Classifications

• • 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 or brazing
  • B23K35/0244—Powders, particles or spheres; Preforms made therefrom
  • B23K35/025—Pastes, creams or slurries
• • 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°C
• • 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°C
  • B23K35/262—Sn as the principal constituent
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • 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 resistors
  • H05K3/32—Assembling printed circuits with electric components, e.g. with resistors electrically connecting electric components or wires to printed circuits
  • H05K3/34—Assembling printed circuits with electric components, e.g. with resistors electrically connecting electric components or wires to printed circuits by soldering
  • H05K3/3465—Application of solder
  • H05K3/3485—Application of solder paste, slurry or powder
• • 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
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
  • H05K2201/00—Indexing scheme relating to printed circuits covered by H05K1/00
  • H05K2201/02—Fillers; Particles; Fibers; Reinforcement materials
  • H05K2201/0203—Fillers and particles
  • H05K2201/0263—Details about a collection of particles
  • H05K2201/0272—Mixed conductive particles, i.e. using different conductive particles, e.g. differing in shape
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • 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 resistors
  • H05K3/32—Assembling printed circuits with electric components, e.g. with resistors electrically connecting electric components or wires to printed circuits
  • H05K3/34—Assembling printed circuits with electric components, e.g. with resistors electrically connecting electric components or wires to printed circuits by soldering
  • H05K3/346—Solder materials or compositions specially adapted therefor
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10W—GENERIC PACKAGES, INTERCONNECTIONS, CONNECTORS OR OTHER CONSTRUCTIONAL DETAILS OF DEVICES COVERED BY CLASS H10
  • H10W72/00—Interconnections or connectors in packages
  • H10W72/071—Connecting or disconnecting
  • H10W72/073—Connecting or disconnecting of die-attach connectors
  • H10W72/07351—Connecting or disconnecting of die-attach connectors characterised by changes in properties of the die-attach connectors during connecting
  • H10W72/07355—Connecting or disconnecting of die-attach connectors characterised by changes in properties of the die-attach connectors during connecting changes in materials
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10W—GENERIC PACKAGES, INTERCONNECTIONS, CONNECTORS OR OTHER CONSTRUCTIONAL DETAILS OF DEVICES COVERED BY CLASS H10
  • H10W72/00—Interconnections or connectors in packages
  • H10W72/30—Die-attach connectors
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10W—GENERIC PACKAGES, INTERCONNECTIONS, CONNECTORS OR OTHER CONSTRUCTIONAL DETAILS OF DEVICES COVERED BY CLASS H10
  • H10W72/00—Interconnections or connectors in packages
  • H10W72/30—Die-attach connectors
  • H10W72/321—Structures or relative sizes of die-attach connectors
  • H10W72/325—Die-attach connectors having a filler embedded in a matrix
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10W—GENERIC PACKAGES, INTERCONNECTIONS, CONNECTORS OR OTHER CONSTRUCTIONAL DETAILS OF DEVICES COVERED BY CLASS H10
  • H10W72/00—Interconnections or connectors in packages
  • H10W72/30—Die-attach connectors
  • H10W72/351—Materials of die-attach connectors
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10W—GENERIC PACKAGES, INTERCONNECTIONS, CONNECTORS OR OTHER CONSTRUCTIONAL DETAILS OF DEVICES COVERED BY CLASS H10
  • H10W72/00—Interconnections or connectors in packages
  • H10W72/30—Die-attach connectors
  • H10W72/351—Materials of die-attach connectors
  • H10W72/352—Materials of die-attach connectors comprising metals or metalloids, e.g. solders
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10W—GENERIC PACKAGES, INTERCONNECTIONS, CONNECTORS OR OTHER CONSTRUCTIONAL DETAILS OF DEVICES COVERED BY CLASS H10
  • H10W72/00—Interconnections or connectors in packages
  • H10W72/30—Die-attach connectors
  • H10W72/381—Auxiliary members
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10W—GENERIC PACKAGES, INTERCONNECTIONS, CONNECTORS OR OTHER CONSTRUCTIONAL DETAILS OF DEVICES COVERED BY CLASS H10
  • H10W74/00—Encapsulations, e.g. protective coatings

