EP4025379A1

EP4025379A1 - Solder alloy and solder paste containing said alloy

Info

Publication number: EP4025379A1
Application number: EP20761287.0A
Authority: EP
Inventors: Daniel BUCKLAND; Ian Wilding; Guang REN; Maurice COLLINS
Original assignee: Henkel AG and Co KGaA
Current assignee: Harima Chemical Group Inc
Priority date: 2019-09-06
Filing date: 2020-08-31
Publication date: 2022-07-13
Also published as: WO2021043708A1; CN114340836A

Abstract

The present invention relates to a lead-free solder alloy which consists essentially of or consists of: from 0.01 to 11 wt.% of zinc (Zn); from 0.01 to 6 wt.% of bismuth (Bi); from 0.01 to 2 wt.% of gallium (Ga); and, tin (Sn), wherein said weight percentages are based on the total weight of the alloy. The invention is also directed to a solder paste comprising the defined lead-free solder alloy in particulate form and a solder flux.

Description

SOLDER ALLOY AND SOLDER PASTE CONTAINING SAID ALLOY

FIELD OF THE INVENTION

The present invention concerns a lead-free solder alloy and a solder paste containing said alloy. More particularly, the present invention is concerned with a solder alloy containing gallium as a micro-alloying element and to a solder paste formed from said alloy and a solder flux.

BACKGROUND OF THE INVENTION

The main circuits of electrical devices are printed circuit boards to which electronic parts are soldered. Within such circuits, solder alloys are used to make permanent electrical connections between two conductors and the associated soldering process is typically effected by heating the solder to above its melting point, surrounding the conductors to be connected with molten solder and allowing the solder to cool. In addition, solder alloys are used to interconnect semiconductor devices, including integrated circuit chips fabricated on a silicon wafer: an array of solder bumps are deposited on the top side of the wafer, the chip is flipped such that the solder bumps align with matching pads on a substrate and the system is heated to flow the solder.

It is recognized that some chips, including integrated circuit chips, may be damaged through exposure of their components to excessive heat. Given that the entire assembly is heated to flow the solder in flip-chip connecting methods, the melting point of the solder must be low enough to prevent temperature-sensitive components - such as plastic connectors - from being damaged. Historically this provoked the use of lead containing solders - in particular tin-lead (Sn-Pb) solders - on account of their low melting points. But problematically both lead and many lead alloys are toxic and environmental regulations have now dictated the replacement of lead solders with less toxic counterparts.

The difficulty in making such a replacement is that the solders must still exhibit low melting points and sufficient conductivity for electronics applications. Whilst lead-free solders are known, these solders typically require processing temperatures that are 30 to 40°C higher than those historically used for production with tin-lead solders. For example, the established lead-free solder SAC 305 - comprising 96.5 wt.% tin (Sn), 0.5 wt.% copper (Cu) and 3 wt.% silver (Ag) - has a minimum processing temperature of approximately 232°C and its use thus requires specialized circuit board materials which can withstand these elevated temperatures. These high temperatures can thermally damage a printed circuit board (PCB) and many components attached thereto.

Moreover, the replacement of Sn-Pb solders can impact the susceptibility of printed circuit boards - even when using circuit boards formed from specialized materials at elevated temperatures - to pad cratering. This is a fracture in the resin between copper foil on the PCB and the outermost fibreglass layer of a PCB and is considered a high strain rate event with minimal creep. Less compliant solders or those solders with higher yield points will increase pad cratering potential as they provide minimal load sharing. Further, many tin- based, lead-free solders have the propensity to grow tin filament whiskers when placed under compressive stress which may cause an electrical shortage: this is of particular concern in applications requiring high reliability, such as medical devices, aerospace applications and military applications.

It is further noted that in recent years, solders installed within electronic circuit boards can be subjected to one or both of: significant temperature changes, for example within the range from -40° to 150°C; and, vibration loads. When subjected to the former, the difference of the coefficients of linear expansion of the mounted electronic components and the substrate can induce stress: additionally, repeated plastic deformation caused by temperature changes tends to cause cracks in a solder joint, and the stress repeatedly applied with the lapse of time concentrates in the vicinity of the tips of the cracks, so that the cracks tend to transversely develop to the deep portion of the solder joint. Where cracks become markedly developed, this can disconnect the electrical connection between the electronic components and the electronic circuit formed on the substrate. Vibration of the electronic circuit board can exacerbate the development of the cracks.

When bismuth (Bi) or antimony (Sb) is added to a lead-free solder alloy composed mainly of Sn, a portion of the Sn crystal lattice is substituted with Bi or Sb. As a result of this, the Sn matrix is reinforced to increase the alloy strength of the solder alloy: this provides an inhibitory effect on the development of cracks but only in the solder bulk. The low thermal conductivity of bismuth (Bi), antimony (Sb) and tin (Sn) as compared to copper (Cu) commonly used in the formation of electrodes, creates a temperature gradient within the solder joints when electric current flows into the electrode: the temperature gradient in turn promotes the thermomigration of Cu, Sn, Bi and Sb within an interfacial layer between the solder and electrode. Holes occur in this region and, with increased time at elevated temperature, the holes tend to continuously connect with each other and finally result in the rupture of the interfacial layer. Numerous authors have attempted to address the difficulties associated with replacing Sn- Pb solders in electronic devices and components thereof.

JP H08-252688 (Fujitsu Ltd.) describes a solder alloy for low-temperature bonding of parts within an electronic apparatus, said solder alloy consisting of: 37 to 58 wt.% Sn; 0.1 to 2.5 wt.% Ag; and, the balance Bi. It has been observed that small electronic devices - such as mobile phones or notebook computers - in which these Sn — Bi based solder alloys are used show low impact strength with fractures commonly occurring at the solder bonding interface.

Liang Zhang et al. Development of Sn-Zn lead-free solders bearing alloying elements, Journal of Materials Science: Materials in Electronics, Volume 21, Issue 1, 1-15 (2010) describes the alloying of particular elements - rare earths, Bi, Ag, Al, Ga, In, Cr, Cu, Sb, Ni and Ge - with Sn-Zn and investigates the effects on the wettability, oxidation resistance, melting behavior, mechanical properties, creep properties and microstructures of the resultant alloys.

Guang Ren et al. Alloying influences on low melt temperature SnZn and SnBi solder alloys for electronic interconnections, Journal of Alloys and Compounds, Volume 665, 251-260 (2016) describes low temperature soldering in the context of consumer and “throw away” electronics: the document focuses in particular on tin-zinc (Sn-Zn) and tin-bismuth (Sn-Bi) metallurgical alloys which are incorporated into solder pastes for lower temperature processed electronic interconnections.

