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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 exposure. If the tack drying process is done correctly, 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 environment, two of the three parameters—temperature 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.
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 repercussions by the developing time.
These repercussions manifest themselves in the necessity of longer dwell times in the developer 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).
To read this entire article, which appeared in the October 2019 issue of PCB007 Magazine, click here.