What is PCB Wave Soldering? A Complete Guide

How well do you understand PCB wave soldering? The wave method is an older process but one of the most popular. It has various benefits, which explain its continued use today.

That said, it also has a few downsides. These drawbacks sometimes limit its application, such as when assembling surface mount components. Here, we explore how it works, its good and bad sides, and more.

What is PCB Wave Soldering?

Wave soldering in PCB assembly is an industrial process that attaches components to printed circuit boards. It typically involves passing the PCB in a solder bath. The solder surges upward to contact the board’s underside, resembling a wave and giving the method its name.

Wave soldering is one of the most used PCB technologies today. It’s also one of the most effective–and straightforward–especially when mounting through-hole components. Like many circuit board procedures, it’s highly automated. Let’s see how it works below.

How Does PCB Wave Soldering Work?

Components are mounted by inserting their legs (or pins) in the assembly holes on the PCB. The assembler first applies solder flux to the board’s underside. The flux cleans the surfaces to be soldered, ridding them of oxidation.

The board then goes through a heating stage before passing over a solder bath. Here, the solder attaches to the leads and pads. Upon cooling, it hardens and firmly holds the assembled parts to the board.

Everything takes place in an automated machine. This machine comprises the following parts: a solder bath, a wave generator, a conveyor system, heaters, and flux applicators.

Difference Between Reflow and Wave Soldering

The two technologies differ in a few ways. As a result, they suit varying circuit board assembly needs, such as component type and lead time. Let’s see how and when to use each:

• The wave process requires through holes for component leads and pins, while reflow utilizes pads located on circuit board traces. This difference makes it best for thru-hole parts and reflow S
  
• In a reflow process, the assembler first holds components on the PCB with solder paste (solder + flux). The other method requires no paste.
  
• The solder wave process is also a quicker technology than reflow and comparably cheaper, given its ability to scale production and lower assembly costs.
  

Wave technology is preferred when the board has more THT parts than SMDs. In most cases, the assembler combines the two techniques, beginning with the wave before assembling SMT parts in a reflow process.

PCB Wave Soldering Process

Wave PCB soldering is a simple procedure with only a few steps: initial preparations, applying flux, and the preheating stage. There’s also the solder application step and cooling down before sending the PCBAs for inspections.

Step 1: Flux Application

• The soldering machine coats the PCB bottom side with controlled amounts of flux
  
• The flux cleans the pads and leads, removing oxides and prepping them for soldering
  
• The flux is either corrosive or non-corrosive. There are also two application methods: foaming and spraying
  
• Foaming is less costly and easier to use but more susceptible to evaporation and decomposition
  
• Spraying uses a complex and expensive system but reduces evaporation and flux degradation
  

Step 2: Preheating

• The board now goes to the preheating stage
  
• Here, it gets warmed to between 90°C and 125°C
  
• The temperature range depends on whether the board is single-sided, double-sided, or multilayered
  
• The heat activates the flux and warms the board to prevent thermal shock during soldering
  
• Heating technologies include infrared and hot air or convection
  
• Infrared red is mainly used on high-density boards to ensure uniform warming
  
• In convection heating, the machine blows hot air to warm the board
  

Step 3: Soldering

• Soldering can now begin
  
• A mechanical or electromagnetic pump creates a high-pressure flow of molten solder
  
• This produces a wave that hits the PCB as it moves on a conveyor
  
• The solder adheres to the leads and pads that flux had cleaned earlier
  
• There are typically two wave types: laminar and turbulent
  
• A laminar wave is smooth and the most used
  
• The turbulent type is rougher. It suits closely packed parts or pins
  

Step 4: Cooling and Inspection

• The board is taken to a cooling stage for the solder to harden and firmly attach the components
  
• The cooling is gradual to prevent warping or other damage that may weaken the joints or ruin the board
  
• After cooling, workers inspect the boards for defects. The inspection methods include manual and automated visual inspection (AOI)
  

PCB Wave Soldering Advantages and Disadvantages

The solder wave process offers both advantages and disadvantages. As such, it favors some cases better than others. Here is a summary of the benefits and limitations.

Advantages

• It’s less complex compared to most other methods.
  
• It’s also fast, making it high-production runs
  
• It creates sturdy joints, making it crucial for bulky parts
  
• It’s one of the most cost-effective PCB assembly methods, especially for higher volumes
  

Disadvantages

• Not suitable for surface mount (SMT) compared to other methods
  
• It wastes solder and flux compared to SMT, selective, and other soldering methods
  

When to Use Wave Soldering in PCB Assembly

The process suits some circuit boards (PCBs) and components better than others. Sometimes, we use it alongside other methods like reflow. The following are example applications that fit it.

• Single-sided boards with thru-hole components on one side and SMDs on the other
  
• It’s also preferred if the SMT components are few or scattered, and the assembler can weld them individually
  
• If most parts are THT and the SMT components are large-sized and discreet, allowing a combination of assembly methods
  

Best Practices for Wave Soldering

Like any other PCB assembly technique, the wave method requires a careful approach. There’s also the need for the designer and assembler to collaborate. That’s because incorrect component placement can cause assembly problems. The following tips can help optimize the process.

1. Do not place SMT parts too close to through holes. It makes the welding process less effective and weakens the joints. Avoid placing small components behind larger ones, too.

2. Space components appropriately. It will help get the solder to the right places and prevent many part mounting problems. For instance, aligning IC chips with the wave direction ensures all pins receive solder.

3. To reduce costs, use a machine with a solder bath of the correct size based on the expected PCB sizes. Smaller pots hold about 1000 lbs, while larger ones can reach 2000 lbs or more.

4. Use high-quality flux types to achieve better wetting. Low-quality flux reduces adherence, leading to unreliable solder joints.

5. Use the correction combination of preheating temperature and period to avoid defects. Higher levels cause shorts, while lower levels lead to skips.

6. Ensure the correct wave height and duration by correctly calibrating the soldering machine. Avoid the extremes of either side.

Conclusion

PCB wave soldering is essential when mounting through-hole components and sometimes surface mount types. It remains popular, even with most PCBs using surface mount components. On the brighter side, it supports mass assembly and requires less complex equipment, among other benefits. However, as discussed in this guide, it’s less helpful when assembling SMT components.