Wave Solder Process
The key profile parameters for the wave soldering process will depend on the type of flux used. OA (Organic Acid) or RMA (Rosin Mildly Active) fluxes typically have such a wide thermal process window that advanced profiling systems like the SlimKIC are often not used. Instead the user simply adjusts the process based on the "look" of the solder joints or perhaps uses a thermal sticker that changes colors when a key temperature is exceeded. The disadvantage of these fluxes is that the board must be cleaned after it is soldered. "No Clean" fluxes have a much tighter thermal process window and are usually profiled regularly. These profil
requirements vary significantly depending on the flux and board type. Usually there is a "flux activation temperature" that the board must rise above for a minimum amount of time. Too low a temperature and/or too short a time at the specified temperature and the flux will not adequately remove the oxides from the board. Too long a time above the specified temperature will cause the flux to be "used up" and cause the board to begin oxidizing. Topside temperatures above 160°C will induce failures in some BGA’s (Ball Grid Arrays).
When the PCB first enters the wave solder machine, it passes through the fluxer and the bottom of the PCB is coated with flux. It then passes over the preheat area, usually made up of 1-3 heated zones. Typically, the PCB should rise no more the 2°C per second in the preheat area, and the temperature of the top side of the board should rise to about 150°C-160°C, or to within at least 100°C of the wave (molten solder) temperature. These are commonly used specifications. We suggest you contact your flux and solder manufacturers for their recommendations.
When passing through the wave solder machine, if the PCB temperature is raised more than 100°C in the wave, the board may warp and component thermal shock may occur. This is especially true of SMT components that are being soldered to the bottom side of the PCB.
The solder in the wave usually has a melting point of 183°C. The wave temperature is usually 249°C-260°C. As the PCB passes over the wave, the bottom of the board is immersed in the solder. The surface tension of the solder causes it to be pulled up into the holes of the PCB, which solders the components into place.
The dwell time over the wave is usually 3-5 seconds. If the dwell time is too short, good wetting will not occur. If the dwell time is too long, the components may overheat. The SlimKIC is capable of taking up to 20 readings per second from each thermocouple on a test board (in data logging mode). Dwell time can be accurately measured by putting a thermocouple through a hole in the board and calculating the time when the TC temperature first starts rising to when it first starts falling. This is a fairly simple task with the Zoom and Pointer tools in the KIC software
When monitoring temperatures with the SlimKIC remote transmitter/data logger, the SlimKIC unit rides over the solder on a board behind the part being profiled, and the SlimKIC itself never touches the solder bath.
After the PCB exits the wave, the temperature drops rapidly and the solder hardens. Some wave solder machines have one or more fans to help cool the PCB before it goes on to the next processing step.
Like solder reflow, when running the first wave solder profile, the machine zone setpoints and conveyor speed are set to some "first guess" temperature, then the board is run through the process. The user then has the initial data for a profile prediction.
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