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What is Wave Soldering? A Complete Guide to the Wave Soldering Process for PCBA

This article was originally published in December 2022 and last updated on June 12, 2026.

For a lot of electronics manufacturers soldering through-hole components is a major issue. SMT assembly methods are currently the main focus of the industry. But parts soldered through the holes, like connectors and big capacitors, are still frequently used because of their mechanical strength and reliability. So the question is: how do you solder hundreds or thousands of through-hole joints on one board and still keep the speed and quality unchanged? The answer is Wave Soldering.
 
As a PCBA manufacturer, FS Technology realizes that fine assembly and reliable soldering techniques are very essential for making trustworthy electronic devices. They use both the SMT and THT processes to cater to the changing demands about components. Both processes include the use of wave and reflow soldering methods.
 
This article is centered on the wave soldering process, which is a key thing of THT assembly. What is wave soldering, its operation, advantages and disadvantages, wave soldering vs reflow soldering are discussed here. Selective wave soldering, flux application, equipment, and common defects are also included.
 
You can also have a quick technical overview of wave soldering by watching the YouTube video below.

What is Wave Soldering?

Wave soldering is a very fast, automated soldering process mainly used to attach through-hole parts onto a printed circuit board(PCB). During this method, the PCB is brought over a moving solder wave – it is a continuous molten solder wave standing upward inside the wave soldering machine. The bottom of the board touches the solder wave, so that solder will flow to exposed metal leads and pads, resulting in the formation of dependable electrical and mechanical connections within seconds.
 
Manual soldering, by contrast, depends on the operator’s ability and the joints are made one at a time. Wave soldering can handle hundreds or thousands of connections at the same time. It is the preferred technique for manufacturing through-hole boards in large quantities, like power supplies, industrial control boards, automotive electronics, and consumer appliances, which mainly use large connectors, transformers or electrolytic capacitors, among other components.
 
Briefly, wave soldering offers speed, consistency, and repeatability – these are the characteristics that hand soldering will not have even at large volumes. And, as it is fully automated, the chances of human error and causing thermal damage to delicate components are less when it is properly controlled.

Working Principle of Wave Soldering Systems

Wave soldering systems operate by creating a continuous wave of molten solder that remains stationary in the machine via a pump and a nozzle. A conveyor belt carries the PCB, which has the components mounted on top while their leads poke through the bottom; the PCB is held at an angle (usually 5 to 7 degrees).
The bottom side and the protruding leads of the components get exposed to the wave of solder as the board is moved over it. This is how solder is made to attach to the metal parts that are not covered.
 
Nowadays, most wave soldering equipment is equipped with a dual-wave system:
  1. Turbulent wave – It is quite thin but very fast. It has a very strong upward as well as sideways flow. It is capable of pushing solder in very small holes and around the leads of components. Because of this, it is unlikely that any joint area will be left unfilled, even if it is very difficult to access.
  2. Laminar wave – The smooth and broad wave that comes after the turbulent wave. It eliminates the excess solder, cuts down the possibility of bridging, and results in a neat and bright fillet.
     
First, the PCB is moved over the turbulent and then the laminar wave. The level of the solder wave is accurately maintained by the change in speed of the pump and the size of the opening of the nozzle. When the wave is at the correct height, both the solder and the underside of the board are in contact. Apart from that, no solder is allowed to run on top of the board, which can bring component damage or solder bridges.
 
Angle and speed of the conveyor also influence how long the board and the solder wave will remain in contact. Normally, the time for contact is 2 to 5 seconds. It is during this time that the solder is allowed to melt, flow and, after solidification, become a dependable joint.

Wave Soldering Process Steps

Technically, the wave soldering process (which is also known as the wave solder process) involves mainly what comes next 4 steps: flux application, pre-heating, wave contact, and cooling. In some cases, assemblies may need to be cleaned if a particular flux type has been used.

Step 1: Flux Application

Flux needs to be applied on the underside area of the PCB where the solder joints are to be formed. The usual ways are: using foam flux, spraying flux, and wave fluxing. Flux on metal acts as a deoxidizer and, at the same time, allows better solder spreading.

