Selective soldering machines fill the critical performance gap between wave soldering and hand soldering operations. Wave soldering is suitable for medium to large runs when all components are through-hole or surface mount while hand soldering is suitable for low volume production and difficult to access parts. Process time for a wave soldered board is usually a few minutes while the time to process a hand soldered board depends on the number of individual connections to be soldered. Hand soldering is often the greatest bottleneck in a production line.

By comparison, selective soldering machines process each solder joint individually, but the process is automated and at least three times faster than hand soldering. They can be precisely programmed to solder specific connections and provide excellent solder joint quality (Figure 4-1). Selective solder equipment is a sound choice when some components cannot be exposed to the heat of molten solder in a wave solder machine.

In recent years, surface mount components have largely replaced through-hole as the components of choice and the reflow oven has largely replaced the wave solder machine. However, not all required components in a design are available in surface mount format. Designers can be forced to mix through-hole components in a surface mount design. There is always a switch, a connector, or some device that may only be available only as a through-hole component. When mixed technology or physical barriers prevents access to components, the only option may be hand soldering or the higher performance of a selective soldering machine.

There are two general configurations for these machines, either the circuit board is fixed and the mini-solder wave is moved under the circuit board, or the mini-solder wave is fixed, and the circuit board is moved from above. This article concentrates on a fixed solder pot and movable circuit board which allows rotation through 90 degrees to access the difficult parts (Figure 4-2).

Selective soldering machines have three major elements, a precise nozzle wave of molten solder with a nitrogen shroud, a fixture to hold and move the circuit board, and the software to control the entire process. The nozzle wave is the heart of the process. Successful soldering always requires clean and oxide free surfaces. First, flux is sprayed onto the component before lowering into nozzle wave. Flux activation and oxide removal is accomplished using a high temperature nitrogen gas enveloped around the nozzle wave (Figure 4-3). The temperature of this nitrogen atmosphere is under tight control and is hotter then the nozzle solder wave. This heats the components, activates the flux, and prevents the parts from reoxidizing. All selective soldering machines use nitrogen.

After fluxing, the board is lowered into solder wave for a specific dwell time and then raised, resulting in a perfect solder connection. With precise computer control of the solder wave height, temperature, and board angle, the key technical and process requirements are achieved to solder the most difficult components reliably and repeatedly.Selective soldering is a mature and effective technology for mixed technology components, heat sensitive components, and low to medium production. For more information regarding selective soldering or any of the other electronics manufacturing services available at the EMPF, please contact the Helpline at 610.362.1320.