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Pre-applied solder ball reinforcement can eliminate the need
for dispensing secondary underfill in board assembly in some
cases. Normally, CSPs or other BGA packages require adhesive
underfill to glue the component rigidly to the printed circuit
board assembly (PCBA) to prevent failures due to shock. The
underfill acts as a structural component, relieving stress
on the balls due to body forces or mechanical flexure.
If the primary failure of an non-underfilled CSP or other
BGA package occurs at the ball-package interface, the pre-applied
solder ball reinforcement reduces the probability of joint
failure at that interface. The cured reinforcement material
relieves the stress at the ball-substrate interface by reducing
the geometric stress concentration factor at that point. Therefore
the solder joint withstands more load during shock-loading
or thermal cycling.
The ability to jet underfill adhesive between balls on BGAs
as part of the semiconductor packaging process enables this
method of BGA solder ball reinforcement. The high rate of
jetting underfill in a small stream provides a faster and
more cost-effective process than previously proposed methods
of solder ball reinforcement. Jetting the adhesive without
contaminating the solder balls is a one-step process. As a
result, this process allows semiconductor packaging companies
to offer their customers a package with improved reliability
at a lower cost than underfilling in the final printed circuit
board assembly line.
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