JP 2002018589 (Senju Metal Industry Co.) describes a lead-free solder alloy which is obtained by adding 0.005 to 0.2 wt.% Ga to Sn which is the main component. In certain embodiments Cu, Sb, Ni, Co, Fe, Mn, Cr or Mo are added to the lead-free solders. Alternatively, Bi, In or Zn are added to the lead-free solders to inhibit the oxidation thereof.

There exists a need to develop alternative lead-free solder alloys having low melting points which do not present the disadvantages identified in the prior art.

STATEMENT OF THE INVENTION

In accordance with a first aspect of the invention, there is provided a lead-free solder alloy which consists essentially of or consists of: from 0.01 to 11 wt.% of zinc (Zn); from 0.01 to 6 wt.% of bismuth (Bi); from 0.01 to 2 wt.% of gallium (Ga); and, tin (Sn), wherein said weight percentages are based on the total weight of the alloy. For completeness, the tin constitutes the majority element of the solder alloy and is present to facilitate the wetting of the molten solder alloy to the metallic substrates to which it is to be adhered in use.

Without intention to be bound by theory, the zinc content in the alloy is not elevated above 11 wt.% as such a level is considered: i) to diminish the oxidation resistance of the alloy, leading to a shorter shelf life; ii) to worsen the wetting performance of the alloy; and, iii) to render the alloy more brittle due to the existence of rich primary Zn phase. The skilled artisan will comprehend that it is important to have a low zinc oxide content in the solder: the melting temperature of the oxides is very high compared to the melting point of the solder alloys and the presence of the oxide is thus deleterious to the reflow and wetting properties of the alloy.

A bismuth content higher than 6 wt.% will result in enlarged pasty ranges and diminished mechanical strength of the alloy. And a gallium content of greater than 2 wt.% is considered deleterious to the ductility and wettability of the alloy.

When Zn, Bi or Ga each constitute less than 0.01 wt.% of the alloy, the levels of these metals are insufficient to promote the desired alloying or micro-alloying effects which benefit thermal, micro-structural, mechanical and wetting properties of the alloy.

In important embodiments of the invention, the defined solder alloy is in particulate form and has an average particle size of from 0.05 to 150 pm, for example from 5 to 75 pm. The particulate alloy may be further characterized in that: from 90 to 100% by volume of said particles are spherical; and, from 0 to 10% by volume of said particles are non-spherical.

In accordance with a second aspect of the present invention, there is provided a solder paste comprising, based on the weight of said paste: from 40 to 95 parts by weight of a particulate solder alloy having an average particle size of from 0.05 to 150 pm; and, from 5 to 60 parts by weight of solder flux, wherein said solder alloy consists essentially of or consists of: from 0.01 to 11 wt.% of zinc (Zn); from 0.01 to 6 wt.% of bismuth (Bi); from 0.01 to 2 wt.% of gallium (Ga); and, tin (Sn), wherein said weight percentages are based on the total weight of the alloy .

Conventionally, the solder flux of the paste comprises: a) at least one binder; b) at least one solvent; and, optionally c) additives. Said at least one binder a) of the solder flux should preferably comprise: at least one rosin resin; at least one thermoplastic polymer; at least one thermosetting crosslinkable resin; or, a mixture thereof. And in important embodiment, the solder flux should comprise either non-activated rosin (R) or mildly activated rosin (RMA).

In accordance with a further aspect, the present invention provides a method of forming a solder joint, said method comprising: (i) providing two or more work pieces to be joined; (ii) providing a solder paste comprising a solder flux and solder alloy particles as defined herein above and in the appended claims; (iii) heating the solder paste in the vicinity of the work pieces to be joined to form a solder joint, wherein upon heating the solder paste the flux forms a residue which at least partially and preferably completely covers the solder joint. The two or more work pieces to be joined typically comprise: an electronic component, such as a chip resistor or a chip capacitor; or, a copper pad typically disposed on a printed circuit board.

DEFINITIONS

As used herein, the singular forms "a", " an " and "the" include plural referents unless the context clearly dictates otherwise.

The terms “comprising", “ comprises ” and “comprised of” as used herein are synonymous with “including”, “includes", “ containing ” or “contains", and are inclusive or open-ended and do not exclude additional, non-recited members, elements or method steps. If used, the phrase "consisting of’ is closed and excludes all additional elements. Further, the phrase "consisting essentially of excludes additional material elements but allows the inclusion of non-material elements that do not substantially change the nature of the invention.

With particular regard to the alloy solder powder of the present invention, the term “consists essentially of’ is intended to mean that impurities may be tolerated within the powder in an amount of up to 0.25 wt.%, based upon the weight of the powder. The skilled artisan will recognize that the constituent metals of the solder alloy may often be secondary or refined metals which are not absolutely (100%) pure. Moreover, even if the constituent metals are sourced as raw materials having a purity of at least 99.9%, as is desirable, impurities can be introduced into the solder powder from holding fixtures or from the solder pot itself. Wthout intention to limit the meaning of the term “impurities”, the typical impurities which might be present include: copper (Cu); gold (Au); cadmium (Cd); aluminium (Al); iron (Fe); sulphur (S); and, phosphorous (P).

For completeness, the term “lead free" means that that the lead content is less than 0.01 wt%, preferably less than 0.001 wt%, based on the weight of the alloy.

When amounts, concentrations, dimensions and other parameters are expressed in the form of a range, a preferable range, an upper limit value, a lower limit value or preferable upper and limit values, it should be understood that any ranges obtainable by combining any upper limit or preferable value with any lower limit or preferable value are also specifically disclosed, irrespective of whether the obtained ranges are clearly mentioned in the context.

The words "preferred", " referably¹', “desirably’ and “particularly’, and synonyms thereof, are used frequently herein to refer to embodiments of the disclosure that may afford particular benefits, under certain circumstances. However, the recitation of one or more preferable, preferred, desirable or particular embodiments does not imply that other embodiments are not useful and is not intended to exclude those other embodiments from the scope of the disclosure.

As used throughout this application, the word “may” is used in a permissive sense - that is meaning to have the potential to - rather than in the mandatory sense.

