Printed circuit boards – in particular multilayer, flexible and rigid-flex ones – are extremely hygroscopic, i.e. they can absorb moisture from the surrounding air and bind it. For example, a dried polyimide film has already reached its moisture saturation point after a few hours.

Problem:

Moisture in a circuit board can lead to explosive vaporization during the soldering process. The sudden evaporation of moisture in a PCB can cause failures due to delamination, blistering, cracks, etc. The problem is aggravated by lead-free soldering processes due to the higher temperatures.

Storage recommendation:

The PCBs should be packed and stored in a dry room at a uniform temperature. Note that a rapid temperature drop of more than 7 °C can already cause condensation on the packed PCB.  

Drying:

Although our products are dried during the manufacturing process, it must be noted that transport conditions in particular are not predictable (PCBs are shipped in all weather conditions and any temperature). What‘s more, storage conditions are not always optimal. Therefore it is recommended to dry PCBs before processing. PCBs must not be dried on the stack, but standing freely. They should be processed immediately after annealing because their hygroscopic properties remain.

It is important to note that due to the heat effect on the immersion tin and OSP surfaces, an artificial ageing process is triggered, which can adversely affect the soldering behaviour. It is generally assumed that a drying period of 2 hours at 120 °C corresponds to an ageing of about 2 months.

Recommended parameters for drying
Single-sided
and double-
sided FR4
Multilayer
FR4
Multilayer
Hybrid
Flexible
up to
2 layers
Flexible
from
3 layers
Rigid-Flex
up to
4 layers
Rigid-flex
5 to 8 layers
Rigid-flex
more than
8 layers
120°C
2 h
120°C
2 h - 4 h**
120°C
2 h – 8 h*
120°C
2 h
120°C
2 h – 4 h**
120°C
2 h – 4 h**
120°C
4 h – 6 h**
120°C
4 h – 8 h**
Maximum processing times after drying
Single-sided
and double-
sided FR4
Multilayer
FR4
Multilayer
Hybrid
Flexible
up to
2 layers
Flexible
from
3 layers
Rigid-flex
up to
4 layers
Rigid-flex
5 to 8 layers
Rigid-flex
more than
8 layers
24 h 8 h 8 h 8 h 6 h 6 h 6 h 6 h

* Due to the use of different materials, the time required for the drying process can greatly vary, as the hygroscopic properties of the materials used in hybrid structures may differ from FR4. This also applies to edge contacts.

** The required time depends on the thickness of the flexible layers, the acrylic adhesive thickness on the cover layer, as well as the layout. If large copper areas are present – especially on the outer layers, but also in the middle layers – moisture removal from the printed circuit board is delayed because the copper seals these areas. This also applies to edge contacts.

ILFA Drying_Storage Recommendations PCB