Wave soldering is a large-scale process by which electronic components are soldered to a printed circuit board (PCB). The term “wave soldering” comes from the fact that the process uses a tank or pot to hold a quantity of molten solder and the components are inserted into the PCB, and passed across a pumped wave or cascade of solder. The solder wets the exposed metallic areas of the board (those not protected with solder mask), creating a reliable mechanical and electrical connection. The process is much faster and can create a higher quality product than the manual soldering of components.

The NU/ERA wave soldering machine (Figure 7-1)  is engineered to maintain exacting operating parameters throughout the production sequence, regardless of the job size or complexity.  Speed and thermal profiling versatility  eliminates problems commonly associated with military specification PCBs.  The printed circuit boards to be soldered are placed on a conveyer system which carries the boards through the solder wave. The fingers of this conveyer system are made of materials that are not wetted by solder and can resist the high temperature.

A standard wave solder machine consists of three zones: the fluxing zone, the preheating zone, and the soldering zone. An additional fourth zone, cleaning, can be used depending on the type of flux applied.

The first zone the board encounters applies flux to the underside of the board using either a spray or foam fluxer.  Precise control of flux quantities is required. Too little flux will cause poor joints, while too much flux can cause cosmetic or other problems. 

The PCB then enters the preheating zone. Convection heaters blow hot air onto the PCB to increase its temperature.  A forced airblast (or “Air Knife”) then smoothly spreads the flux, removes the excess, and assures full penetration into difficult to reach recesses.  It also warms the PCB, evaporates volatiles, and conditions it for the higher temperatures of the preheat unit.  For thicker or densely populated PCBs, an upper preheater, such as an infrared heater, might also be incorporated.

Preheating is necessary to activate the flux and remove any flux carrier solvents. Preheating is also necessary to prevent thermal shock which occurs when a PCB at ambient temperature is suddenly exposed to the high temperature of the molten solder wave.

The next zone is the soldering zone.  The tank of molten solder has a pattern of standing waves (or in some cases, intermittent waves) on its surface.  When the PCB is moved over this tank, the solder wave contacts the bottom of the board and adheres to the solder pads and component leads via surface tension.  Precise control of the wave height is required to make sure solder is applied to all areas without splashing to the top of the board or other undesired areas.  This process is sometimes performed in a nitrogen (N2) atmosphere to reduce oxidation (and the formation of solder dross) and increase the quality of the joints.

The Technical Devices NU/ERA machine features solder pots that can be swapped to quickly change between leaded and lead-free solder.  The entire pot assembly is mounted on a roll out frame which provides access to the pumps. The wave  is produced using a propeller drive pump to provide a gentle movement of molten solder at a low velocity to the discharge duct.  This eliminates turbulence and provides a precise, even lamina flow solder wave.  Molten solder is then drawn from the bottom of the insulated solder pot, eliminating contaminants in the solder wave.

The reduction and elimination of solder dross is a growing industry concern as lead (Pb) soldering is being replaced by Pb-Free alternatives at a significantly higher cost.  Dross eliminators are entering the market and may hold some solutions.  There are also solder recovery machines on the market that can further separate useful solder from dross.   Due to the precise requirements needed for wave soldering, most soldering equipment must be closely monitored.  While many systems are kept in constant alignment by microprocessor control, there are also subsystems which have on/off and automatic modes for most operations and are programmable for accurate and reliable process repeatability.  

Wave soldering continues to be used as a cost-effective, mass soldering method for many printed circuit board assembly applications.  If you would like a demonstration of Technical Devices NU/ERA Wave soldering system, please contact Ken Friedman at the EMPF by phone (610) 362-1200 x 279 or email kfriedman@aciusa.org.