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, brazing
  • B23K35/0244—Powders, particles or spheres; Preforms made therefrom
  • B23K35/025—Pastes, creams, 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 degrees C
  • B23K35/262—Sn as the principal constituent
• • 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
  • B23K35/264—Bi as the principal constituent
• • 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
• • 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
  • C22C13/02—Alloys based on tin with antimony or bismuth as the next major constituent
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C30/00—Alloys containing less than 50% by weight of each constituent
  • C22C30/02—Alloys containing less than 50% by weight of each constituent containing copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C30/00—Alloys containing less than 50% by weight of each constituent
  • C22C30/04—Alloys containing less than 50% by weight of each constituent containing tin or lead
• • 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
  • H05K3/3485—Applying solder paste, slurry or powder
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
  • B22F1/00—Metallic powder; Treatment of metallic powder, e.g. to facilitate working or to improve properties
  • B22F1/10—Metallic powder containing lubricating or binding agents; Metallic powder containing organic material
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C9/00—Alloys based on copper
  • C22C9/02—Alloys based on copper with tin as the next major constituent

Definitions

• the present invention relates generally to the composition of mixed alloy solder pastes, and more particularly, some embodiments relate to the composition of each alloy constituent in a solder paste for high temperature solder joint applications.
• Pb-bearing solders to replace the traditional eutectic Pb-Sn have been widely investigated.
• SnAg, SnCu, SnAgCu and SnZn solders are becoming the mainstream solders for interconnection in the semiconductor and electronics industries.
• high temperature Pb-free solders to substitute the conventional high lead ones, i.e. Pb-5Sn & Pb-5Sn-2.5Ag, is still in its infancy.
• High temperature solders are used to keep the internal connection within the components in an assembly when the assembly is being soldered onto a printed wiring board (PWB).
• a common use of high temperature solder is for die-attach.
• the assembly is formed by soldering a silicon die onto a lead-frame using the high temperature solder. Then, the silicon die/lead-frame assembly, either encapsulated or not, is attached onto PWB by soldering or mechanical fastening.
• the board may be exposed to a few more reflow processes for surface mounting other electronic devices onto the board. During the further soldering processes, the internal connection between the silicon die and the lead-frame should be well maintained. This requires that the high temperature solder resist the multiple reflows without any functional failure.
• the major requirements for the high temperature solders include (i) a melting temperature around 260°C and above (in accordance with typical solder reflow profiles), (ii) good thermal fatigue resistance, (iii) the high thermal/electric conductivity, and (iv) low cost.
• solder candidates have been proposed recently for the high temperature die-attach applications, such as (1) Sn-Sb, (2) Zn-based alloys, (3) Au-Sn/Si/Ge and (4) Bi-Ag.
• Sn-Sb alloys with less than 10wt% Sb, maintain good mechanical properties without forming massive intermetallic compounds. But their solidus temperature is no higher than 250°C, which can not satisfy the 260°C requirement for reflow resistance.
• Zn-based alloys including eutectic Zn-Al, Zn-Al-Mg, and Zn-Al-Cu, have melting temperature above 330°C.
• Zn-(20-40wt%)Sn solder alloys proposed to be one of the high temperature lead-free substitute solder, have liquidus temperature above 300°C, but the solidus temperature is only around 200°C.
• the semi-solid state of Zn-Sn solder at around 260°C is supposed to maintain a good interconnection between the components during subsequent reflows.
• Eutectic Au-Sn composed of two intermetallic compounds, has been experimentally shown to be a reliable high temperature solder because of its melting temperature of 280°C, good mechanical properties, high electrical & thermal conductivities, and excellent corrosion resistance.
• extremely high cost restricts its application within the fields where cost overcomes reliability considerations.
• Bi-Ag alloys with solidus temperature of 262°C satisfy the melting temperature requirement for high temperature die-attach solders.
• the poor wetting of a solder originates from (1) poor reaction chemistry or (2) the oxidation of the solder. Weak bonding is always associated with the poor wetting.
• the poor wetting of Bi-based solder on different metallization surfaces results mainly from the poor reaction chemistry between Bi and the substrate materials (i.e. Cu) or the oxidation of Bi during reflow.