As used herein, “ambient conditions” refers to a set of parameters that include temperature, pressure and relative humidity of the immediate surroundings of the element in question. Herein ambient conditions are: a relative humidity of from 30 to 100% percent; a temperature in the range from 20 to 40°C; and, a pressure of 0.9 to 1.1 bar.

As used herein “room temperature ” is 23°C ± 2°C.

As used herein “boiling point’ is the atmospheric boiling point, unless indicated otherwise, with the atmospheric boiling point being the boiling point as determined at a pressure of 100 kPa (0.1 MPa). As used herein, initial boiling point and boiling point range of the high boiling point hydrocarbon mixtures are as determined by ASTM D2887.

As used herein “solidus temperature ” refers to the temperature below which the alloy exists as a solid phase and above which there is incipient melting of the alloy. As used herein “liquidus temperature ” is the lowest temperature at which the alloy is completely liquid. The range between the liquidus and solidus temperature is assigned the term "pasty range" in this application.

The liquidus and solidus temperatures mentioned herein refers to temperatures measured using Differential Scanning Calorimetry (DSC). Specifically DSC measurements were performed on a Netzsch DSC 204 F1 Phoenix (Netzsch Geratebau GmbH, Selb/Bayern, Germany) at a heating rate of 10 K/min for specimens of 4 mm x 2 mm x 0.5 mm. Samples were put into an AI₂O₃ pan in the DSC apparatus at room temperature and cooled to -40°C at the highest possible rate and equilibrated for 10 minutes while employing a nitrogen gas flow of 20 mL.min-¹. Measurements were performed between -40°C and 700°C. A baseline correction was performed during experiments which comprised a single DSC run using empty AI₂O₃ pan(s) as a reference. Further the solidus temperature was obtained from mid pan thermocouple data on the basis that end-plate thermocouples are subject to a significant conduction error.

As used herein, the term "average particle size" refers to the D₅₀ value of the cumulative volume distribution curve at which 50% by volume of the particles have a diameter less than said value. The average particle size or D₅₀ value is measured in the present invention through an automated image analysis technique with a Malvern Morphologi G (Malvern Instruments Ltd). In this technique, the size of particles in suspensions or emulsions is measured using the diffraction of a laser beam, based on the application of either Mie theory or a modified Mie theory for non-spherical particles. The average particle sizes or D₅₀ values relate to scattering measurements at an angle of from 0.02° to 135° relative to the incident laser beam.

As used herein, "spherical" is used to denote particles that have an aspect ratio (i.e. , ratio of major axis to minor axis) of from 1 : 1 to 1.5: 1 , for example from 1.1 to 1.4: 1. The shape of the particles may be determined by any suitable method known in the art including but not limited to optical microscope, electron microscope and suitable image analysis software.

The term "solder flux" as used herein encompasses a composition which is used to prevent oxidation during a soldering process but which may also provide some form of chemical cleaning prior to soldering. The term "solder residue" as used herein encompasses the residue formed as a result of heating of a solder flux during a soldering process. Without being bound by theory, it is considered that the solder flux residue forms when at least some of the solvent is evaporated from the solder flux.

As used herein, the term "epoxide" denotes a compound characterized by the presence of at least one cyclic ether group, namely one wherein an ether oxygen atom is attached to two adjacent carbon atoms thereby forming a cyclic structure. The term is intended to encompass monoepoxide compounds, polyepoxide compounds (having two or more epoxide groups) and epoxide terminated prepolymers. The term “monoepoxide compound’ is meant to denote epoxide compounds having one epoxy group. The term “polyepoxide compound’ is meant to denote epoxide compounds having at least two epoxy groups. The term “diepoxide compound’ is meant to denote epoxide compounds having two epoxy groups.

The epoxide may be unsubstituted but may also be inertly substituted. Exemplary inert substituents include chlorine, bromine, fluorine and phenyl.

DETAILED DESCRIPTION OF THE INVENTION Lead-Free Alloy Solder

As noted above, the first aspect of the present invention provides a lead-free solder alloy which consists essentially of or consists of: from 0.01 to 11 wt.% of zinc (Zn); from 0.01 to 6 wt.% of bismuth (Bi); from 0.01 to 2 wt.% of gallium (Ga); and, tin (Sn) wherein said weight percentages are based on the total weight of the alloy. The tin constitutes the majority element of the solder alloy and is present to facilitate the wetting of the molten solder alloy to the metallic substrates to which it is to be adhered in use.

In an important embodiment, the lead-free solder alloy either consists essentially of or consists of: from 5 to 10 wt.%, preferably from 7 to 9 wt.% of zinc (Zn); from 1.5 to 5 wt.%, preferably from 2 to 4 wt.% of bismuth (Bi); from 0.01 to 1 wt.%, preferably from 0.1 to 0.5 wt.% of gallium (Ga); and, tin (Sn) wherein said weight percentages are based on the total weight of the alloy.

An illustrative lead-free solder alloy either consists essentially of or consists of:

8 wt.% of Zn;

3 wt.% of Bi; from 0.1 to 0.5 wt.% of Ga; and,

Sn wherein said weight percentages are based on the total weight of the alloy. Further exemplary alloys include: Snss.g/Zns^h/Gao.i; Snsss/Zns/Bh/Gao^; Snss s/Zns/Bh/Gao^s; Sn88 /Zn8//Bi3/Gao.3; Snss.e/ZnsBh/Gao.^ and, Snss.s/Zns/Bh/Gao.s-

The above-defined solder alloys may be produced by melt-blending the stated metals in accordance with known methods. Multi-stage melt-blending is not precluded, wherein a master batch of Sn with an amount of at least one of the stated metals is first prepared by melt-blending, which master match is then optionally cooled before being melt blended in further stage(s) with either the remaining stated metals or the remaining amounts of the stated metals necessary to yield an alloy of the desired composition. An illustrative multi stage melt-blending process might entail the first preparation of a master batch consisting of Sn, Zn and Bi, followed by the melt-blending of the micro-alloying element Ga into said master batch. Irrespective of the number of blending stages, the or each means of melt blending must serve to homogeneously mix the constituent metals. Moreover, the composition of the melt - present in a bath or the like - should be verified through analytical means before the alloy is solidified to a particular morphology: the use of optical emission spectrometry might be mentioned in this context.

