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Today we're going to talk about the storage and handling of moisture
sensitive devices.
Any plastic IC package will absorb moisture from the air
When the package is exposed to the sudden high temperatures of solder reflow
this moisture can expand rapidly and damage the component.
Sometimes the damage is visible such as cracks in the package
other times its internal and it's not visible, but it can mean early product failure.
The smaller, thinner packages with less space around the die
are more prone to this type of damage.
Moisture sensor devices can be ICs,
LEDs and even connectors. They require special handling and record keeping.
These parts arrive at our facility in special moisture barrier bags.
The bags are labeled with the moisture sensitivity level and other information.
We inspect the bags for tears and store them until we need the parts for assembly.
We don't open the bag until we are ready to use the parts.
Each bag should contain a humidity indicator card
which we use to confirm that the parts have not been exposed to moisture
during storage.
Then we have to complete all assembly and solder reflow operations
within the floor life window of the part. The floor life is the amount of time
a part can remain outside the bag
before it has a chance to absorb enough moisture to make it susceptible to
damage during soldering.
This can be as little as 24 hours or as long as a year
depending on the part's moisture sensitivity level as well as the
temperature
and humidity in the factory. If a second reflow operation is needed
it also has to be done within the floor life window.
In a high-mix manufacturing environment we often use a batch of parts in multiple
production runs which can span over months or years.
So between runs we return the parts to the moisture barrier bag
add fresh desiccant and the humidity indicator card
then we heat seal the bags with a light vacuum.
Floor life is cumulative, so we have to keep careful records of
every production run to ensure that we don't exceed the total floor life.
The bag is marked with a seal date
and returned to storage. If the floor life is exceeded
the part has to be baked at a constant temperature and five percent relative
humidity
The bake oven gently and slowly evaporates, or "bakes off"
any moisture inside the package.
Temperature and duration of baking depend on the part's moisture sensitivity level,
the package type and thickness, and by how much it has exceeded its floor life.
If you supply moisture sensitive devices to your contract manufacture
be sure to follow proper handling procedures and provide accurate
documentation.
We have received parts which were over-vacuumed during sealing
compressing the reel and causing some of the parts to flip
and twist inside the tape.
We've received moisture sensitive devices on reels that couldn't handle the
specified bake temperature.
And we have received moisture sensitive parts in loose bags without desiccant
which means we have to bake them before we can use them. Baking can cause oxidation
that reduces component solderablity. And it impacts our material flow
and production schedules. So we reduce the need for baking
and the risk of part damage by storing parts properly,
keeping careful records of cumulative floor time, and scheduling production
accordingly.
You can learn more about
handling moisture sensitive devices in the article in the links.