• Ge-doped BiAg aimed at preventing the excessive formation of dross on alloy surface during melting have been developed.
• this doping will not change the reaction chemistry between Bi and the metallization surface finish of the substrate.
• Bi and Cu will not form EVICs at a Bi/Cu interface, which is the dominant reason for the poor wetting and the weak bonding interface.
• Bi and Ni will form IMC layer between a Bi/Ni interface, but the brittle IMCs (Bi3Ni or BiNi) weaken the joint strength because cracks always grow along either the interface between Bi3Ni and the solder matrix or the interface between BiNi and the Ni substrate. Accordingly, the reaction chemistry between Bi and substrate materials result in the poor wetting and the weak bonding strength.
• the present invention claims a new technology for designing and preparing mixed alloy solder pastes, which deliver the combined merits from the constituent alloy powders.
• the mixed alloy solder pastes are suitable for high-temperature solder applications such as die attach because the constituents provide the desired merits including improved reaction chemistry, well-controlled IMC layer thickness, and the enhanced reliability accordingly from the second alloy and the high melting temperature and good thermal/electrical conductivity from the first alloy.
• the invention also presents the method of preparing the mixed alloy solder pastes and the methodology of joining the electronic components or mechanical parts with the mixed alloy solder pastes.
• the invented technology provides a method of designing mixed alloy powder pastes, in which the additive powders are present in the paste to improve the reaction chemistry at a relatively lower temperature or together with the melting of the first alloy solder powders.
• the mixed alloy powder pastes include two or more alloy powders and a flux.
• the alloy powders in the paste are composed of one solder alloy powder as a majority and an additive alloy powder as a minority.
• the additives provide the superior chemistry to wet on various metallization surface finishes of the substrates, namely the commonly used Cu and Ni surface finishes, etc.
• the additives will melt before or together with the melting of the majority solder.
• the molten additives will wet on and adhere to the substrate before or together with the partially or completely molten first alloy.
• the additives are designed to dominate the formation of IMCs along the substrate metallization surface finish and be completely converted into IMCs during the reflow process.
• the thickness of the IMC layer will thus be well controlled by the quantity of the additives in the paste because of the dominant roles the additives play in the IMC formation.
• the first alloy solder will have a strong affinity to the IMC layer formed between the additives and the substrate. This strong affinity will enhance the bonding strength between the solder body and the IMCs.
• the desired reaction chemistry and the well controlled thickness of the IMC layer not only improves the wetting performance but also enhances the bonding strength associated with the wetting performance.
• Figure 1 illustrates a reflow solder process implemented in accordance with an embodiment of the invention.
• Figure 2 shows the wetting performance of an example solder paste consisting of 90wt%Bil0.02Ag3.74Sn+10wt% flux on a Cu coupon and an Alloy 42 coupon.
• Figure 3 shows the wetting performance of an example of the mixed alloy powder solder paste consisting of 84wt%Bil lAg+6wt%Bi42Sn+10wt% flux on a Cu coupon and an Alloy 42 coupon.
• Figure 4 shows the wetting performance of an example of the mixed alloy powder solder paste consisting of 84wt%Bil lAg+6wt%52In48Sn+10wt% flux on a Cu coupon and an Alloy 42 coupon.
• Figure 5 is a DSC chart for a mixed alloy powder solder paste consisting of
• Figure 6 is a DSC chart for a mixed alloy powder solder paste consisting of
• Figure 7 is a DSC chart for a mixed alloy powder solder paste consisting of
• Figure 8A and B are cross-section images of the joints made of a mixed alloy powder solder paste on Cu and Ni coupons.
• the mixed alloy powder paste consists of
• the present invention is directed towards solder pastes comprising mixtures of different solder alloys in flux.
• Two or more solder alloys or metals are incorporated into a flux material.
• a first solder alloy or metal (the "first alloy") will form the main body of the solder joint during reflow.
• the remaining second solder alloy or metal, or further additional solder alloys or metals, (the "second alloy") is selected according to reaction chemistry with the metal substrate or affinity to the first alloy.