Step 2: Preheating

First, the board is taken through a preheating section where usually infrared heaters or hot air convection are used. Gradually increasing the heat of the board not only wakes up the flux but also changes the difference in the thermal expansion between the board and the solder wave to a minimum extent when the board comes into contact with the molten solder wave. Usually, the preheat temperatures range from 90°C to 120°C, according to the board thickness and component sensitivity.

Step 3: Soldering Wave Contact

Pre-heated PCB is passed over the solder wave (or dual waves – turbulent followed by laminar). Solder gets deposited on the bottom side of the board and component leads as the molten solder wets them. The conveyor angle (5–7°) and speed (1.2–1.8 m/min) are adjusted to give proper contact time of 2–5 seconds.

Step 4: Cooling

Once the board has passed the solder wave, it is placed in a cooling section. Cooling can be done by forced air or by an inert gas jet to quickly solidify the solder joints to hinder any unintended movement and to strengthen the joints.

Optional Step: Cleaning

If water-soluble or rosin-based flux (not no-clean) was used, cleaning of the board is a must which can be done by using deionized water or a suitable solvent to remove the flux residues.
Every step needs to be well controlled to make flawless joints. Temperature deviations, alteration of conveyor speed, or wave height will negatively affect solder quality.

Wave Soldering Machine & Wave Soldering Flux

A wave soldering machine consists of several interconnected zones that perform the soldering operation. In sequence, the typical layout of the machine is below:
  • Flux applicator – Different methods like foam, spray or wave are used to apply wave soldering flux
  • Preheat zone – The temperature of the board is gradually increased by the use of infrared or convection heaters.
  • Solder pot with wave nozzle – The solder pot contains molten solder, and a pump is used to create the solder wave.
  • Cooling zone – The joints are solidified by the use of forced air or inert gas.
  • Conveyor system – The PCB is moved through all the zones at a controlled angle and speed by the conveyor.
Tips: Wave soldering machine is the main equipment that cannot be missed to accomplish the PCBA successfully. Since different projects have different requirements, it is necessary to look at the equipment capability of the PCBA suppliers that you select. It will be better to assess the PCBA service provider’s equipment portfolio as your project’s requirements when you choose a PCBA service provider.
 
The wave soldering flux performs three critical roles: it cleans metal surfaces by removing oxides, it stops metals from oxidizing again during the warming phase, and it lowers the surface tension of liquid solder to make it flow better.
 
Some of the common varieties of wave soldering flux are:
Flux TypeCharacteristicsCleaning Required
Rosin-basedGood activity, leaves residuesYes (solvent)
Water-solubleHigh activity, corrosive residuesYes (deionized water)
No-cleanLow residues, minimal activityNo
 
Applying flux must be done evenly and precisely. A shortage of flux results in poor wetting and the formation of voids; an excess of flux produces splattering, leftovers, or solder balls.
 
The solder pot typically contains leaded or lead-free solder alloys (e.g., Sn63/Pb37 or SAC305). The temperature is constantly kept at 250°C to 260°C for leaded solder, or 260°C to 280°C for lead-free. A pump is used to generate a solder wave, the height of which is controlled by the position of the nozzle and the speed of the pump. An accurately set-up wave soldering machine is capable of producing the same high-quality joints for thousands of boards.

Selective Wave Soldering for Mixed-Technology Boards

Selective wave soldering is a derivative of the traditional wave soldering method aimed at PCB assemblies that are not suitable for taking the whole PCB through the solder wave.
 
Typically, this kind of problem arises on mixed-technology boards that have SMDs soldered by reflow or where the heat-sensitive components are on the bottom side. In that case, sending the boards through a full wave will probably result in washing off the SMDs, damaging the components, or forming solder bridges.
 
Selective wave soldering employs a small, mobile solder nozzle or mini-wave, which is only pointed at the specific through-hole pins that require soldering. The PCB is either kept stationary or it moves on a precision carrier, while the nozzle passes under the board. Only the targeted pins come into contact with the solder wave.
 
Selective wave soldering is a good solution if one wants to:
  • Protect SMDs from both thermal shock and mechanical damage
  • Reduce the amount of solder and flux used
  • Eliminate the need for expensive carrier pallets used for the covering of certain areas
  • Perfect for low-to-medium volume manufacturing and prototyping
     
The modalities usually consist of power modules, industrial control boards, automotive electronics, and any PCB with the combination of through-hole connectors and fine-pitch SMDs.
 
