With the rise of hybrid or ‘mixed technology’ PCBs, (boards that combine surface-mount and through-hole components), wave soldering remains an important process in modern electronics manufacturing. While reflow soldering is the standard method for surface-mount components, wave soldering plays a key role in mixed-technology assemblies, especially for through-hole parts that demand robust, high-reliability joints. In industries like automotive, industrial controls and power systems, this technology is indispensable for producing durable connections that can withstand extreme conditions. As devices become more compact and performance-driven, wave soldering delivers the strength, scalability, and consistency needed for high-stakes applications.
Ready to see the impact of our wave soldering process firsthand? Request a tour of our production floor or contact us today to explore how it can take your projects to the next level.
What is SMT Wave Soldering?
Before we get into explaining SMT wave soldering, it’s essential to understand what Surface Mount Technology (SMT) is and how wave soldering fits into the broader world of PCB assembly.
What is Surface Mount Technology (SMT)?
Surface Mount Technology is a method for assembling electronic circuits in which components are mounted directly onto the surface of printed circuit boards (PCBs). Unlike traditional through-hole methods, SMT allows for more compact, lightweight and high-density PCB designs. It’s the go-to approach for modern electronics manufacturing – from smartphones to automotive Electronic Control Units (ECUs) – due to its speed, scalability and compatibility with automated production lines.
What is wave soldering?
Wave soldering is a soldering technique used in PCB assembly, especially when dealing with through-hole components or a combination of SMT and through-hole (known as mixed technology). During the wave soldering process, the PCB is passed over a wave of molten solder. The solder migrates to the exposed metal pads and component leads, forming solid and reliable connections.
Compared to hand soldering, wave soldering is more consistent and scalable. And while reflow soldering is dominant for SMT components, wave soldering remains essential for robust joints in power or large-format boards where reflow alone may fall short.
How SMT and Wave Soldering Work Together
In mixed-technology PCB designs, both surface-mount and through-hole components are often used on the same board, but they typically require different soldering methods. Surface-mount components are usually soldered using a reflow process, while wave soldering is reserved for through-hole parts.
In some rare cases, certain robust SMT components may be wave soldered if they’re placed on the underside of the board and can tolerate the heat and mechanical stress. However, this approach is limited and not commonly used due to component sensitivity and design constraints.
The more common and reliable method, and the one used at MPE, is to reflow solder the SMT components first, then wave solder the through-hole parts. This dual-process approach is especially effective in industries like automotive and industrial electronics, where durable and cost-effective mixed-technology assemblies are critical.
How does the SMT wave soldering process work?
Step 1: PCB Preparation
First, the PCB must be prepared. Adhesive is applied to the underside of where components will be placed. This adhesive is then heat cured, ensuring a secure connection between components and the board before soldering.
Step 2: Placing the Components
Next, the surface-mount components are positioned on the board using automated pick-and-place machines. In rare cases where wave soldering will be used for SMT parts, they must be placed on the bottom side of the PCB and must be capable of withstanding wave soldering temperatures.
Step 3: Preheating the PCB
The assembled PCB moves into a preheating section, where it’s gradually warmed up to a temperature that prepares it for soldering. This step ensures that the board and components won’t experience thermal shock, which could lead to damage or poor solder joints. This step also cures the component adhesive.
Step 4: Wave Soldering
The PCB is passed over a wave of molten solder generated by a pump. This solder bonds with the exposed leads of through-hole components and, in limited cases, the pads of specially selected surface-mount parts.
Step 5: Inspection and Quality Control
After cooling, the board is inspected for any defects such as cold solder joints (weak, uneven and unreliable connections) or solder bridges (unintended connection between pads). A mix of Automated Optical Inspection (AOI) systems, visual or X-ray inspections can be used to ensure high-quality, reliable results.
What are the advantages of SMT Wave Soldering?
Wave soldering offers several key benefits that make it a popular and effective process in modern electronics manufacturing, especially for boards combining through-hole and surface-mount components:
- Speed and Efficiency: Automated component placement and wave soldering ensures faster production rates, reducing overall assembly time.
- Consistency and Reliability: Provides highly uniform solder joints, minimising human error compared to hand soldering.