• the second alloy's melting temperature Tm(B) is lower than the first alloy's melting temperature Tm(A).
• the second alloy melts first, and spreads onto the substrate.
• the presence of the second alloy facilitates the setting of the molten first alloy on the substrate.
• the second alloy is designed to be completely converted into IMCs, resulting in minimal or absent low-melting phase in the final joint.
• the additives in the pastes modify the reaction chemistry during reflow, improve the wetting, control the thickness of IMCs, and thus enhance the bonding strength.
• the design process can be extended into many other soldering applications wherever a poor wetting solder is in use.
• Pb-Cu alloys have a high melting temperature but poor wetting on various metal substrates. Thus, they are difficult to use in soldering.
• small additives such as Sn or Sn-containing alloys, will help Pb-Cu to wet various metal surfaces.
• Figure 1 illustrates a reflow process using a mixed solder paste according to an embodiment of the invention.
• the mixed solder paste comprises first alloy solder particles 118 and second alloy solder particles 115 suspended in a flux.
• the second alloy is selected according to its superior reaction chemistry to the substrate, or to a range of common substrates.
• the mixed solder paste is applied to a substrate 124. (For sake of explanation, the flux is omitted from the figure.)
• the temperature of the assembly rises above the second alloy's melting temperature, Tm(B).
• Tm(B) the second alloy's melting temperature
• the second alloy melts and spreads 112 over the substrate 124 and around the still-solid first alloy particles 118.
• the superior surface reaction chemistry of the second alloy will facilitate the wetting of the molten solder alloy 112 on the substrate 124. This leads to the formation of IMC layer 109 between the molten second alloy 112 and the substrate 124.
• the IMC layer is mainly controlled by the quantity of the second alloy 115 in the initial paste.
• the second alloy is designed to have a good affinity to the first alloy.
• This affinity may be determined by (1) the negative mixing enthalpy between the first alloy and the second alloy, or (2) the formation of a eutectic phase composed of the constituent elements from the first and second alloys. In some embodiments, this affinity results in some of the first alloy 118 dissolving into the molten second alloy 112 to form a mixture 106 of the first and second alloys.
• the first alloy finishes melting, forming a solution 103 of the first and second alloys, which wets to the IMC layer 109.
• the second alloy is removed from the solution 103, increasing the IMC layer 109, and leaving the molten first alloy 100.
• excess constituents from the second alloy may be incorporated into IMC with constituents from the first alloy. The affinity between the first alloy and the second alloy assists in improving the wetting of the first alloy 100 onto the IMC layer 109, thereby enhancing the bonding strength.
• solder bump 121 or joint is formed of the substrate 124 bonded to the IMC 109, which is bonded to the solidified first alloy. After solidification, a homogenous solder joint with the improved bonding interface has been achieved.
• solder joint resulting from use of the mixed solder paste shows large improvements of the use of a solder paste containing a single solder alloy, even when the single solder alloy is composed of the elements of the first and second solder alloys.
• Figure 2 illustrates solder bumps 201 and 207 formed using a solder paste consisting of 86.24wt%Bil0.02Ag3.74Sn+10wt% flux on a Cu substrate 200 and an Alloy 42 substrate 205, respectively. As these results show, significant dewetting 202 and 206 occurs with the use of a single solder alloy.
• Figure 3 illustrates solder bumps 211 and 216 formed using a mixed solder paste consisting of
• the mixed solder paste comprises BiAg as the first alloy and SnSb as the second alloy.
• Sn is chosen for its superior reaction chemistries over Bi with various substrates.
• SnSb has a lower melting temperature than BiAg.
• Sn and Bi exhibit a negative mixing enthalpy and form the eutectic phase in a wide composition range, according to the binary phase diagram.
• Sb and Bi also show a negative mixing enthalpy and infinite solubility to each other.
• SnSb melts first and forms a Sn-containing IMC layer on the substrate surface.
• all alloy powders in the paste melt.
• the good affinity between Bi and Sn/Sb guarantees good adhesion of molten Bi on the Sn-containing IMC layer.
