This article is about wave soldering vs. reflow soldering.
Hand soldering is ideal for repairs and assembling boards during prototyping. But at the commercial level, you should rely on wave or reflow soldering methods.
Both processes use machines to enable automatic soldering during PCB assembly, but they couldn’t be more different.
So we’ll compare wave soldering vs. reflow soldering to determine the best application for each process. Read on to learn more!
What Is Wave Soldering?
Wave soldering is a PCB component welding method that uses a liquid wave to sip into the joints and solidify.
The liquid solder is usually dissolved Tin (50%) with Lead (49.5%) and Antimony (0.5%).
But lead-free solder is safer for the environment, so most assemblers use tin-copper-nickel alloys.
Motor/pump agitation creates waves of molten metal to contact the circuit board.
But manufacturers/assemblers must ensure the metal does not flow to the top side.
So the conveyor belts moving the boards must hold them firmly using metal fingers.
And they must adjust the wave height to not splash above the board.
The wave-PCB contact time is usually 2-4 seconds to prevent excessive heating.
Manufacturers can calibrate this period using the conveyor speed and wave height.
Wave Soldering Process
This process comprises four steps.
Flux Spraying
Flux plays a crucial role in soldering because it has the following functions.
- Removing the oxidized barrier on the component pin and board metal surfaces
- Cutting off secondary oxidation from the board during the thermal process
- Minimizing solder paste surface tension
- Improving heat transmission
So this stage ensures you get high joint quality.
Soldering flux
Preheating
Preheating activates the flux and prevents thermal shock.
Wave Soldering
Soldering occurs in an open-top wave soldering bath in the wave soldering machine.
A conveyor moves the boards above this bath with components on the upper side.
A heated element heats the solder to its melting point then an agitator forms waves of the molten metal.
So the board’s underside encounters molten solder waves. It does not contact the pool of liquid metal throughout.
A vector image of a wave solder machine
On contact, the waves melt all exposed metal parts (pins and pads), enabling them to bond with the solder.
The process creates sturdy mechanical and electrical connections between the two.
Manufacturers can adjust this process to accommodate different flux types, lead/lead-free solder, or circuit board sizes.
Cooling
Once the metal is in place, the final step is to cool it to form a sturdy mechanical joint.
After that, the boards go through cleaning stations to remove the excess flux. Inspection and packaging are the last steps.
Advantages of Wave Soldering
Compared to other soldering techniques, wave soldering has the following advantages.
Unparalleled Efficiency
Wave soldering is considerably more efficient than hand or reflow soldering because it takes a short time.
After placing the electronic components on the board, you can heat the solder and wave it quickly under the board.
So the process is ideal for mass production because you can produce hundreds of boards per day in one machine.
Soldered component pins
And you can produce these many boards reliably because the process has minimal chances for mistakes or inconsistencies.
Cheaper
In addition to being efficient, wave soldering is cheaper than the other methods. The process does not need stencils.
Also, wave soldering machines are relatively cheaper than reflow ovens and easy to use.
Induces Less Warpage
Since flux and solder get applied on one side, the PCB is less likely to experience warpage.
So the circuit board will spend less time during post-processing, making wave soldering even more efficient.
Creates Sturdy Solder Joints
As stated earlier, wave soldering creates sturdy mechanical joints almost effortlessly.
So the electrical connections will not corrode or break easily over time.
Disadvantages of Wave Soldering
The primary disadvantage of wave soldering is it is not ideal for short component pitches.
So you cannot use the process to solder components with leads that are close to each other.
Application of Wave Soldering
Wave soldering is best for through-hole technology assembly because it is not ideal for parts with short pitches.
Through-hole components are usually sizable with long pin pitches.
A soldered resistor (through-hole component)
What Is Reflow Soldering?
Although not as efficient as wave soldering, reflow soldering is the most prevalent in the electronics industry.
Consumers globally want small electronic products, so manufacturers have to make compact boards to run these devices.
And the best way to achieve that is to use surface mount technology.
