Patent Description:
Lead-free solder alloys are well known and provide non-toxic alternatives to the most widely used solder alloy - eutectic <NUM>%Pb-<NUM>%Sn alloy. Examples of such lead-free alloys include the binary eutectic <NUM>%Bi-<NUM>%Sn alloy (see, for example, <CIT> B) and the binary <NUM>%Bi-<NUM>%Sn alloy (see, for example, <CIT>). Such alloys exhibit a loss of ductility at high strain rates, which can be improved by the addition of small amounts of additives, such as up to <NUM>% by weight silver (see, for example, <CIT>). However, the impact energies exhibited by these alloys, measured using the Charpy Impact Test, are relatively low. Accordingly, there is a need to develop lead-free solder alloys which exhibit improved impact toughness.

In order for such lead-free alloys to be used in soldering methods such as wave and reflow soldering, the alloys must exhibit good wettability in relation to a variety of substrate materials such as copper, nickel and nickel phosphorus ("electroless nickel"). Such substrates may be coated to improve wetting, for example by using tin alloys, silver, gold or organic coatings (OSP). Good wetting also enhances the ability of the molten solder to flow into a capillary gap, and to climb up the walls of a through-plated hole in a printed wiring board, to thereby achieve good hole filling.

Furthermore, solder compositions need to exhibit a good thermal fatigue life and decreased high temperature creep. Improved ductility and thermal and electrical conductivity are also desirable. These properties can be achieved by selecting a specific solder alloy if one is known, or through the use of specific additives. However, it would be advantageous if the properties of existing commonplace solders could be adapted to provide these benefits without requiring alternative solder alloys to be developed. <CIT> relates to a soldering composition, soldering method, and electronic component. <CIT> relates to a material and method of manufacturing of a solder joint with high thermal conductivity and high electrical conductivity. <CIT> relates to variable melting point solders and brazes. <CIT> relates to a solder paste composition, solder precoating method and mounted substrate. <CIT> relates to a lead-free solder material, electronic circuit board and their production method. <CIT> relates to a process for preparing powdered silversolder compositions. <CIT> relates to an electrical solder and method of manufacturing. <CIT> relates to a solder material, its manufacturing method and soldering method.

Accordingly, there is a desire for a solder composition that will overcome, or at least mitigate, some or all of the problems associated with the solders of the prior art or at least a useful or optimized alternative.

According to a first aspect, the present invention provides a solder composition according to claim <NUM>.

The present disclosure will now be further described. In the following passages different aspects of the disclosure are defined in more detail. Each aspect so defined may be combined with any other aspect or aspects unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous.

The term "solder alloy" used herein refers to a fusible metal alloy with a melting point in the range of from <NUM> - <NUM> degrees C.

The "Charpy impact test" referred to herein, also known as the Charpy v-notch test, is a standardized high strain-rate test which determines the amount of energy absorbed by a material during fracture. This absorbed energy is a measure of a given material's toughness and acts as a tool to study temperature-dependent brittle-ductile transition. Further details regarding this test can be found in Charpy Impact Test: Factors and Variables, J. Holt, ASTM STP <NUM>, the contents of which is hereby incorporated by reference.

The term "wettability" used herein refers to the degree to which solder spread on a wettable surface. Wettability is determined by surface tension of the liquid solder and its ability to react with the wettable surface. Wetting can also be described in terms of the contact angle of the molten and subsequently frozen solder alloy on a substrate, with lower contact angles being favoured over high contact angles.

The term "wave soldering" used herein refers to the large-scale soldering process by which electronic components are soldered to a printed circuit board (PCB) to form an electrical assembly.

The term "reflow soldering" used herein refers to the process where solder paste is printed or dispensed, or a solder perform is placed on the surface of a printed circuit board, components are placed in or near the deposited solder, and the assembly is heated to a temperature above the liquidus of the solder alloy.

The term "rare earth element" used herein refers to an element selected from Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu.

All percentages herein are by weight unless expressly stated otherwise.

Preferably at least one of the powders is spherical, preferably both. That is, at least <NUM>% of the particles have a length-to-width ratio of less that <NUM>. Preferably at least <NUM>%, more preferably at least <NUM>%, of the particles have a length-to-width ratio of less that <NUM>. For most applications this high degree of "sphericity" is preferred, the major advantage being lower surface area, which minimizes oxidation, and better load acceptance (less tendency for clogging and interlocking), which assists dispensability and release through a stencil aperture. In an alternative embodiment, at least one of the powders may be irregular.