• the existence of Ag in the first alloy can convert any extra Sn into Ag3Sn IMCs residing in the solder body. Therefore, there is minimal or no low melting BiSn phase left because Sn is completely consumed by forming (1) IMC layer between the solder and the metal substrate and (2) Ag3Sn inside the BiAg solder bump.
• Figure 5 illustrates a DSC curve for the joint resulting from the use of
• the top curve illustrates the heat flow profile after reflow on a ceramic coupon.
• a spike at around 138°C illustrates the presence of the second alloy.
• the bottom curve illustrates the heat flow profile of the paste after reflow on a Cu coupon. The absence of this spike in the bottom curve verifies the disappearance of the low melting phase in the BiAg+SnSb system.
• Figure 6 illustrates the disappearance of the low melting phase in the BiAg+SnAg system. The experiment of Figure 6 used
• FIG. 7 illustrates the disappearance in the BiAg+BiSn system.
• the experiment of Figure 7 used 84wt%Bil lAg+6wt%Bi42Sn+10wt% flux on ceramic and Cu coupons, as in Figures 5 and 6.
• the top curve, illustrating the heat flow profile after solder reflow on ceramic shows a lack of low melting phase. This is likely due the small quantity of the reactive agent, Sn, in the mixed solder paste and the high affinity between Sn and Ag, resulting in the Sn of the second alloy being incorporated with some Ag of the first alloy into IMC in the final solder bump.
• Figure 8A is a micrograph of a solder joint resulting from use of a mixed solder paste consisting 84wt%Bil lAg+6wt%Sn3.5Ag+10wt% flux.
• the mixed solder paste was applied to a Cu coupon 300.
• An IMC 301 forms between the Cu 300 and the second alloy.
• the size of this IMC 301 is dependent primarily on the amount of second alloy in the paste.
• 6wt% of the second alloy Sn3.5Ag resulted in an IMC that is only a few microns thick.
• the bulk of the solder joint is made up of Ag 303 in a Bi-rich phase 302. Aging at 150°C for 2 weeks did not significantly increase the IMC thickness.
• Bi and Cu do not form intermetallics, so Bil lAg alone forms a weak bond because no IMC layer is presented between the solder and substrate.
• a method of designing a mixed solder paste comprises selecting a first alloy according to a desired trait of a finished solder joint, and then selecting a second alloy according to applicable substrates and affinity with the selected first alloy.
• the relative amounts of the first alloy, second alloy, and flux may be determined according to factors such as desired IMC layer thickness, required application conditions, and reflow processes.
• the IMC layer thickness is related to amount of second alloy in the solder paste, the reflow profile, and the aging conditions following application. Acceptable thicknesses of IMC layer may vary with different application conditions and different IMC compositions. For example, for a Cu6Sn5/Cu3Sn IMC layer, 10 microns may be about as thick as is acceptable.
• the amount of second alloy in the paste As the amount of second alloy in the paste is increased, there may be low melting phase remaining gin the final joint. If the amount of second alloy is reduced in the solder paste, desired wetting performance may be difficult to achieve. As the amount of second alloy is reduced, good wetting requires the use of a larger total amount of paste printed or dispensed on the substrate. However, increasing the total amount of paste may interfere with the geometry constraints from the soldering packages.
• the first alloy must be chosen from various high melting solder alloys.
• Bi-rich alloys whose solidus temperature is around 258°C and above, i.e. Bi-Ag, Bi-Cu, and Bi-Ag-Cu, are used.
• the second alloys are selected from the alloys that have shown superior chemistry to wet on and adhere to various metallization surface finishes and good affinity to the molten Bi.
• Group C has the solder alloys with solidus temperature lower than 200°C, i.e.
• the first alloy is an alloy from the Bi-Ag system and has a solidus temperature around 260°C and above.
• the first alloy comprises from 0 to 20wt% Ag with the remainder being Bi.
• the first alloy comprises from 2.6 to 15wt% Ag with the remainder being Bi.
• the first alloy is selected from the Bi-Cu system and has a solidus temperature around 270°C and above.
• the first alloy comprises from 0 to 5wt% Cu with the remainder being Bi.
• the first alloy comprises from 0.2 to 1.5wt% of Cu with the remainder being Bi.
