|
Hello,
Welcome to the assembly forum!
Popcorn cracking simply refers to package cracking of a
surface mount device caused by excessive moisture
ingress into the package and extremely high board
soldering peak temperature. All packages absorb moisture
from the air around it, with the absorption rate
depending on a multitude of factors: package
characteristics, relative humidity, ambient temperature,
etc. It turns out that some packages, especially the
thinner ones, are more prone to absorb moisture from its
surroundings than others.
Excessive moisture inside a package won't be a problem
until the package undergoes board soldering. During
board soldering (usually by IR reflow), the high
temperature vaporizes the moisture inside the package.
The violence with which the trapped vapor will try to
escape from the package will result in internal
thermomechanical stresses that are strong enough to
delaminate the package.
Package delamination is the only way these internal
stresses may be relieved, so delam is almost always
expected in moisture-sensitive units after
board-mounting. The delam occurs at the die paddle-mold
interface, and may cover the entire die paddle in
extreme cases.
Some packages are structurally weak that this
delamination extends into package cracking before the
internal stresses are relieved. This cracking mechanism
is so fast, that it even becomes audible, which is why
this has been known as 'popcorn cracking.' The cracking
originates from the corners of the die paddle and
propagates downward at a 45-degree angle.
Prevention of excessive moisture absorption prior to
board-mounting is necessary to preclude packages from
exhibiting popcorn cracking. Moisture-sensitive devices
must undergo bake and dry-packing prior to shipment to
customers, and must be board-mounted by manufacturers
within a prescribed number of hours once taken out of
the bag. Thus, the moisture sensitivity level of any new
package must be determined prior to mass-production and
shipment to customers.
More power to you!
EE |
