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 stuff it with electronic (semiconductor) devices, it becomes susceptible according to each of the individual’s susceptibility.
However, there is another risk factor many operators forget: moisture.
So, today’s blog post is going to address both issues and will explain how you can protect your PCBs from both when storing them.

The problem with moisture
By now you will be well aware of the problems ESD damage can cause.
Moisture, on the other hand, may be a new issue to you. Surface Mounted Devices (SMDs), for example, absorb moisture and then during solder re-flow operations, the rapid rise in temperature causes the moisture to expand and the delaminating of internal package interfaces, also known as “pop corning.” The result is either a circuit board assembly that will fail testing or can prematurely fail in the field.

The problems moisture causes in SMDsMoisture from air diffuses inside the plastic body & collects in spaces between body & circuit, lead frame and wires. Expanding vapour can crack (popcorn) the plastic body or cause delamination.

Storing PCBs
All PCBs should be stored in a moisture barrier bag (MBB) that’s vacuum sealed. In addition to the bags, Desiccant Packs and Humidity Indicator Cards must be used for proper moisture protection. This ‘package’ is also known as a dry package.
Most manufacturers of the Moisture Sensitive Devices (MSD) will dictate how their product should be stored, shipped, etc. However, the IPC/JEDEC J-STD-033B 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. The ESD Handbook ESD TR20.20 mentions the importance of moisture barrier bags in section 5.4.3.2.2 Temperature: “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 WVTR.

Components of a dry package
A dry package has four parts:

1. Moisture Barrier Bag (MBB)
2.
Desiccant
3.
Humidity Indicator Card (HIC)
4.
Moisture Sensitive Label (MSL)

Moisture Barrier Bag (MBB)

Moisture Barrier Bags (MBB) 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.

204519
Desiccant is a drying agent which is packaged inside a porous pouch so that the moisture can get through the pouch and be absorb by the desiccant. Desiccant 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. 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.

Humidity Indicator Cards (HICs)


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. 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.

The Moisture Sensitive Level (MSL) label


The Moisture Sensitive Level (MSL) label tells you how long the devices can stay outside the bag before they should be soldered onto the board. This label is 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’re 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 air tight container until it used.
Dry Packaging - Step 1

Place the MSL label on the bag and note the proper level on the label.
Dry Packaging - Step 2

Place the tray stack (with desiccant and HIC) into the moisture barrier bag.
Dry Packaging - Step 3

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.
Dry Packaging - Step 4

Now your devices are safe from moisture and ESD.
Dry Packaging - Step 5

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

And that’s a wrap! Just to let everyone know that we will be taking a little summer break over the next few weeks so there won’t be any new posts going up until the end of September. Don’t miss us too much…

 

 

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About descoeurope

Desco Europe is the newest brand in the Desco Industries family, consolidating our two previous UK-based brands, Charleswater and Vermason. Charleswater, originally located in W. Newton, MA, was acquired in 1990. After several years of offering both Desco and Charleswater products in the US, Charleswater was set up to be the leading brand in the UK with the acquisition of Conductive Products in 1998. Vermason was a manufacturer of ESD protection products and was founded in Letchworth in 1979. In April 2007 Desco Industries, Inc. of Chino California acquired 100% of the shares of Vermason Ltd. Vermason is now a division of Desco Industries, Inc. Today, as Desco Europe, we retain our manufacturing capability in Letchworth, Hertfordshire, with sales and administration being split between Letchworth and our Crowborough, East Sussex office. Desco Europe sells the full range of DII products, manufactured in our facilities in the USA and UK, servicing the European market via trained distributors who will sell the Desco Europe value package and complete ESD solution to all ESD users in their territory, leading with hi-end solutions that mark us out from the competition. Desco Europe’s UK-based inventory coupled with that of our distributors means that we are in a unique position to support our Customers across Europe no matter what their needs.

Posted on 2017-08-17, in Articles, Bags and Labels, ESD Information, Packaging, Handling and Storage, Resources. Bookmark the permalink. Leave a comment.

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