Mixed technology wave soldering is a critical process step for assemblies that combine surface mount components and through-hole parts. For product designers and sourcing managers, knowing when and how this process is applied is essential to avoid late-stage redesigns or unnecessary rework. Through-hole components, such as connectors, transformers and high-power devices, still require robust solder joints and wave soldering remains the most practical method to achieve them at scale. Understanding its parameters, fixtures and limitations allows you to design with confidence, specify realistic requirements and select the right assembly partner.
If you need a reliable route to PCB builds with both SMT and THT parts, talk to our team about how mixed technology wave soldering can support your project.
What is Mixed Technology Wave Soldering?
Mixed technology wave soldering refers to using a solder wave to attach through-hole components to a board that already carries surface mount components. It is part of the wider category of mixed-technology PCB assembly, where Surface Mount Technology (SMT) and Through Hole Technology (THT) devices are combined to balance size, cost and mechanical strength.
In practice, surface mount components are placed and soldered first in a reflow oven. Once that stage is complete, the through-hole components, such as connectors, relays, transformers and power devices, are inserted into the board. The populated PCB is then fitted into a pallet or carrier, which masks sensitive areas and stabilises the assembly. This pallet passes across a controlled wave of molten solder, forming consistent joints on all exposed through-hole leads in a single pass.
For a wider explanation of how SMT and THT processes are combined in production, see our overview of mixed technology PCB assembly.
Why Wave Solder Through-Hole Components?
Through-hole components remain necessary in many assemblies because they provide mechanical strength and reliable current handling. Connectors, power devices and large transformers are almost always supplied in THT packages. For these parts, wave soldering is the most efficient way to achieve consistent joints across multiple leads at once.
In wave soldering, the PCB is passed over a standing wave of molten alloy after fluxing and preheating. The solder contacts all exposed leads in one motion, filling the plated through-holes and forming joints in seconds. This bulk process makes it faster and more consistent than hand soldering for volume production.
Read our Guide to PCB soldering for an overview of soldering methods and how wave soldering compares.
Step-by-Step Through-Hole Wave Soldering Process
The through-hole wave soldering process in a mixed PCB follows a controlled sequence to ensure reliable joints:
Step 1: Component placement
Surface mount devices are placed on the board first and soldered in the reflow oven. Once the SMT stage is complete, through-hole components, such as connectors or transformers, are inserted into the PCB. Leads are trimmed where needed in order to fit correctly in the plated holes and achieve the right protrusion for soldering.
Step 2: Board loading and fixturing
The PCB is secured in a pallet, which masks sensitive SMT areas, controls solder immersion depth and keeps the assembly stable as it moves through the soldering line.
Step 3: Flux application
A layer of flux (a chemical that removes oxides and helps solder flow into the plated holes) is applied to the underside of the board, usually by spray or foam.
Step 4: Preheating
The PCB passes through a preheater to activate the flux, evaporate solvents and reduce thermal shock when it reaches the solder bath.
Step 5: Wave soldering
The board is conveyed across one or more solder waves. The molten alloy makes contact with the leads and plated holes, producing solder joints simultaneously across the full underside.
Step 6: Cooling and inspection
Controlled cooling ensures joints solidify without disturbance. Assemblies are then inspected to IPC-A-610 standards, with attention to fill level, fillet shape (i.e. the external contour of the solder joint where the lead meets the pad) and overall joint quality.
Advantages and Disadvantages
Wave soldering offers clear productivity gains for mixed-technology assemblies, but it also introduces constraints that must be considered at the design and sourcing stages.
| Advantages | Challenges |
| High throughput – hundreds of joints soldered in one pass | Risk of bridging on fine-pitch or closely spaced leads if pallet design and process settings are not optimised |
| Consistent quality – stable parameters give repeatable joints across batches | Thermal load – heavy boards or heat-sensitive parts may be stressed by the solder bath |
| Lower unit cost for volume builds compared with manual soldering | Copper dissolution – too much wave contact or poor alloy control can erode copper from pads and holes |
| Reliable mechanical joints for connectors, transformers and other large parts | Limited suitability when bottom-side SMT is dense or tall |
Equipment, Fixtures and Carriers
Mixed technology wave soldering relies on more than the solder pot – the heated reservoir that pumps molten solder into a controlled wave. Each stage of the soldering line requires supporting equipment and carriers to ensure reliable results.
