Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C12/00—Alloys based on antimony or bismuth
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C13/00—Alloys based on tin
• • 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
• • 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
  • B23K35/262—Sn as the principal constituent
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L2924/00—Indexing scheme for arrangements or methods for connecting or disconnecting semiconductor or solid-state bodies as covered by H01L24/00
  • H01L2924/0001—Technical content checked by a classifier
  • H01L2924/0002—Not covered by any one of groups H01L24/00, H01L24/00 and H01L2224/00
• • 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 resistor
  • H05K3/32—Assembling printed circuits with electric components, e.g. with resistor electrically connecting electric components or wires to printed circuits
  • H05K3/34—Assembling printed circuits with electric components, e.g. with resistor electrically connecting electric components or wires to printed circuits by soldering
  • H05K3/341—Surface mounted components
  • H05K3/3431—Leadless components
  • H05K3/3436—Leadless components having an array of bottom contacts, e.g. pad grid array or ball grid array components
• • 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 resistor
  • H05K3/32—Assembling printed circuits with electric components, e.g. with resistor electrically connecting electric components or wires to printed circuits
  • 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
  • H05K3/3463—Solder compositions in relation to features of the printed circuit board or the mounting process

Definitions

• the present invention relates to an alloy and, in particular, a lead-free solder alloy.
• the alloy is particularly, though not exclusively, suitable for use in ball grid arrays and chip scale packages in the form of solder spheres.
• a ball grid array joint is a bead of solder between two substrates, typically circular pads. Arrays of these joints are used to mount chips on circuit boards.
• solder alloy there are a number of requirements for a solder alloy to be suitable for use in ball grid arrays and chip scale packages.
• the alloy must exhibit good wetting characteristics in relation to a variety of substrate materials such as copper, nickel, nickel phosphorus, nickel boron ("electroless nickel'') .
• Solder alloys ' tend to dissolve the substrate and to form an intermetallic compound at the interface with the substrate. For example, tin in the solder alloy will react with the substrate at the interface to form an intermetallic.
• the substrate is copper, then a layer of Cu ⁇ Sns will be formed. Such a layer typically has a thickness of from a fraction of a micron to a few microns.
• an intermetallic compound of Cu 3 Sn may be present. Such an intermetallic compound may result in a brittle solder joint. In some cases, voids occur, which may contribute to premature fracture of a stressed joint.
• the present invention aims to address at least some of the problems associated with the prior art and to provide an improved solder alloy. . Accordingly, the present invention provides an alloy suitable for use in a ball grid array or chip scale package, the alloy comprising
• Copper forms an eutectic with tin/ lowering the melting point and increasing the alloy strength.
• a copper content in the hyper-eutectic range increases the liquidus temperature but further enhances the alloy strength.
• the alloy preferably comprises from 0.15 to 1 wt.% Cu, more preferably from 0.5 to 0.9 wt.% Cu, still more preferably from 0.6 to 0.8 wt.% Cu. Silver lowers the melting point and improves the wetting properties of the solder to copper and other substrates.
• the alloy preferably comprises from 0.1 to 3 wt.% Ag, more preferably from 0.1 to 2 wt.% Ag.. Most preferably, the alloy comprises from 0.1 to 1 wt.% Ag.- Preferred ranges within this range are from 0.1 to 0.5 wt.% Ag, more preferably from 0.1 to 0.4 wt.% Ag, still more preferably from " 0.1 to 0.3 wt.% Ag. The lower limit for the Ag range may be raised to 0.2 wt.%.
• a low silver content has been found to be beneficial because it provides reduced alloy stiffness with the corollary of improved drop shock resistance.
• drop shock or other high strain rate testing the stiffness and acoustic impedance play a primary role in determining how stress is transferred through the solder alloy to the interface (i.e. the solder/IMC/substrate) .
• Preferably such stress or stress waves are damped by the alloy.
• low silver contents improve the alloy characteristics in this respect.
• a Ag3Sn intermetallic typically forms as high aspect ratio laths and plates. In forming a solder joint the Ag3Sn IMC has a tendency to nucleate at the interfaces (i.e.