Selective wave soldering is slower than full-wave soldering. Still, it is really capable of giving the flexibility and the exactness required for assembling the complicated modern devices.

Wave Soldering for SMD Components

Wave soldering is a very effective and efficient way to solder a large number of components on a circuit board at one time, but is only applicable to through-hole components. Although wave soldering is mainly intended for the soldering of through-hole parts, it can also be used with limited number of surface mount devices (SMD).
This kind of SMD wave soldering is usually referred to as glue-attached SMD wave soldering.
 
Essentially, SMD components are soldered using the reflow method. Yet, those few SMDs that are put on the bottom side of the board and adhered before sending the board through the wave soldering machine, will be soldered by wave soldering. The adhesive is acting like a safety belt that will hold the components in place when the PCB is exposed to the wave of molten solder.
 
The major points to consider for successful SMD wave soldering are:
  • Adhesive selection – if done properly, the correct dosed epoxy or acrylic adhesive is the thing that will hold the SMD in place during the wave without any movement.
  • Component alignment – The transport direction is set by the longer axis of the SMD. It is the orientation that will result in minimal or no shadowing, and solder will get to the terminations very smoothly.
  • Pad layout – Pairs of solder pads need to be spaced so as not to allow them to bridge each other. Besides the distances between pads, the shortening of the lengths of the pads is also to be considered.
  • Types of components – Basic passive components like chip resistors & capacitors (0402, 0603, 0805, 1206) are suitable. Fine-pitch ICs, connectors, and BGA packages are incompatible.
     
Disadvantages of wave soldering for SMDs:
  • It is restricted to components having exposed terminations on the underside of the board only.
  • The introduction of adhesive materials means both cost of materials and process.
  • With increasing component density the possibility of solder bridging is raised.
  • Not all SMDs are capable of withstanding the thermal shock of the solder wave.
     
Actually, many PCBA manufacturers are only using wave soldering of SMDs on the bottom side with simple and low-density designs. Selective wave soldering of complex mixed-technology boards is much a better option. Another way is manual soldering of through-hole components after reflow.

Wave Soldering vs. Reflow Soldering

The first step to select a PCBA process is to compare wave soldering and reflow soldering.
FeatureWave SolderingReflow Soldering
Primary UseSoldering THT (DIP) components.Soldering Surface Mount Device (SMD) components.
Solder ApplicationSolder is applied via a dynamic wave during the process.Solder is pre-applied as paste (a mixture of powder and flux) before component placement.
Process PrincipleThe board’s underside is exposed to a wave of molten solder.The entire board passes through a controlled, multi-zone oven where the pre-applied solder paste melts (reflows).
Thermal ProfileHigh thermal mass process; the board is heated from below by the solder itself.A carefully managed temperature profile (preheat, soak, reflow, cooling) heats the entire assembly.
Typical ApplicationBoards with mostly THT components, or mixed technology with SMDs on top side only.Boards with mostly or exclusively SMD components.
CostLower for mass THTHigher due to paste printing and ovens
Common IssuesBridging, solder ballsTombstoning, voids
 
Key points:
  • Wave soldering is suitable for thru-hole components while reflow soldering is suitable for surface mounted components.
  • For mixed technology boards, manufacturers typically do a reflow step first and follow it with wave (or selective wave) soldering for thru-hole components.
  • Since only the bottom of the board in wave soldering is exposed to heat, the upper/top side components which are sensitive to heat get protection.
  • Reflow soldering needs a careful temperature profile; in wave soldering the main parameters are wave height, conveyor speed, and preheating.
 
If your board has mainly through-hole components, then wave soldering is the right choice. For most SMDs in your design, reflow is the better option. Mixed assemblies need both techniques to be used one after the other.

