Surface-mount technology has taken over most high-density PCB designs, but through-hole technology (THT) is far from obsolete. Components with heavy leads, high-voltage terminals, or mechanical stress exposure such as automotive connectors and industrial power relays need the structural bond that a plated through-hole provides. The lead passes completely through the board and is soldered on both sides, distributing mechanical and thermal loads far more effectively than a surface pad alone.
This is why industries ranging from transportation and new energy to security and medical devices continue to specify through-hole components on mixed-technology boards. A board might carry SMT chips on top while connectors, electrolytic capacitors, and transformers sit on the same assembly as through-hole parts. Welding all of those through-hole joints efficiently and consistently is where a well-controlled wave soldering line earns its place.
Wave soldering is a bulk soldering method in which the entire bottom side of a PCB contacts a standing wave of molten solder. The wave wets every exposed pad and component lead at once, forming hundreds of joints in a single pass lasting only seconds. The process runs on a conveyor system and follows a tightly controlled sequence of stages.
Before the board reaches the solder wave, flux is applied to the underside. Flux removes oxide layers from copper pads and component leads and prevents new oxidation from forming during heating. Spray fluxing and foam fluxing are the two common methods. Applying the right amount of flux is critical: too little leads to poor wetting and cold joints, while too much leaves corrosive residue that can degrade long-term reliability.
The board then passes through a preheating zone where hot air or infrared heaters raise its temperature gradually. Preheating serves two purposes: it activates the flux and it prevents thermal shock when the board contacts the molten solder wave. Sudden temperature spikes can crack ceramic capacitors, delaminate the board substrate, or warp large panels. A controlled preheat profile keeps the temperature ramp gentle and uniform.
This is the heart of the process. The conveyor carries the fluxed, preheated board over a molten solder wave generated by a pump in the solder pot. Most modern wave soldering machines use a dual-wave design: a turbulent first wave wets the leads and pushes solder into the holes, and a smooth second wave flattens the joints and removes excess solder to prevent bridging. Contact time, wave height, conveyor angle, and solder temperature all determine joint quality.
After the solder wave, the board enters a cooling zone where forced air solidifies the joints quickly enough to prevent grain growth but slowly enough to avoid thermal stress. Proper cooling locks in the mechanical and electrical integrity of each joint and prepares the board for downstream inspection and testing.
Even on a well-tuned line, several defects can appear if process parameters drift. Knowing what to watch for helps both the manufacturer and the customer evaluate quality.
Catching these defects requires inspection at multiple stages. Farway's published DIP process includes plug-in AOI, IPQC sampling during production, and QA sampling at line exit, supported by 24 rear-welding stations where trained operators perform targeted repair welding on any flagged joints before the board advances.
The quality of a through-hole soldering service depends heavily on the equipment behind it. Farway Electronic operates two DIP plug-in production lines equipped with Nitto wave-soldering machines at its 2,000-square-metre facility in LongGang, Shenzhen. The line configuration includes:
This setup supports both prototype runs and medium-to-large volume orders, and it integrates directly with the company's SMT lines, conformal coating line, and testing stations to provide a one-stop smt + dip assembly service for mixed-technology boards.
Soldering is only half the job. Verifying that every through-hole joint meets acceptance criteria is what separates a reliable PCBA from a field-failure risk. Farway assembles to IPC-A-610 as its PCBA assembly standard and holds four management-system certifications that underpin its process discipline:
| Standard | Scope |
|---|---|
| ISO 9001 | Quality Management System |
| ISO 13485 | Medical Device Quality Management |
| IATF 16949 | Automotive Industry Quality Management |
| ISO 14001 | Environmental Management System |
After wave soldering and board washing, each board moves into the testing phase. Farway's published testing capabilities include plug-in visual inspection, AOI, X-ray inspection for hidden joints, thermal imaging, high- and low-temperature reliability testing, ICT circuit testing, and FCT functional testing. This multi-layer inspection catches soldering defects that visual checks alone would miss, such as insufficient hole fill or internal voids in thick through-hole joints.
Most production boards today are not purely SMT or purely through-hole; they are mixed-technology assemblies. The typical workflow places and reflows SMT components first, then inserts through-hole parts, and finally runs the board through wave soldering while masking or gluing protects the already-soldered SMT joints. Getting this sequence right requires tight coordination between the SMT line, the DIP line, and the test station.
Farway runs two SMT production lines alongside its two DIP lines, plus a conformal-coating spraying line, four low-pressure injection moulding machines, and two finished-product assembly lines, all under one roof. This vertical integration means a customer can hand over a BOM and Gerber files and receive a tested, coated, and boxed product without managing multiple subcontractors. The company also offers RoHS-compliant processing for customers shipping into regulated markets.
A wave soldering line cannot produce reliable joints from oxidized or counterfeit components. This is why dip plug-in welding service quality starts upstream in component management. Farway sources from authorised brand agents and distributors, runs incoming quality inspection, and stores materials under controlled temperature and humidity with anti-static packaging and first-in-first-out rotation through its ERP system. BOMs are screened for sourcing risk before any order is placed.
For through-hole components specifically, lead oxidation is a frequent cause of solderability problems. Components stored too long or in poor conditions develop oxide layers that even active flux struggles to remove. Controlled storage and timely processing directly reduce the defect rate on the wave soldering line.
When evaluating a high quality through-hole soldering partner, the following criteria matter more than price alone:
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