In certain embodiments, the molten solder alloys may be extruded into wires or cast into molds to form ingots or bars, which ingots may be hot compressed to convert the cast structure into a wrought microstructure. The appropriate size and shape of the molds can be determined based on the intended use of the ingots and also on the alloy's density once cooled and solidified. The bars or ingots provide for the convenient storage of the alloy and may subsequently be melted to produce solder baths which are useful where solder is to be applied by coating or through complete or partial immersion of a substrate therein: mention may in particular be made of hot-air leveling and wave soldering as described in inter alia US2006/0104855 (Rothschild). The extrusion of solder alloys into wires, which may be hollow and which may in turn be molded or shaped, provides forms which are useful in conventional manual soldering applications.

The solder alloy of the present invention may also be presented in the form of a powder. The aforementioned ingots, bars, wires and others solid forms of the alloys can be transformed into powder by grinding, milling and other conventional diminution techniques. In the alternative, the powder may be formed from the molten alloy by granulation or atomization. Granulation is conventionally performed by free-fall of the molten alloy through a plate having sized apertures or nozzles into a cold liquid, typically water: during the free- fall and by impact with the liquid surface, the melt stream is broken up into droplets which then solidify. Atomization techniques which may be mentioned include but are not limited to: jet atomization of the melt, wherein the molten liquid is dispersed into droplets by the impingement of jets of gas, water or oil; centrifugal atomization; impact atomization; ultrasonic atomization; impulse atomization; and, vacuum atomization. Neikov Granulation and Atomization in Handbook of Non-Ferrous Metal Powders: Technologies and Applications, Chapter 5 (2009) provides an instructive reference in this regard.

Particularly when they are to be employed within a solder paste, particles of the solder alloy should have an average particle size of from 0.05 to 150 pm, preferably from 5 to 75 pm and more preferably from 15 to 45 pm. Very fine particles having an average particle size of less than 0.05 pm can be difficult both to produce and to process into solder pastes: such fine particles may increase cohesive force within the pastes and as a corollary increase viscosity and tackiness of the paste. Very fine particles can moreover be easily oxidized. Conversely, particles having an average particle size of greater than 150 pm can impede blending operations in the formation of a solder paste and moreover have limited utility where printing of fine circuit patterns on a substrate is required.

It is preferred that the solder, whether or not meeting the above described particle size parameters, should comprise or consist of spherical solder particles. Whilst the presence of some non-spherical particles in a solder paste may be tolerated and might, for instance, limit slump and outflow of the paste upon application, it is preferred that spherical particles constitute from 90 to 100% by volume of the solder particles and non-spherical particles constitute from 0 to 10% by volume of the solder particles. Importantly, spheres represent the physical object with the lowest surface area-to-volume ratio and hence, for a given oxide layer thickness, will contain the smallest amount of oxide. Further, non-spherical particles may jam the screen or the stencil through which the solder paste is printed or clog a needle through which solder paste is dispensed.

Solder Pastes and Solder Fluxes

In the present invention, the solder flux and particles of solder alloy may be provided separately. For example, the solder flux may be applied separately in liquid, paste or film form, whereas the solder particles may be provided in the form of a powder, sheet, stick or wire, for instance. Alternatively, the solder flux and solder particles may be provided together. It is not precluded, for instance, that the solder flux may be retained within the solder particles, for example within the voids in acicular particles or fibers of the alloy. However, it is preferred that the solder alloy particles be provided together with the solder flux in form of a solder paste.

As is known in the art, a solder paste is a suspension of solder alloy particles in a flux- containing vehicle in which the shape and size of the particles and the rheology and tackiness of the flux vehicle are matched to the method of paste application most appropriate to the design of electronic component assembly. The solder paste flux lowers the surface tension of solder to improve capillary flow and optimizes fillet geometries by promoting wetting; it also protects surfaces from re-oxidation during reflow. The weight ratio of the solder particles to the flux is not particularly limited but it is conventional that the amount of the solder particles ranges from 40 to 95 parts by weight and that of the flux ranges from 5 to 60 parts by weight. In particular, the solder paste may consist of, based on the weight of the paste: from 70 to 95 wt.% of solder alloy particles; and, from 5 to 30 wt.% of solder flux. It is noted that, as a result of the density difference, the flux may comprise a higher percentage by volume of the paste than it does by weight.

Solder Flux

The solder flux of the present invention comprises: a) at least one binder; b) at least one solvent; and, optionally c) additives.

Said at least one binder a) may comprise: at least one rosin resin; at least one thermoplastic polymer; at least one thermosetting crosslinkable resin; or, a mixture thereof. The term rosin resin in intended to encompass rosin acids, gum rosin, wood rosin, tall oil rosin, rosin esters, hydrogenated rosins, dimerized rosins, disproportionated rosin, polymerized rosin and modified rosin resin, the term “modified rosin" being used here to denote a rosin resin that is changed in some way other than by esterification, hydrogenation, or dimerization, for example by the Diels-Adler reaction and/or by ene-type reactions with a,b-olefinically unsaturated carbonyl compounds. Exemplary modified rosins thus include rosin esters modified with maleic acid. Exemplary thermoplastic polymers include but are not limited to polyamides, polybutylenes, polyimides and polyacrylates. Exemplary crosslinkable thermosetting resins include but are not limited to epoxy resins, phenolic resins, polyesters and styrenated polyesters: such thermosetting resins may, of course, require an appropriate curative or hardener to be present in the solder flux.

As will be recognized by the skilled artisan, pure rosin - abietic acid and the isomers and oligomers thereof - is a very weak acid that does not exhibit activity at ordinary temperatures but, under heating at 90°C or more, melts and exhibits activity to remove oxide films on a metallic base. The activity of rosin can be enhanced by addition of an activator, a material that decomposes or otherwise changes upon heating to an activation temperature to produce a by-product that reacts with oxide on the substrate.

Depending upon the use and type of activator present, rosin fluxes are designated as non- activated (R), mildly activated (RMA), fully activated (RA) and super-activated (RSA). Herein it is preferred that the flux is a either a Type R flux (non-activated) or a Type RMA flux (Rosin Mildly activated) on the basis that that these fluxes leave virtually no flux residue. As regards an RMA flux, the flux residue is noncorrosive, tack free and exhibits a high degree of freedom from ionic contamination after cleaning. In contrast, RA flux residues should be removed from printed circuit boards, and RSA residues must be removed since they are corrosive in electronics applications.

