Wave Soldering Explained

Wave soldering was invented in the mid-50s, and it was the kickstart of the Electronics revolution that changed the world, enabling the mass production of PCBs (Printed Circuit Boards). Since its invention, relatively few changes have been made, and it’s an exciting technology, as you are about to discover.

What is Wave Soldering?

Wave Soldering is an automated process used for soldering through-hole components onto a PCB (Printed Circuit Board).

Despite the electronics industry’s shift towards SMT (Surface Mount Technology), wave soldering remains vital. Certain components, such as large capacitors, lack from SMT alternatives, keeping wave soldering relevant for tasks like attaching through-hole components to SMT boards.

Wave Soldering Process

The process is straightforward: once the components are mounted onto the PCB, the board is placed in the wave soldering machine. The components’ leads protrude from the lower side of the PCB, where the circuits are printed.

Wave soldering devices use a conveyor to transport the board through several stages and have tanks that house molten solder and a pump that elevates the solder toward the PCB’s lower side, creating molten solder waves, hence the term “wave soldering”.

When the PCB reaches the molten solder wave, heat, flux, and capillary action create precise and reliable solder joints adhering just the components’ leads to the printed circuit.

Parts of a Wave Soldering Machine

Over the years, the wave soldering process was refined, but the basics of the process remain the same. Waveforms changed and were refined and can be regulated, depending on the process’ needs. However, many parts are common to all wave soldering machines, regardless of the generation.

Let’s take a look at these parts and their functions.

• Conveyor System: This system is in charge of transporting the PCBs through the different stages of the wave soldering process. Depending on the machine’s design, the conveyor belt or chain is propelled by gears, rolls, or other mechanisms.
• Support Frames or Fixtures: These items hold the PCB in place as it moves through the different stages. They prevent movement or shifting of the board during transportation and soldering.
• Solder Tank and Heater: This container holds the solder, which is melted by a heating element. Then, the molten solder is kept at a stable temperature and ready to create the solder wave.
• Fluxing Section: Some basic and old models have a spot before the pre-heating zone for flux to be manually applied. However, these days, almost all wave soldering machines have a sprinkler that sprays the flux onto the PCB’s lower part and component leads, getting them ready for soldering.
• Pre-Heating Zone: This part heats the PCB before entering the solder wave, reducing thermal shock and enhancing solder adhesion. Some devices use resistors as heating elements, and others use infrared panels.
• Solder Pump: A mechanical or electromagnetic pump inside the solder tank pushes the molten solder upwards, creating a molten solder wave. The waveform, height, and size can be adjusted depending on the PCB layout and size of the components being soldered.
• Cooling Section: After the soldering area, there is an array of fans. Cool air reduces the PCB and components’ temperature to help the solder dry and prevent component damage due to excessive heat.
• Fume Extraction System: Wave soldering machines have fume extraction systems to extract the fumes produced during the soldering process, ensuring a safe working environment.
• Control Panel: Wave soldering machines have control panels to set parameters such as the conveyor speed, solder temperature, flux application rates, etc. Depending on the model and the machine’s complexity, control panels may be more complex and allow the regulation of items like the waveform, height, disturbance, and other settings.

How it Works

The wave soldering process is straightforward. The components are placed into the board, and a robotic arm or operator places the assembly into the support frame on the conveyor that transports it through the process’s different stages.

It’s important to mention that the conveyor has a slight inclination between 3 and 7 degrees to prevent the solder joints from sticking to the molten solder. This inclinations is called “desoldering angle”. Besides, variations in this angle regulate the size of the solder joints; small angles result in larger solder joints and vice versa.

1. Flux Application: This is the first stage of the PCB’s journey through the wave soldering machine. A nozzle sprays flux to the lower part of the PCB to remove oxide and debris and prepare the board and the components for soldering.
2. Preheating: The board then goes through the heating element. This area is often closed, resembling a tunnel for even heat distribution. Preheating activates flux, prevents thermal shocks that could lead to the PCB’s deformation, and removes moisture. It also prepares the board for proper solder adhesion.
3. Wave Soldering: This is the most crucial stage of the process. The board goes through molten solder that wets its lower part. Thanks to capillary action, solder effectively creates solid and reliable joints. Proper adjustment of the wave’s height, flow rate, and contact time (partially set by the desoldering angle) is critical to prevent soldering bridges and ensure the solder doesn’t stick to undesired sections of the board.
4. Cooling: This is the final stage of the process. Coolers located at the exit of the machine cool down the PCB.
5. Cleaning: It’s essential to clean the boards to remove debris, excess flux, and flux residues that could result in corrosion and degradation over time. Some wave soldering machines include an additional section to do this, while others require operators to do it manually.

Pros and Cons of Wave Soldering

Like every process, wave soldering has its pros and cons. Let’s see the pros and cons of this practical and fast soldering method.

Pros

• Speed and Efficiency: Wave soldering is great for medium-scale and large-production PCB assembly.
• Consistency: Wave soldering creates consistent and even joints, which look great and ensure that all boards have the same quality.
• Compatible with Lead-Free Solder: Wave soldering is compatible with lead-free solder. This enables companies that must meet the demanding RoHs regulations to manufacture electronic components using this technique.
• Cost Effective: Wave soldering reduces labor costs because solders have multiple connections simultaneously and tens of PCBs per hour. Besides, it consumes less solder than manual soldering.

 

Cons

• Not Ideal for SMT: Although techniques like masking and the improvement of the wave-control allow soldering of some SMD components, it’s not suitable for small or high pin density SMD components.
• Excessive Heat: Molten solder can be too hot for some through-hole and SMD components. That’s why some high-precision PCBs can’t be soldered with this technique.

Common Applications

Wave soldering is broadly used in several industries and is practical for many applications. Below, you will find some of the most common uses of this wonderful soldering technique.

• Through-Hole Components: Wave soldering is broadly used for soldering through-hole components onto PCBs.
• Mixed Technology PCBs: Some PCBs that combine through-hole and SMT technologies use wave soldering to connect the through-hole components to the board.
• Mass Production: Wave soldering is used for high-volume production lines, simultaneously enabling rapid and accurate soldering of multiple boards.
• Electronics Assembly: Wave soldering is widely used to assemble many electronic devices, such as consumer electronics, industrial and automotive electronics, and telecommunication devices that use through-hole components. It’s especially useful in power electronics, which require soldering large components like capacitors and resistors.