Metal fusion bonding – Process – Plural joints
Reexamination Certificate
1998-08-21
2001-02-27
Heinrich, Samuel M. (Department: 1725)
Metal fusion bonding
Process
Plural joints
C228S253000, C228S254000
Reexamination Certificate
active
06193139
ABSTRACT:
The invention relates to a method for joining electrically conductive metal-coated electrodes and to the application of the method to the production of microjoints in electronics industry.
Design and manufacturing of electronic components and their high density assemblies are developed intensively, so that the performance of functionally more advanced devices can be produced more reliably and cost-effectively. In particular, increasing usage of consumer electronics and portable telecommunication devices is enforcing electronics industry to research and develop microjoining technologies and substrate materials.
Since ever lighter and smaller electronic components and devices should have more functions, the number of the (I/O)-contacts of microcircuits is increasing continuously. Concurrently, the number of microjoints between integrated circuits and the contact areas of the substrates is growing remarkably. Consequently, the interdistances between the contact areas are reducing, which makes the microjoining more difficult. Furthermore, problems are produced by the increasing number of passive components of various sizes on printed circuit boards, the alignment of smaller contacts areas between integrated circuits and substrates as well as by printing the pastes on substrate contact pads. A more fundamental materials and microjoining problem arises from the fact that high density assemblies imply the use of ever smaller solder volumes, and therefore greater emphasis has to placed on the properties of the materials to be joined and especially on their metallurgical compatibility.
Recently, now direct chip attachment and packaging technologies have been developed in order to overcome difficulties produced by higher density joint structures in microelectronics assemblies. For example, “flip chip”-technique is one of the most promising one among the direct chip attachment techniques. Its merits include, for example, area array design and production capabilities, smaller footprint on printed wiring boards, lower cost and better reliability on board level. Area array bumps provide better electrical performance and thermal control by increasing signal propagation rates by shortening signal paths as well as by permitting the employment of available surface mount facilities and assembly lines. On the other hand, presently used direct chip attachment technologies differ significantly from those used in assembling other components, and therefore they increase, in part, the number of different assembly technologies in electronics production.
In general, small metal or alloy bumps are deposited on the contact metallisations of an integrated circuit, most generally using PbSn or PbIn solder alloys, gold or nickel. There can be several hundreds of such 20-50 &mgr;m high bumps in one microcircuit. Material, generally used for bonding, is a paste, which is composed of solder particles and a flux and which has been printed on the contact pads of a substrate. After the alignment procedure the bumps are pressed on the contact pads of the substrate and the assembly is heated over the melting point of solder alloy or, in some cases, over that of solder bumps. While melting the solder alloy or solder bump reacts chemically with the conductor surfaces and generate intermetallic layers, e.g. Cu
6
Sn
5
, Cu
3
Sn, Au
2
Sn, AuSn tai Ni
3
Sn
4
, and thus a conventional solder joint is formed. After joining the contact area is generally cleaned from flux residues and shielded against mechanical and chemical influences of the environment, for example, by filling the gap between an integrated circuit and a substrate with a proper protection polymer. As a rule, the solder pastea are heated about 40-50° C. above the melting points of solder alloys. Then the flux removes the oxide layers of the solder paste particles as well as the conductors, protects liquid solder and the surfaces to be joined and thereby permits the formation of joints by metallurgical reactions. The great majority of commercial solders are Sn-based alloys such as Sn37Pb solder, and therefore the reaction products formed in the joint region are some of the above-mentioned intermetallic compounds. This type of soldering technique is viable, when the solder volumes are relatively large and when the flux residues are not harmful for the functional joints or when the cleaning of the residues is technically and economically possible.
Even though the development of direct bonding techniques of integrated circuits and fine-pitch passive components has eliminated some the joining problems, it has revealed new production and commercial problems, which affect the feasibility of the techniques.
A fundamental problem which is due to the increasing density of electrical contacts, is related to the decreasing solder volumes used in microjoints, i.e. amounts of solder alloy per joint or the thickness of solder coatings on conductors. Consequently, the whole solder volume can take part in the chemical reactions between the solder alloy and conductor metallisations. Because of this the compositions and microstructures of solder joints can change even completely from the original microstructure of the solder alloy during the manufacturing and/or in use. These microstructural transformations weaken the mechanical properties of miniaturised and relatively unstable joints. The problem is even pronounced, if lead cannot be used in solder bumps, in metallisations or in solder filler materials in future. With increasing contact densities, i.e. with decreasing contact areas, the washing of flux residues will become more difficult and expensive. Another problem is related to the effects which alternating mechanical stress, humidity or oxygen can have on the properties of ever smaller microjoints.
When finding out solutions to the above-mentioned problems in the assembly of electronic devices, for example, electrically conductive adhesives are being employed more widely. Conductive adhesives are composite materials, which are composed of electrically insulating polymer matrix and more or less uniformly distributed electrically conductive particles; in general silver, graphite, nickel or metal-coated polymer balls. With all the additives or filler materials mentioned the electrical conductivity is based on mechanical contact. Solder alloy particles which melt during the adhesive joining can be used also as filler materials. As a matter of fact, when the adhesive joining and soldering is combined, we may speak about “adhesive soldering”. The adhesive itself can be, for example, an epoxy blend or a thin thermoplastic film in which small spherical low-melting solder alloy particles are finely dispersed. The joining of components is carried out by placing such a thin film between the contact pads of components and the substrate and by aligning the contact pads accurately. Temperature is then raised above the melting temperature of metal particles, but, for example, below the curing temperature of an epoxy blend, and the component and substrate are compressed together. Liquid solder particles will flatten and react metallurgically with the mating contact pads producing electrically conductive solder joints between metal particles and contact pads. The polymer locating between the conductors as well as the extruded polymer in the interconductor regions contract and therefore metal particles will experience a compressive stress, which stabilises electrically conductive joints. The composition, size distribution and volume fraction of metal particles dispersed in polymer matrix can alter significantly as it appears for example in the following patents EP 0 147856 A3 and EP 0 265077 A3.
It has been found experimentally that the resistivity values in mechanical joints produced with the solder adhesives and the joining methods based on known technology remain relatively high due to small contact areas and the values increase gradually during long-term high temperature and high humidity testing. This may cause serious functional failures especially in demanding applications.
Althou
Heinrich Samuel M.
Wolf Greenfield & Sacks P.C.
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