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Limited or Reduced Underfill Fillet Size for Improved Flip-Chip PBGA Reliability

IP.com Disclosure Number: IPCOM000009370D
Original Publication Date: 2002-Aug-20
Included in the Prior Art Database: 2002-Aug-20
Document File: 2 page(s) / 457K

Publishing Venue

Motorola

Related People

James Guajardo: AUTHOR [+4]

Abstract

Abstract—Encapsulation of flip chip packages with underfill epoxy is critical for package reliability. However, underfill materials crack during stressing and cause electrical failures. The underfill fillet is essential in lowering the stress on the die and improving moisture resistance in the package, but is mainly where the underfill material cracks originate. By reducing the size of the underfill fillet below a specified limit, the stress reduction on the die and moisture resistance are maintained. The reduced fillet results in fewer cracks and electrical failures thereby improving reliability.

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Limited or Reduced Underfill Fillet Size for Improved Flip-Chip PBGA Reliability

James Guajardo, Brian Sawyer, Bill Stone, Trent Uehling

Abstract—Encapsulation of flip chip packages with underfill epoxy is critical for package reliability. However, underfill materials crack during stressing and cause electrical failures.  The underfill fillet is essential in lowering the stress on the die and improving moisture resistance in the package, but is mainly where the underfill material cracks originate.  By reducing the size of the underfill fillet below a specified limit, the stress reduction on the die and moisture resistance are maintained.  The reduced fillet results in fewer cracks and electrical failures thereby improving reliability.

A typical Flip-Chip Plastic Ball Grid Array (FC-PBGA) assembly process consists of an encapsulation step in which an epoxy is dispensed in a manner that allows it to flow between the silicon die and the substrate.  Often a fillet is added around the perimeter of the die with an additional dispense pass or by "self filleting" properties of the underfill material. 

For a reliable package, underfill materials for Flip-Chip Plastic Ball Grid Array (FC-PBGA) packages must possess a perfect balance of adhesion strength, Coefficient of Thermal Expansion (CTE), Modulus of Elasticity (E) and Fractural Toughness.   In addition, the geometry of the cured material -- especially the fillet around the die plays an important role in the reliability of the assembly.

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