Selecting the right soldering process and why the decision should be made early

After the components have been inserted, three soldering processes are available. The choice depends directly on the layout.

Wave soldering

In wave soldering, the PCB passes over a solder wave. Although the process is fast and efficient, it affects the entire bottom side. Bottom-side SMD components often make wave soldering impractical without extensive protective measures.

In wave soldering, typical process parameters for lead-free alloys include a solder temperature of 250–260°C and a contact time of 2–4 s. The component-side preheat temperature is typically set to 100–150°C in order to prevent thermal shock and activate the flux. Although the process window may seem forgiving, it is not. High thermal mass in individual pads or unfavorable spacing can be enough to compromise process stability.

Selective soldering

Selective soldering is a localized process: Only the individual THT solder joints are soldered. It is slower but more flexible—and the right choice when component geometry, spacing, or bottom-side population prevents the use of wave soldering.

Hand soldering

Hand soldering remains an option for low production volumes or special requirements. At EPnP, it is also used when THT components are placed too close to SMD components. As a general guideline, machine processing is often limited when spacing falls below 4 mm.

It is precisely this question that should be addressed early in the design phase. Involving EPnP only during manufacturing can lead to additional manual effort that could have been avoided through earlier planning.

THR: the often-overlooked hybrid approach

Between THT and SMD, there is a third option: through-hole reflow (THR). However, this is often considered too late in a project. By using shortened leads, inserting components into prepared paste-filled holes, and processing them together with SMD components during reflow soldering, THR eliminates a separate soldering step and can greatly simplify manufacturing.

As a technical guideline, the hole diameter should exceed the lead diameter by at least 0.2–0.25 mm (according to IPC-2221) to ensure that sufficient solder paste can be forced into the hole. Because approx. 50% of the paste volume consists of flux that escapes during reflow, the paste volume must be carefully matched to the hole diameter and board thickness. Because not every component geometry is suitable, evaluating THR early in the design phase is worthwhile whenever it is a viable option.

EPnP in THT manufacturing

Through-hole technology processes are highly sensitive to layout decisions. When EPnP is involved only during manufacturing, the consequences often become apparent only when it is too late. Hole diameters, pad geometries, thermal mass, spacing, and bottom-side population determine which soldering processes can be automated and where hand soldering becomes unavoidable. Addressing these factors early on helps avoid additional effort that cannot be eliminated through optimization.

Work at EPnP therefore begins with project clarification and design for manufacturing (DFM) before a soldering strategy is defined.

Quality assurance, documentation, and traceability are not optional extras but rather integral parts of the manufacturing configuration—regulated sectors such as medical devices, aerospace, and vehicle manufacturing.

Throughout the product lifecycle, we take variants, component changes, and revisions into account: THT processes remain stable only when approvals, alternatives, and process parameters are maintained in a traceable manner.

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