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Figure 2: PCB temperature profile during platen thermal cycling.
The temperature profiles in Figure 2 include the chamber temperature, and it can be clearly seen that this does not change as the platen and PCBs cycle up and down by 2°C, with a nominal chamber temperature of 40°C. This is an important advantage of this approach in that there is no attempt to use transitioning in the humidity chamber condition to create temporary condensing conditions. Furthermore, it can be seen that the transition is rapid in the cooling and heating phase of the cycle, and additionally the low temperature part of the cycle can be sustained indefinitely, or as long as set in the program cycle.
The effect on surface insulation resistance (SIR) on a test PCB with 4 SIR patterns with a 400 μm track and 200μm gap in an ambient condition of 40°C/85% relative humidity (RH) is now explored. This PCB is not conformally coated, the ENIG finished copper is fully exposed. In Figure 3, the effect of cycling to increasingly lower temperatures is presented.
Five sets of data are shown, in which the minimum temperature diminishes by 0.5°C from set to set. In each set the temperature returns to the ambient 40°C. These results show that it is not until the bottom temperature of the cycle reaches 37°C does the reduction in SIR become particularly significant. For the 1.5 to 2.5°C depression of the PCB temperature the level of condensation is only having a minimal effect, but for 3.0 and 3.5°C the condensation is clearly having a significant impact. From this it is clear that the 0.5°C steps provide a fine level of control and that a range of condensing conditions can be readily controlled. The interaction between the ambient condition and the platen temperature can be seen in the comparative results given in Figure 4. At the ambient condition of 40°C/93%RH a platen temperature of 38.5°C now causes significant condensation, which at 85%RH took a platen temperature of 36.5°C. Hence with this experimental arrangement the ambient temperature and humidity and the platen temperature are all independently controlled, and the impact of these factors can be assessed independently. It is also pertinent to note that the platen temperature at which condensation occurs closely agrees with the dew points given in Table 1.
Figure 3: SIR response as the platen was cycled to cooler temperatures.
Figure 4: SIR response at 40oC/93%RH under condensing conditions.
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