Metal fusion bonding – Process – With protecting of work or filler or applying flux
Reexamination Certificate
1999-09-13
2001-03-06
Ryan, Patrick (Department: 1725)
Metal fusion bonding
Process
With protecting of work or filler or applying flux
C228S042000
Reexamination Certificate
active
06196446
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to soldering methods for electronic devices, and more particularly to methods of fluxless soldering.
BACKGROUND OF THE INVENTION
During a typical solder operation, typically two (2) components are mechanically attached to each other with a metal material called solder. The process requires that the components are placed together with the solder placed in the area where the attachment is to occur. The components are heated to a temperature to melt (reflow) the solder. When the solder melts the liquid solder attaches metallurgically to the components. Liquid solder (like all metals) instantaneously forms an oxide. Oxide layers form on the exterior of the solder forming a “crust” or film which in some cases is very difficult to penetrate or break. If the oxide layer is not removed or broken the solder joint will be very poor. The components and solder are cooled to a temperature below which the solder solidifies, thus creating a solder joint.
Typically, soldering processes include three basic steps: (1) pre-cleaning and deoxidation of surface oxides; (2) solder reflow and/or reflow joining; and (3) post-soldering cleaning. Different flux materials are used in the pre-cleaning step to prepare the surfaces for the soldering step by removal of contaminants and metal oxides from the solder surface (flux is a chemical formulated to remove oxides and prevent oxidation prior or during the soldering process). For example, activated fluxes, such as zinc, ammonium chloride, mineral acid-containing materials, and the like, are typically used in “coarse” soldering applications, e.g., repairing coarse wiring in motors or houses. The solder joining step can occur only after the oxide coating is removed because the high melting point oxides prevent wetting of the two surfaces to be joined by reflow of solder. The third step, post-soldering cleaning, removes flux residue remaining after the reflow.
Highly acidic fluxes are used for the soldering of aluminum layers. Aluminum has a tenacious oxide layer which is chemically very inert and difficult to remove. Thus, mild rosin fluxes are ineffective with aluminum and special fluxes containing acid compounds which are highly corrosive, such as inorganic acids in a cadmium fluoroborate vehicle, must be used. Fluxes used with aluminum can also contain metal chlorides, fluorides, and ammonium compounds.
Because of the gross corrosive nature of these fluxes, and the high attack rates on metals in microelectronic assemblies, such fluxes cannot be used in microelectronics. For microelectronic devices, the standard practice is to reduce the acid activity of the flux to a mildly activated or non-activated grade in an attempt to minimize the adverse effects of the flux on the components. Typical soldering processes for copper layers in microelectronic applications use rosins which form a very mild organic acid when melted at the soldering temperature but which are relatively inert at room temperature.
Although corrosion and other risks can be minimized in copper soldering applications using mild flux agents, flux is necessary to keep the solder from oxidizing, allow it to flow and wet the parts being soldered. In addition, with the shrinking size of all electronic components and bonding pads, the rapidly growing use of surface mount technology, and the increasing demand for flip-chip device bonding, the post reflow cleaning of flux residues is becoming increasingly difficult. The small gaps between assembled parts, and solidification cavities in mixed soldered joints are very resistant to penetration by cleaning liquids. Inefficient post-soldering cleaning can reduce the long term reliability of the whole assembly. Further, there can be other problems associated with non-activated or mildly activated flux processes, such as higher defect levels and high rework costs. Optoelectronic devices are also very sensitive to flux residues due to absorption and bending of the optical signals.
In a typical soldering procedure, the flux residue needs to be removed through a cleaning process. Many previous cleaning solvents such as Freon can no longer be used due to environmental concerns. Great efforts have been made to develop replacement solvents but the ultimate solution is to solder without the use of flux, i.e., fluxless soldering. Fluxless soldering is a method of soldering components together using a variety of different solders without the use of a flux.
An exemplary method to perform fluxless soldering typically involves a mechanical “scrubbing” of the components after the solder has melted to mechanically break the oxide on the solder. This method can be fixture-intensive, mechanically stresses the components, and provides a marginal solder joint. Another method is the use of batch-type equipment such as a DAP furnace which requires the components to be assembled or fixtured and then placed into a chamber which is sealed, evacuated, and back-filled with an inert (oxygen-free) gas. The assemblies are heated and cooled and then removed from the chamber. This method is usually capital and floor space intensive, as well as a batch operation.
SUMMARY OF THE INVENTION
An apparatus for performing fluxless soldering in accordance with the invention includes an enclosure having a gas inlet through which an inert gas is introduced to provide an inert gas-rich environment within said enclosure and a gas outlet which allows inert gas to exit from the system, and provides access for the components to be soldered.
REFERENCES:
patent: 3680200 (1972-08-01), Terrill et al.
patent: 4034468 (1977-07-01), Koopman
patent: 4564135 (1986-01-01), Barresi et al.
patent: 4568277 (1986-02-01), MacInnes et al.
patent: 4832249 (1989-05-01), Ehler
patent: 4836434 (1989-06-01), Takenaka et al.
patent: 4921157 (1990-05-01), Dishon et al.
patent: 4979664 (1990-12-01), Lyons et al.
patent: 5139193 (1992-08-01), Todd
patent: 5227604 (1993-07-01), Freedman
patent: 5255840 (1993-10-01), Nowotarski
patent: 5265788 (1993-11-01), Ozawa et al.
patent: 5407121 (1995-04-01), Koopman et al.
patent: 5427303 (1995-06-01), Nowotarski
patent: 5436202 (1995-07-01), Miura
patent: 5499754 (1996-03-01), Bobbio et al.
patent: 5560531 (1996-10-01), Ruszowski
patent: 5604831 (1997-02-01), Dittman et al.
patent: 5735451 (1998-04-01), Mori et al.
patent: 5785237 (1998-07-01), Lasto et al.
patent: 5829665 (1998-11-01), Yoneyama et al.
patent: 5852257 (1998-12-01), Dittman et al.
patent: 5858312 (1999-01-01), Sindzingre et al.
patent: 5881193 (1999-03-01), Anigbo et al.
Fang Lu
Potteiger Brian Dale
Scrak Shaun P.
Yeagle Frederick Arthur
Daune, Morris & Heckscher LLP
Lucent Technologies - Inc.
Ryan Patrick
Stoner Kiley
LandOfFree
Automated fluxless soldering using inert gas does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Automated fluxless soldering using inert gas, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Automated fluxless soldering using inert gas will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2494078