Best Storage Conditions for PCBs

Most people are aware of the dangers ElectroStatic Discharge (ESD) can pose on a Printed Circuit Board (PCB). A standard bare PCB (meaning that it has no semiconductor components installed) should not be susceptible to ESD damage. However, as soon as you pack it with electronic (semiconductor) devices, it becomes susceptible according to each of the individual component’s susceptibilities. However, there is another risk factor many operators forget: MOSITURE! So, in today’s blog post we are going to address both issues and will explain how you can protect your PCBs from both when storing them.

The problem with moisture

We know by now the problems ESD damage can cause, however Moisture could be a whole new area for you to explore. Let’s take Surface Mounted Devices (SMDs) as an example of why moisture causes a problem. SMDs will absorb moisture, then during the re-flow operation the rapid rise in temperature causes the moisture to expand and delaminating of internal packaging interfaces occur; an issue also known as “pop corning.” Unfortunately, the result is either a circuit board assembly failing testing or prematurely failing in the field.

How to Protect from ESD and Moisture when Storing PCBs

To protect PCBs from both ESD and Moisture they should be stored in a vacuum sealed moisture barrier bag (MBB). To complete what is known as a dry packaging system, Desiccant Packs and Humidity Indicator Cards should be used in addition.

Most manufacturers of the Moisture Sensitive Devices (MSD) will advise how their product should be stored, shipped, etc. However, the IPC/JEDEC J-STD-033D standard describes the standardised levels of floor life exposure for moisture/reflow-sensitive SMD packages along with the handling, packing and shipping requirements necessary to avoid moisture/reflow-related failures.

Both the North American ESD Handbook ESD TR20.20 and European User Guide IEC TR 61340-5-2 mention the importance of moisture barrier bags.

Per section 5.3.4.5.3 Temperature of IEC TR 61340-5-2: “While only specialized materials and structures can control the interior temperature of a package, it is important to take possible temperature exposure into account when shipping electronic parts. It is particularly important to consider what happens to the interior of a package if the environment has high humidity. If the temperature varies across the dew point of the established interior environment of the package, condensation may occur. The interior of a package should either contain desiccant or the air should be evacuated from the package during the sealing process. The package itself should have a low moisture vapour transmission rate (MVTR).”

4 Steps to a Completely Dry ESD Packaging System

1. Moisture Barrier Bag (MBB)

Moisture Barrier Bags (MBBs) work by enclosing a device with a metal or plastic shield that keep moisture vapour from getting inside the bag. They have specialised layers of film that control the Moisture Vapour Transfer Rate (MVTR). The bag also provides static shielding protection.

2. Desiccant

Desiccant is a drying agent which is packaged inside a porous pouch so that the moisture can get through the pouch and be absorbed by the desiccant. It absorbs moisture vapour (humidity) from the air left inside the barrier bag after it has been sealed. Moisture that penetrates the bag will also be absorbed. The desiccant remains dry to the touch even when it is fully saturated with moisture vapour. The recommended amount of desiccant is dependent on the interior surface area of the bag to be used. Use this desiccant calculator to determine the minimum amounts of desiccant to be used with Moisture Barrier Bags.

3. Humidity Indicator Card (HIC)

Humidity Indicator Cards (HICs) are printed with moisture sensitive spots which respond to various levels of humidity with a visible colour change from blue to pink or yellow to brown. The humidity inside barrier bags can be monitored by the HIC inside. Examining the card when you open the bag will indicate the humidity level the components are experiencing so the user can determine if baking the devices is required.

4. Moisture Sensitive Label (MSL)

The Moisture Sensitive Level (MSL) label tells you how long the devices can last outside the bag before they should be soldered onto the board. The label is designed to be applied to the outside of the bag. If the “level” box is blank, look on the barcode label nearby.

Creating a dry package

Now that you know the components of a dry package, you are probably wondering: but how do I put it all together? Not to worry – we’ve got you covered! If you follow these steps, you will create a secure dry package and your PCBs will be protected against ElectroStatic Discharge and moisture.

Place the desiccant and HIC onto the tray stack. Trays carry the devices. Remember to store desiccant in an airtight container until it used.

Place the MSL label on the bag and note the proper level on the label.

Place the tray stack (with desiccant and HIC) into the moisture barrier bag.

Using a vacuum sealer, remove some of the air from the bag, and heat seal the bag closed. It is not good to take all the air out of the bag. Only slight evaluation is needed to allow the bag to fit inside a box.

Now your devices are safe from moisture and ESD.

Conclusion

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

Find the right protective packaging for your sensitive components! Check out the Moisture Barrier Bag Selection Guide and desiccant calculator to find the right packaging products for your application. Still need help creating a dry packaging system? Contact us at Service@DescoEurope.com.

Do you use moisture barrier bags in your facility? What are your experiences? We’d love to hear from you in the comments!