• the first alloy is selected from the Bi-Ag-Cu system and has a solidus temperature around 258°C and above.
• the first alloy comprises from 0 to 20wt% Ag and from 0 to 5wt% Cu with the remainder being Bi.
• the first alloy comprises from 2.6 to 15wt% Ag, and from 0.2 to 1.5wt% Cu with the remainder being Bi.
• the second alloy is from the Sn-Sb system and has a solidus temperature between around 231°C and around 250°C.
• the second alloy comprises from 0 to 20wt% Sb with the remainder of Sn.
• the second alloy comprises of from 0 to 15wt% Sb with the remainder being Sn.
• the second alloy comprises from 0 to 20wt% Sb and from 0-20wt% of X with the remainder being Sn.
• the second alloy comprises from 0 to 10wt%Sb and from 0 to 5wt% X with the remainder being Sn.
• the second alloy comprises Sn-Ag and has a solidus temperature around 221°C and above.
• the second alloy comprises from 0 to 10wt%Ag with the remainder being Sn. In a further embodiment, the second alloy comprises from 0 to 5wt%Ag with the remainder being Sn. In a seventh embodiment of the invention, the second alloy comprises Sn-Cu and has a solidus temperature around 227°C and above. In a particular embodiment, the second alloy comprises from 0 to 5wt%Cu with the remainder being Sn. In a further embodiment, the second alloy comprises from 0 to 2wt%Cu with the remainder being Sn.
• the second alloy comprises from 0 to 10wt%Ag and from 0 to 20wt% X with the remainder being Sn.
• the second alloy comprises from 0 to 5wt%Ag and from 0 to 5wt% X with the remainder being Sn.
• the second alloy comprises Sn-Zn and has a solidus temperature around 200°C and above.
• the second alloy comprises from 0 to 20wt%Zn with the remainder being Sn.
• the second alloy comprises from 0 to 9wt%Zn with the remainder being Sn.
• the second alloy comprises a Bi-Sn alloy with solidus temperature around 139°C and above.
• the second alloy comprises from 8 to 80wt% Sn with the remainder being Bi.
• the second alloy comprises from 30 to 60wt% Sn with the remainder being Bi.
• the second alloy comprises a Sn-In alloy with solidus temperature around 120°C and above.
• the second alloy comprises from 0 to 80wt% In with the remainder being Sn.
• the second alloy comprises from 30 to 50wt% In with the remainder being Sn.
• the second alloy comprises a Bi-In alloy with solidus temperature between around 100 and around 200°C. In a particular embodiment, the second alloy comprises from 0 to 50wt% In with the remainder being Bi. In a particular embodiment, the second alloy comprises from 20 to 40wt% In with the remainder being Bi. In a thirteenth embodiment of the invention, the second alloy comprises an In-Cu alloy with solidus temperature between around 100 and around 200°C. In a particular embodiment, the second alloy comprises from 0 to 10wt% Cu with the remainder being In. In a particular embodiment, the second alloy comprises from 0 to 5wt% Cu with the remainder being In.
• the second alloy comprises an In-Ag alloy with solidus temperature between around 100 and around 200°C.
• the second alloy comprises from 0 to 30wt% Ag with the remainder being In.
• the second alloy comprises from 0 to 10wt% Ag with the remainder being In.
• the second alloy comprises from 0 to 20wt% Ag, 0 to 20wt% X with the remainder being In.
• the second alloy comprises from 0 to 10wt% Ag, 0 to 5wt% X with the remainder being In.
• Further embodiments of the invention provide methods for making mixed solder pastes.
• particles of the first alloy are formed and particles of the second alloy are formed.
• the particles of the first and second alloys are then mixed with a flux to form a solder paste.
• the final paste comprises the first alloy powder, the second alloy powder, with the balance flux.
• the first alloy particles are of an alloy having a solidus temperature of at least about 260°C.
• the second alloy comprises an alloy having a solidus temperature between about 230°C and about 250°C, an alloy having a solidus temperature between about 200°C and about 230°C, or an alloy having a solidus temperature below about 200°C.
