Browse Prior Art Database

QFN Solder Joint Life Improvement Innovation

IP.com Disclosure Number: IPCOM000012188D
Original Publication Date: 2003-Apr-16
Included in the Prior Art Database: 2003-Apr-16
Document File: 3 page(s) / 555K

Publishing Venue

Motorola

Related People

Norman L. Owens: AUTHOR

Abstract

Surface mount packages present challenges in order to obtain sufficient solder joint life for many applications. Leadless packages, such as the Quad Flat No-Lead (QFN) packages, present special challenges since there is no lead (flexible metal connection) to absorb the stress associated the co-efficient of thermal expansion (CTE) mismatches. Consequently, all the stress is conveyed directly through the solder joints, typically leading to earlier failure compared to leaded components. This innovation proposes that addition of non-electrically functional mechanical connections to relieve the stress on the electrical connections. Furthermore, by adjusting the size and location of these mechanical connections, a significant improvement in the solder joint life performance may be obtained.

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QFN Solder Joint Life Improvement Innovation

Norman L. Owens, P.E.

Abstract:

Surface mount packages present challenges in order to obtain sufficient solder joint life for many applications.� Leadless packages, such as the Quad Flat No-Lead (QFN) packages, present special challenges since there is no lead (flexible metal connection) to absorb the stress associated the co-efficient of thermal expansion (CTE) mismatches.� Consequently, all the stress is conveyed directly through the solder joints, typically leading to earlier failure compared to leaded components.� This innovation proposes that addition of non-electrically functional mechanical connections to relieve the stress on the electrical connections.� Furthermore, by adjusting the size and location of these mechanical connections, a significant improvement in the solder joint life performance may be obtained.

Problem Description:�

The QFN is a relatively new package that is gaining rapid market acceptance.� This package is characterized by a lead frame that is exposed on the bottom side of the package to provide electrical connections and, optionally, heat sinking thermal features.� The die is attached to the topside of the lead frame, interconnected to the lead frame, and encapsulated with mold compound.

The QFN package is attached to the larger circuit board by a thin conductive layer of paste or solder.� When the electronic device inside the package is activated it generates heat. Then the conductive layer is subject to mechanical stress because of the mismatch of the CTE of the materials used in the package and the larger circuit board.� This stress leads to an eventual fatigue failure of the conductive layer, limiting the component’s life in its application.�

Measures taken to mitigate this fatigue failure would result in a more robust product.

The State of the Art:

In QFNs all terminals on the package are connected to the die either through wires or the die attachment.� Therefore, any failure in any terminal solder joint will result in a functional failure of the device. However, it has been recognized in similar situations that this is not necessary.

In the case of mechanical stress, the corner pads typically have the greatest concentration of stress because they are the greatest distance from the “neutral point,” typically the center of the bottom surface of the package.� In order to mitigate this problem in flip chip applications, it has been disclosed that fatter corner joints could provide mechanical interconnection but not connected to any active or passive pad on the chip.� The opening of the electrical connection is of no consequence in a non-electrical joint.

There is also the case of Ball Grid Arrays (BGA).� In this package family a solder ball is present on the bottom surface of the package. The solder ball provides some stand-off and a...