Particle roundness influences paste viscosity and the tendency to shear. Spheres offer less resistance to viscous flow compared with particles of irregular shape. Accordingly, paste made from the same flux and spherical powder will have lower viscosity than those of the same weight percent and particle size range but of irregular shape. One possible advantage with pastes of the latter appearance is that they are less likely to shear thin when screen/stencil printed at high speed and constant squeegee motion. Interlocking of the powder reduces paste flow out. Reduction of shear thinning is important because it will prevent slumping and smearing which can result in solder bridging and solder balling.

Preferably the solder powder particles are from <NUM> to <NUM> microns in mean diameter. More preferably the particles are from <NUM> to <NUM> microns in mean diameter, most preferably <NUM> to <NUM> microns. The diameter measurement refers to the longest diameter of the particle. Preferably the powder particles of both the first and second components are substantially the same.

Preferably the solder composition consists of the blend of the first powder component and the second powder component, together with unavoidable impurities. It will be appreciated that the composition according to the present invention may contain unavoidable impurities, possibly as part of the first and/or second components, although, in total, these are unlikely to exceed <NUM> wt% of the composition. Preferably, the alloys contain unavoidable impurities in an amount of not more than <NUM> wt% of the composition, more preferably not more than <NUM> wt% of the composition, still more preferably not more than <NUM> wt% of the composition.

Preferably the solder composition is lead-free. This allows the composition to comply with regulatory requirements.

The inventors have discovered that it is possible to engineer the effective melting temperature, and the mechanical, electrical and thermal properties of a reflowed solder using standard solder alloy and/or metal powders.

In particular, the inventors have discovered that a mixture of two or more solder alloys is particularly useful. In particular, where the first and second solder alloys have different melting points, during the first reflow, which goes up to a peak temperature of above the liquidus of the lower melting alloy but below the solidus of the other powder, the high temperature alloy powder particles dissolve quickly into the liquid phase of the low temperature alloy.

As the mixing progresses, the composition of the solder is quickly changing. This makes the solidification process highly nonlinear because the liquidus temperature of the mixed composition is also continuously increasing until the alloys are completely mixed.

Preferably the melting points differ by at least <NUM>. The greater the difference in melting points, the more pronounced the improved characteristics are that can be obtained from the known solder compositions.

Preferably the first and second solder alloys comprise at least one common element. This facilitates a quick dissolution of one alloy into the other at temperatures close to or sometimes even below the melting point of one of the alloys. Preferably the at least one common element is tin.

For example a <NUM>% of SAC305 powder mixed with <NUM>% of eutectic 42Sn58Bi results in increasing the liquidus of the final composition to approximately <NUM> from <NUM> of the original 42Sn58Bi. This is because the addition of Sn shifts the alloy composition away from the SnBi eutectic. In addition, small amounts of Ag and Cu coming in from SAC305 change the microstructure of the alloys providing additional improvements in the final solder properties.

These forgoing process changes occur during the first reflow. Thus, the first reflow can be carried out at a lower temperature than would be required for the final blend. Before reflow it is a mixture of two separate alloys.

As a consequence of the forgoing composition it has been found that the presence of the higher melting component leads to an increase in liquidus temperature and, thus, a decrease in the homologous temperature at same operating temperatures. This means an automatic increase in thermal fatigue life and a decrease in high temperature creep. The homologous temperature allows comparison of different solder compositions.

For example, a solder having a working temperature range of -<NUM> to <NUM> and a melting (liquidus) temperature of <NUM> (<NUM>) is working at from <NUM>. 53Tmp to <NUM>. Increasing the melting temperature to <NUM> reduc es this range to from <NUM> Tmp to <NUM> Tmp Thus, the tensile strength, shear strength and modulus of elasticity are improved.

In addition, the decrease in fraction of Bi content in the solder improves its ductility. The presence of small amount of Ag and Cu improves ductility, thermal and electrical conductivity and refines the solder microstructure, leading to enhanced mechanical properties.

In a preferred embodiment, the first powder component forms about <NUM>% by weight of the solder composition and is 42Sn 58Bi, and wherein the second powder composition forms about <NUM>% by weight of the solder composition and is SAC305 (<NUM>% Sn, <NUM>% Cu, <NUM>% Ag). Preferably the solder composition consists of the forgoing components. It will be appreciated that while this example represents a preferred composition, the final composition may be selected from any alloys and mixtures in suitable proportions.