Without intention to limit the activators which may be present in combination with the rosin resins, mention may be made of: monocarboxylic acids, such as formic acid, acetic acid, propionic acid, capronic acid, enanthic acid, caprilic acid, pelargonic acid, capric acid, undecanoic acid, lauric acid, myristic acid, palmitic acid and stearic acid; dicarboxylic acids, such as oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid; oxycarboxylic acids such as oxysuccinic acid, citric acid, tartaric acid, hydroxyacetic acid, salicylic acid, (m-, p-) hydroxybenzoic acid, 12- hydroxydodecanoic acid, 12-isobutyric acid, (o-, m-, p-) hydroxyphenylacetic acid, 4- hydroxyphthalic acid and 12- hydroxy stearic acid); salts of the aforementioned mono-, di- or oxyacarboxylic acids with amine, ammonia or alkali metals; and, amine salts of halogen containing acids such as hydrochloric acid and bromic acid, for example, diethyl amine hydrochlorate triethylamine hydrochloride and ethylamine bromated. The use of activators possessing halide ions is not preferred in the present invention.

In certain embodiments, the binder a) comprises: a1) an epoxy resin; a2) a hardener; and, optionally a3) a catalyst. Upon heating the solder flux based on such a binder a), the epoxy resin may undergo cross-linking, meaning that the solder flux residue may comprise cross- linked epoxy resin.

Said epoxy resins a1) may be pure compounds but equally may be mixtures of epoxy functional compounds, including mixtures of compounds having different numbers of epoxy groups per molecule. An epoxy resin may be saturated or unsaturated, aliphatic, cycloaliphatic, aromatic or heterocyclic and may be substituted. Further, the epoxy resin may also be monomeric or polymeric.

The use of polyepoxide compounds having an epoxy equivalent weight of from 100 to 700 g/eq, for example from 120 to 320 g/eq may be desirable and, generally, diepoxide compounds having epoxy equivalent weights of less than 500 or even less than 400 are preferred: this is predominantly from a costs standpoint, as in their production, lower molecular weight epoxy resins require more limited processing in purification. As examples of types or groups of polyepoxide compounds which may be utilized in the present invention (a1), particular mention may be made of: glycidyl ethers of polyhydric alcohols and polyhydric phenols; glycidyl esters of polycarboxylic acids; and, epoxidized polyethylenically unsaturated hydrocarbons, esters, ethers and amides. Without intention to limit the present invention: the hardener a2) may comprise a phenolic group containing hardening agent and/or may be an anhydride-based hardener, typically a liquid anhydride-based hardener; and, the catalyst a3) may comprise a substituted aromatic amine catalyst, and / or a phosphene-based salt catalyst and / or an amide-based catalyst.

Broadly the solvents of the solder flux may be selected from: ketones; alcohols; ethers; acetals; esters; glycol ethers; amines; amides; and, hydrocarbons. The solvents mentioned above may be used singly or in combination of plural kinds. Examples of the ketones include acetone, methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, cyclopentanone, methyl n-amyl ketone, acetonylacetone, isophorone and acetophenone. Examples of the alcohols include ethyl alcohol, isopropyl alcohol, n-butanol, ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, hexylene glycol and texanol. Examples of the ethers and acetals include n-butyl ether, n-hexyl ether, ethyl phenyl ether, 1,4-dioxane, trioxane, diethyl acetal, 1 ,2-dioxolan, tetrahydropyran and tetrahydrofuran. Examples of the esters include methyl formate, ethyl formate, propyl formate, isobutyl formate, methyl acetate, ethyl acetate, propyl acetate, n-butyl acetate, benzyl acetate, isoamyl acetate, ethyl lactate, methyl benzoate, diethyl oxalate, dimethyl succinate, dimethyl glutamate, dimethyl adipate, methyl carbonate, ethyl carbonate, propyl carbonate, butyl carbonate, ethylene glycol monomethyl ether acetate, ethylene glycol monoethyl acetate, ethylene glycol monopropyl acetate, ethylene glycol monobutyl ether acetate, ethylene glycol diacetate, propylene glycol monomethyl ether acetate, propylene glycol monoethyl ether acetate, propylene glycol monompropyl ether acetate, propylene glycol monobutyl ether acetate, propylene glycol diacetate, diethylene glycol monomethyl ether acetate, diethylene glycol monoethyl ether acetate and diethylene glycol diacetate. Examples of the glycol ethers include ethylene glycol monomethyl ether, ethylene glycol dimethyl ether, ethylene glycol monoethyl ether, ethylene glycol diethyl ether, ethylene glycol monobutyl ether, ethylene glycol dibutyl ether, propylene glycol monomethyl ether, propylene glycol dimethyl ether, propylene glycol monoethyl ether, propylene glycol diethyl ether, propylene glycol monobutyl ether and propylene glycol dibutyl ether. Examples of the amines and the amides include dimethylformamide, N,N-dimethylacetamide, N-methyl-2-pyrrolidone, pyridine and pyrazine. These amines and amides can be used singly or in combination. Examples of the hydrocarbons include n-heptane, n-octane, n-decane, cyclohexane, benzene, toluene, xylene, ethylbenzene, diethylbenzene and pinene.

A preference is noted for the use in the solder flux of high boiling solvent(s) having a boiling point greater than 120°C. Based on the total solvent content, it is preferred that said solvent(s) having a boiling point greater than 120°C constitute at least 60 wt.% thereof. And as such high-boiling solvents, particular mention may be made of: butyl carbitol; ethylene glycol monohexyl ether; diethylene glycol mono hexyl ether; triethylene glycol monomethyl ether; ethylene glycol mono-2-ethylhexyl ether; diethylene glycol mono-2-ethylhexyl ether; diethylene glycol dibutyl ether; triethylene glycol butyl methyl ether; and, tetraethylene glycol dimethyl ether.

The solder flux or solder paste of the present invention may, of course, also contain additives and adjunct ingredients. Such additives and adjunct ingredients are necessarily minor components of the present compositions and may conventionally be selected from: fillers; oxide sequestrants; catalysts; pigments, such as titanium dioxide, iron oxides or carbon black; rheological agents; thixotropic agents, such as hardened castor oil, amides and waxes; antioxidants; surfactants; impact modifiers; adhesion regulators; degassing agents; stress modifiers; tackifiers; and, mixtures thereof. The choice of appropriate additives is limited only in that these must be compatible with the other components of the solder flux or solder paste and cannot be deleterious to either the use of the flux or paste in printing applications or the physical properties of the product obtained upon heating (re flowing) the paste or flux.