Definitions

• This invention relates to a solder composition and more particularly to the use of a multi-component solder paste that has a low melting eutectic first component and a high melting second component that does not melt but does react with the eutectic component during reflow soldering of electronic components or packages to a metallised surface of a thermoplastic or polymer substrate to thereby form a solder interconnect that has a melting temperature higher than the reflow soldering temperature.
• reflow soldering is one of the last steps in the assembly of the package.
• the assembly is usually comprised of a substrate prepared with surface metallisation to receive components; the components are constructed with pins, leads or surface metal pads for mounting to the surface metallisation of the substrate.
• Reflow soldering is a specialised type of soldering using a solid powder in a paste form; the solder paste is deposited at the joint of an assembly to be soldered and the assembly is thereafter heated, such as in an oven to reflow the solder paste as a fluid which, upon cooling, forms a metallurgical bond with the parts to be joined.
• Reflow soldering is predominately used to attach components to printed circuit boards because of component density, process ease, and improved through-put.
• the circuit board or substrate is comprised of a low cost thermoplastic material (such as polypropylene (PP) , nylon, polycarbonate (PC) , acrylonitrile-butadiene-styrene (ABS), polyethylene terephthalate (PET), polypropylene oxide (PPO) , or polymethylstyrene (PS) ), the chemistry of the solder must be modified to form a reliable solder joint during reflow soldering at a temperature below the heat deflection temperature of such thermoplastic material to avoid damage to such substrate.
• a low cost thermoplastic material such as polypropylene (PP) , nylon, polycarbonate (PC) , acrylonitrile-butadiene-styrene (ABS), polyethylene terephthalate (PET), polypropylene oxide (PPO) , or polymethylstyrene (PS)
• the heat deflection temperature of such plastic ('defined herein to mean the- temperature at which a specified deflection occurs in a plastic exposed to a relatively low stress level under controlled conditions) is usually in the range of 100-150°C.
• Eutectic solder compositions are known which will melt at such lower temperatures; examples of such solders include Sn-Bi (melting at 139°C) , Sn-In (melting at 118°C) , and Sn- Bi-In (having eutectics melting respectively at 77.5°C, 59°C and 56.5°C) .
• Sn-Bi melting at 139°C
• Sn-In melting at 118°C
• Sn- Bi-In having eutectics melting respectively at 77.5°C, 59°C and 56.5°C
• solder Therefore there is a need to design a solder and devise a fabrication technique using such solder, that forms a solder ]omt between the component and substrate at a temperature below the heat deflection temperature of the thermoplastic substrate, but which solder has a post-reflow solder melting temperature which is much higher to preserve the mechanical integrity of the assembly particularly m an automotive service application.
• the solder must possess multiple components that are different m composition and melting temperature.
• Two component solders that have been used by the prior art have been designed to accelerate the wetting of the joint area by early melting of one or more low temperature components, but requires all components to fully melt during completion of the reflow soldering process.
• the reflow soldering may take place in stages where the low temperature component is melted in a first stage, and then a second reflow soldering stage is undertaken to melt the high temperature component and thereby complete the total reflow soldering sequence.
• such solders fail to provide both a low temperature reflow melting temperature (below that of the heat deflection of the thermoplastic substrate.) and a post reflow melting temperature (well above the heat deflection temperature of the substrate) that is never experienced during fabrication or service.
• the invention in a first aspect, is a solder composition formed as a paste by use of a fugitive carrier medium including up to 5-15 A (by weight of the composition) flux, the solder composition being useful for soldering electronic components and packages to a circuit on a substrate consisting of low cost thermoplastic or polymer having low heat deflection temperatures.
• the composition comprises: (a) a first ternary or binary eutectic powder or alloy of the system Sn-Bi-In, (b) a tin based second powder that reacts with the first powder at or below 150°C m 15 minutes or less, said powders containing less than .lo Pb as an impurity and the first powder being weight proportion to the second powder m a ratio of 3:1 to 1:3, and (c) other alloying additives selected from the group of Cu, Ni, Ag, Ce, In, Bi, and Au added in an amount that enhances the mechanical properties of the soldered joints at elevated temperatures and does not inhibit metallurgical interactions of the first and second powders.
• the invention m another aspect is a method of interconnecting electronic components and packages to circuits on a substrate by reflow soldering, the substrate being comprised of a low cost thermoplastic or polymer having a pre-determmed heat deflection temperature.