In aspect reference example, the present inventors have discovered that a mixed powder of a metal and a solder alloy powder has surprising benefits. Without wishing to be bound by theory, it is believed that during reflow, solder forms intermetallic bonds with the metal particles. Under a single long reflow or multiple reflow cycles, some of the metal from the metal particles is dissolved into the bulk solder while the rest remains in its original form. This results in a mixture of solder and metals particles forming a composite structure. Thus, the metal powder addition can improve compliance of the resultant solder joint, and improve thermal and electrical conductivity. An example is copper powder mixed with SnBi alloy. SnBi is a brittle alloy with relatively poor thermal and electrical conductivity. Addition of Cu particles in solder bulk improves it electrical and thermal conductivity. Another example is addition of nano and micro sized Ag particles to improve its mechanical strength and enhance electrical and thermal conductivity.

However, during the initial reflow step, the composition melts at the melting temperature of the solder alloy. As a consequence, unique properties of the solder can be achieved while still having an easy to melt and handle solder composition.

In the foregoing compositions, the second powder component can have a particle size ranging from smaller than to larger than the first solder powder. In one preferred embodiment, the particle size of the second powder is substantially the same size as the first solder powder. That is, the second powder comprises particles are from <NUM> to <NUM> microns in mean diameter. More preferably the particles are from <NUM> to <NUM> microns. Still more preferably the particles are from <NUM> to <NUM> microns. In certain situations, particle size between <NUM> and <NUM> microns are preferred. In one embodiment, the particles, in particular the metal particles, are preferably from <NUM> to <NUM> microns, more preferably from <NUM> to <NUM> microns. The metal particles may be from <NUM> microns to <NUM> microns. Alternatively, the metal particles may have a mean diameter of from <NUM> microns to <NUM> microns.

In a reference example, the present inventors have discovered that a mixed powder of two immiscible alloys with similar melting temperature but a different Solid-Liquid phase transition is advantageous. Thus the first and second solder alloys have similar melting temperatures and are immiscible. For example, some Bi containing alloys expand during liquid-solid transition (negative Coefficient of thermal expansion (CTE)) while many others shrink (positive CTE). The inventors have discovered that by mixing particles of a positive CTE alloy that has similar melting temperature with a negative CTE SnBi they can obtain a low-stress solder joint formation. They have further discovered that this will happen when the two alloys do not mix on heating (i.e. are immiscible). If they dissolve in each other then the resultant alloy could have its own characteristic transition.

In the foregoing compositions, the second powder component preferably has particle sizes that are comparable with those of the first powder component. That is, the second powder comprises particles are from <NUM> to <NUM> microns in mean diameter. More preferably the particles are from <NUM> to <NUM> microns in mean diameter, most preferably <NUM> to <NUM> microns. Preferably the particle sizes of the first and second powders are substantially the same since this facilitates easy handling and mixing.

By similar melting temperatures it is preferably meant that the first and second solder alloys have a melting temperature within at most <NUM>. More preferably the first and second solder alloys have a melting temperature within at most <NUM> and most preferably within <NUM>.

Preferably the coefficient of thermal expansion of the first solder alloy is positive and the coefficient of thermal expansion of the second solder alloy is negative.

In order to ensure that the powders are immiscible with each other, preferably at least the second powder component has a non-reactive coating layer. This allows for the use of known powders to achieve the advantageous benefits of the present invention.

Preferably the solder composition further comprises a further powder component selected from materials such as carbides, nitrides, oxides and carbon nanotubes, preferably selected from Al<NUM>O<NUM>, SiO<NUM>, TiO, NiO and carbon nanotubes. These components are preferably sized in accordance with the solder and metal particles described herein. That is, preferably having a longest average diameter on the micron scale; preferably from <NUM> to <NUM> microns.

These components have surprisingly been found to allow for alloy microstructure modification after reflow. As a consequence, the mechanical properties and thermal fatigue life of the alloy can be improved.

According to a further reference example, there is provided a solder composition comprising a blend of a first powder component and a second powder component, wherein the first powder component is a first solder alloy and the second powder component is selected from materials such as: carbides, nitrides, oxides and carbon nanotubes, preferably selected from materials such as: Al<NUM>O<NUM>, SiO<NUM>, TiO, NiO and carbon nanotubes. Preferably, the second powder component is one or more of Al<NUM>O<NUM>, SiO<NUM>, TiO, NiO and carbon nanotubes. The components of this aspect correspond to those in the foregoing aspects. For example, the first powder component for use in this aspect may be the same as any first powder component described herein.

