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62 PCB007 MAGAZINE I OCTOBER 2019 from the technical datasheet. Typical dwell times are in the range 40–70 minutes, depend- ing on the type of solder mask. If the air temperature is higher, however, things are completely different. Sometimes, for example, the air temperature can be much higher than the recommended temperature of 80°C. This can happen in tunnel tack dry ovens of a certain design. In such cases, the ideal belt speed (dwell time) must be deter- mined experimentally. The optimal transporta- tion speed (conveyor speed) for a good drying depends on the board size, amount of copper, and panel thickness. In practice, the conveyor speed is set depending on panel thickness. The panel thickness can be measured very easily; it is often stated in the accompanying papers. Looking at some real-world examples, let's start with these assumptions: • The PCB is built with FR-4 and copper – Copper density: 8,900 kg/m³ – FR-4 density: 1,850 kg/m³ – Average density of a PCB: 3,965 kg/m³ (assumption: 30% copper) • The heat capacities are as follows – Copper heat capacity: 385 J/kgK – FR-4 heat capacity: 600 J/kgK – Average heat capacity of a PCB: 536 J/kgK (assumption: 30% copper) • We will consider three boards – 0.05 mm, 0.5 mm, and 5 mm – The size is 508 x 609 mm (approximately 20" x 24") We will further assume that the airflow is laminar over the PCB surface and flows over both sides: on the top and bottom of the PCB. The heat transfer coefficient depends strongly on the airflow over the PCB surface. Of course, if the airflow is turbulent, the heat transfer co- efficient is higher, and drying is better, but that turbulent airflow could cause surface defects to appear on the wet solder mask surface. At laminar airflow Re < 10 5 , the calculated heat transfer coefficient is 3 W/m²K at an air- speed of 0.3 m/s and 9 W/m²K at an airspeed of 3m/s. Later, I'll show you how to calculate the heat transfer coefficient. With the given data, it is possible to calculate the time until the board reaches 80°C, for example, if the air- flow temperature is 120°C and the temperature of the board at the beginning is 25°C: Where: C = specific heat capacity of the PCB, Jkg -1 K -1 V = volume, A·d, m³ ρ = density, kgm -3 α = heat transfer coefficient, Wm -2 K -1 θ = temperature of the PCB after time t, K θ L = air temperature (i.e., 120°C, 393 K) θ 0 = temperature of PCB at t=0, 25°C, 298 K A = surface, m² d = board thickness, 0.5 mm/5 mm (0.5; 5 · 10 -3 m ) t = time, seconds And: Where: With the known values, the heating time can be calculated: Table 1 shows the results of these calcula- tions. Table 1: The results of calculations to determine the time to heat a room-temperature PCB to 80°C.

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