• the method comprises the steps of: (a) mixing a solder paste formulation consisting of a first ternary or binary eutectic powder of the alloy system Sn-Bi-In, a tin-based second powder that reacts with the first powder at or less than 150°C in 15 minutes or less, and other alloy powder additions that do not inhibit the metallurgical interactions of such first and second powders, the weight ratio of first to second powders being generally 3:1 to 1:3; (b) applying the solder paste onto a faying surface of the substrate circuit; and (c) heating the assembly to a temperature above the melting point of the low temperature eutectic powder to react Sn with the melted eutectic to form solid solutions mtermetallics and thereby raise the
• Figure 1 is a ternary phase diagram of the Sn-Bi-In alloy system illustrating the eutectics useful as the low melting component of the solder compositions;
• FIG. 2 is a highly enlarged schematic illustration of a solder connection made in accordance with this invention.
• Figure 3 is a schematic diagram of the multi-component solder material of this invention illustrating its condition prior to reflow heating
• Figure 4 is a schematic diagram, similar to that of Figure 3, illustrating the solder material condition during and as a result of the reflow heating
• Figure 5 is a schematic diagram similar to Figure 3, illustrating one of the possible microstructures in the solder after completion of reflow heating and cooling.
• a solder paste 10 in accordance with this invention is employed to form a tin-based essentially lead-free solder connection for mounting electronic components or packages 11 onto a printed circuit or metallised surface 12 of a thermoplastic or polymer substrate 13.
• the substrate is preferably selected from PP, nylon, PC, ABS, PET, PPO and PS plastics, each of which present the problem solved by this invention.
• Each of these examples has a heat deflection temperature which is in the range of 100- 150°C. It is important that during reflow soldering, temperatures should not be experienced that will exceed such heat deflection temperature.
• solders for electronic interconnects must possess certain mechanical and physical properties after the soldering steps have been completed. The interconnect must withstand higher temperatures due to subsequent processing or due to service temperatures under which, the assembly is used. The solder should not soften under thermomechanical loading in service, otherwise the electronic connects can be ruptured.
• a soldering paste according to this invention that will reflow at 150°C or lower and yet achieve a higher post reflow soldering melting temperature, comprises intermixed powder components and a fugitive carrier and fugitive flux.
• One powder component 15 is a low temperature melting eutectic or alloy of the Sn-Bi-In system and the second component 16 is a high temperature melting powder metal selected for example from Sn or alloys of Sn-3.5,. Ag, or Sn-lOV, In-9.53, Bi-0.5% Ag (by weight) .
• the multi-component may also contain (preferably up to 3 wt.V,) other alloying elements selected from the group of Cu, Ni, Ag, Ce, In, Bi and Au; such elements should be selected to enhance the mechanical properties of the solder joint at elevated temperatures as long as they do not inhibit metallurgical interactions of the first and second powders (at or less than 150°C in 15 minutes or less) .
• the weight ratio of the first eutectic component to the second component can range generally from 3:1 to 1:3; i.e., the weight Vi. of the first component to the solder can be 25-75%.
• the powders are sized to a mesh of - 200/ +325 and contain no more than 0.1 f 6 by weight lead.
• Such paste has the powder components dispersed in a liquid vehicle or carrier 17 that contains a flux.
• One advantage of paste is that it may be conveniently applied to the faying surface 18, for example by stencil printing. After the faying surfaces are arranged in contact with the paste, the assembly is heated to melt the eutectic powder component, whereupon the partly molten solder coalesces to form a liquid body sufficient to complete the interconnect. In order to obtain a strong bond, it is essential that the molten solder flow in intimate contact onto the faying surfaces 18, which phenomena is referred to as wetting.- Wetting is enhanced by the presence of the flux, which reacts to remove oxides from the metal surfaces, particularly the faying surfaces.
• Tne flux may be comprised of one or more solvents sucn as high boiling point alcohol, glycol or water, activators such as a rosin or synthetic resin components, weak organic acids, ethoxylated amide compounds, fatty amide compounds, and halides. It may also contain a small percentage of surfactants.
• the solvents will escape during the reflow heating stage through volatilisation, decomposition or reaction. Depending on the nature of the flux, the flux residue can either be removed during the subsequent cleaning steps or be left on the board for the life of the product.
• solder paste can be used such as screen printing, syringe dispensing, ink jet printing.
• An alternative method is to coat the second component onto one or both of the parts to be joined and then apply the first component m a paste form.
• Figure 1 shows the various ternary and binary eutectics of the alloy system Sn-Bi-In. Melting temperatures for the eutectics (shown in parentheses) are in degrees Kelvin.
• the low temperature powder component is a ternary eutectic, such as Ej having a melting temperature of 77.5°C
