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60 PCB007 MAGAZINE I OCTOBER 2019 Article by Nikolaus Schubkegel Tack drying is a key process step, as it makes the solder mask surface suitable for exposure. Tack drying happens after coating the board surface with a solder mask, but before expo- sure. If the tack drying process is done cor- rectly, the solder mask surface will not adhere to the base plate or the artwork. The tack drying process is controlled through three parameters: temperature, dwell time, and airflow. Practically speaking, in a factory environ- ment, two of the three parameters—tempera- ture and dwell time—are easily controlled. Generally, there is little opportunity to change the airflow. Let's look at temperature and dwell time more in detail. Temperature As a general rule, the tack-dry temperature should be as low as possible; in other words, it should only be as high as necessary. If the temperature is too low, the evaporation rate for the solvent will be to slow, and the solder mask will not dry in a reasonable amount of time. If the temperature is too high, however, the dry time certainly will be excellent, but it could create a solder mask lock-in with reper- cussions by the developing time. These repercussions manifest themselves in the necessity of longer dwell times in the de- veloper and potential solder mask residues on pads and in holes. For example, the LPI solder mask for rigid products from Taiyo recommends temperature range for tack dry between 65–80°C (150– 176°F) at the surface of the PCB. The elevated temperature leads to a higher vapor pressure of the solvent in the diffusion interface on the immediate solder mask surface. Air circulation then takes the solvent vapor away. Also, as a general rule, increasing the temperature by 20°C will double the evaporating speed of the solvent (valid in the range of 20–100°C). Dwell Time At any given temperature, the hold time in the tack-dry oven should be as short as possi- ble. In a well-ventilated convection dryer with an air temperature of 80°C, the dwell time should not exceed the recommended hold time Solder Mask Tack Dry