Reflow Soldering vs Wave Soldering: Difference and Process

There are two mainstream PCB assembly soldering methods: reflow soldering (the standard for SMT) and wave soldering (primarily for through-hole). Reflow uses solder paste printed on pads and a multi-zone oven profile; wave soldering uses a molten solder wave in a dedicated machine to solder many through-hole joints at once.
In mixed-technology assemblies, manufacturers often combine both: reflow for SMT first, then wave (or selective wave) for remaining through-hole leads.

What is Reflow Soldering?

Reflow soldering joins components by melting solder paste deposited on PCB pads and allowing it to wet the metallized terminations during a controlled thermal profile (preheat/soak/reflow/cool). No mechanical 'pressure' is required.

The main difference between reflow soldering and wave soldering is the method used to apply heat to the parts being joined together. Wave soldering requires dedicated equipment (fluxer, preheater, solder pot with a pumped wave, and cooling to pick up each piece as they are placed into position on top of each other and then press down firmly until they are melted together.

Reflow is the most common method for attaching surface-mount components; through-hole leads are generally handled by wave or by selective wave in mixed assemblies.

Reflow Soldering Process

Therefore, Reflow soldering is a process where SMD components are assembled on a bare circuit board using solder paste. The components may or may not be mounted on their own pads, so the soldering process begins with assembling the component onto the board and then applying the solder paste to both components and their pads.

1. Pre-Heating

Of utmost importance in PCB manufacture is the preheating process. It has two major purposes during reflow soldering:

• The PCB assembly can now easily reach the desired and necessary temperature to ensure thermal profiling.
• Preheat pushes out any volatile solvents within the solder and helps to expel them. If you don't do this right, then it can affect your soldering quality.

Preheat not only reduces thermal shock but also drives off solvents in the paste to prepare for flux activation and even heating.

2. Thermal Soak

Soldering can depend on the type of solder flux that is in the solder paste. The solder must heat up significantly to activate this flux. If the solder doesn't activate the flux, then it won't be effective in soldering.

The soak stage activates the flux and helps equalize temperatures in shadowed regions, lowering risks of insufficient wetting or tombstoning later.

3. Reflow Soldering

The point of the peak temperature is to help the solder paste melt and reflow properly. Too low a temperature may prevent this action entirely, while too high a temperature can ruin a board or even damage components. Temperature control is key in all aspects of the process.

Without proper soldering, BGAs can end up with uneven ball-to-ball connections and a higher chance of coming undone during reflow soldering. This prevalence led manufacturers to focus on developing better reflow soldering processes. 

For BGA/QFN packages, monitor for voids or head-in-pillow defects and verify hidden joints with X-ray (AXI) or sample cross-sections.

4. Cooling

Once the temperature reaches the peak, the curve of the temperature will start to fall. Once it cools down, the solder paste and parts are permanently fixed onto their contact pads.

Controlled cooling helps stabilize the joint and limit excessive intermetallic growth, improving mechanical reliability.

What is Wave Soldering?

Wave soldering is a bulk soldering process primarily used for through-hole components: the bottom of the PCB passes over a pumped wave of molten solder that forms the joints in one pass.

While SMT parts are typically reflowed, manufacturers may still use selective wave or adhesive-bonded SMD over wave in special cases of mixed assembly.

Wave Soldering Process

A standard wave line consists of flux application → preheat → solder wave contact → cooling, with dedicated equipment for each stage.

Wave soldering is a process where the components are placed on the board, then a wave of molten solder flows over them.  Unlike reflow, wave soldering does not use solder paste; it relies on a molten solder bath to form joints during the brief 1–5 s contact with the wave. This is done by the use of a wave soldering machine in the above video. It's more expensive and less efficient than reflow soldering because the temperature during this process is not controlled as well as it needs to be in order for it to work properly. 

1. Flux Spraying

Choosing the right flux and deposition method (spray or foam) is crucial to remove oxides and lower surface tension before wave contact.

Soldering performance mainly depends on the cleanliness of the metal surface. It also depends on the functions of the solder flux. The use of a solder flux is key for seamless soldering operations. Some important abilities that solder flux has are:

• Oxides are a type of residue left on metal surfaces. Eliminating oxides reduces the drag coefficients between pins and boards and allows for more power transfer.
• You'll also find a shorter fabrication process that minimizes the slow oxidation of the circuit board during the thermal process.
• Withstanding a higher breakaway force.
• The heat is properly transmitted.

NOTE:  It's important that you use a compatible flux for your application because some solvents may damage components if they are not compatible with them. 

2. Pre-Heating

Preheat activates flux and reduces thermal shock to components and laminates.

PCBs travel through a heat tunnel in a pallet along a chain which is similar to a conveyor belt. The flux needs to be activated, and the PCBs need to be preheated before they are sealed.

3. Wave Soldering

As the temperature rises, solder paste melts, creating a wave of liquid solder that travels across the board before hardening. This wave bonds components to the board to give them a solid grip and create an insulated joint.