Wave Soldering vs Hand Soldering

AspectWave SolderingHand Soldering
SpeedHundreds of joints per secondOne joint at a time
ConsistencyHigh – machine-controlledOperator-dependent
Operator skillMinimal (setup only)Critical for quality
Cost per jointVery low (high volume)High (labor-intensive)
FlexibilityFixed processAdaptable for rework, prototypes
Key differences:
  • Speed – An entire PCB can be soldered in a wave soldering machine in seconds. If hand soldering is done on the same board, it might take minutes or even hours, according to the number of joints.
  • Consistency – The wave solder process delivers identical results across thousands of boards. Hand soldering quality varies with operator fatigue, technique, and tip condition.
  • Best use cases – Wave soldering is designed for through-hole board mass production. Hand soldering is still a must for prototyping, small batches, rework, and components that do not tolerate wave soldering (e.g. heat-sensitive parts, odd-form components).
  • Cost – For volumes beyond a few hundred boards, wave soldering is the most cost-effective way by far. Hand soldering is only economically feasible for low-volume or specialized work.
To sum up, wave soldering is a definite winner on performance and uniformity for production. Meanwhile, hand soldering is still a good option for versatility and rework.

Pros and Cons of Wave Soldering

Pros
  • High throughput – solders hundreds or thousands of joints per board in seconds.
  • Low cost per joint for mass production.
  • Solder joint quality remains the same all the time.
  • Fully automated, reducing labor dependency.
  • Suitable for a wide range of through-hole component sizes.
  • Can also solder certain SMD components (with adhesive).
Cons
  • Possibility of thermal shock for delicate components or very thin boards
  • Not suitable for fine-pitch SMDs or double-sided SMT boards.
  • High cost of initial equipment and need for large floor space.
  • Irregular board shapes need solder pallets or fixtures.
  • Lead-free soldering involves higher temperature and more maintenance.
  • Flux residues may require cleaning depending on flux type.
  • Problems like bridging, insufficient fill, or voids will need very good process tuning.

Common Defects & Troubleshooting Tips

DefectDescriptionPossible CausesTroubleshooting Tips
BridgingSolder connects two adjacent pins– Excessive wave height
– Conveyor speed too slow
– Poor pad design
– Insufficient flux		Reduce wave height, increase speed, check flux coverage, adjust conveyor angle
Insufficient fillSolder does not fully fill the plated through-hole– Low preheat temperature
– Poor hole wetting
– Conveyor speed too fast		Increase preheat, slow down conveyor, check flux activity, verify hole quality
Pinholes / BlowholesSmall voids or holes in solder joint– Trapped moisture or outgassing
– Contaminated board or leads
– Excessive flux		Bake boards before soldering, clean boards, adjust flux amount
Solder ballsSmall solder spheres on board surface– Solder splashing from wave
– Excess flux
– High conveyor speed		Lower wave height, reduce flux, adjust speed, check flux preheating
IciclesSharp, cone-shaped solder protrusions– Insufficient preheat
– Conveyor speed too fast
– Poor wave separation		Increase preheat, reduce speed, ensure proper laminar wave separation
No solder (skip)Missing solder on some pins– Warped board
– Insufficient flux
– Low wave height		Use board support tooling, check flux coverage, raise wave height

Conclusion

Wave soldering continues to be one of the major methods for through-hole PCBA assembly. It offers fast production speed, consistent quality, and the lowest cost per solder joint mainly for the manufacture of large runs. Selective wave soldering is the technique of choice for mixed-technology boards as it is highly precise and ensures that the SMD components are kept intact and safe from any damage. Knowing how to perform the wave soldering process, setting up the machine correctly, choosing the right flux and understanding what defects look like through troubleshooting will finally let manufacturers make solder joints that are dependable.
 
If you want to go for full-wave, selective or even hand soldering of your prototypes, FS Technology can supply you with expert PCBA assembly services which will be very well adjusted to your project requirement. Contact us to discuss your next assembly project.

FAQs about Wave Soldering

Wave soldering is primarily used for high-volume soldering of THT components and some wave-compatible SMDs.
Wave soldering uses a molten solder wave for THT, while reflow soldering uses solder paste and a temperature profile to assemble SMDs.
Yes, but only certain SMDs. They require adhesive and special design considerations to prevent defects.
A wave soldering machine, including a solder pot, flux sprayer, preheating stage, conveyor system, and solder wave pump.
Automotive electronics, power supplies, industrial equipment, consumer devices, and any application involving THT components.
The choice depends on your board’s component mix. If your design is predominantly through-hole (THT), a wave soldering company is essential. If it’s mostly surface-mount (SMD), a provider with advanced reflow ovens is better. For mixed-technology boards, you need a partner like FS Technology that is proficient in both processes and can integrate them seamlessly.

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