Reflow, Wave, Selective and Vapor Phase Soldering: How They Compare

Solder is solder, but how it is melted makes a real difference to joint quality, to which parts can sit where on the board, and to cost. There are four methods you will come across when talking to an assembler. Most boards need one or two of them.

Reflow soldering: the default for SMT

How it works: solder paste is printed on the pads, components are placed on it, and the board passes through a convection oven with several heating zones. The paste melts, wets the pads and leads, and solidifies into joints as the board cools.

The temperature profile decides joint quality: preheat, soak, a short time above the melting point (about 217 °C for common SAC305 lead-free solder), then controlled cooling. A good profile is measured with thermocouples on the actual board, not copied from a datasheet.

Best for: virtually all surface-mount assembly, from 0201 passives to BGAs, on one or both sides of the board.

Watch out for: boards with very different thermal masses, such as a large transformer next to tiny passives. The small parts can overheat before the big one is properly soldered. That is where profile tuning, or vapor phase, comes in.

Wave soldering: fast for through-hole

How it works: after through-hole parts are inserted, the board is fluxed, preheated and passed over a standing wave of molten solder. The wave touches the underside and solders every through-hole joint in one pass.

Best for: boards with many through-hole joints and little or no SMT on the solder side. It is fast and economical at volume.

Watch out for: SMT parts on the underside. They either need to be glued and designed for wave soldering, or protected with a pallet, or the board should use selective soldering instead.

Selective soldering: through-hole with precision

How it works: a small, programmable solder fountain (a mini wave) moves under the board and solders only the joints it is told to, one by one or in small groups.

Best for: mixed-technology boards where through-hole connectors or power parts sit close to surface-mount parts on the underside. It gives consistent, repeatable joints, uses flux only where needed, and avoids exposing the whole board to a wave.

Watch out for: speed. It is slower than wave soldering on boards with hundreds of through-hole joints, so it is chosen for control, not throughput.

Vapor phase soldering: even heat for difficult boards

How it works: the board is lowered into a vapour of an inert heat-transfer fluid (a perfluoropolyether). The vapour condenses on the cooler board and gives up its heat. Because the vapour has a fixed boiling point, the board can never get hotter than that temperature, however long it stays in, which is typically around 230 °C for lead-free work.

Best for:

• boards mixing very heavy and very light components;
• heavy copper or metal-core boards that are hard to heat evenly in convection;
• some BGA and bottom-terminated components where even heating reduces defects.

The oxygen-free vapour blanket also helps wetting.

Watch out for: availability and cost. It is a specialist process and is not needed for most boards.

Comparison

Which does your board need?

• All surface mount: reflow only.
• SMT plus a few connectors: reflow, then selective or hand soldering.
• Lots of through-hole, simple underside: reflow (if any SMT) plus wave.
• Very uneven thermal mass or heavy copper: ask about vapor phase, or about a carefully tuned reflow profile.

Our article on SMT vs through-hole covers the design side of this choice.

Talk to us about your board

Polyfit's EMS division builds SMT, through-hole and mixed-technology assemblies in Chennai, built to IPC-A-610. Send us your board files and we will come back with a quote.