4. Cooling

In wave soldering, the specified temperature reaches its peak right before the process is complete. This gives us a flat slope along the top of the profile while also providing a cooling zone at the bottom. By keeping it in this cool environment afterward, we can successfully assemble the board and keep it from warping during production. 

Difference Between Wave Soldering & Reflow Soldering

There are many things to consider when it comes to soldering. Among them: are the pad shapes, the time you have available, component orientations, and PCB type. For these reasons, wave soldering is more complex than other soldering methods. Monitoring board temperature and time spent in the solder wave are critical if you want to avoid defects.

Therefore, Reflow soldering is a process that uses a temperature gradient to melt the solder paste and attach components to the board. This can be done using either wave or reflow soldering techniques, depending on how you want your finished product to look. The basic idea behind both methods is that you start off with a component placed on top of your board (or PCB), then heat up one side of it until it melts into place, allowing you to place more components underneath it until they're attached too!

1. Popularity Difference

Reflow soldering is a highly preferred method for surface mounting electronic components because it offers numerous advantages over the less popular wave-soldering process. As the years pass, the popularity of wave-soldering decreases for good reason, making reflow soldering the most commonly used option.

Reflow dominates SMT production, while wave (or selective wave) remains the workhorse for through-hole interconnects in mixed assemblies.

2. Efficiency Difference

Wave soldering is a continuous, high-throughput process well-suited to large volumes of through-hole joints; reflow throughput is constrained by print-place-reflow sequencing. This means that you'll spend less time on assembly and more time on your project.

Wave Soldering is cheaper than Reflow Soldering.

3. Time Management

The reflow process takes place over a period of time while there isn't just one wave of molten flux in reflow soldering like there is in the wave soldering process.

In mixed assemblies, manufacturers typically reflow SMT first and then wave the remaining through-hole leads to prevent remelting SMT joints.

In the wave process, the metal being soldered is heated to its melting point and then quickly cooled by passing through a nozzle where it's ejected at high velocity onto the workpiece. This causes damage to both sides of your joint because you're heating them up and then cooling them down quickly again—and this can lead to unevenness or other problems with your board's finish. It also doesn't allow for much control over how fast or slow you need to do this; if you're not careful, your board could end up looking like it was sprayed with hair spray instead.

4. Cost Difference: Wave Soldering is Cheaper than Reflow

Wave soldering is cheaper than reflow soldering when it comes to fabricating printed circuit boards: In addition to being faster, this method also results in less material waste because there's no need to discard unused solder paste after each step of the process. In fact, many companies will pay extra money if they want their engineers to use this type of solder since it means they'll save money over time if they don't have any leftovers lying around at any given moment during construction (which could lead to delays).

Although there are more variables to control, it's considerably more cost-effective. Wave soldering also tends to be faster and cheaper than reflow soldering; however, this method is typically only applied in small-scale manufacturing processes as it can't compete with rapid, cheap production speeds.

5. Soldering Reliability

Soldering defects in PCB can be inevitable as long as soldering takes place. It's not reasonable to assume that one soldering technology has more soldering defects than another, even though the conclusion is based on piles of experimental data. After all, every situation is different. Therefore, it doesn't matter to compare solder reliability between wave soldering and reflow soldering.

Even though there is always a chance of mistakes occurring, the risk of errors can be reduced when assemblers follow professional manufacturing regulations and get fully aware of the characteristics and performance of the equipment. In addition, qualified engineering staff should be trained and updated in order to keep up with advancements in modern technology.

Typical reflow risks: tombstoning, voids, head-in-pillow under BGAs; typical wave risks: bridging, solder skips, icicles—robust profiles and AOI/AXI inspection help mitigate both.

6. Wave Soldering is Less Time Consuming

Wave can be less time-consuming for boards with many through-hole joints processed in one pass; for dense SMT designs, reflow remains the most efficient and controllable route. However, wave soldering still requires special equipment such as irons which cost significantly more than traditional ironing tools like flat irons or steamers; these products typically cost around $20-$50 depending on what model you buy (a general example)!

7. Production Process

Wave solder is a great way to mass-produce printed circuit boards. We'll help by manufacturing the boards at a rapid rate and in small batches.

Use reflow for SMT (single- or double-sided), apply wave or selective wave for remaining through-hole; consider selective wave where keep-out areas or thermal constraints make full-wave impractical.

Most of the time, we use solder screen printing, but when we need to produce a small number of PCBs, we'll use "reflow soldering." We only use this technique when we have a little bit of extra time.

Why NextPCB?

Need to solder your PCBs? Look no further than NextPCB.

For hidden joints (e.g., BGAs), we perform sample X-ray inspection (AXI) in addition to AOI to validate wetting and void levels.

For mixed assemblies with tight keep-out zones, we support selective wave soldering to control heat input and avoid reflowed SMT areas.

NextPCB is the best solution you'll find for smart devices. We have a large production capacity to perform both wave soldering and reflow soldering. In addition, we have a large manufacturing setup that can be used to perform any type of soldering technique in bulk. 

Conclusion

Quick guide:

Use reflow for SMT; use wave/selective wave for through-hole; in mixed builds, reflow first, then wave.

Choose based on part geometry, density, throughput, and inspection needs.