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Browse Prior Art Database

Pseudo Brewster Angle Of Incidence For Joining Or Removing A Chip By Laser Beam

IP.com Disclosure Number: IPCOM000052089D
Original Publication Date: 1981-Apr-01
Included in the Prior Art Database: 2005-Feb-11
Document File: 2 page(s) / 22K

Publishing Venue

IBM

Related People

Chastang, JC: AUTHOR [+3]

Abstract

Individual chip heating by polarized laser beam of the appropriate wavelength, i.e., a wavelength at which silicon is opaque, for the purpose of joining a chip to or removing a chip from a chip carrier may be more efficiently done by tilting the laser beam so that it strikes the chip at the pseudo-Brewster angle.

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Pseudo Brewster Angle Of Incidence For Joining Or Removing A Chip By Laser Beam

Individual chip heating by polarized laser beam of the appropriate wavelength, i.e., a wavelength at which silicon is opaque, for the purpose of joining a chip to or removing a chip from a chip carrier may be more efficiently done by tilting the laser beam so that it strikes the chip at the pseudo-Brewster angle.

One technique used for joining an integrated circuit chip 10 to a chip carrier 12 is by heating the back of the chip until solder balls 14 melt and join the chip at selected locations to conductive lands on the carrier. A chip can also be removed from a chip carrier by heating the back of the chip until the solder joining the chip to the carrier melts. One method of heating a chip is with a laser
16.

Normally the laser beam is directed onto the back side of the chip at normal incidence or at some convenient angle not far from normal. For a silicon chip, normal incidence results in about 30% of the laser light being reflected and thus lost. A more efficient angle of incidence is the pseudo-Brewster angle. For silicon this angle is about 74 degrees from normal. At the pseudo-Brewster angle virtually all of the laser light is absorbed by the chip.

Laser beam 18 could be directed at a chip at the pseudo-Brewster angle by tilting the laser 16 as shown or by using one or more mirrors. It is also possible to use a prism. A fused silica prism of index 1.45 and a 33 degree apex angle,...