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Method for a compliant BGA perimeter gasket to reduce mechanical stress and thermal interface material spillage

IP.com Disclosure Number: IPCOM000007245D
Publication Date: 2002-Mar-06
Document File: 4 page(s) / 37K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method for a compliant ball grid array (BGA) perimeter gasket to reduce mechanical stress and thermal interface material (TIM) spillage. Benefits include improved quality and improved thermal performance.

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Method for a compliant BGA perimeter gasket to reduce mechanical stress and thermal interface material spillage

Disclosed is a method for a compliant ball grid array (BGA) perimeter gasket to reduce mechanical stress and thermal interface material (TIM) spillage. Benefits include improved quality and improved thermal performance.

Background

              Two problems occur with a BGA package on a board assembly that has a heatsink attached to it (see Figure 1):

•             Mechanical stresses

•             Grease spillage

              Shock and vibration are mechanical stresses that are imparted to the BGA substrate, the BGA solder joints, and the printed circuit board (PCB) at the package perimeter during shipping and handling. This problem is becoming more of a challenge as BGA pitch scales to smaller solder-ball diameters.

              TIM, typically a thermal grease, spills over the edge of the BGA substrate, possibly onto the board. This spillage creates cosmetic defects. If the thermal material is electrically conductive and bridges not-common conductors on the board, the functionally of the board assembly is compromised.

              Mechanical stress is conventionally mitigated by board stiffeners and dampeners placed on the underside of the board when mounting in a chassis. However, no package-specific methods exist that mitigate these stresses.

              Grease spillage is conventionally mitigated by strict control of the amount of grease applied to the interface between the die (or package top) and the heatsink bottom. However, spillage is not caused only by excess grease or thin bond-line thickness (BLT). Grease spillage and migration may be driven by gravity or by the oil can effect during thermal or power cycling. Previous studies have shown that controlling the amount of dispensed grease does not prevent spillage. The reliability of the BGA is affected by spillage because it creates shorts between two solder ball joints.  

              Compliant materials include rubber and elastic. Because they are applied after the high-temperature board assembly processes, thermal stability above 100°C is not required.

General description

      The disclosed method utilizes a compliant material framed under and over the substrate of a BGA package, such as an organic land grid array (OLGA). This compliant material fills the interface between the package substrate and the PCB as well as the package substrate and the heatsink attached to the BGA pac...