You cannot solder surface mount components using wave soldering because the pins are too close to each other.
So reflow soldering is the best option.
Instead of using molten solder, this process uses solder paste applied via stencils to the contact pads on circuit boards.
After that, a pick-and-place machine temporarily attaches the components to the solder pads.
A circuit board moving into a reflow oven
The temporarily assembled board then goes into a reflow soldering machine.
This reflow oven melts the solder to create molten metal that will attach the component to the pad.
Reflow Soldering Process
The reflow soldering technique also has four steps.
Preheating
After applying the solder paste and placing the components, you must preheat the board for two reasons.
- To expel volatile solvents from the solder paste
- Allowing assembled boards to attain the temperature compatible with thermal profiling consistently
Thermal Soak
Solder paste contains flux that you must activate first before soldering.
So thermal soaking places the board in an environment where the temperature is high enough for flux activation.
Reflow Soldering
After thermal soaking, the reflow soldering oven increases the temperature to the peak level to melt the solder.
It is critical to regulate the temperature at this stage. If too low, the paste will not reflow sufficiently.
And if too high, the heat will damage the components.
An infrared reflow oven
Cooling
After sufficiently reflowing and melting evenly, the board must undergo cooling to solidify the metal and strengthen the joints.
Advantages of Reflow Soldering
- Reduced thermal shock
- Less wasteful (precise soldering)
- Avoids bridging defects
- Easy to control and monitor
- Trusted by most PCB assemblers
- Ideal when you have limited soldering options
Disadvantages of Reflow Soldering
- Not as quick as wave soldering
- Costly machines
- Higher chances of board warpage
- Limited joint mechanical strength
Application of Reflow Soldering
The reflow technique introduces precision into the soldering process. So it is ideal for surface mount technology soldering.
Surface-mount components on a PCB
Wave Soldering vs. Reflow Soldering: Process
With wave soldering, a wave crest attaches the components to the board, hence the name.
The process is not as precise and can be wasteful.
On the other hand, reflow soldering relies on hot air or vapor to heat the already applied solder paste.
These heating elements come from either infrared convection or vapor phase ovens.
Wave Soldering vs. Reflow Soldering: Soldering Environment
Although wave soldering is a more efficient process in terms of time, it requires careful monitoring of the environmental conditions.
For instance, you must manage the preheating process carefully to prevent board damage.
Also, the molten solder must attain the required temperature to attach the components without defects or imperfections.
Circuit boards with through-hole components heading into a wave solder machine
But reflow soldering does not require a highly controlled operating environment.
The oven only needs to heat the board to different temperatures to prep, thermally soak, and solder the board.
These conditions are easier to manage.
Wave Soldering vs. Reflow Soldering: Complexity
The wave soldering technique is more complex because it requires careful monitoring of the solder wave.
So you should have an experienced technician to handle the process.
But with reflow soldering, the process is simple because you don’t have to keep tabs on multiple steps.
Wave Soldering vs. Reflow Soldering: Speed
Wave soldering occurs faster than reflow soldering because the steps are almost standard for all boards.
And the PCB spends less time in the wave soldering oven.
This quick assembly makes the process ideal for mass production.
But with reflow soldering, you must handle the solder paste application using a stencil, which is unique for each board.
After that, the board spends more time in the reflow oven.
A laser-cut stainless steel solder paste stencil
Wave Soldering vs. Reflow Soldering: Cost
In addition to being faster, wave soldering is cheaper than reflow soldering.
It does not require stencil production before the solder paste application.
And wave soldering equipment is more affordable than reflow ovens.
Wave Soldering vs. Reflow Soldering: Applications
Wave soldering is ideal for welding through-hole components on circuit boards, while reflow works best for surface mount components.
Wrap Up
As you can see, wave and reflow soldering differ immediately from the start.
The latter requires more work and is more expensive because it needs solder paste stencils for each unique board design.
And the reflow ovens are more expensive than their wave soldering counterparts. The welding steps are also different.
That’s it for this article. We hope it clarifies the difference between the two component assembly solutions.