PCB storage, shelf life and baking before assembly
Boards absorb moisture, and moisture and reflow do not mix. How to store boards, how long they keep and how to bake them.
Why storage matters
FR4 laminate absorbs moisture from the air. During reflow soldering the trapped water turns to steam, and it can cause delamination, blisters or other damage between the layers. The surface finish also ages: oxide builds up on the surface, and solder wets worse the longer a board has been stored in poor conditions.
How to store bare boards
- Keep the boards at a constant temperature and humidity, at most 30 °C and at most 65 % humidity, until the components are mounted.
- Use vacuum-sealed, antistatic, heat-shrink polyethylene film. Optionally add humidity indicators and/or moisture-absorbing sachets; a mechanical support (plywood, cardboard) can be used during storage.
- Keep the storage time as short as possible; use the boards on a FIFO basis (first in – first out) and repack unused boards in vacuum film.
- Open the package only when you are ready to assemble.
- Handle boards with clean gloves, so that the pads are not contaminated by skin oils.
- Do not stack unpackaged boards without interleaving paper or foam.
Boards stored for a long time, or whose transport conditions cannot be established exactly, should be checked with a solderability test. The storage conditions the warranty relies on are the ones on the page Claims, returns and warranty (point 4).
Shelf life
The storage period in vacuum packaging and a controlled environment depends on the surface finish:
| Surface finish | Storage period |
|---|---|
| HASL | 6 – 12 months |
| HAL with lead (Pb) | 6 – 12 months |
| Organic passivation (OSP) | under 6 months |
| Ni/Au | 12 months |
| Immersion tin | under 6 months |
Storing the boards in conditions other than those recommended greatly shortens the maximum storage period; for immersion tin or organic passivation it can drop below 4 weeks. Check the date code on the board and plan assembly accordingly; for large series, order in lots that you will use within that period.
When and how to bake
Baking removes absorbed moisture; it is recommended before the components are mounted, especially when:
- the board has been stored open or for a long time;
- the humidity indicator shows that the limit was exceeded;
- the board is thick or has many layers and will go through lead-free reflow, which is hotter than leaded reflow.
Recommended parameters, in a forced-convection or vacuum drying oven; stand the boards on edge, 10 mm apart, not on top of each other:
| Material | Temperature | Drying time | Time until component mounting |
|---|---|---|---|
| FR4 (Tg 135 °C) | 120 °C | at least 120 min | at most 24 hours |
| FR4 (Tg above 135 °C), rigid-flex, flex, multilayer with 6 layers or more | 130 °C | at least 120 min | at most 8 hours |
In a vacuum oven at 50 mbar the process runs 20 °C lower, and the drying time is at least 60 minutes. Boards with heat-sensitive surfaces (for example immersion tin) are preferably dried in a vacuum oven. Also follow your assembler’s procedure and the laminate data sheet: baking too hot or too long can oxidise the surface finish, so do not bake without a reason.
What to check on arrival
- Packaging intact, humidity indicator in range.
- Date code matches the order.
- Boards flat and free of visible blisters or discolouration.
If something is wrong on arrival, tell us at once and keep the packaging. For the production tests applied before shipping, see electrical test and AOI; for the finish, see lead-free HASL and RoHS. To start an order, use the price calculator.
Frequently asked questions
How long can I store bare PCBs?
It depends on the surface finish, in vacuum packaging and a controlled environment: HASL 6–12 months, Ni/Au 12 months, organic passivation and immersion tin under 6 months. In conditions other than the recommended ones (at most 30 °C and 65 % humidity) the period drops sharply.
Do I always have to bake PCBs before soldering?
Baking before the components are mounted is recommended, especially when the boards have been stored for a long time, the packaging was opened or the humidity indicator shows exceeded limits. Boards stored correctly in vacuum packaging rarely need it.
At what temperature do I bake?
FR4 with Tg 135 °C: 120 °C for at least 120 minutes, mounting within 24 hours. FR4 with Tg above 135 °C, flex or multilayer with 6 layers or more: 130 °C for at least 120 minutes, mounting within 8 hours. In a vacuum oven at 50 mbar, 20 °C lower and at least 60 minutes.
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