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64 PCB007 MAGAZINE I OCTOBER 2019 The conclusion is that a thin board heats up very quickly while a thick board heats up slowly. Consequently, in a tunnel tack dry, the belt speed needs to be set high for thin boards and low for thick boards. While this is rele- vant for the processing of boards with different thicknesses, it is also very important for the construction of tunnel ovens. Airflow The most important criterion for tack dry is the airflow on the solder mask surface. For a giv- en piece of equipment, there is no possibility to increase the airspeed on the surface of the PCB. It is possible to open or to close ventilation flaps on the machinery, and this will affect the airflow and airspeed on the surface of the PCB. Mainly, adjusting the flaps will simply affect the temper - ature distribution on the PCB surface, and it is not advisable to change the flap settings. Even at a long dwell time at an elevated tem- perature, the solder mask will not dry properly if there is insufficient airflow. Higher airflow results in a higher airspeed on the PCB surface, has a strong influence on the heat transfer co- efficient (i.e., the heating-up speed of PCB), and has a strong influence on the evaporating speed of solvent due to a higher difference of partial pressure of solvent in the air and in the diffusion interface on the solder mask surface. The heat transfer coefficient can be calcu- lated from the Nusselt criteria: Which is valid if: And: Nusselt and Reynolds are dimensionless numbers. Thus, the Reynolds criterion is as follows: Where: w = airspeed 0.3 m/s l = length of the PCB (0.6m) ρ = air density at 120°C, 1-bar pressure 0.8878 kg·m -3 η = dynamic viscosity of air at 120°C, 22,792 · 10 -6 Pa.s λ = heat conductivity of air 32,73·10 -3 W/m·K If the airspeed is 3 m/s, then a similar cal- culation gives an increased heat transfer coef- ficient of 9.5 Wm-2K-1. The position of the air- flow relative to the PCB itself can be important. Figure 1 illustrates the differences in evapora- tion efficiency. In the first example, boards are parallel to the airflow resulting in both sides of the board receiving nearly equal airflow. In the second case, only one side of one board is surrounded by good airflow, which means that evaporation will be at different rates. The evaporation speed is different for pure solvents compared to solvent blends and com- pletely different if evaporating a complex sol- vent resin mixture (as in solder mask). The Figure 1: The importance of PCB positioning in a tack-dry oven.