If you need to avoid baking your surface mount devices (SMD) before solder reflow, storing them in proper dry packaging will be required. A dry package typically consists of three components; an MBB, desiccant, and humidity indicator card (HIC). To determine how dry the package should be, you need to determine your MSL (moisture sensitivity level) and storage times. Information about this and all other factors concerning dry packaging, can be found in the IPC/JEDEC J-Std-033D standard.
1. Moisture Barrier Bag (MBB)
Moisture Barrier Bags (MBBs) control the amount of moisture or moisture vapor that passes through them via heavy metalized or solid foil layers. Unlike a static shielding bag that is metalized also, you cannot see through an MBB. Solid foil layer bags will be dryer or allow less moisture to pass through. The process to quantify the amount is what is called the Moisture Vapor Transmission Rate (MVTR). This is a measurement of grams of moisture through 100 square inches of MBB film. The lower the number the better. The JDEC standard allows for a maximum MVTR of </=0.02 grams. As an example of a bag’s performance, a 3.5 mil metalized MBB is 0.02 whereas a 4.0 mil solid foil bag has an MVTR of 0.0003 or about 100 times dryer.
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 vapor (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 vapor. The recommended amount of desiccant is dependent on the interior surface area of the bag to be used. Use this desiccant form 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 color 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.
Now that you know the components of a dry ESD package, follow these steps to create a secure dry package to ensure 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.
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 or 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 “How to use Desiccants with Moisture Barrier Bags” guide to find the right packaging products for your application. Still need help creating a dry packaging system? Contact us at Service@Desco.com.
Do you use moisture barrier bags in your facility? What are your experiences? We’d love to hear from you in the comments!