The inclusion of a filler - in an amount up to 5 wt.%, based on the total weight of the solder flux - may permit control of the mechanical and/or thermo-mechanical properties of the solder flux and/or the flux residue. In particular, the presence of a filler may serve to decrease the disparity in the Coefficient of Thermal Expansion (CTE) between the solder flux residue and the solder joint, thereby increasing the resistance to thermal cycling fatigue. And it is acknowledged that certain fillers may also be included: to promote surface lubrication; to reduce isotropic shrinkage; to modify electrical and / or magnetic properties; to modify optical properties; and, for radiation absorption. These reasons aside however, the inclusion of higher levels of filler may deleteriously increase the viscosity of the solder flux.

Exemplary fillers include but are not limited to: silica; aluminum oxide; aluminum nitride; zinc oxide; barium sulphate; calcium carbonate; polytetrafluoroethylene (PTFE); polyhedral oligomeric silsesquioxanes; glass fibers; glass flakes; glass beads; glass spheres; synthetic and natural fibers; mica; talc; kaolin; wallastonite; nanoclays; graphene; functionalized graphene; diamond; carbon nanotubes (CNT); graphite and carbon fibers; ferromagnetic metals and, boron nitride. Fillers which impart thixotropy to the composition may also have utility: such fillers are also described as rheological adjuvants and include hydrogenated castor oil, fatty acid amides, or swellable plastics such as PVC.

The solder flux or solder paste may comprise at least one additive which acts to assimilate or sequester oxide from the molten solder, whether said oxide is disposed originally on the surface of the solder powder or is dispersed within the solder. By assimilating the oxide, the interference of this compound with wetting is minimized. Suitable additives in this regard are broadly organic molecule having nucleophilic and/or electrophilic end groups. Specific mention may be made of organic compounds having terminal carboxylic acid groups and more particularly of dimer acids. The preparation and structure of the dimer acids by addition dimerization or polymerization of unsaturated fatty acids is described in inter alia Journal of American Oil Chemists Society, 39, 534-545 (1962) and US Patent No. 3,157,681.

It is noted that the presence of liquid rubber in the solder flux - in an amount up to 5 wt.%, based on the weight of the solder flux - may also serve to negate the effects of the differences in Coefficient of Thermal Expansion (CTE) between work pieces and the solder joint, thereby increasing the resistance to thermal cycling fatigue. Moreover, the liquid rubber may increase the ductility of the solder flux residue, thereby increasing the mechanical properties of the solder joint and providing improved impact or “drop shock” resistance. Suitable rubbers which may be mentioned include acrylonitrile-butadiene rubbers having one or more terminal groups selected from carboxyl, hydroxyl and / or amine groups, which copolymers may be obtained by emulsion polymerization.

Whilst the concentration of tackifier compounds in toto is not rigidly limited and may be adjusted depending on inter alia solubility, the concentration thereof is preferably from 0.05 to 20 wt.%, based on the weight of the solder flux.

Aside from the aforementioned rosins, the solder fluxes of the present invention are not precluded from containing other resin acids and the esters thereof. “Resin acicf’ is a term of art that is used to refer to monocarboxyl ic diterpene acids, as discussed inter alia in Simonsen et al. The Terpenes, Vol. Ill, Cambridge University Press, Cambridge (1952); Exemplary resin acids include, without limitation: communic acid (CAS # 1231-35-2); isopimaric acid (CAS # 5835-26-7); levopimaric acid (CAS # 79-54-9); neoabietic acid (CAS # 471-77-2); palustric acid (CAS # 1945-53-5); pimaric acid (CAS # 127, 27-5); and, sandaracopimaric acid (CAS # 471-74-9).

Further tackifier compounds which may be present in the solder flux include but are not limited to: naphthotriazole-based derivatives; benzotriazole-based derivatives; imidazole- based derivatives; benzoimidazole-based derivatives; mercaptobenzothiazole-based derivatives; and, benzothiazolthio fatty acids. These tackifier compounds exhibit a strong adhesive effect particularly to copper and are capable of imparting tackiness to other conductive substances. For completeness, it is noted that the solder flux should comprise less than 1 wt.% of water, based on the weight of the composition, and is most preferably an anhydrous composition that is essentially free of water.

In an embodiment of the present invention, the composition of the solder flux is selected such that the solder flux residue has a Coefficient of Thermal Expansion (CTE) which differs from that of the material of solder joint by less than 100%, preferably less than 60%, more preferably less than 50%. By minimizing the differences in the coefficient of thermal expansion, thermal cycling resistance reduced.

In a further embodiment which is not mutually exclusive of that above, the composition of the solder flux is selected such that the solder flux or solder paste has a viscosity of from 250000 to 2000000 mPa.s, preferably from 300000 to 1500000 mPa.s.

To form a solder flux or solder paste, the above described parts are brought together and mixed. To ensure homogeneity, it will often be preferred that the elements are not mixed by hand and, as such, suitable mixing devices might include: static mixing devices; magnetic stir bar apparatuses; wire whisk devices; augers; batch mixers; planetary mixers; C.W. Brabender or Banburry® style mixers; and, high shear mixers, such as blade-style blenders and rotary impellers.

As noted above, the present invention also provides for the use of a solder paste as defined herein to form or strengthen a solder joint and / or interconnection. Within the constraints of the above defined method steps and without intention to limit the present invention, the solder joint may be formed from said solder paste during a manufacturing method selected from: a surface mount technology (SMT) method; a die and component attach method; a package on package (POP) method; a chip scale package (CSP) method; a ball grid array (BGA) method; a flip chip method; a can shield attachment method; and, a camera lens attachment method.

The following examples are illustrative of the present invention and are not intended to limit the scope of the invention in any way.

EXAMPLES

The following materials were utilized in the Examples:

HSA21231-45: Halide/halogen containing Solder flux based on gum rosin, polymerized gum rosin and glycol solvent, available from Henkel AG & Co. KgaA. HSA20732-91 : Halide/halogen-free Solder flux based on gum rosin post-reacted with acrylic acid, disproportionated resin and glycol ether solvent, available from Henkel AG & Co. KgaA.

HSA21235-36A: Halide/halogen containing Solder flux based on gum rosin post-reacted with acrylic acid, disproportionated resin and glycol ether solvent, available from Henkel AG & Co. KgaA.

The following properties were measured for the solder alloy: i) Tension Testing

Ultimate tensile strength, yield strength, and elongation were measured as part of one continuous test. The test was performed in accordance with ASTM E8 on a Satec HVL 60 Tension Testing Unit. The lead-free solder specimen tested was a rod measuring 3 inches (7.62 cm) long and 0.250 inches (0.64 cm) in diameter. For the purpose of elongation calculations upon completion of the test, the specimen was marked in the gauge length with ink and scribed with dividers. Elongation gauge marks were 4 times the diameter. The gauge marks for measuring elongation were approximately equidistant from the center of the length of the reduced section.