• the paste is composed of between about 60 and about 92wt% of the first alloy powder, an amount of the second alloy powder greater than 0% but less than or equal to about 12wt%, with the balance being flux.
• the second alloy powder is between 2 and 10 wt% of the mixed solder paste.
• the first alloy comprises a Bi-Ag alloy, a Bi-Cu alloy, or a
• the alloy having a solidus temperature between about
• the alloy having a solidus temperature below about 200°C comprises a Sn-Bi alloy, a Sn-In alloy, or a Bi-In alloy.
• the second alloy powder comprises a powder composed of a plurality of alloy powders.
• the second alloy powder may comprise a mix of different alloys selected from the alloys described herein.
• the relative amounts of the first and second alloys in the mixed solder pastes are determined according to the solder application. In some cases, when the amount of second solder alloy in the paste is increased past a certain threshold, the chances of retaining some low melting phase in the final solder joint may be increased. In some cases, when the amount of second solder alloy in the paste is below a certain threshold, the wetting to substrate may be reduced. In one embodiment, the amount of second solder alloy in the paste is determined such that the low melting phase may be completely converted into high melting IMCs after reflow. In a further embodiment, the second alloy in the paste varies between an amount greater than 0wt% but less than about 12wt%. In a particular embodiment, the second alloy in paste is greater than about 2wt% but less than about 10wt%.
• the reflow profile used for soldering with a mixed solder paste is designed to heat up quickly to above the melting temperature of the first alloy.
• a shorter soaking time at low temperature may allow the reactive agent, such as Sn, to flow quickly toward the substrate and react with the substrate in a completely melted pool rather than a semisolid melted pool.
• the melting of both first and second alloy will facilitate the diffusion of the second alloy elements from the molten solder towards the substrate and the part and "sink" onto the surface to form the IMC layer.
• Table 1 describes formulations of example mixed solder pastes made using a first alloy comprising Bil lAg or Bi2.6Ag, a second alloy comprising SnlOAg25Sb or SnlOAglOSb, and flux.
• Table 2 describes formulations of example mixed solder pastes made using a first alloy comprising Bil lAg, a second alloy comprising Sn3.8Ag0.7Cu, Sn3.5Ag, Sn0.7Cu, or Sn9Zn, and flux.
• Table 3 describes formulations of example mixed solder pastes made using a first alloy comprising Bil lAg, a second alloy comprising Bi42Sn, Bi33In, or In48Sn, and flux.
• Each paste described by Tables 1, 2, and 3, was prepared and printed onto a coupon using a three -hole stainless steel stencil.
• Cu, Ni, Alloy 42, and Alumina coupons were used.
• Each paste was printed onto each coupon.
• the holes were 1/4 inch in diameter.
• the printed coupons were reflowed through a reflow oven with a profile designed for the mixed alloy powder solder pastes. The reflow was performed in a three-zone reflow oven, at 380°C, 400°C, and 420°C at a belt speed of 13" per minute under an N 2 atmosphere.
• Figures 3 and 4 are pictures that are representative of typical results.
• Figure 3 shows the wetting performance of an example of the mixed alloy powder solder paste consisting of 84wt%Bil lAg+6wt%Bi42Sn+10wt% flux.
• the left image shows the paste reflowed on a Cu coupon; the right image shows the paste reflowed on an Alloy 42 coupon.
• Figure 4 shows the wetting performance of an example of the mixed alloy powder solder paste consisting of 84wt%Bil lAg+6wt%52In48Sn+10wt% flux.
• the left image shows the paste reflowed 401 on a Cu coupon 400; the right image shows the paste reflowed 402 on an Alloy 42 coupon 405.
• the reflowed solder balls were peeled off from the alumina coupons for DSC testing.
• the solder bumps formed on the Cu coupons and Ni coupons were also punched off for DSC testing.
• DSC measurement was carried out using a differential scanning calorimeter at a heating rate of 10°C/min.
• the representative DSC curves are shown in Figs. 5-7.
• Figure 5 illustrates a DSC curve for the joint resulting from the use of 84wt%Bil lAg+6wt%Snl5Sb+10wt% flux.
• the top curve illustrates the heat flow profile after reflow on a ceramic coupon.
