A desirable property of plastics is to be impermeable to water. However, this is only true to a limited extent and depends on the type of plastic, so that multilayer composite systems or metalized films are used, for example, for packaging of moisture-sensitive goods with a long storage time (pharmaceuticals, food).
The reason for the conduction of moisture through films is the ability of the materials to absorb water molecules and pass them on by diffusion.
In the case of printed circuit boards, however, moisture does not only play a role in packaging. In the case of the base materials themselves, water absorption leads to the undesirable side effect of moisture escaping during the soldering process. If the water content is too high and the temperature rises too quickly, delamination of the base material occurs. Lower adhesive strengths additionally promote these failures.
Base materials can be roughly divided into three classes with regard to water absorption:
The amount of water absorbed can be easily determined by weighing the printed circuit boards before and after drying. In this way, temporal phenomena can also be investigated. When weighing, it is important to use sufficiently accurate scales and to ensure that the dried printed circuit boards have cooled sufficiently, since measurement errors are caused by air turbulence during cooling in the scales.
The aim of correct storage and drying is to ensure that PCBs are sufficiently dry during the soldering process. In addition, the solderability of the surfaces must be maintained.
In 2008, the ZVEI/VdL issued guidelines for the storage of printed circuit boards. The following storage temperatures, humidity and packaging are recommended:
The ZVEI/VdL also issued guidelines for the drying of printed circuit boards in 2008.
The following drying parameters are recommended:
(Drying in convection/air-circulation oven or in vacuum drying oven, not in a stack)
|Time until soldering process
FR4 Tg 135°C
< 24 h
FR4 Tg >135°C, Starrflex, Flex, ML ≥ 6Lg.
130 – 150 °C
< 8 h
Vacuum drying at 50 mbar allows 20K lower temperatures and 60 minutes shorter times and is recommended for thermally sensitive surfaces (chem. tin).
During drying, the glass transition temperatures Tg should not be exceeded. Otherwise, this is referred to as annealing, in which the PCBs are subjected to significantly higher thermal and thermomechanical stresses.
The data are rough guide values, since there are hardly quantifiable dependencies on
For further questions regarding storage and drying of printed circuit boards, please contact our technology manager Dr. Lehnberger