• the silver content in the alloy according to the present invention is advantageously ⁇ 1 wt.%, preferably ⁇ 0.4 wt.%, more preferably ⁇ 0.35 wt.%, still more preferably ⁇ 0.3 wt.%.
• the alloy preferably comprises from 0.02 - 0.2 wt. % of at least one of nickel, cobalt, iron and chromium, more preferably from 0.02 - 0.1 wt.% of at least one of nickel, cobalt, iron and chromium.
• the alloy preferably comprises from 0.005 - 0.3 wt.% magnesium.
• improved properties can be obtained by the presence of from 0.02 - 0.3 wt% Fe.
• the alloy preferably comprises from 0.01 - 0.15 wt% manganese, more preferably from 0.02 - 0.1 wt% manganese.
• Nickel, cobalt, chromium, manganese, iron, antimony and zirconium may act as intermetallic compound growth modifiers and grain refiners.
• nickel forms an intermetallic with tin and with the copper to form a CuNiSn intermetallic and the presence of ' the low solubility elements in the intermetallic slows the diffusion of Cu and thereby reduces the amount of IMC that forms over time.
• Manganese, cobalt and chromium each have low solubility in tin and are also believed to form intermetallics with copper and tin.
• the presence of the intermetallics affects the microstructure developed on cooling the alloy from the molten to the solid state. A finer grain structure is observed, which further benefits the appearance and strength of the alloy.
• the alloy comprises from 0.005 to 0.3 wt% chromium, preferably from 0.01 to 0.3 wt% chromium, more preferably from 0.02 to 0.2 wt% chromium, still more preferably from 0.03 to 0.1 wt% chromium.
• alloys containing the Cr addition have a reduced ball pull force for the so called Mode 1 failure.
• Mode 1 is the preferred failure mode. It is necking and tensile failure in the solder, not at the interface. Favourable results are also obtained by the presence of Ni in addition to the Cr.
• Chromium has also been found to s.often the alloy and improve oxidation resistance.
• Zirconium has been found to reduce solder interface growth.
• Indium, zinc, magnesium, calcium, gallium and aluminium may act as diffusion compensators.
• the addition of appropriate fast diffusing species can be effective in balancing what otherwise would be a net atom flux away from, for example, the solder-substrate interface, resulting in void formation (Horsting or Kirkendall) .
• Indium has been found to have a beneficial effect on solder wetting. Indium lowers the melting point of the solder. Indium may also act to reduce the formation of voids in the solder joint. Indium may also improve the strength of the Sn-rich matrix. Zinc has been found to act in a similar manner to indium.
• Aluminium has been found to alter the shape of the metallic phases. Aluminium may also combines with any antimony that may be present. Aluminium (as well as chromium, germanium, silicon and phosphorous) may also be beneficial in terms of oxidation reduction.
• Phosphorus, germanium and gallium may act as dross reducers.
• the alloy preferably comprises up to 0.05 wt. % of one or more rare earth elements.
• the one or more rare earth elements preferably comprise two or more elements selected from cerium, lanthanum, neodymium and praseodymium.
• the alloys according to the -present invention are lead- free or essentially lead-free.
• the alloys offer environmental advantages over conventional lead-containing solder alloys.
• the alloys according to the present invention will typically be supplied as a solder sphere for CSP applications but may also be supplied as bar, stick or ingot, optionally together with a flux.
• the alloys may also be provided in the form of a wire, for example a cored wire, which incorporates a flux, a sphere or a preform cut or stamped from a strip or solder. These may be alloy only or coated with a suitable flux as required by the soldering process.
• the alloys may also be supplied as a powder blended with a flux to produce a solder paste.
• the alloys according to the present invention may be used in molten solder baths as a means to solder together two or more substrates and/or for coating a substrate.
• the alloys according to the present invention may contain unavoidable impurities, although, in total, these are unlikely to exceed 1 wt.% of the composition.
• the alloys may contain unavoidable impurities in an amount of not more than 0.5 wt.% of the composition, more preferably not more than 0.3 wt.% of the composition, still more preferably not more than 0.1 wt.% of the composition.
