Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L25/00—Assemblies consisting of a plurality of individual semiconductor or other solid state devices ; Multistep manufacturing processes thereof
  • H01L25/50—Multistep manufacturing processes of assemblies consisting of devices, each device being of a type provided for in group H01L27/00 or H01L29/00
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
  • B23K1/00—Soldering, e.g. brazing, or unsoldering
  • B23K1/20—Preliminary treatment of work or areas to be soldered, e.g. in respect of a galvanic coating
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
  • B23K35/00—Rods, electrodes, materials, or media, for use in soldering, welding, or cutting
  • B23K35/001—Interlayers, transition pieces for metallurgical bonding of workpieces
  • B23K35/007—Interlayers, transition pieces for metallurgical bonding of workpieces at least one of the workpieces being of copper or another noble metal
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
  • B23K35/00—Rods, electrodes, materials, or media, for use in soldering, welding, or cutting
  • B23K35/02—Rods, electrodes, materials, or media, for use in soldering, welding, or cutting characterised by mechanical features, e.g. shape
  • B23K35/0222—Rods, electrodes, materials, or media, for use in soldering, welding, or cutting characterised by mechanical features, e.g. shape for use in soldering, brazing
  • B23K35/0244—Powders, particles or spheres; Preforms made therefrom
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
  • B23K35/00—Rods, electrodes, materials, or media, for use in soldering, welding, or cutting
  • B23K35/22—Rods, electrodes, materials, or media, for use in soldering, welding, or cutting characterised by the composition or nature of the material
  • B23K35/24—Selection of soldering or welding materials proper
  • B23K35/26—Selection of soldering or welding materials proper with the principal constituent melting at less than 400 degrees C
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  • H01L24/01—Means for bonding being attached to, or being formed on, the surface to be connected, e.g. chip-to-package, die-attach, "first-level" interconnects; Manufacturing methods related thereto
  • H01L24/26—Layer connectors, e.g. plate connectors, solder or adhesive layers; Manufacturing methods related thereto
  • H01L24/28—Structure, shape, material or disposition of the layer connectors prior to the connecting process
  • H01L24/29—Structure, shape, material or disposition of the layer connectors prior to the connecting process of an individual layer connector
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  • H01L24/80—Methods for connecting semiconductor or other solid state bodies using means for bonding being attached to, or being formed on, the surface to be connected
  • H01L24/83—Methods for connecting semiconductor or other solid state bodies using means for bonding being attached to, or being formed on, the surface to be connected using a layer connector
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
  • B23K2101/00—Articles made by soldering, welding or cutting
  • B23K2101/36—Electric or electronic devices
  • B23K2101/40—Semiconductor devices
• • B—PERFORMING OPERATIONS; TRANSPORTING
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  • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
  • B23K35/00—Rods, electrodes, materials, or media, for use in soldering, welding, or cutting
  • B23K35/22—Rods, electrodes, materials, or media, for use in soldering, welding, or cutting characterised by the composition or nature of the material
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  • B23K35/26—Selection of soldering or welding materials proper with the principal constituent melting at less than 400 degrees C
  • B23K35/264—Bi as the principal constituent
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  • H01L2224/29101—Material with a principal constituent of the material being a metal or a metalloid, e.g. boron [B], silicon [Si], germanium [Ge], arsenic [As], antimony [Sb], tellurium [Te] and polonium [Po], and alloys thereof the principal constituent melting at a temperature of less than 400°C
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  • H01L2224/29109—Indium [In] as principal constituent
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  • H01L2224/29111—Tin [Sn] as principal constituent
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  • H01L2224/29298—Fillers
  • H01L2224/29299—Base material
  • H01L2224/293—Base material with a principal constituent of the material being a metal or a metalloid, e.g. boron [B], silicon [Si], germanium [Ge], arsenic [As], antimony [Sb], tellurium [Te] and polonium [Po], and alloys thereof
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  • H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
  • H05K3/00—Apparatus or processes for manufacturing printed circuits
  • H05K3/30—Assembling printed circuits with electric components, e.g. with resistor
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  • H05K3/34—Assembling printed circuits with electric components, e.g. with resistor electrically connecting electric components or wires to printed circuits by soldering
  • H05K3/3457—Solder materials or compositions; Methods of application thereof
  • H05K3/3473—Plating of solder

Definitions

• 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.
• small metal or alloy bumps are deposited on the contact metallisations of an integrated circuit, most generally using PbSn or Pbln solder alloys, gold or nickel. There can be several hundreds of such 20-50 ⁇ 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.
• solder joint 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.
• 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.
• solders 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.
• 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 metal lurgically with the mating contact pads producing electrically conductive solder joints between metal particles and contact pads.
