Since the introduction of automated PCB assembly and component lead soldering, wave soldering has been the mainstream method of applying solder automatically to the bottom The PCB is passed though the tunnel at a constant speed, with the temperature of each zone precisely controlled. Temperature profiling is a key factor in the operation of a reflow machine to ensure consistent and high reliability solder joints. As part of component failure analysis, it is good practice to request a reflow profile from the machine operator to reveal the actual thermal history of the process. Temperature profiling is achieved by attaching thermocouples to a test PCB and connecting them to a data recording instrument. Temperature monitoring thermocouples must be fixed firmly to the component leads and PCB, because the circulating heated air will be approximately 30°C hotter than the monitored surfaces. A previously assembled PCB or test board is used to measure and store the time and temperatures of each thermocouple as the PCB passes through the oven. This process is repeated several times with adjustments to conveyor speed, heater energy and air flow until the desired profile is achieved. Profile settings are stored in the oven controller for specific PCBs and reactivated when required. Figure 8-7 shows a general target temperature profile shape for three heating zones, plus a cooling zone.
Zone 1 preconditions the component and PCB, with a ramp rate of not greater than 4°C per second, to a maximum temperature of 150°C. Too rapid heating will thermally shock the components and PCB, causing delamination, bond wire stress/breaking and micro cracking of sensitive ceramic based components.
Zone 2 is the activation zone, since it activates the flux in the solder paste and allows unwanted binder and solvents to evaporate. It also allows time for temperature Homogenisation. The heating rate should be low if the size and colour of the components vary, but with uniformity, it may be 1 – 2 ºC per second. If the soaking temperature is too high, premature evaporation and oxidation of the solder paste can impair solder-ability.
In Zone 3, the reflow zone, the paste is taken above the solder melting point allowing the component and PCB to form solder joints. The ramp rate from soak temperature to peak reflow again should not exceed 4°C per second. Maximum reflow temperature is set at approximately 25°C above the melting point of the solder ensuring that the component soldering specification, maximum temperature and time at maximum temperature are not exceeded, most components are specified in a reflow process with an absolute maximum temperature of 260°C for 10 seconds, with two passes permitted. The reflow maximum temperature window is set by the component and PCB specifications, and the minimum is determined by the solder paste to produce a satisfactory joint.
Ideally in Zone 4, the cooling rate should mirror the heating rate. Forced cooling is preferred, rather than natural cooling to ambient. Again, there is a compromise between rapid cooling to optimise solder joint strength and reliability, and avoiding thermal shock.