• the alloys according to the present invention may consist essentially of the recited elements. It will therefore be appreciated that in addition to those elements which are mandatory (i.e. ' Sn, Cu,- Ag and at least one of Ni, Co, Mn, Fe, Zr and Cr) other non-specified elements may be present in the composition provided that the essential characteristics of the composition are not materially affected by their presence.
• the alloys will typically comprise at least 90 wt.% tin, preferably from 94 to 99.5 % tin, more preferably from 95 to 99 % tin, still more preferably 97 to 99 % tin.
• the present invention further provides an alloy for use in a ball grid array or chip scale package, the alloy comprising
• the alloys according to the present invention are particularly well suited to applications involving ball grid arrays or chip scale packages. Accordingly, the present invention also provides for the use of a solder alloy as herein described in a ball grid array or chip scale package.
• the present invention also provides for a ball grid array or chip scale package joint comprising the solder alloy composition as herein described.
• An alloy was prepared by melting Sn in a cast iron crucible (alternatively a ceramic crucible can be used) . To the molten Sn was added an alloy of Sn-3wt% Cu, and alloys of Sn-5 wt% Ag and Sn-O .35wt%Ni. ' These additions were made with the alloy bath temperature at 35O 0 C. The bath was cooled to 300 '0 C for the addition of phosphorus in the form of an alloy Sn-O.3%P.
• the alloy was sampled to verify the composition of
• the alloy composition was then jetted as a metal stream into an inerted vertical column.
• the metal stream was spherodised by the application of magnetostrictive vibrational energy applied through the melt pot and at or near the exit orifice.
• alloy composition could be punched and then spherodised as a sphere.
• the alloy provided in the form of a sphere, can be used in a ball grid array joint or chip scale package. Flux is printed or pin transferred to the pads of a CSP. The spheres are then pick and placed or shaken through a stencil onto the fluxed pads. The package is then reflowed in a standard reflow oven at a peak temperature of between 24O 0 C and 260 0 C.
• Alloy and solder joint performance was assessed in packages aged at 15O 0 C for up to 1000 hours. IMC growth was measured by standard metallographic techniques. Mechanical •ball pull testing was used to assess solder joint failure mode (brittle or ductile) .
• Example 2 The following alloy composition was prepared in a similar manner to Example 1 (all wt.%) .
• This alloy may be provided in the form of a sphere and used in a ball grid array joint or chip scale package.
• This alloy may be provided in the form of a sphere and used in a ball grid array joint or chip scale package.
• This alloy may be provided in the form of a sphere and used in a ball grid array joint or chip scale package.
• Alloys have been prepared corresponding to the compositions of Examples 1 to 4 where Ge is substituted for the phosphorus content.
• This alloy may be provided in the form of a sphere and used in a ball grid array joint or chip scale package.
• This alloy may be provided in the form of a sphere and used in a ball grid array joint or chip scale package.
• This alloy may be provided in the form of a sphere and used in a ball grid array joint or chip scale package.
• This alloy may be provided in the form of a sphere and used in a ball grid array joint or chip scale package.
• This alloy may be provided in the form of a sphere and used in a ball grid array joint or chip scale package.
• This ' alloy may be provided in the form of a sphere and used in a ball grid array joint or chip scale package. . .
• This alloy may be provided in the form of a sphere and used in a ball grid array joint or chip scale package.
• This alloy may be provided in the form of a sphere and used in a ball grid array joint or chip scale package.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Electric Connection Of Electric Components To Printed Circuits (AREA)
• Conductive Materials (AREA)

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• 2006
  • 2006-08-24 JP JP2008527511A patent/JP2009506203A/ja active Pending
  • 2006-08-24 WO PCT/GB2006/003167 patent/WO2007023288A2/en active Application Filing
  • 2006-08-24 EP EP06779198A patent/EP1924394A2/en not_active Withdrawn
  • 2006-08-24 KR KR1020087007074A patent/KR101339025B1/ko active IP Right Grant

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