- Cost-effective: Reduces labour costs and material waste by automating the process.
- Versatility: Effective for mixed-technology PCBs, enabling reliable soldering of through-hole parts alongside SMT components soldered via reflow.
- Scalability: Ideal for mass production, supporting large volumes of components with minimal adjustments.
These advantages make wave soldering highly efficient for industries that require precision and high-volume output, such as automotive and medical device electronics.
What are the disadvantages of SMT Wave Soldering?
While wave soldering is efficient and widely used for through hole components in hybrid PCB assemblies, it also has limitations that manufacturers must consider when designing for quality and reliability:
- Component Limitations: Only certain surface-mount parts can withstand the thermal and mechanical stresses of wave soldering. The height of the components also has to be less than the depth of the wave.
- Shadowing Issues: Taller components can block the solder wave, leading to incomplete joints.
- Design Constraints: Requires careful PCB design layout to ensure compatibility with the solder wave.
- Less Ideal for Fine-Pitch Components: Not suitable for very small, densely packed SMT parts compared to reflow soldering.
MPE’s Expert Insight:
Paul Carter, Director of Sales at MPE explains: “Wave soldering is sometimes overlooked in discussions about modern assembly techniques, but it remains essential for through-hole components in mixed-technology PCBs, especially in sectors like automotive or power electronics. When you’re dealing with high current or harsh environments, wave soldering provides unmatched joint strength and long-term reliability.”
At MPE, we see SMT wave soldering thriving in sectors like automotive, industrial controls and power systems, where durability and high current handling are essential. A common misconception is that wave soldering is only used for through-hole parts. In reality, it’s often the most cost-effective, reliable solution for mixed-technology boards. At MPE Electronics, SMT components are always soldered using a reflow oven. We only use wave soldering for the through-hole components that follow. In mixed-technology boards, the SMT components are reflow-soldered first, then the through-hole parts are wave soldered in a dedicated process. Many sourcing managers and product designers are surprised to learn how efficient it is for high-volume production, especially when both surface-mount and through-hole components are needed on the same board.
What to Look for in a CEM Partner offering SMT Wave Soldering
Choosing the right Contract Electronics Manufacturer (CEM) for wave soldering isn’t just about capability. It’s about precision and reliability. Look for a partner that emphasises cleanliness and flux control, as both are critical to preventing defects like solder bridging or residue buildup. Effective preheat management also helps protect components from thermal stress.
Also essential, is to ensure that their equipment is regularly calibrated, and that they use advanced inspection tools like AOI or X-ray to detect hidden issues. Finally, confirm that they hold certifications like IPC-A-610 and ISO 9001 – clear indicators of quality standards and process discipline.
SMT Wave Soldering FAQs:
What’s the difference between wave soldering and reflow soldering?
Wave soldering involves passing a PCB over a wave of molten solder to connect through-hole or mixed-technology components. Reflow soldering, in contrast, uses solder paste applied to pads, then heats the entire board in a reflow oven to melt the solder and secure surface-mount components. Wave is ideal for large joints and through-hole parts, while reflow is preferred for fine-pitch SMT components due to its precision and thermal control.
Can all surface-mount components be used in SMT wave soldering?
Not all surface-mount components are suitable for SMT wave soldering. While many components can handle the heat and mechanical stress of the wave process, smaller or more delicate components might be better suited for reflow soldering. It’s important to choose components designed for wave soldering to ensure they don’t get damaged during the process.
How does SMT wave soldering compare to hand soldering in terms of efficiency?
In the rare cases that wave soldering is used to attach surface mount parts to the board, it is more efficient than hand soldering, especially in high-volume production. It automates the process, ensuring consistent, reliable solder joints at a faster rate than manual methods, while minimising the risk of human error. This makes it ideal for industries where scalability and quality are critical.
MPE Electronics is an established and experienced contract electronics manufacturer specialising in PCB assemblies and full box build assembly for a wide range of commercial and industrial businesses.
To find out how MPE Electronics’ PCB manufacturing and assembly services can benefit your business, contact our expert and friendly team on +44 (0)1825 764822 or enquiries@mpe-electronics.co.uk.
