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Moisture-Sensitive Components Need Sensible Handling

Moisture-Sensitive Components Need Sensible Handling

Michael Allen, President, Z-Axis 25 november 2013, Ebnonline.com

As moisture-sensitive components become more prevalent in the supply chain, low- and mid-volume manufacturers need to adjust their materials handling and manufacturing processes to prevent manufacturing defects and early-life failures.

With smaller package sizes becoming mainstream in all levels of product design, more mid-volume and low-volume manufacturers are encountering moisture-sensitive parts. We are seeing ICs, LEDs, and even connectors that require special handling.

Moisture absorbed from the air by a plastic IC package expands rapidly during solder reflow. Smaller, thinner packages with less space around the die are most prone to damage. Sometimes it is visible, such as when packages burst open (sometimes called the popcorn effect). Other times, internal damage such as delamination or cracking is invisible to inspection but can cause early-life product failure.

The JEDEC J-STD-020 standard defines moisture sensitivity levels (MSLs) and corresponding floor life, or how long a part with a given MSL can remain outside dry storage before reflow. Levels range from MSL 2 (a floor life of one year) to MSL 5A (a floor life of 24 hours). MSL 1 parts are not moisture-sensitive, and, in the extreme, MSL 6 parts must always be baked before use.

Parts that exceed their floor life must have the moisture baked out slowly before solder reflow. Default bake cycles range from 48 hours at 125°C to 79 days at 40°C, but they can be reduced depending on MSL and package thickness, according to J-STD-020.

The floor life limit adds complexity to manufacturing and component storage. Your contract manufacturer needs procedures to monitor cumulative floor time and environmental conditions, as well as equipment such as vacuum systems for resealing bags, dry-box storage areas, and bake ovens for parts and boards. At Z-AXIS, moisture-sensitive parts arrive sealed in moisture barrier bags with desiccant and a humidity indicator card. The MSL is printed on the bag label (see photo). We inspect the bags for tears and store them unopened until needed.

Upon opening the bag, we check the humidity indicator card to confirm the parts have not been exposed to moisture during storage. We then assemble and reflow the boards within the floor life window. If a second reflow is needed, it must be done within the floor life window, or the entire board must be baked between reflow operations.

Floor life is cumulative. Tracking cumulative floor life is easier in high-volume manufacturing, where entire reels of parts are used in a single run. But in high-mix manufacturing, we build products in many smaller runs over the course of a year or more. It is very common for us to return partially used reels to storage after each run. Partial reels are resealed in moisture barrier bags with a humidity indicator card and fresh desiccant. The bags are heat sealed with light air evacuation. Every time a bag is opened and resealed, we record the time and date and track cumulative exposure time.

MSL ratings are at ≤ 30°C and 60% relative humidity, so we have to derate the floor life if parts are exposed to higher levels. Conversely, lower factory temperature and humidity will extend floor life by an amount depending on package size and thickness, per the J-STD-033C standard. We could conceivably extend the floor life of MSL 2a parts from 28 days to indefinite by maintaining relative humidity of less than 30%, but this would conflict with the need to maintain relative humidity more than 30% for ESD protection.

By now, you might be wondering why we don’t just bake moisture-sensitive parts before use. Unfortunately, this hurts solderability, material flow, and production time. It is better to store moisture-sensitive parts properly, keep careful records of cumulative floor time, and schedule production accordingly.

If you supply moisture-sensitive parts to your contract manufacturer, make sure that your own handling procedures are compliant, and that you provide accurate documentation to your partner. We have received moisture-sensitive parts in loose bags without desiccant, requiring us to bake before use. We have received partial reels of parts sealed in moisture-barrier bags with a vacuum high enough to crack the reel. Improper handling leads to production delays, higher costs (if your manufacturer passes them on to you), and potential product failures.

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