Desiccant and humidity indicator card: how much does a moisture barrier bag need?
A moisture barrier bag for MSD to J-STD-033 needs enough desiccant to absorb residual air moisture and the water vapour permeating the film over the whole storage period. The rule of thumb is 0.011 units per square inch of bag area; a precise calculation uses WVTR, storage time and interior humidity. The indicator card confirms it.
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Why are moisture sensitive devices dry packed?
Plastic SMD packages absorb moisture from the air. During reflow this water turns to steam almost instantly, and the vapour pressure can crack the package or delaminate internal layers (popcorning). IPC/JEDEC J-STD-033 therefore defines how moisture sensitive devices (MSD) are dry packed with barrier bag, desiccant and indicator card, stored and baked when necessary.
How long a component may stay exposed is set by its moisture sensitivity level (MSL) to J-STD-020. Dry packing stops that clock during transport and storage.
What is a unit of desiccant?
Desiccant is specified in units to MIL-D-3464, not by weight. One unit is the quantity that adsorbs a defined amount of water under set conditions and weighs roughly 28 to 33 g depending on the material. How much water it actually holds depends on the highest humidity allowed inside the bag.
This limit is the maximum interior humidity (MIH). The stricter it is, the less water one unit holds:
| Maximum interior humidity | Capacity per unit |
|---|---|
| 10 % RH | 3.0 g |
| 20 % RH | 4.8 g |
| 30 % RH | 5.8 g |
| 40 % RH | 6.2 g |
Kraft paper pouches are economical; spunbonded polyethylene pouches are cleaner, sulphur free and lint free, so they suit assemblies with silver surfaces.
Moisture barrier bagsMetallised barrier bags with low water vapour transmission for the dry pack.
View productsHow much desiccant does a moisture barrier bag need?
For a quick estimate, use the MIL-P-116 rule of thumb: units = 0.011 x bag area in square inches. To include storage time and film quality, calculate to EIA-583 with water vapour transmission rate (WVTR), months and permitted interior humidity. Both give similar results for a typical bag; always round up.
Worked example: 10 x 20 inch bag
Bag area counts both sides: 10 x 20 x 2 = 400 square inches (about 254 x 508 mm). The detailed calculation assumes a WVTR of 0.02 g per 100 square inches per 24 h, twelve months and 20 % RH maximum interior humidity.
| Method | Formula | Result |
|---|---|---|
| Rule of thumb MIL-P-116 | 0.011 x 400 | 4.4 units, use 4.5 |
| Calculation EIA-583 | 0.231 x 400 x 0.02 x 12 / 4.8 | 4.6 units, use 5 |
- At a 10 % RH target, capacity drops to 3.0 g per unit and the example needs more than 7 units.
- Trays, foam and cardboard bring their own moisture: dry them first or add desiccant.
Film, bag size and seal are covered in the guide Moisture barrier bags for MSD.
How do you read a humidity indicator card?
A humidity indicator card (HIC) carries spots for defined humidity levels, to J-STD-033 for 5, 10 and 60 % RH. A spot changes colour once the bag humidity exceeds its value; the actual humidity is at the spot showing the transition shade. Read the card immediately after opening.
- Cobalt chloride cards change from blue to pink (transition lavender), cobalt-free cards from brown to yellow (transition tan).
- The 5 % spot serves as a reference. If the 10 % spot has changed, components of MSL 2a to 5a must be baked before soldering. A changed 60 % spot affects MSL 2 as well.
- The spots respond within eight hours and are most accurate at 23 °C.
- Before packing, the highest spot must still show the dry colour.
Cards with other steps, such as 30/40/50 % RH, are for general humidity monitoring. Why room humidity also matters for charge dissipation is explained in the guide Humidity in ESD control.
How should desiccant, cards and bags be stored and reactivated?
Desiccant, indicator cards and barrier bags have no fixed shelf life, but poor storage reduces their effect. Keep all three in closed original packaging in a dry, ventilated room at a steady 20 to 24 °C, away from sunlight and heat. Check stock older than one year before use.
Desiccant
- Use stock on a FIFO basis and record the delivery date as reference.
- Saturated desiccant can be regenerated by drying to the supplier instructions; weighing shows whether it has regained capacity.
Humidity indicator cards
- Store in the original can with the enclosed desiccant, which is replaced after three openings.
- Keep away from water, steam and ammonia.
- Changed spots return to the dry colour within four to 24 hours in a sealed container with one unit of fresh desiccant, or faster after three hours at 180 °F (about 82 °C) without lid and desiccant pouch.
Barrier bags
The aluminium layer of metallised bags can oxidise under UV, humidity and heat. Discard bags with tears, creases or scratches. The differences between barrier and shielding bags are explained in the guide Shielding bag or pink poly.
Vacuum or nitrogen when sealing?
Neither replaces the desiccant. A light vacuum reduces the trapped air and moisture, makes the pack compact and reveals later leaks through loss of tension. Nitrogen displaces room air before sealing. In both cases a continuous, crease-free heat seal is what counts.
- No hard vacuum: reel and tray edges can puncture the film.
- Take the desiccant out of its container just before sealing, otherwise it already absorbs moisture from the room air.
- Check the seal for creases and channels; a vacuum pack must not slacken after hours.
Frequently asked questions
Can desiccant be reused?
Yes, saturated desiccant can be regenerated by drying. It should not move unchecked from an opened MSD bag into a new dry pack, because its remaining capacity is unknown. Weighing or fresh stock removes the doubt.
How long does a humidity indicator card last?
There is no fixed expiry date. Keep cards in the sealed original can with desiccant, replace it after three openings and check that the highest spot shows the dry colour before use. Discard cards that reached 60 % RH.
Why are there cobalt-free indicator cards?
Traditional cards contain cobalt dichloride, a hazardous substance with an occupational exposure limit. Cobalt-free cards use another indicator, change from brown to yellow and serve the same function.
Does vacuum packing replace desiccant?
No. Vacuum only reduces the trapped air. Water vapour still permeates the film slowly and has to be absorbed by the desiccant. The indicator card belongs in vacuum packed bags as well.
Looking for desiccant for your dry packs?
Desiccant pouches in several unit sizes for barrier bags to J-STD-033. Questions about selection? Write to us.
Standards based
Dry packing to J-STD-033, units to MIL-D-3464.
Traceable
Desiccant quantity by rule of thumb and WVTR calculation.
Verifiable
The indicator card shows humidity on opening.
Selection help
We help you choose the right products; technical details are clarified with the manufacturer.