Plated Through Hole Soldering: Wave, Selective and Reflow

Through hole parts never disappeared. Connectors, transformers, relays and power devices are still mounted through the board, because a plated through hole gives mechanical strength and a solder joint that can be inspected from both sides. The challenge is that wave soldering, the process these parts were designed for, is a poor fit for a modern board that carries dense surface mount components on both faces.

Why Through Hole Assembly Survives

The joint itself is the reason. A barrel of plated copper filled with solder resists the mechanical load of a cable being plugged and unplugged, and it survives vibration that would fatigue a surface mount joint on a large connector body.

Serviceability matters as well. A part that can be desoldered and replaced without damaging the board is valuable in industrial equipment with a long service life, and the visible fillet on both sides of the board makes inspection straightforward.

The Limits of Wave Soldering

Wave soldering passes the board over a flowing solder wave so that every exposed joint is soldered at once. It is fast and inexpensive, but it requires the solder side of the board to be free of fine pitch surface mount parts, which is rarely true today.

The process also damages what it touches. The board sees a large thermal shock as it crosses the wave, bridging and incomplete fills occur where the geometry is complex, and the flux application affects every joint whether it needs flux or not.

Selective Soldering

Selective soldering applies solder only where it is needed. A small nozzle produces a local wave, or a fountain of solder is directed at a defined area, so surface mount components elsewhere on the board stay cool and dry.

The board is still heated, but locally and in a controlled sequence. Flux is applied only to the sites being soldered, which reduces residue and eliminates the need to clean around parts that cannot tolerate moisture.

Dip and Drag Parameters

A dip process works well for parts with leads that are difficult to reach with a nozzle. Typical settings run a solder temperature between 275 and 300 degrees, a penetration speed in the range of 22 to 25 mm per second, a dwell of one to three seconds and a withdrawal speed of about 3 mm per second.

Those numbers are starting points rather than universal settings. They depend on board thickness, thermal mass and the alloy in use, and the joint is judged by the fillet it produces rather than by the machine settings that produced it.

Through-Hole Reflow

Through-hole reflow puts the paste into the hole during stencil printing and solders the part in the same reflow cycle as the surface mount components. The board then passes through one thermal cycle instead of two, and no wave or selective soldering is required at all.

The technique needs much more paste than a surface mount pad. The volume required is typically around thirty times that of a conventional pad on the same board, and the paste has to fill the hole and form a fillet without leaving voids.

Getting the Paste Volume Right

Stencil printing can deliver that volume with a stepped stencil, where the area over the through hole connectors is thicker than the rest of the tool. The step is built into the stencil and gives the extra paste without affecting the fine pitch regions.

Automated dispensing is the alternative, and it handles parts that a stencil cannot reach. The nozzle should be slightly larger than the hole so the paste is pressed against the barrel wall; a smaller nozzle pushes paste through and out of the hole rather than leaving the volume required.

Thermal Profile and Component Protection

The reflow profile has to heat the board enough to melt paste inside a hole, which is a larger thermal mass than a surface pad. That requirement pushes peak temperature and time above liquidus upward, and the profile has to be balanced against the limits of the most sensitive component on the assembly.

Moisture sensitivity is the usual constraint. Parts that have absorbed moisture need baking before assembly, and the placement process has to keep them within their floor life, or the higher temperature of a through hole profile will cause popcorning and internal cracks.

Inspection and Defect Prevention

Inspection looks for a fillet on the top and bottom of the joint, an absence of voids and full wetting of the barrel. X-ray helps where the joint is hidden, and cross sectioning a coupon is the definitive check on barrel fill percentage.

Most defects trace back to solder paste volume, hole size or pad geometry, and the three are linked: a hole that is generous for its lead needs less paste, while a tight hole demands more accurate deposition. A hole that is too large for the lead, or a pad that is too small to hold the paste, produces an incomplete joint no matter how the profile is tuned, which is why the pad design should be reviewed alongside the process.

Choosing Between the Processes

Use selective soldering when a small number of through hole parts sit among dense surface mount circuitry, and through hole reflow when the parts can tolerate a reflow profile and the volume of paste can be delivered reliably. Wave soldering remains valid for boards that are mostly through hole.

Whichever route is chosen, the decision belongs in the design phase. The process determines the pad sizes, the paste volumes and the keepout areas, and a board designed for wave soldering and then assembled with a lead-free selective process will need compromises in both directions.

Voiding and Barrel Fill Quality

A plated through hole joint is accepted on the basis of how much of the barrel is filled with solder and how much voiding is present. Both are affected by the flux, the paste chemistry and how quickly the volatiles can escape while the solder is molten.

Slow heating drives volatiles out before the solder flows, and a hole that is not vented traps them. Voiding above an accepted limit is a reliability concern rather than a cosmetic one, because a large void concentrates thermal and mechanical stress at the joint.

Cleaning and Residue Control

Residue management has moved up the agenda as no-clean chemistries became common. A residue that is harmless on an open board can become a leakage path when it sits between fine pitch pads or under a component that traps moisture against the surface.

Where cleaning is required, the process must match the flux. A water-soluble flux demands thorough washing and drying, while a no-clean flux should not be washed unless the chemistry is designed for it, since cleaning can activate residues that were inert.

Design Rules That Help the Process

Hole size relative to lead diameter decides how much solder the joint can hold, and a hole that is too large produces a thin, weak fillet regardless of process settings. Thermal relief connections on planes keep the hole from acting as a heat sink that never reaches temperature.

Spacing is the other lever. Sufficient clearance between through hole pads prevents bridging during a dip or a selective pass, and keeping tall parts away from the path of a nozzle removes a mechanical obstruction that would otherwise force a compromise in the program.

FAQ

Is wave soldering obsolete? No, but its use has narrowed to boards that are predominantly through hole. Mixed technology boards generally move to selective soldering or through hole reflow.

How much paste does a through hole joint need? Around thirty times the volume of a surface mount pad of similar area, because the paste has to fill the barrel and still form a fillet on both faces.

Can through hole parts be reflowed with the rest of the board? Yes, provided the part can survive the profile and enough paste can be deposited. Many connectors are now rated for reflow for exactly this reason.