• a spike at around 138°C illustrates the presence of the second alloy.
• the bottom curve illustrates the heat flow profile of the paste after reflow on a Cu coupon.
• the absence of this spike in the bottom curve verifies the disappearance of the low melting phase in the BiAg+SnSb system.
• Figure 6 illustrates the disappearance of the low melting phase in the BiAg+SnAg system.
• the experiment of Figure 6 used 84wt%Bil lAg+6wt%Sn3.5Ag+10wt% flux on ceramic and Cu coupons, as in Figure 5.
• Figure 7 illustrates the disappearance in the BiAg+BiSn system.
• the experiment of Figure 7 used 84wt%Bil lAg+6wt%Bi42Sn+10wt% flux on ceramic and Cu coupons, as in Figures 5 and 6.
• Figure 8a is a cross section of a solder bump using
• Figure 8b is a cross section of a solder bump using the same solder paste on a Ni coupon. As these results show, the IMC layer thickness was restricted to a few ⁇ on both coupons.
• the second solder alloy is a Bi-Sb-Sn alloy composed of greater than 0wt% to 40wt% Sb, greater than 0wt% to 40wt% Sn, and Bi.
• the Bi- Sb-Sn-X alloy is composed of greater than 0wt% to 40wt% Sb, greater than 0wt% to 40wt% Sn, greater than 0wt% to 5wt% X, and Bi.
• the first solder alloy is a Bi-Ag-Y alloy, a Bi-Cu-Y alloy, or a Bi-Ag- Cu-Y alloy, where Y is Al, Au, Co, Ga, Ge, In, Mn, Ni, P, Pd, Pt, Sb, Sn, or Zn.
• Y is Al, Au, Co, Ga, Ge, In, Mn, Ni, P, Pd, Pt, Sb, Sn, or Zn.
• Y ranges from greater than 0wt% to 5wt% of the first solder alloy.
• the solder paste is composed of a higher weight percentage of the second alloy powder, a lower weight percentage of the first alloy powder, or some combination thereof.
• the solder paste is composed of an amount of a first solder alloy powder between 44wt% and less than 60wt% of the first alloy powder, an amount of the second alloy powder greater than 0 wt% and less than 48 wt%, and an amount of flux between 0wt% and 15wt%.
• the amount of the second alloy powder may be greater than 2 wt% and less than 40 wt%.
• the solder paste is composed of an amount of a first solder alloy powder between 44wt% and 87wt%, an amount of a second solder alloy powder between 13wt% and 48wt%, and an amount of flux between 0wt% and 15wt%.
• the amount of the second alloy powder may be between 26wt% and 40wt%.
• the higher weight percentage of the second alloy powder, the lower weight percentage of the first solder alloy powder, or the combination enhances the homogeneity of the solder paste. This improved homogeneity permits the application of the mixed alloy solder paste to smaller joint formation, which provides both wetting and high temperature performance even for bonding joints at least as small as 0.2mm in diameter.
• this combination of the solder alloy powders permit an even higher temperature solder joint temperature after reflow.
• the formed joint remelting temperature may be higher than 270°C.
• the formed joint remelting temperature may be up to 280°C or even higher.
• the various diagrams may depict an example architectural or other configuration for the invention, which is done to aid in understanding the features and functionality that can be included in the invention.
• the invention is not restricted to the illustrated example architectures or configurations, but the desired features can be implemented using a variety of alternative architectures and configurations. Indeed, it will be apparent to one of skill in the art how alternative functional, logical or physical partitioning and configurations can be implemented to implement the desired features of the present invention. Also, a multitude of different constituent module names other than those depicted herein can be applied to the various partitions. Additionally, with regard to flow diagrams, operational descriptions and method claims, the order in which the steps are presented herein shall not mandate that various embodiments be implemented to perform the recited functionality in the same order unless the context dictates otherwise.
• module does not imply that the components or functionality described or claimed as part of the module are all configured in a common package. Indeed, any or all of the various components of a module, whether control logic or other components, can be combined in a single package or separately maintained and can further be distributed in multiple groupings or packages or across multiple locations.

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• 2016
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