It is the oldest method of soldering, it is still popular method in certain kinds kinds of applications:

• Development of prototype boards
• Low volume production
• Soldering of extremely temperature sensitive components
• Solder reflow of fine pitch components using hot bar
• Rework or repair of machine soldered boards

The main disadvantages are operator training, speed, and consistent quality.

1.4.2 Machine Soldering:

Two prominently used machine soldering types are:

A. Wave Soldering - Primarily used for soldering through-hole components on to PCBs.

B. Reflow Soldering. - Used for soldering SMD components on to PCBs. Reflow soldering of SM components have the following advantages over manual soldering:

• Mass soldering
• Consistency in manufacture through precise control of process parameters.
• flexible for small production runs as well.

In reflow soldering, the solder is applied as a 'solder paste' typically by using a stencil mask and a squeege. The solder paste contains both a solder flux and the solder in the form of minute solder balls. The solder paste operation is done BEFOREadding the components to the board. The board is then passed through a reflow oven at which time, the solder paste melts to complete the soldering of the components to the circuit board. The reflow oven will have several heating zones designed to preheat, activate the flux, reflow the solder, and cool the board. The oven is setup so the components receive as little thermal shock as possible, and spend only a few seconds above the solder melting temperature.

In wave soldering, the components are placed on the board. THEN the board is placed in the wave soldering machine. Typically the first step in the wave solder, the bottom of the board passes through a fluxing operation which either sprays flux on the bottom or passes the bottom of the board through a flux foam. After the flux, the board will usually pass a pre-heater to activite the flux and pre-heat the board for the soldering. Then, the bottom of the board is passed through a molten solder wave (looks like a smooth waterfall only consisting of molten solder). Afterwards, there will be a stage of cool-down.

Some details of the process vary by how expensive the oven or wave-solder machine is, the complexity of the circuit board involved, for what kind of volume the production line is intended for, and how 'mil or space qualified' the circuit board is.