Why you should worry about desiccant and humidity indicator cards!
Let’s talk “desiccant” and “humidity indicator cards” this week! “Desi-what” I hear you say???
Seems like we chose the right subject…!
But let’s rewind a little at this point: A few months ago we published a post on Moisture Barrier Bags (MBB) and ESD Control. The article described that Moisture Barrier Bags shield (no pun intended here!) ESD sensitive devices from 2 potential risks:
- The faraday cage created when using these bags correctly protects contents from ESD Damage.
- Specialised layers of film controlling the Moisture Vapour Transfer Rate (MVTR) also protect contents from moisture.
Moisture Barrier Bags
The blog post went on to explain that “desiccant” and “humidity indicator cards” must be used for proper moisture protection.
So what are “desiccant” and “humidity indicator cards” and how are they used? These are the questions we’d like to clarify with 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.
A desiccant pouch
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?
A word of warning before we move forward – the following calculations may look a bit daunting at first sight. But honestly, once you’ve read through it all and calculated a couple of examples, you’ll be fine! Promise!!!
There are 2 different methods you can use – so let’s dive right in:
1. 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
2. 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
Phew – told ya’ it wasn’t that bad! Now that we’ve covered the desiccant, we’ll take a quick look at humidity indicator cards. And I really mean “quick” because they’re much easier to explain/use….
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 colour 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.
Example of a humidity indicator card
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.
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.
Well, that’s it – that’s all we have for today! Did we answer all your questions? If not, let us know your questions in the comments below!