According to a further aspect of the present invention, there is provided a solderable paste comprising the solder composition as described herein. That is, the paste comprises the powder blend of the present invention together with a flux. Suitable fluxes are well known in the art.

The compositions of the present invention may then be processed into the form of a bar, a stick, a solid or flux cored wire, a foil or strip, a pre-form, a pre-applied or free standing film or solder spheres for use in ball grid array joints, or a pre-formed solder piece or a reflowed or solidified solder joint.

According to a further aspect of the present invention, there is provided a method of forming the solder composition as described herein, the method comprising mixing a first powder component with a second powder component.

According to a further aspect of the present invention, there is provided the use of the composition as described herein or the solderable paste as described herein in a soldering method.

According to a further aspect of the present invention, there is provided the use of the composition as described herein or the solderable paste as described herein to form a soldered joint.

In a further aspect, the present invention provides a soldered joint comprising an alloy of the present invention.

In a further aspect, the present invention provides the use of an alloy of the present invention in a soldering method. Such soldering methods include, but are not restricted to, wave soldering, Surface Mount Technology (SMT) soldering, die attach soldering, thermal interface soldering, hand soldering, laser and RF induction soldering, and rework soldering.

In a reference example, the present invention provides a solder composition comprising a blend of a first component and a second component, wherein the first component is a first solder alloy and the second component is a second solder alloy or a metal. The preferable features of the above described aspects of the present invention are also preferable with regard to this reference example. The first and/or second component may be in the form of a powder, paste, strip, foil, sphere, disc or a preform. Preferably the first component is in the form of a paste.

In a further reference example, the present invention provides a method of forming the above described solder composition, the method comprising mixing. Preferably the first component is a paste and/or the second component is in the form of a powder, paste, strip, foil, sphere, disc or a preform.

A further reference example provides a method of forming a solder joint comprising:.

The work pieces to be joined may be, for example, a circuit board and a circuit component. The method may be used, for example, in the manufacture of printed circuit boards. The first solder component may be a first alloy component, and may be in the form of a powder, paste, strip, foil, sphere, disc or a preform, preferably a paste. The second solder component may be a second solder alloy or a metal, and may be in the form of a powder, paste, strip, foil, sphere, disc or a preform. Once the solder components have been mixed, they may be heated at a temperature lower than the reflow temperature of the first solder component. An example of the above described method is as follows:
A method of assembly, comprising:.

A solder composition was prepared comprising a 42Sn 58Bi powder component in an amount of about <NUM>% by weight of the solder composition and about <NUM>% by weight of a SAC305 powder (<NUM>% Sn, <NUM>% Cu, <NUM>% Ag). On testing it was found that the alloy had improved ductility, thermal fatigue and creep resistance compared to the 42Sn 58Bi powder alone.

A solder composition was prepared comprising a 42Sn 58Bi powder component in an amount of about <NUM>% by weight of the solder composition and about <NUM>% by weight of a Copper metal powder. On testing it was found that the alloy had improved ductility, thermal fatigue resistance and electrical conductivity compared to the 42Sn 58Bi powder alone.

A solder composition was prepared comprising two bismuth-containing alloys. One of the alloys selected expands during liquid-solid transition (-ve CTE) and the other shrinks (+ve CTE). This composition was found to give rise to a low-stress solder joint.

Two solder compositions were prepared. The first comprised <NUM> wt% SAC305 and <NUM> wt% Sn58Bi, and the second contained <NUM> wt% SACX0307 (Sn0.3Ag0.7Cu0.1Bi) and <NUM> wt% Sn58Bi. Measurement of the chip shear resistance and the pin pull resistance indicated that the values were comparable with the benchmark alloy Sn57.6Bi0.4Ag.

The present application includes, by way of example, the following figures:.

The presently claimed compositions are useful for applications including, but not limited to, LED assembly, photovoltaic cell tabbing and stringing, semiconductor backend process, and Die attachment. Final form factor is application dependent but the solder can be made into any form, including, but not limited to, paste, perform, film, and wire, and can be combined with cleanable or no-clean flux chemistry.

When introducing elements of the present disclosure or the preferred embodiments(s) thereof, the articles "a", "an", "the" and "said" are intended to mean that there are one or more of the elements.

Claim 1:
A solder composition comprising a blend of a first powder component and a second powder component, wherein the first powder component is a first solder alloy and the second powder component is a second solder alloy, and wherein the first and second solder alloys have melting temperatures within at most <NUM> and do not mix on heating.