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Method for a Z-axis force-distribution elastomeric gasket for BGA devices with heatsinks

IP.com Disclosure Number: IPCOM000007709D
Publication Date: 2002-Apr-16
Document File: 5 page(s) / 267K

Publishing Venue

The IP.com Prior Art Database

Abstract

The disclosed method is for a Z-axis force-distribution elastomeric gasket for BGA devices with heatsinks. Benefits include improved thermal performance and improved reliability.

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Method for a Z-axis force-distribution elastomeric gasket for BGA devices with heatsinks

The disclosed method is for a Z-axis force-distribution elastomeric gasket for BGA devices with heatsinks. Benefits include improved thermal performance and improved reliability.

Background

              Ball grid arrays (BGAs) can have reliability issues during shock and vibration. Solder balls crack at the corners and outer-row pads. The affected areas are the weakest because these areas are furthest away from the package center. They experience the largest deflection between BGA package and board compared to the inner center package. This failure creates electrical discontinuity from BGA package to the board resulting in board system failure.

              Smaller solder balls on BGAs pose much a greater challenge to reliability. Conventional solder balls are 0.72 mm with a BGA pitch of 1.27 mm. BGAs of 1-mm pitch with packages up to 42.5-mm square use 0.6-mm solder balls. This configuration creates higher stress concentration at the edge of the BGA package due to the smaller solder-joint area and lower solder-ball height.

              With the increasing heat generated from higher-speed CPUs and buses, a larger and heavier heatsink is required to dissipate heat.  Situated near the BGA package, the CPU/heatsink creates a fulcrum at the BGA outer row next to the CPU. This fulcrum causes solder-joint cracks next to the BGA, increasing the reliability issue.

              Solder balls that are under greater compressive force are less likely to fail during vibration or shock because they experience less net tensile force than joints that are not preloaded with compressive force. For flip-chip BGAs (FC-BGAs) using retained heatsinks, the balls below and near the die have greater strength and resistance to cracking. However, balls far from the die have little, if any, compressive force due to the heatsink retention method where the heatsink is sitting on the die and not the package. The perimeter balls are the most highly stressed. The expansion/contraction is due to thermal mismatch and is greatest at the points farthest from the center of the package. The disclosed method adds preload force to highly stressed outer solder balls by placing a elastomeric gasket between the heatsink and FC-BGA package. This gasket delivers the preload of the heatsink evenly across the BGA substrate or at areas that require the most compression.

              A conventional method uses bolts that pull the heatsink down onto the die of the FC-BGA, transferring the preload pressure downward onto the balls underneath the die (see Figure 1). This solution transfers the preload pressure through the die to the center of the package.

              Another conventional method includes a retention spring that crosses the BGA-package top surface and locks using 2 hooks soldered to the board on opposite sides of the BGA package (see Figure 2). The retention spring can be released and reinstalled by pressing at both ends of the spring. However, this conventional solution...