Cleaning and liquid contact with solids – Processes – Using sequentially applied treating agents
Reexamination Certificate
2002-05-31
2003-11-25
Carrillo, Sharidan (Department: 1746)
Cleaning and liquid contact with solids
Processes
Using sequentially applied treating agents
C134S002000, C134S019000, C134S025400, C134S025500, C134S030000, C134S032000, C134S034000, C134S035000, C134S036000, C134S042000
Reexamination Certificate
active
06652665
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a method of removing cured silicone polymer deposits from the surface of electronic components to provide product rework, recovery, and defect repair in microelectronics fabrication. The invention is particularly concerned with a novel and highly efficient method of removing cured Sylgard™ (Trademark of Dow Corning Corp.) and related elastomeric silicone adhesives from the surface of ceramics, metals, cured epoxy resins, and polyimides for reclamation and reuse of the recovered semiconductor assembly parts.
The present invention describes a new method of removing cured elastomeric silicone adhesive, particularly, Sylgard and related silicone polymers which are commonly used in electronic module assembly. Silicone polymers are widely used in microelectronics fabrication processes as sealants and adhesives. For example, one major application in non-hermetic ceramic module assembly includes Sylgard seal band attachment of a protective metal cap onto a ceramic chip carrier to provide protection of the semiconductor device against mechanical damage, moisture ingress, and environmental corrosion.
Other applications of the silicone polymers include: device encapsulation, top seal between the silicon device chip and the substrate to provide an &agr;-particle barrier, passivation coatings on printed circuit boards, coatings on various metallic, plastic, and thermoplastic components to provide protection against mechanical and environmental damage, and use of conductive silicones to attach a heat spreader or a heat sink to the backside of a flip chip for heat dissipation. In addition, thermally and electrically conductive adhesives based on a silicone matrix in conjunction with various types of fillers such as silica, quartz, alumina, aluminum nitride, and metals such as Cu, Ag, Au, silver plated Al, In—Sn on Cu or Ni, and carbon black find applications as adhesives for direct attachment of heat sinks or heat slugs to device chips for heat dissipation and also as die bond adhesive in wire bonded packages. Commonly used heat slug materials include Al—SiC, anodized Al, SiC, metal matrix composite, Cu and Mo.
Microelectronics fabrication processes often require disassembly of assembled components. Typical reasons include carrying out diagnostic tests, to replace or repair the semiconductor device, or to recover electrically good substrates from test vehicles or early user hardware used to assess product performance and reliability prior to actual product release. Removal processes for various assembly materials must be selective for a particular material set and cause no detriment to the substrate integrity and electrical performance. It is also required that the removal method be environmentally and chemically suitable for use in a manufacturing environment.
Sylgard formulation is a primer-less organosiloxane based two component system comprising a vinyl-functionalized (CH2═CH—) siloxane, typically vinyl-terminated-poly(dimethylsiloxane) as part A, and dihydro-dimethyl siloxane as part B, along with a curing catalyst and inorganic fillers such as silica and quartz. The adhesive composition is prepared by mixing the two components in a specified ratio and the mixture is de-aireated to remove any trapped air bubbles prior to dispensing on the components bonding sites.
The adhesive is applied onto the surfaces to be bonded and the component parts are aligned and assembled followed by curing up to 170° C. to 175° C. for 45 to 60 minutes or by stepwise cure up to 150° C. involving: (a) ramp from 25° C. to about 70° C. at 2 to 3° C./minute, hold for about 90 minutes, (b) ramp up to 150° C. at 2 to 3° C./minute and hold at 150° C. for about 30 minutes.
Equation (I) is an illustration of the Sylgard chemistry in terms of the reactive components and the curing reactions involved. The crosslinking reactions between the precursors are heat-accelerated resulting in a cured hydrophobic polymer of flexible/elastomeric matrix having special stress absorbing properties. These characteristic features of elastomeric silicones are particularly useful for providing protection from moisture ingress and maintenance of adhesive joints between diverse materials having different thermal coefficients of expansion (TCE) under high stress conditions during thermal cycling and other reliability stress test exposures.
Thermally and electrically conductive silicones are obtained by incorporating conductive fillers such as alumina, silica, aluminum nitride, and metal powders or carbon black for electrical conductivity when necessary. Typically, the adhesive formulation comprises Al
2
O
3
and SiO
2
/quartz filled polydimethyl siloxane/dimethyl vinyl terminated glycidoxypropyl trimethoxy silane and dimethyl methyl hydrogen siloxane components and a curing catalyst. Electrically conductive alkyl silicone such as methyl silicone and fluorosilicone resins for bonding chips to lead frames may contain metal powder or metal coated inorganic or organic polymer particles.
A major problem in the use of crosslinked elastomeric silicone adhesives such as Sylgard in electronic assembly products has been the difficulty in removing the cured polymer material and obtaining residue-free surfaces for module assembly rework, repair of defective components, and for reuse or recycling of assembly parts. Among the various known methods for removing cured silicones, mechanical scraping of the bulk of the coating followed by media blast and water rinse using pressurized spray, is labor intensive and has additional problems of surface damage and incomplete removal which invariably requires an additional cleaning operation with organic solvents which again does not result in a silicone-free surface. Yet another problem is that it is limited to removal of flat or planar surface residues only.
Another method is the chemical-mechanical removal which is based on the use of a strongly alkaline solution comprising NaOH, KOH, or tetramethylammonium hydroxide (TMAH) in lower boiling alcohols such as methanol, isopropanol, or mixture thereof, which causes a base induced chemical degradation of a —Si—O—Si— chain resulting in removal/dissolution of silicone residue from surfaces. Use of a low boiling solvent with strong alkali has chemical safety and flammability issues. There are also concerns about the compatibility of component materials with such high pH aqueous or alcoholic alkaline environments. An alternate method employs organic solvents without any reactive reagent, typically, toluene, dichloromethane, or dimethylformamide, to cause swelling of the silicone polymer which can then be removed by peeling or other mechanical means. This method is not considered practical because of incomplete removal which also requires the manual operation of peeling off the swelled polymer and because the required solvents are unacceptable for industrial applications due to strict regulations on the use of such solvents arising from associated environmental and health issues.
There are a number of solutions proposed by others for stripping cured elastomeric silicone adhesives from various surfaces. Minetti et al. U.S. Pat. No. 3,969,813, the disclosure of which is incorporated by reference herein, describes a high pressure water jet technique to remove room temperature vulcanization (RTV) silicone encapsulant under the chip to lift off the chip by mechanical impact of a directed high pressure H
2
O jet stream at 12,000 to 20,000 psi pressure. This method, however, leaves silicone residue which is removed by subsequent solvent-based cleaning with isopropanol (IPA).
Corby, U.S. Pat. No. 3,673,099, the disclosure of which is incorporated by reference herein, describes a method for stripping cured silicones and vinyl polymers as polyvinyl cinnamates from substrates using an organic or inorganic base in N-methylpyrrolidone (NMP) with or without another solvent. Specific stripping compositions claimed to be effective for removing methyl-phenyl polysiloxane resins comprise guanidine carbonate or quaternary ammoni
Ahmad Umar M.
Lei Chon C.
Sachdev Krishna G.
Carrillo Sharidan
Cioffi James J.
International Business Machines - Corporation
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