Browse Prior Art Database

Pad Reflow in a Closed System

IP.com Disclosure Number: IPCOM000035666D
Original Publication Date: 1989-Jul-01
Included in the Prior Art Database: 2005-Jan-28
Document File: 2 page(s) / 77K

Publishing Venue

IBM

Related People

Meyer, A: AUTHOR

Abstract

Solder balls, by means of which semiconductor chips are fastened to a substrate, are formed by evaporating PbSn pads on a semiconductor wafer through metal masks, yielding cylindrically shaped pads. In a reflow process, the pads are melted in the form of semispheres while the oxide skin on the pad surface is reduced.

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Pad Reflow in a Closed System

Solder balls, by means of which semiconductor chips are fastened to a substrate, are formed by evaporating PbSn pads on a semiconductor wafer through metal masks, yielding cylindrically shaped pads. In a reflow process, the pads are melted in the form of semispheres while the oxide skin on the pad surface is reduced.

Such a single wafer pad reflow process is carried out in a system as illustrated in the attached sectional drawing. This system provides for the wafer to be placed in an H2 filled chamber. Halide lamps, for example, are used to heat the wafer to reflow temperature through a quartz window covering the chamber. A problem with this process is that the high thermal conductivity of the hydrogen causes a considerable amount of heat to be dissipated from the pads and the wafer surface so that only the lower third of the pads is melted without any reduction of the oxide skin. As a result, the pads act as cooling ribs which, in addition to preventing the required surface temperature from being reached, lead to a non-uniform temperature distribution across the wafer.

This problem is solved by providing a Cu plate at a distance of 0.5 to 1.0 mm above the wafer surface. The Cu plate is heated by radiation, heating also the hydrogen which, rather than dissipating its heat, transfers it to the pads and the wafer surface. A further advantage is that the high thermal conductivity of the Cu plate leads to a uniform heat distribution acro...