Electric heating – Metal heating – By arc
Reexamination Certificate
2001-11-19
2004-02-24
Van, Quang T. (Department: 3742)
Electric heating
Metal heating
By arc
C219S121590, C219S121450, C219S121540
Reexamination Certificate
active
06696661
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains generally to plasma enhanced bonding of metallic materials, and more particularly, to a method and apparatus for using hot plasma gas to simultaneously heat, clean, and activate the bonding surfaces before and during bonding of a circuit package to a circuit board.
BACKGROUND OF THE INVENTION
Various techniques exist for bonding metals together, including welding, brazing, soldering, and more recently, direct contact and diffusion bonding.
Welding is a metal joining process wherein coalescence is produced by heating the metal to suitable temperatures, with or without the application of pressure and with or without the use of filler metals. Coalescence is a growing together into one body. A weld is the junction of members or the edges of members which are to be joined or have been joined by melting and refreezing at the contact interfaces of members themselves. Filler metal is the material to be added in making a welded, brazed, or soldered joint. Base metal is the material to be welded, soldered, or cut.
The term arc welding applies to a large and varied group of processes that use an electric arc as the source of heat to melt and join metals together as the molten metal re-freezes. In arc welding processes, the joining of metals, or weld, is produced by the extreme heat of an electric arc drawn between an electrode and the work-piece, or between two electrodes. The formation of a joint between metals being arc welded may or may not require the use of pressure or filler metal. The arc is struck between the workpiece and an electrode that is mechanically or manually moved along the joint, or that remains stationary while the workpiece is moved underneath it. The electrode is either a consumable wire rod or a nonconsumable carbon or tungsten rod which carries the current and sustains the electric arc between its tip and the workpiece. When a nonconsumable electrode is used, a separate rod or wire can supply filler material, if needed. A consumable electrode is specially prepared so that it not only conducts the current and sustains the arc, but also melts and supplies filler metal to the joint, and may produce a slag covering as well.
Gas welding processes are a group of welding processes in which a weld is made by heating with a gas flame. Pressure and/or filler metal may or may not be used. Also referred to as oxyfuel gas welding, the term gas welding is used to describe any welding process that uses a fuel gas combined with oxygen, or in rare cases, with air (20% Oxygen), to produce a flame having sufficient energy to melt the base metal. The fuel gas and oxygen are mixed in the proper proportions in a chamber, which is generally a part of the welding tip assembly. The torch is designed to give the welder complete control of the welding flare, allowing the welder to regulate the melting of the base metal and the filler metal. The molten metal from the work-piece edges and the filler metal intermix in a common molten pool and join upon cooling to form one continuous piece.
Brazing and soldering are welding processes in which materials are joined by heating to a suitable temperature and by using a filler metal with a melting point below that of the base metal. The filler metal is distributed to the closely fitted surfaces of the joint by capillary action.
Resistance welding consists of a group of processes in which the heat for welding is generated by the resistance to the electrical current flow through the parts being joined, using pressure. It is commonly used to weld two overlapping sheets or plates which may have different thicknesses. A pair of low resistance electrodes conducts electrical current through the sheets, forming a weld. The Key aspect of the resistance weld is that most of the resistance in the welding circuit is in the contact resistance where the metals are pressed together, so that most of the (1{circumflex over ( )}2 *R) heat is formed at the surfaces that are to be welded together.
The properties of a welded joint depend partly on the correct preparation of the edges being welded. Cleanliness is of key concern. All mill scale, rust, oxides, and other impurities must be removed from the joint edges or surfaces to prevent their inclusion in the weld metal. The edges should be prepared to permit fusion without excessive melting. Care must be taken to keep heat loss due to radiation into the base metal from the weld to a minimum. A properly prepared joint will keep both expansion on heating and contraction on cooling to a minimum.
Diffusion bonding is a method of joining metallic or non-metallic materials. This bonding technique is based on the atomic diffusion of elements at the joining interface. Diffusion process actually is the transport of mass in form of atom movement or diffusion through the lattice of a crystalline solid. Diffusion of atoms proceeds by many mechanisms, such as exchange of places between adjacent atoms, motion of interstitial atoms or motion of vacancies in a crystalline lattice structure. The latest is the preferable mechanism due to low activation energy required for atom movement. Vacancy is referred to an unoccupied site in a lattice structure. Both Diffusion and Direct contact bonding are the preferable bonding methods. Direct Contact/Fusion bonding occurs when the surface atoms are brought within atomic distances with the application of pressure and heat. Any surface contamination will inhibit bonding.
Diffusion bonding involves diffusion of atoms via a thermodynamic process where temperature and diffusibility of the material are considerable parameters. In general, the diffusion rate, in term of diffusion coefficient D, is defined as D=D
o
−Q/RT
, where D
o
is the frequency factor depending on the type of lattice and the oscillation frequency of the diffusing atom. Q is the activation energy, R is the gas constant and T is the temperature in Kelvin.
The activation energy for atomic diffusion at the surface, interface and grain boundaries is relatively low compared to the bulk diffusion due to a looser bond of the atoms and higher oscillation frequency of the diffusing atom. This enhances the atomic diffusion, and thus eases the diffusion bonding of two metal pieces assuming that a perfect interface contact exists.
Since diffusion bonding is driven by the diffusion of atoms, diffusion bonding process can be used to bond dissimilar materials that are difficult to weld, such as, steel and copper alloys. When bonding metals together, direct contact/fusion and diffusion bonding causes micro-deformation of surface features, due to the pressure and temperature applied, which leads to sufficient contact on an atomic scale to cause the materials to bond. However, before the materials can be joined they undergo an extensive preparatory treatment.
The interface contact can be optimized by a treatment of the surface to be bonded through a number of processes, such as mechanical machining and polishing, etching, cleaning, coating, and material creeping under high temperature and loading.
All of the above-mentioned bonding methods are problematic due to the inability to prevent contaminants from adhering to the bond site surfaces. Surface contamination causes poor bond adhesion, resulting in less robust bonds and therefore less reliable interfaces. Accordingly, much effort has gone into cleansing the surfaces of metals prior to being bonded.
Low temperature plasmas of various ionized gases can be used to reactively etch/ash organic materials found on the surface of materials. In this procedure, typically the material is placed in an RF cavity with a flowing reactive gas. The nature of the gas selected is chosen based upon the desired effect. Oxygen or argon is generally used, however, specific gases (CF
4
) may be used to tailor the reaction for the desired effect. By removing surface contamination, plasma cleaning increases the bonding or adhesive properties of the bondsite surface.
A plasma is a collection of positive, negative, and neutral particles wherein typically the
Costa Jessica
GeoMat Insights, LLC
Law Office of Jessica Costa
Van Quang T.
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