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Novel designs for increased solder volumes for flip chip attach

IP.com Disclosure Number: IPCOM000015767D
Original Publication Date: 2002-Apr-01
Included in the Prior Art Database: 2003-Jun-21
Document File: 6 page(s) / 117K

Publishing Venue

IBM

Abstract

IBM Invention Disclosure (YOR8-1999-0319) Title of Invention Novel designs for increased solder volumes for flip chip attach FIELD OF INVENTION

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Novel designs for increased solder volumes for flip chip attach

IBM Invention Disclosure (YOR8-1999-0319)

# Title of Invention

Novel designs for increased solder volumes for flip chip attach

# FIELD OF INVENTION

This invention relates to the use of surface tension of molten solder to produce larger volumes of solder than traditionally available on linear arrays.

# BACKGROUND

Surface tension is the attractive force within a liquid which defines the shape a volume of liquid will take when unconstrained. When the liquid is placed on a surface, how the liquid interacts with the surface will contribute to the shape the liquid will form. For smaller volumes of liquid sitting on a hydrophobic surface, the shape of the liquid will approach the shape of a semi-sphere. Described here is an applications for surface tension induced shape changes of molten solder.

Flip chip attach is a method for electrical connection of silicon chips to packages which facilitate connection to circuit boards. It is higher speed and more versatile than conventional wire bond techniques. Flip chip attach utilizes small columns of solder to electrically and mechanically connect pads on a wafer to pads on a ceramic module. The diameter of the solder column is approximately equal to the chip pad size. The ceramic module facilitates direct high speed connection to a circuit board (see figure 1).

In achieving this electrical and mechanical connection, the solder columns must make up for some imperfections in the ceramic module. Due to the heat cycling during the fabrication process of the ceramic modules, thermal mismatches cause movement of pad locations and warping of the surface of the module. The solder columns, being thick and ductile, are able to compensate for waviness in the modules, and movement in pad locations. However, in order to achieve these corrections, the solder columns must be of a certain volume to ensure reliability. As dimensions on chips continue to shrink, and the need for increase I/O on a pad increase, so will the diameter of these solder columns shrink. With the current process not being able to achieve better than a 1:1 height to width aspect ratio, a decrease in diameter results in a cubed decrease in solder volume.

Described here is a design by which surface tension induced shape changes of solder can achieve an increased solder volume for particular pad arrangements. Specifically, the solder can deposited over a larger area, extending beyond the limits in which it will eventually be used, to the point that the required volume can be achieved (see figure 2a). After raising the solder above the reflow temperature, the surface tension of the solder will cause the solder to relocate and center over the pad on which the metal capture pad is located (see figure 2b).

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Evaporation through a molybdenum shadow mask has been the traditional process used for deposition of solder. A molybdenum mask is produced which has holes correspond...