The specimen was affixed to the testing unit and stretched by gradual increase of force at a strain rate of 10 mm/min. When the sample started to lose its elasticity the load applied was noted and the yield strength (MPa) was determined. The specimen was further stretched with gradual force until breaking. The load at breaking was also noted and the ultimate tensile strength (MPa) was calculated, together with the Percentage Elongation at break based on the following formula:

Percentage Elongation = (Elongation at Break / Initial Gauge Length) ^c 100% ii) Microstructure

The microstructure of a given alloy was determined by Scanning Electron Microscopy (SEM) using a Hitachi TM1000 in back scatter electron (BSE) mode with a 15kV voltage. In the appended drawings:

Figure 1a is an SEM micrograph illustrating the microstructure of a comparative alloy Sn- 8Zn-3Bi.

Figure 1b is an SEM micrograph illustrating the microstructure of an alloy in accordance with an embodiment of the present invention Sn-8Zn-3Bi-0.1Ga. Figure 1c is an SEM micrograph illustrating the microstructure of an alloy in accordance with an embodiment of the present invention Sn-8Zn-3Bi-0.25Ga.

Figure 1d is an SEM micrograph illustrating the microstructure of an alloy in accordance with an embodiment of the present invention Sn-8Zn-3Bi-0.5Ga.

All micrographs were obtained under identical conditions and indicate that the presence of gallium influences the microstructure of the solder alloy particles.

Examples 1 to 3

Given a base alloy composition of Sn-8Zn-3Bi, four alloys were prepared having the percentage of the microalloying element Ga indicated in Table 1 herein below. The solidus temperature, pasty range, ultimate tensile strength and elongation of the alloys, as determined by the above described tests, are also provided in that Table.

Table 1

The above results indicate that the solidus temperature is gradually reduced with the increasing content of Gallium. There is not a significant change in the pasty range caused by the compositional difference. The presence of the micro-alloying element Gallium contributes to an increased tensile strength and elongation. The mechanical strength and ductility can both be enhanced through the inclusion of Gallium.

The following properties were evaluated for solder pastes. iii) Solder Paste Viscosity

The viscosity of the solder pastes was determined in accordance with IPC-TM-650 Test Methods Manual using a Malcom Spiral Pump Viscometer (available from Malcom Instruments Corporation). The pastes to be tested were introduced at a volume sufficient to fill the viscometer receptacle to about 60% of its depth. Prior to testing, receptacles were introduced into the temperature controlled unit of the viscometer and allowed to stabilize at 25 ± 0.25°C for 15 minutes. The viscosity of the pastes was evaluated at the shear rates stated below at a temperature of 25 ± 0.25°C.

The Thixotropic Index (Tl) of the pastes was calculated as a ratio of a material’s viscosity measured at two different speeds different by a factor of ten. iv) Joint Shear Performance

A two-factor factorial design with nine replications was selected in the experiment. The input variables are the solder alloy and the component substrate. The peak temperature (205°C) and the duration of time above solder liquidus temperature (30 seconds) were not variables.

Test boards were assembled with a singular size (1206) of pure silver (Ag), tin (Sn) and copper (Cu) plated surface mount chip resistors. Herein “1206” means a component with a nominal length of 0.12 inch (3.0 mm) and a nominal width of 0.06 inch (1.5 mm). There were x resistors on each board. Y boards were assembled for each experimental run so a total of Z boards were assembled (3 Component Substrate x 2 Solder Alloy x 9 replication).

Each board was cut into two identical pieces. The first half of the board represented the initial time zero and the components on this half of the board were sheared right after assembly. The experimental results on the effect of reflow profile on shear force immediately after assembly were reported. The other half of the test vehicles were then subjected to air-to-air thermal shock conditioning from -40 to 125°C with 30 minute dwell times (or 1 hour per cycle) for 500 cycles. The experimental matrix is listed in Table 2.

The components were sheared using a Dage Series 4000 shear tester according to the parameters and conditions identified in Table 2 herein below.

Table 2 Example 4

The alloy of Example 2 above (Sn-8ZN-3Bi-0.25Ga) was blended with the solder flux HSA20732-91 at an alloy loading of 86% by weight. The initial viscosity of the obtained solder paste was measured. A portion of the paste was retained at 26.5°C for four days, after which the viscosity of the paste was measured again. The results of these tests are provided in Table 3 below:

Table 3

The only fractional change in the viscosity of the pastes after four days is noteworthy as it can provide significant advantages in terms of storage stability and transport logistics. In contrast, equivalent pastes based on SnZn solder alloy are known to exhibit a short-shelf life, increasing in viscosity to above a workable range within the order to hours. The presence of both Bi and Ga in the alloy of the present invention is thus considered to markedly improve paste stability.

Example 5

The alloy of Example 2 above (Sn-8ZN-3Bi-0.25Ga) was blended with the solder flux HSA21231-45 at an alloy loading of 86% by weight. A comparative paste was obtained based on SnZn with the solder flux HSA21231-45 at an alloy loading of 86%. Joint shear performance was measured in accordance with the test defined above and the results are indicated in Table 4 below.

Table 4 The above-described embodiments of the invention are intended to be exemplary. It will be understood that various changes or modifications can be made thereto without departing from the scope of the present invention as set forth in the appended claims.

Example 6

Particle size and shape analysis was performed for the SnZnBiGa0.25 Type 4.5 powder. Particle size and shape analysis was performed by stenciling a small solder paste sample onto a glass slide. A drop of pine oil/resin solution was then gently mixed into the stenciled solder paste, spreading the resultant mixture over an area of approx. 20mm diameter. A glass cover slip was then gently placed on top of the mixture in order to remove any air pockets. The image analyzer system was switched on and focused on the samples under x20 magnification. An image of a representative area was then captured. After instructing the software to select all particles spurious ellipses were deleted or resized where possible. The particle measurements were repeated until >200 particles were measured. The particle size analysis numerical results are in tables 5 and 6 below and illustrated in figure 2. Similarly, the particle shape analysis numerical results are in table 7 below and illustrated in figure 3.

