Method for a through-hole preload retention apparatus for BGA packages

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...