• solder adhesives Although the progress in solder adhesives has removed a significant part of the weaknesses associated with known anisotropically conductive adhesives, there is still a problem, which is difficult to overcome also with solder adhesives. This problem is due to the fact that the areas of contact pads as well as their interdistances are continuously diminishing. Accordingly, uniformly distributed filler particles must also become smaller, if the short- circuits between the neighbouring contact pads are to be avoided. On the other hand, because of electrical conductivity the contact areas must be large enough, which implies relatively large number of solder particles. Smaller average particle sizes and their standard deviations produce difficulties, which are related to the fabrication of alloy powder as well as to the production of soldered microjoints. Another problem is the storage of extreemly small solder particles, so that they could remain relatively unoxidized. If the particles were markedly oxidized their electrical conductivity is decreased and the melting point increased.
• the principal object of the present invention is to provide a new fluxless joining method and, related to the method, also an applicable solder adhesive without the drawbacks of the joining methods of the known art.
• the method can be applied equally well to the assembly of ultra fine-pitch components and substrates as to the joining of components with relatively large contact areas.
• the electrical, mechanical and chemical properties of high density microjoints produced with the joining method of the present invention are superior to those produced with the joining methods of the known art.
• the joining method permits also more comprehensive usage of different filler materials in adhesives.
• the present invention concerning the joining method as well as the solder adhesive are characterised by what is stated in the novelty part of the first and sixth claims, respectively.
• the principal object of the present invention is to select the materials becoming into contact with each other in such a way that they are metallurgically as compatible as possible. In other words, they fuse together by mutual dissolution without the formation of any intermetallic compounds.
• This can be achieved by coating the electrical contact areas of both the component and the substrate with adequately thick base coating layer, for example tin (Sn), and by plating them with thin metallurgically compatible topcoat layers or by using between the base layers a polymer film containing metallurgically compatible solder particles.
• base coating layer for example tin (Sn)
• Both the top coat material as well as solder particles should have adequate solubility and diffusion rate in the base coating, and they should not oxidise too strongly.
• a metal such as bismuth which dissolves readily into the base coating - for example tin (Sn) - at the joining temperature precipitates as finely dispersed particles during cooling and while diffusing deeper into the base coating with time it will improve the properties of the base coating as microjoining material.
• Useful solder metals which are compatible with the base coating such as tin (Sn) are, for example, bismuth (Bi) and indium (In) and many of their alloys as well as alloys they form with tin.
• the joining can be performed also under normal atmosphere (in air) and without fluxes.
• the coating materials such as tin and bismuth melt together transiently at low temperatures; the second melting of the joint region will occur at higher temperatures than the first melting.
• the thickness of the base coatings of component leads as well as the contact areas of substrate are large enough to prevent the melting of the whole base coatings during the joining operation. This retards effectively the chemical reactions between the solid base metal coating and the underlying conductor metal - being generally copper or nickel. In laboratory tests the thickness of about 10 ... 50 ⁇ m have been sufficient.
• thermosetting or thermoplastic film before the joining a thin thermosetting or thermoplastic film is placed between the areas to be connected.
• solder metal or alloy is not in form of particles but it has been brought into the contact areas by plating one or both of the base coatings chemically or electrochemically with a thin layer of the corresponding solder metal, most preferably with pure bismuth or tin-bismuth alloy.
• the thickness of the top coat is generally in the range of 1 - 3 ⁇ m.
• microjoints produced according to the fundamental idea of the present invention is based on the formation of solid solution or of solid solution containing finely dispersed precipitates, and it can be realised in the two different ways:
• the contact areas to be joined have been plated with the base coating - for example tin - and subsequently with a thin metallurgically compatible over- or topcoat layer - for example bismuth - which dissolves rapidly into the tin coatings, when the contact region is heated up to the bonding temperature, say 180 °C.
• the metal producing the interfusion can be brought between the base metal coated contact areas also in the form of metallurgically compatible particles with help of polymer matrix.
• the temperature of joining depends in each case on the combination of metals used. Due to the small thickness of topcoat layer or small volumes of the particles the dissolution and subsequent melting and solidification of the joint regions occur rapidly. Because of the mutual dissolution of thin topcoat layers - or particles - and the base coatings the melting temperature of the dissolution affected contact region decreases rapidly much below those of the original materials now participating the joints and thus a thin liquid layer is formed in the contact region.
• the melting temperature of tin is 232 °C and that of bismuth is 271 °C, while the lowest melting point of thin Sn-Bi-containing liquid layer 139 °C.
• the joining temperature is somewhat above 139 °C, e.g. in the range of 160 - 180 °C. Since the tin base coating is plated chemically or electrochemically with bismuth, there is fully metallic interface between the two coatings and this enhances the melting of the contact region. While bismuth is diffusing from thin topcoat layers deeper into the tin coating its concentration decreases rapidly in the thin liquid layer and therefore the melting point of the liquid increases with the result that the liquid region can solidify partly or completely already at the joining temperature.