Table 5

Table 6

Table 7

Example 7

Direct in-application performance comparison of Sn-Zn-Bi vs Sn-Zn-Bi-0.25Ga solder pastes with the HSA21235-36A flux vehicle under equivalent conditions (equivalent flux, substrate and reflow condition).

Reflow performance of Sn-8Zn-3Bi (comparative example) and Sn-8Zn-3Bi-0.25 (according to the present invention) at 88% metal loading (vacuum treated) were evaluated through stencil printing surface mount assembly process on a standard SPTV1.1 test board across ENIG, ImmSn & OSP-Cu metalisations. All boards were reflowed through a P100 Sn-Pb type reflow profile (Figure 4) under aneorobic conditions (O2 ppm <1000ppm). Peak temperature 205°C, Ramp Rate 0.86°C/s with a time to peak of 202 seconds.

Figure 5 illustrates HSA21235-36A Sn-8Zn-3Bi 0805 P100 anaerobic 5a) ENIG 5b) ImmSn 5c) OSP-Cu, whereas Figure 6 illustrates HSA21235-36A Sn-8Zn-3Bi-0.25Ga 0805 P100 Anaerobic 6a) ENIG 6b) ImmSn 6c) OSP-Cu. The results and figures exemplify the solderability improvement arising from the Gallium addition to the alloy. The SnZnBi alloy demonstrates much poorer solderability in terms of substrate and component wetting compared against Sn-Zn-Bi-Ga0.25 under equivalent conditions. The Ga addition to SnZnBi creates a more roboust solder paste from an industry applications perspective.

Example 8

MUSTIII wetting balance

HSA21235-36A flux formula’s wetting balance performance was evaluated on the GEN3 MUSTIII wetting balance with a SnZnBiGa0.25 solder bath held at 200°C. 1mm diameter wire of Cu (99.9% annealed), Ni (99.0%, annealed) & Zn (99.9% extruded) were evaluated against industry standard Acteic 2 & 5. The wetting test had a 10s duration, 5mm immersion depth and a 20mm/s immersion speed.

Figure 7 illustrates MUSTIII Wetting Balance at 200 °C Ni 99.0%.

Figure 8 illustrates MUSTIII Wetting Balance at 200 °C Zn 99.9%.

Figure 9 illustrates MUSTIII Wetting Balance at 200 °C Cu 99.9%.

Figures 7 and 8 displays the relative wetting speeds and forces of the SnZnBiGa0.25 alloy on Cu, Ni and Zn substrates. The SnZnBiGa0.25 alloy displayed poor wetting performance on Ni substrates as only negative wetting forces were oberserved irrespective of flux. The wetting performance on Zn shows some level of mutal compatibility between the two metals that can be significantly enhanced via flux chemistry optimisation. Finally, the wetting performance on Cu appears poorer than what would be expected from conventional SAC305 alloys. However, there is evidence that this can be improved via flux chemistry optimisation.

Halide/halogen activators from flux formula HSA21231-45 promote wetting accross a range of substrates, whereas resin/solvent system of flux formula HSA20732-91 appeares to promote coalscence at expense of wetting. Hydribising these two into flux formula HSA21235-36A improves balance of wetting/coalescence performance and enhances overall reflow performance for the SnZnBiGa0.25 alloy system.

Claims

1. A lead-free solder alloy which consists essentially of or consists of: from 0.01 to 11 wt.% of zinc (Zn); from 0.01 to 6 wt.% of bismuth (Bi); from 0.01 to 2 wt.% of gallium (Ga); and, tin (Sn), wherein said weight percentages are based on the total weight of the alloy.

2. The lead-free solder alloy according to claim 1 which consists essentially of or consists of: from 5 to 10 wt.% of zinc (Zn); from 1.5 to 5 wt.% of bismuth (Bi); from 0.01 to 1 wt.% of gallium (Ga); and, tin (Sn).

3. The lead-free solder alloy according to claim 2 which consists essentially of or consists of: from 7 to 9 wt.% of zinc (Zn); from 2 to 4 wt.% of bismuth (Bi);

0.1 to 0.5 wt.% of gallium (Ga); and, tin (Sn).

4. The lead-free solder alloy according to claim 3 which consists essentially of or consists of:

8 wt.% of Zn;

3 wt.% of Bi; from 0.1 to 0.5 wt.% of Ga; and,

Sn.

5. The lead-free solder alloy according to claim 4 which is selected from the group consisting of: Snss.g/ZnsiBb/Gao.i; Snsss/Zns/Bb/Gao^; Snssjs/Zns/B /Gao^; Sn88.7/Zn8//Bh/Gao.3; Snss.e/ZnsBh/Gao.^ and, Snss.s/Zns/Bb/Gao.s.

6. The lead-free solder alloy according to any one of claims 1 to 5, wherein said alloy is in particulate form and has an average particle size of from 0.05 to 150 pm.

7. The lead-free solder alloy according to claim 6, wherein said particulate alloy has an average particle size of from 5 to 75 pm.

8. The lead-free solder alloy according to claim 6 or claim 7, wherein said particulate alloy is characterized in that: from 90 to 100% by volume of said particles are spherical; and, from 0 to 10% by volume of said particles are non-spherical.

9. A solder paste comprising, based on the weight of said paste: from 40 to 95 parts by weight of the solder alloy as defined in any one of claims 6 to 8; from 5 to 60 parts by weight of solder flux.

10. The solder paste according to claim 9 comprising: from 70 to 95 parts by weight of the solder alloy as defined in any one of claims 6 to 8; from 5 to 30 parts by weight of solder flux.

11. The solder paste according to claim 9 or claim 10, wherein the solder flux comprises: a) at least one binder; b) at least one solvent; and, optionally c) additives.

12. The solder paste according to claim 11, wherein said at least one binder a) comprises: at least one rosin resin; at least one thermoplastic polymer; at least one thermosetting crosslinkable resin; or, a mixture thereof.

13. The solder paste according to claim 11 or claim 12, wherein said at least one binder a) comprises a rosin resin selected from the group consisting of: rosin acids; gum rosin; wood rosin; tall oil rosin; rosin esters; hydrogenated rosins; dimerized rosins; disproportionated rosin; polymerized rosin; modified rosin resin; and, mixtures thereof.

14. The solder paste according to any one of claims 9 to 13, wherein said solder flux comprises a non-activated rosin (R) or a mildly activated rosin (RMA).

15. The solder paste according to any one of claims 9 to 14 having a viscosity of from 250000 to 2000000 mPa.s, preferably from 300000 to 1500000 mPa.s as determined at 25°C.