Drying and storage of PCBs

Technical informations

Introduction

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:

High water absorption

(>1%)
esp. flexible and rigid polyimide material

Medium water absorption

(>0,1%)
Epoxy systems FR4, FR5

Low water absorption

(<0,1%)
PTFE, LCP

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.

Further informations

Storage conditions

In 2008, the ZVEI/VdL issued guidelines for the storage of printed circuit boards. The following storage temperatures, humidity and packaging are recommended:

More informations

Drying of PCBs

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)

Material	Temperature	Time	Time until soldering process
FR4 Tg 135°C	120 °C	≥120 min	< 24 h
FR4 Tg >135°C, Starrflex, Flex, ML ≥ 6Lg.	130 – 150 °C	≥120 min	< 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

Number and size of copper areas
Moisture can also diffuse in between copper surfaces over a longer period of time, making drying correspondingly more difficult. In these cases, greater attention must be paid to dry storage conditions.
Humidity, processing time and storage conditions after drying directly before soldering
Dry PCBs absorb a large part of the storable water quantity within a few hours under unfavorable conditions.
Soldering process and profile
Soldering ovens with high temperature gradients (especially non-adapted vapor-phase systems) or with a high proportion of radiation (IR soldering ovens) are, from experience, more critical than convection ovens with well-controlled temperature zones.
Material types, resin content, degree of resin filling, copper treatment, processes for copper roughening, pressing process, etc.

These informations are available as PDF download

For further questions regarding storage and drying of printed circuit boards, please contact our technology manager Dr. Lehnberger