• bismuth diffuses in solid state further away from the original contact interface and the supersaturated part of dissolved bismuth precipitates during cooling or subsequently at room temperature as finely dispersed bismuth precipitates in ductile tin matrix. This will increase the strength of microjoints without reducing significantly the toughness of tin-rich solid solution. It should be noticed that the melting point of the Bi-depleted joint region is raised irreversibly close to the melting point of pure tin. Consequently, the operational temperatures can be markedly above the temperatures of joining. The fact that bismuth is only weakly oxidising, relatively noble semimetal, permits the joining without any flux.
• the joining is accomplished by using between the tin base coatings the alloy - either as a topcoat or in a form of particles - whose melting temperature is without any diffusion contact already below the joining temperature - as is the case with the tin base coatings and the SnBi-, Snln- or Biln-alloy top coatings.
• a thin solder alloy layer between the contact surfaces melts already during the heating and the liquid layer dissolves a thin layer of solid tin base coatings.
• bismuth atoms diffuse rapidly from the liquid deeper into the solid tin coatings of both contact areas.
• the liquidus temperature of the contact region increases and the liquid can solidify partly or completely already at the joining temperature or during the subsequent cooling.
• the melting point of the contact region is raised irreversibly close to that of tin.
• the overcoat still with a very thin noble metal layer, for example with gold or silver and/or locate a thin thermosetting or thermoplastic polymer film between the mating contact areas of component and substrate.
• the polymer foil distributes also stress more uniformly over the joint region and protects from the effect of humidity. Since the contact surfaces are heated above the melting temperature of solder alloy during the joining the polymer can remove some of the oxide layers while flowing away from the contact region.
• Fig. 1(a) shows schematically the case where the conductors (1) are firstly plated with the base metal or alloy coating (2) and subsequently a thin metallurgically compatible layer of metal (3) is deposited on the base coating.
• the conductors coated with above- mentioned layers are compressed together at an elevated temperature the coatings fuse together without forming intermetallic compounds between the conductors, as shown in Fig. 1(b).
• tin as the base coating together with metallurgically compatible top coat metal, say bismuth
• bismuth there is no intermetallic reactions between these two pairs of metals. Instead, bismuth dissolves into tin and precipitates during the cooling or after as very fine particles when the solubility limit of bismuth is exceeded, as shown in Fig. 1(b).
• Fig. 2 presents the case where the conductors (1) plated with the base coating (2) are joined with polymer (4) containing metallurgically compatible solder particles (3).
• Fig.2(a) and Fig. 2(b) illustrate the situation before and after the joining, respectively.
• a solder particle has dissolved into the base coating (2) and precipitated there as finely dispersed particles during the cooling or after.
• Fig. 3(a) shows a scanning electron microscope image of a solder joint which has been produced without flux in air.
• tin-coated (2) copper conductors (1) have been plated with a very thin layer of pure bismuth and then lightly pressed together and heated up to the temperatureof 180 °C for 10 seconds in air and without any flux. Accordingly, the intermixing of tin and bismuth gives rise to the seamless interfusion of the coated conductors and the bismuth exceeding the solubility limit has precipitated in the joint region (3).
• Fig. 3(a) shows a scanning electron microscope image of a solder joint which has been produced without flux in air.
• tin-coated (2) copper conductors (1) have been plated with a very thin layer of pure bismuth and then lightly pressed together and heated up to the temperatureof 180 °C for 10 seconds in air and without any flux. Accordingly, the intermixing of tin and bismuth gives rise to the seamless interfusion of the coated conductors and the bismuth exceeding
• 3(b) shows a scanning electron microscope image taken from the joint region, where Bi-particles (3) have melted between the tin coatings (2) and the bismuth which has dissolved during the joining have precipitated from the tin-rich (Sn,Bi)-solution of the joint region (3).
• One embodiment of the present invention is also that fine particles, for example Bi (Figs. 1, 2 and 3), disperse with time over larger volumes of the base coatings, e.g. tin, and therefore the joint region is gradually changing into practically pure tin.
• the rate of Bi diffusion over the whole of the base coatings can be increased by separate annealing treatment.
• the joining method of the present invention is easy to use. It does not imply the application of flux, and thus the joints need no cleaning. Furthermore, since the method is lead-free, it is environmental-friendly also from that point of view.

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• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Microelectronics & Electronic Packaging (AREA)
• Computer Hardware Design (AREA)
• Power Engineering (AREA)
• Manufacturing & Machinery (AREA)
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• Condensed Matter Physics & Semiconductors (AREA)
• General Physics & Mathematics (AREA)
• Electric Connection Of Electric Components To Printed Circuits (AREA)
• Connections Effected By Soldering, Adhesion, Or Permanent Deformation (AREA)

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• 1994
  • 1994-10-28 FI FI945101A patent/FI98899C/fi not_active IP Right Cessation
• 1995
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  • 1995-10-30 WO PCT/FI1995/000599 patent/WO1996013353A1/en not_active Application Discontinuation
• 1996
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