Publishing Venue
The IP.com Prior Art Database
Abstract
Disclosed is a method for a through-hole preload retention apparatus for BGA packages. Benefits include improved solder joint and package reliability.
Method for a through-hole preload retention apparatus for
BGA packages
Disclosed is a method for a through-hole preload retention
apparatus for BGA packages. Benefits include improved solder joint and package
reliability.
Background`
BGA
packages and boards have had reliability issues during shock and vibration
testing with their corner and outside row pads creating solder ball cracks at
the pad/solder ball interface. The affected areas are the weakest areas because
they are furthest away from the package center and experience the most
deflection between the BGA package and board compared to the inner center
package. This failure creates electrical discontinuity from the BGA package to
the board and vice versa and results in board system failure.
For
example, a desktop product has an issue with heat-sink anchors pulling out of
the PCB during temperature cycling. This failure occurs because of a large
downward preload that is applied to decrease solder ball stress between the BGA
package and board that contributes to board flex during shipping. A preload of
47 lbs is conventionally applied to the BGA chip set so that the solder joints
do not crack during shock and vibration testing. This preload causes board
warping and reliability issues with the spring anchors that hold the heatsink
to the PCB. The preload is inefficient because it pushes on each solder ball in
the same amount even though the most stressed balls are in the corner and/or
edge of the BGA.
Smaller
solder balls on BGAs pose a greater challenge to reliability. Smaller 1-mm
pitch BGAs with a package size of 42.5-mm square use even smaller balls of
0.6-mm in diameter compared to the conventional 0.72-mm, 1.27-mm pitch BGA
package. Greater reliability issues result from the higher stress concentration
at the edge of the BGA package due to the smaller joint area and lessened
solder-ball height.
With
higher heat generated by a higher CPU speed, a larger and heavier heat sink is
required to dissipate heat. The reliability issues on the BGA package increase
because the heat sink is situated near to the BGA package. This situation also
creates a fulcrum at the BGA outer row next to the processor adding stress to
those specific solder balls. This fulcrum then causes solders joint cracks on
the corner and outer row of the BGA.
The conventional
method for addressing heat-sink anchor pullout is a retention spring that
presses across the BGA package top surface and locks on both sides at two hooks
soldered to the board next to the edge of the BGA package. Figure 1 shows a
retention spring that can be released and installed by pressing at both end of
the spring. However, this conventional method focuses the retention force on
the center of the BGA package and does not apply the retention force at the correct
high stress point at the edge and corners of the BGA package. This approach
makes the conventional solution an inefficient method of applying preloaded
downward force on the BGA package an...