The bridge forms at separation
Wave soldering works by contact: the board passes over a standing wave of molten solder, which wets the pads and the protruding pins. A bridge forms at the moment the board leaves the wave, when the solder between two pins has to separate.
If it separates cleanly, surface tension pulls the solder back into two fillets. If it does not, the solder stays as a web between the pins. Everything below changes whether that separation succeeds.
Cause one: geometry
- Pin protrusion. Too long and the pin drags a tail of solder out of the wave; about 1.5–2 mm is the working range.
- Pad size and spacing. Oversized pads hold more solder than the joint needs, and the excess has nowhere to go. Dense pin fields with wide pads bridge first.
- Exit direction. The last pad the solder touches is the one that decides whether a web forms. Pad extensions or solder thieves on the exit side give the solder somewhere to go.
Cause two: flux
Flux has to be present, active and uniform. Insufficient quantity, uneven foaming, a clogged foaming tube, or flux that has lost activity all reduce wetting, and poor wetting means the solder does not pull back to the pads.
The board also has to be clean: oxidised pads and pins resist wetting, and the solder stays where it landed rather than where surface tension wants it.
Cause three: thermal
The wave has a fixed heat input. A board with heavy copper planes, large thermal reliefs or a dense ground pour pulls heat out of the contact zone, so the solder is cooler and less fluid exactly where it needs to drain.
Preheat, pallet design and the orientation of heavy copper relative to the wave are the levers; if the assembly is dominated by thermal mass, selective soldering is often the better process.
Cause four: pot chemistry
Solder picks up copper, and copper raises the melting point and the viscosity of the bath. Impurities also build oxides and dross, which change how the wave behaves. Pot analysis, dross removal and correct alloy topping are scheduled maintenance, and they fix a class of bridging that no layout change will.
Fixing it in the right order
Check pot chemistry and flux delivery first — they are cheap to verify and cheap to correct. Then pin length and board direction, then pad geometry, then process change (selective soldering or a pallet) if the connector field is simply too dense for a wave.
Workmanship is judged to IPC-A-610. The assembly process and its inspection gates are described on the PCBA manufacturing page and in the inspection note.