• the high temperature powder component is pure Sn having a melting temperature of 232°C
• the intermixed powders will have a reflow melting temperature of about 120°C and a post reflow melting temperature of about 160-180°C, provided the weight ratio between the powders is m the range of 3:2 to 2:3.
• the differential between the reflow and post-reflow melting temperatures is due to the interaction between unreflowed powder particles and melted powder which forms a single mass consisting of solid solutions 20 and mtermetallics 21.
• the powder particles 15 prior to reflow heating the powder particles 15 (low temperature powder) can be clearly distinguished as independent, but uniformly mixed with the powder particles 16 (high temperature component) , in a carrier or solvent along with the flux. Notice that the particles size of each of the powders will vary (-200/ +325 mesh) .
• the low temperature particles will melt and form a liquid 19 and the carrier solvent and flux will escape; the solid particles 16 will interact with the liquid 19.
• the microstructure of the solder will consist of uniformly distributed Bi-phase particles 20 m a ⁇ phase eutectic matrix 21.
• the ⁇ -phase is a Sn-In mtermetallic phase with Bi m solid solution.
• the post reflow melting temperature is determined by the ratio of Sn/Ei to achieve a temperature of about 160°C, the ratio is generally about 1 (E,)/l (Sn) .
• Table 1 lists other eutectics of the ternary and binary alloy system that may be used for the eutectic powder component; the table also lists the accompanying melting temperature, composition by weight percent, and the phases for each of said eutectics.
• Table 1 illustrates that (l) a solder paste consisting of Sn and E_, E ⁇ or e 7 or e ⁇ powders, will reflow in the temperature range of 80-150°C, (ii) a solder paste consisting of pure Sn and one of the Bi-In binary eutectics, such as e>, e_, or e, will have a reflow temperature in the range of 80-150°C, (m) a solder paste consisting of pure Sn and Sn-In eutectic (e.J , will have a reflow temperature in the range 140-180°C, (iv) a solder paste consisting of Sn-Bi (e ) and Sn-In (e, , eute
• solder pastes may be formulated to substitute Sn-Ag eutectic particles (96.5?. Sn-3.5°s Ag by weight) or Sn-10oln- 95° B ⁇ -0.5° Ag. in place of the Sn particles.
• any of such previously mentioned soldered pastes may also be alloyed with additions of copper, nickel, silver, cerium, bismuth, indium and gold powders in small quantities which can be added to any of such combinations to further improve mechanical properties of the solder at elevated temperatures.
• These additional elements should preferably be incorporated in an amount only up to 3 1 by weight of the solder, but not in an amount that would inhibit metallurgical interactions of the first and second powders at or less than 150°C in 15 minutes or less at such temperature .
• a method of using the unique solder composition of this invention m a reflow soldering process to interconnect electronic components or packages to circuits on a thermoplastic substrate would comprise the following steps: (a) The solder composition is formulated and prepared by admixing metal powders with a flux medium containing 85-95° 0 by weight of the metal powders to form a paste.
• the solder will consist of dispensable material containing a low melting powder component comprised of a ternary or binary eutectic or alloy of the Sn-Bi-In system, and a high melting component comprising Sn. They are admixed m a ratio with the first component being 25-75° by weight of the admixture, to attain the desired post reflow melting temperature.
• the solvents, activators and surfactants may be as previously described,
• the prepared solder paste is applied onto the faying surfaces of the electronic assembly to be soldered
• the faying surfaces being typically a copper trace which is overlaid or cast m place on or in a 3-d ⁇ mens ⁇ onal plastic moulded component or other polymeric or ceramic substrate
• the dispensing is carried out by screen printing, stencilling or by syringe application or other dispensing methods.
• the assembly, containing the deposited solder is then heated in an oven to a temperature above the melting point of the low temperature metal powder constituent while being essentially below 150°C.
• the second powder component (Sn) interacts with the melted eutectic during reflow which promotes a higher post reflow melting temperature of the solder and thereby improve the mechanical properties of the solder joint.

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• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Manufacturing & Machinery (AREA)
• Microelectronics & Electronic Packaging (AREA)
• Electric Connection Of Electric Components To Printed Circuits (AREA)

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• 1997
  • 1997-04-18 TW TW086105044A patent/TW369451B/zh active
  • 1997-05-01 WO PCT/GB1997/001186 patent/WO1997043081A1/en not_active Ceased
  • 1997-05-01 CA CA002253483A patent/CA2253483A1/en not_active Abandoned
  • 1997-05-01 EP EP97918275A patent/EP0946330A1/de not_active Withdrawn
  • 1997-05-01 JP JP09540613A patent/JP2000511466A/ja active Pending
  • 1997-05-01 EP EP00122960A patent/EP1084790B1/de not_active Expired - Lifetime

Non-Patent Citations (1)

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