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A SOLUTION USING UNDERFILL MATERIAL TO PROVIDE A STANDOFF FOR EUTECTIC BUMPED FLIP CHIP DEVICES

IP.com Disclosure Number: IPCOM000009033D
Original Publication Date: 1999-Jan-01
Included in the Prior Art Database: 2002-Aug-02
Document File: 2 page(s) / 150K

Publishing Venue

Motorola

Related People

Craig Beddingfield: AUTHOR [+2]

Abstract

The attachment of flip chip die with eutectic sol- der bumps onto organic printed wiring boards hav- ing metal landing pads results in smaller die-to- board "gap" distances (typically 50 - 70 microns depending on the wettable area of the board die site) as compared to those obtained with the high-lead, non-reflowing C4 bump on eutectic solder cladded boards (-100 microns). The wettable area of the board die site is determined by the openings in the solder mask layer. Boards having larger openings would have a larger wettable area for each bump, thereby resulting in more area for the eutectic solder to wet (i.e., greater collapse of the bump).

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MOTOROLA Technical Developments

A SOLUTION USING UNDERFILL MATERIAL TO PROVIDE A STANDOFF FOR EUTECTIC BUMPED FLIP CHIP DEVICES

by Craig Beddingfield and Bill Stone

DEFINITION OF PROBLEM

  The attachment of flip chip die with eutectic sol- der bumps onto organic printed wiring boards hav- ing metal landing pads results in smaller die-to- board "gap" distances (typically 50 - 70 microns depending on the wettable area of the board die site) as compared to those obtained with the high-lead, non-reflowing C4 bump on eutectic solder cladded boards (-100 microns). The wettable area of the board die site is determined by the openings in the solder mask layer. Boards having larger openings would have a larger wettable area for each bump, thereby resulting in more area for the eutectic solder to wet (i.e., greater collapse of the bump).

  The present capability of board suppliers for holding solder mask opening size is +/- 25 microns, and for holding the solder mask layer registration to the metal pads on the board is +/- 50 microns. Based on these tolerances, larger solder mask open- ings are desirable for higher assembly yields. However, reliability of the structure is reduced as a result of the excessive collapse. It has been demon- strated that with a higher gap between the die (CTE - 2.5 ppm/"C) and the organic board (CTE - 16 ppm/"C), reliability is improved because of the increased distance between the two materials having different CTE properties. This distance also allows the undertill material to have a greater effect.

  An additional consequence of smaller gap heights is how it affects the underfill process and material selection. Due to varying gap heights because of inconsistent solder mask opening sizes, undertill flow time (determined by capillary action from the die/board surfaces) will vary. Underfill

materials with smaller silicon particle sizes are required for smaller gap height applications. However, these materials typically flow slower, thereby reducing cycle time.

SOLUTION

  Refer to Figure 1, on next page, for the descrip- tion of the solution. The solution involves the use of a modified underfill material 10 being applied onto an organic board 20 (this embodim...