Moisture Barrier Bags, Desiccant and Humidity Indicator Cards

SCS Guest Blog

Are your static and moisture sensitive components protected by your packaging? Learn how to minimize potential product failures by protecting your products from Electrostatic Discharge (ESD) and moisture during the manufacture, transportation, and storage process.

Why are Moisture Barrier Bags important?

Moisture Barrier Bags (MBB) shield ESD sensitive devices from 2 potential risks:

The Faraday Cage created when using these bags protects contents from ESD Damage.
Specialized layers of film controlling the Moisture Vapor Transfer Rate (MVTR) also protect contents from moisture.

“Desiccant” and “humidity indicator cards” must be used for proper moisture protection.

But what exactly are “desiccant” and “humidity indicator cards” and how are they used? These are the questions we will clarify in today’s blog post.

What is desiccant?
Desiccant is a drying agent that absorbs moisture from its surrounding area. Desiccant will stay dry to the touch even when it is fully saturated with moisture.

In a Moisture Barrier Bag it is used to ‘soak up’ moisture from the air inside the bag AFTER it has been sealed. Any moisture that gets through the bag from the outside will also be absorbed.

How is desiccant purchased?
Desiccant is available as a “unit” or fractional “unit”. A unit of desiccant absorbs a specific amount of moisture. One unit of desiccant weighs about 28g.

How is desiccant packaged?
Desiccant is packed in small sealed pouches made from a white plastic called “Tyvek” or brown “Kraft” paper. Tyvek pouches are very clean and Sulphur free. Kraft pouches are economical.

Pouches of desiccant are placed into metal pails – this ensures the desiccant is kept dry during transport and storage.

How much desiccant do you need?
There are 2 different methods you can use:

Method 1 per MIL-P-116
Formula: Unit = 0.011 x bag area in square inches
What you need: Bag area (2 times the surface area of your bag as there are 2 sides to a bag)
Example: 10” x 20” MBB bag
Apply formula: 0.011 x (10” x 20” x 2) = 4.4 rounded up to 4.5 units of desiccant
Method 2 per EIA 583 (allows you to tailor desiccant to your specific needs)
Formula: Unit = 0.231 x Bag Area x Bag MVTR x Months divided by Moisture Capacity
What you need: Bag area, Bag MVTR, Months of Storage, Maximum Interior Humidity (MIH), Moisture capacity table below:

10% MIH
3.0 g/unit
20% MIH
4.8 g/unit
30% MIH
5.8 g/unit
40% MIH
6.2 g/unit

Example: 10” x 20” bag with a 0.02 MVTR, a 12 month storage time and a MIH of 20%
Apply formula: 0.231 x (10? x 20? x 2) x (0.02) x (12/4.8) = 4.62 rounded down to 4.5 units of desiccant

What is a humidity indicator card?

A humidity indicator card allows for quick visual inspection of the relative humidity levels within its surrounding area. They are printed with moisture sensitive spots which respond to various levels of humidity with a visible color change from blue to pink.

In a Moisture Barrier Bag they provide a low-cost method of verifying the effectiveness of the moisture barrier packaging. If you are using Moisture Barrier Bags, moisture will be an issue in your application so you’re obviously aiming for as little moisture as possible. However, if you happen to open your MBB and the humidity indicator card shows a relative humidity of 60%, you’ll know that the contents of your bag have been exposed to moisture and may not be safe for use anymore.

How are humidity indicator cards purchased?
Humidity indicator cards come in many shapes and forms. Some will show relative humidity from 10% – 60%; others from 5% to 15%. Depending on the sensitivity of your application to moisture, the correct type of card should be chosen.

Bear in mind that not all humidity indicator cards are reversible. Some cards will measure the relative humidity only once and then halt at that reading. These types of humidity indicator cards are NOT re-usable. This is important to know so make sure you check before purchasing!

How are humidity indicator cards packaged?
Humidity indicator cards are sold in containers. It is recommended that cards are stored in their original un-opened canister in a dry, well ventilated room with a reasonably consistent temperature of 20°C. Humidity indicator cards should not be stored in ultraviolet sunlight, moisture or heat.

How many humidity indicator cards do you need?
One humidity indicator card per MBB is needed for proper verification of relative humidity.

Conclusion
Moisture Barrier Bags, desiccant and humidity indicator cards all play a very unique and important role when protecting ESD sensitive devices from moisture.

They should always be used together to ensure maximum protection. However, remember that all three tools need to be used correctly as otherwise all your efforts have been in vain. And don’t forget: your Moisture Barrier Bag must be heat sealed with a vacuum sealer to eliminate the amount of “moisture laden air” within the package.