- Wave soldering machines
These include fluxing units, preheaters and solder pots with single or dual waves. Dual-wave systems use a turbulent wave followed by a smooth laminar wave to improve hole fill and reduce bridging. - Pallets (carriers)
Custom pallets made from heat-resistant composite materials hold the PCB during soldering. They expose only the through-hole leads while shielding bottom-side SMT parts from the solder. Pallets also keep the board rigid, control immersion depth and improve repeatability across batches. - Tooling and accessories
Other carrier types can be used to stabilise thin boards or support assemblies with unusual shapes. Options such as different nozzles, nitrogen covers to reduce oxidation at the solder surface, and systems for removing dross (oxidised metal waste from the solder) help improve process stability and lower defect rates.
Together, this equipment defines how well a mixed-technology board can be wave soldered. Correct pallet design in particular is essential for protecting SMT parts and avoiding unnecessary rework
| See our xxxxxx Case Study to learn how we delivered reliable through-hole assembly for a customer operating in a demanding environment. |
Process Parameters and Best Practices
The quality of wave-soldered joints depends heavily on how process parameters are set and controlled. For mixed-technology boards, the following points are critical:
- Solder temperature
Lead-free alloys typically run at 250-260 °C. Alloys with higher tin content may require slightly higher temperatures, which increases stress on the PCB and components. - Contact time
The PCB should stay in contact with the solder wave long enough to achieve full hole fill but not so long that copper dissolution becomes a risk. Conveyor speed and immersion depth must be meticulously monitored and adjusted to achieve this balance. - Flux type and application
Flux chemistry must match the alloy and board finish. Spray fluxing is common for consistency, but volume and coverage must be controlled to avoid residue issues or insufficient wetting (i.e. how well molten solder spreads and adheres to a PCB pad). - Preheat profile
Preheating should evaporate flux solvents and activate the flux chemistry while raising the board to a safe top-side temperature. Too little preheat causes spattering at the wave, while too much can damage laminates or components. - Nitrogen atmosphere
Using nitrogen around the wave reduces oxidation, which improves solder wetting and reduces dross build-up in the pot. This is especially helpful for dense boards or when yield targets are tight. - Inspection criteria
Final joints should be inspected to IPC-A-610, checking for hole fill, fillet quality and wetting. Inspection can involve the use of AOI, X-ray or manual visual checks. Agreeing methods and acceptance criteria early avoids later disputes.
Following these practices helps teams design boards that are manufacturable, specify the right requirements to contract electronic manufacturing partners, like MPE Electronics, and avoid delays caused by rework or yield loss.
Applications and Benefits of Mixed Technology Wave Soldering
Mixed technology wave soldering is most valuable on assemblies that combine SMT and THT parts. It allows large or mechanically strong devices such as connectors, transformers and relays to be soldered efficiently after the SMT stage. Typical applications include power supplies, industrial controllers and medical systems, where both component types are present on the same board.
For product designers, the benefit is having confidence that mixed assemblies can be manufactured repeatedly without excessive rework. For sourcing managers, the ability to wave solder through-hole devices at scale helps control unit costs and maintain consistent quality across batches.
Ready to evaluate mixed technology wave soldering for your next PCB build? Contact our team to discuss design rules, pallet options and inspection standards that support a reliable outcome.
FAQs about wave soldering
What is wave soldering used for?
Wave soldering is used to solder through-hole components on both fully through-hole and mixed-technology PCBs. In modern production it is most often applied to mixed builds, where parts such as connectors, relays and power devices require strong mechanical joints that reflow cannot provide.
Why use wave soldering instead of hand soldering?
Hand soldering is flexible but slow and inconsistent for high volumes. Wave soldering allows hundreds of joints to be soldered in one pass, reducing unit cost and improving repeatability across batches.
Can all PCBs be wave soldered?
Not always. Boards with dense or tall bottom-side SMT parts may not be suitable because the solder wave could damage or bridge those components. In those cases, manual methods are used instead.
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.
