Drying / Tempering
Printed Circuit Boards are hygroscopic and can therefore absorb moisture from the air during transport or storage. This moisture can lead to delamination and other damage when soldering the components.
In order to avoid failures, the circuit boards should be dried immediately before processing to remove moisture.
The temperature and the holding time during drying are determined by the base materials / layer constructions used. High-Tg, multilayer boards as well as Flex- and Rigid-Flex PCBs have a higher moisture absorption.
Please note: All data on the drying of circuit boards are of a general nature. If you have done without drying / tempering, you do not have to change your specifications. The last word is always at the assembly partner!
Drying of Printed Circuit Boards
Drying the circuit boards in the stack is not recommended (better slit board or similar). For a continuous drying process it is also important to ensure that the TOP and BOT side is not covered with plane copper. It is recommended to rasterize the copper area.
Details: ZVEI - " Recommendations for drying bare PCBs"
The specifications of the assembly company should always take precedence.
| 1- / 2- layer PCBs | Multilayer | Flex / Rigid-Flex | |||||
|---|---|---|---|---|---|---|---|
| Drying in convection oven | 1- / 2- layer PCBs | 100°-120° 2h - 4h | Multilayer | 100°-120° 2h - 4h | Flex / Rigid-Flex | 130°-150° 2h - 4h | |
| Drying in vacuum oven | 1- / 2- layer PCBs | 80°-100° 2h - 3h | Multilayer | 80°-100° 2h - 3h | Flex / Rigid-Flex | 130°-150° 2h - 3h | |
| Time to soldering | 1- / 2- layer PCBs | max. 24h | Multilayer | max. 8h | Flex / Rigid-Flex | immediately after | |
| Drying recommendations | 1- / 2- layer PCBs | acc. to storage (see below) | Multilayer | recommended | Flex / Rigid-Flex | absolutely necessary |
Tempering of Printed Circuit Boards
The terms drying and tempering are often colloquially equated, but mean different concepts. Tempering stands for the heating of the circuit board with elevated temperatures, usually Tg value + 10%. It is used to take mechanical stresses out of the material, which shows in twist & bow.
In the circuit board production, thick circuit boards (e.g. > 3.2mm) are tempered to about 150°C prior to the HAL process, to get close to the process temperature during tinning, thus achieving a short dipping time.
Definition of drying and tempering:
Definition Drying: heat treatment below Tg
Definition Tempering: heat treatment above Tg
DRYING < Tg-value < TEMPERING
The Tg value of a circuit board indicates the glass transition temperature of the base material, see also Printed Circuit Board materials
Drying of Flex- / Rigid-flex Circuit Boards
Flex and Rigid-flex circuit boards (polyimide materials) have increased moisture absorption compared to rigid circuit boards. Drying is therfore absolutely recommendable.
Please note
Internal layers dry more slowly than external layers.
Vacuum drying at a lower temperature is not recommended.
Storage of Printed Circuit Boards
Printed Circuit Boards should be stored in an airtight condition in a dry place of relative humidity less than 50%, at a temperature of 18 - 22 degrees Celsius.
The maximum storage times of the respective surface finish must be considered. If the permissible storage time is exceeded, a soldering test must be carried out before assembly and soldering.
The storage time of printed circuit boards should be as short as possible, taking into account the FiFo rule ("first-in", "first-out"). Ideally, unprocessed circuit boards are welded back into a protective bag.
Moisture absorption
When stored at 22°C and 50% rel. humidity, the saturation is 0.2% to 0.3%
When stored at 40°C and 96% rel. humidity, the saturation is 0.3% to 0.7%
High-Tg and halogen-free material as well as multilayers absorb much more moisture.
Notes
The drying process must be qualified by the assembly company. The recommendations for drying can only be used as a reference or rough guideline.
Both the circuit board design as well as the specific ambient, drying and soldering conditions play an important role.
Multi-CB can give no guarantee for the specified values.
