering

Reflow soldering is the most popular mechanical method of attaching surface mount components to PCBs. The process aims to form solder joints by preheating the parts and then melting the solder without creating damage by overheating.

We create a solder paste out of powdered solder and flux in reflow soldering. Then electrical components are temporarily attached to contact pads before soldering with solder paste.

Afterwards, we heat the entire assembly in a reflow soldering oven or under an infrared lamp to melt the solder and attach the joint. Here is a brief overview of the steps:

Step 1: Preheating
This supports assembly boards to reach the required temperature and complies with thermal profiling. Second, it eliminates volatile solvents in the solder paste.

Step 2: Thermal Soak
This is the phase where the temperature rises to the alloy molten point. The flux in the solder paste has to achieve the correct temperature level, or it fails to be activated, producing weak metallurgical bonds.

Step 3: Reflow Soldering
The reflow soldering process takes place at the highest temperature. The soldering paste goes through a eutectoid reaction, and the solder alloy becomes a liquid or molten metal. Temperature control plays a crucial role in the reflow soldering process. Overheating may cause damage to surface mount technology (SMT) components or boards.

Step 4: Cooling
Cooling means solder paste will solidify, and parts are fixed contact pads onto the PCB.

The reflow soldering technique is used in surface mount technology to solder surface mount components on a PCB. It is mostly used for assembling PCBs in smaller-scale manufacturing products that do not need fast, cheap mass production.

SMT machines (also known as pick-and-place machines or SMT component placement systems) is used for attaching surface mount components onto PCBs for assembly lines that employ this process.

Of course, there might be mistakes on the assembly line, so a repair solution should be in place. Engineers usually repair defective surface-mount components (such as faulty PCBs) with the use of soldering irons or SMD soldering stations utilising the hand/manual soldering method.

Pros

• Less thermal shock because components are not submerged directly.
• Solder paste is used as required, generating less waste.
• Cut down on production costs since fewer human resources are required.
• Easier for production to be scaled, great for low-volume manufacturing.

Cons

• If the temperature is not controlled, it can result in poor soldering.
• Not the most suitable technique for complex boards.
• Compared to the wave solder process, it can be more expensive.

Wave soldering

Wave soldering is another mechanical soldering method that allows mass manufacturing circuit boards in a concise amount of time. The machine creates “waves” of solder that join the components to the board. The PCB is then cooled, fixing the parts in place safely.

Here is a brief overview of the wave soldering process:

Step 1: Flux Spraying
Cleaning metal surfaces ensures soldering performance. Flux aids in smoother soldering.

Step 2: Preheating
Circuit boards travel through a heat tunnel for preheating and activating flux.

Step 3: Wave Soldering
As the temperature increases gradually, solder paste turns liquid with waves formed from edge boards above. Components are solidly bonded on boards.

Step 4: Cooling
After being cooled to room temperature, the machine will successfully assemble the board.

We recommend wave soldering for higher speed and efficiency of PCB assembly if you need to meet increased demand for electronic products. It is a faster process with both through-hole PCB assembly and surface mount technologies.

Pros

• Exposes components to heat for a shorter time. As a result, air exposure and oxidation are lesser.
• The process creates better quality joints.
• Suitable for high-volume production.

Cons

• Temperature inconsistencies in the process mean temperature must be well controlled.
• The process requires high environmental maintenance.

Is there a difference between wave soldering vs reflow soldering?

Many people do not know whether to select wave or reflow soldering when purchasing PCB assembly services, as the processes seem similar. However, choosing a suitable soldering method can affect manufacturing efficiency, cost, time to market, revenue, etc. so knowing the difference is essential.

Essentially, the difference between the 2 types of soldering methods lies in flux spraying. You might notice that wave soldering contains this step, but reflow soldering does not.

Flux aids dioxide elimination and surface tension reduction in the material to be soldered. This results in strong joints on the PCB. But flux requires rigorous temperature and time control adherence. Since flux is inherent in solder paste with reflow soldering, we need to manage temperature control so that flux is activated correctly.

Which mechanical soldering process do I use?

So, you might ask which type of soldering and when you should use it?

Overall, wave soldering is far more complex, and without careful monitoring and the right environment, it can lead to board defects.

Controlling the environment is not so important when using reflow soldering to create your PCB. However, wave soldering is faster and cheaper than reflow soldering.

Remember that you may use both reflow soldering and wave soldering. For example, you might decide to reflow solder parts on one side and then wave solder them on the other. It may depend on factors such as your time and budget, component orientations, type of PCB required, etc. Ultimately, it’s best to consult an expert before purchasing any machine you need for your assembly line.