Solder joint comprising a tin alloy and an encapsulant comprising silica particles in an epoxy resin, and electronic device

Abstract

A solder joint at least partially encapsulated with an encapsulant, wherein the solder joint comprises a tin alloy; the encapsulant comprises silica particles dispersed in an epoxy resin; and the tin alloy comprises: from 2.8 to 4.5 wt.% silver, from 2.8 to 4 wt.% bismuth, from 1.0 to 6.5 wt.% antimony, from 0.3 to 1.2 wt.% copper, from 0.001 to 0.4 wt.% nickel, from 0.001 to 0.3 wt.% titanium, and the balance tin together with unavoidable impurities.

Description

[0001] SOLDER JOINT COMPRISING A TIN ALLOY AND AN ENCAPSULANT COMPRISING SILICA PARTICLES IN AN EPOXY RESIN, AND ELECTRONIC DEVICE

[0002] The present invention relates to a solder joint. The solder joint is particularly, though not exclusively, suitable for use in high reliability electronic assemblies,

[0003] 5 such as those having automobile applications.

[0004] Lead-free solders were initially developed due to environmental and health concerns, and as replacements for conventional soft solder alloys. Many conventional lead-free solder alloys are based around the Sn-0.7 wt.% Cu

[0005] 10 eutectic composition. The tin-silver-copper system has also been embraced by the electronics industry as a lead-free alternative for soldering materials. For example, the near-eutectic 96.5Sn3.0Ag0.5Cu (known as SAC305), exhibits superior fatigue life compared to the eutectic Sn-Pb solder, while having a melting point in the range of about 217 to 220 °C.

[0006] 15

[0007] As use of lead-free soldering materials becomes widespread, either due to environmental directives or pressure from the end users, so does the range of applications for such materials. In some fields, such as automotive, high power electronics and energy, including LED lighting for example, it is desirable for 20 solder alloys to operate at higher temperatures, for example at 150 °C or higher, for a relatively longer time. However, SAC305 does not perform well at such temperatures.

[0008] A number of attempts have been made to find better-performing alternatives to 25 SAC305. US10,376,994B2 relates to a soldering material based on Sn, Ag and

[0009] Cu. US2016 / 0325384A1 relates to high reliability, lead-free solder alloys for harsh environment and electronic applications. EP3321025A1 relates to a lead- free solder alloy, a flux composition, a solder paste composition, an electronic circuit board and an electronic control device. US10,195,698B2 relates to lead- 30 free, high reliability solder alloys. US10,300,562B2 relates to a solder alloy, a solder paste and an electronic circuit board. WO2019 / 094242A1 relates to a low- silver tin-based alternative solder alloy to Standard SAC alloys for high reliability applications. WO2019 / 094243A1 relates to a high reliability lead-free solder alloy for electronic applications in extreme environments. However, none of these alternatives provide a favourable combination of high temperature reliability and favourable mechanical properties.

[0010] The commercial solder alloy “Innolot” (MacDermid Alpha) has the composition Sn-3.8Ag-0.7Cu-3Bi-1.5Sb-0.15Ni and exhibits superior performance at high temperatures in comparison to SAC305. Innolot also exhibits superior room temperature mechanical properties in comparison to SAC305.

[0011] WO2021 / 043437A1 discloses solder alloys exhibiting superior high temperature performance and mechanical properties in comparison to both SAC305 and Innolot. For example, Alloy 10 of WO2021 / 043437A1 , which has the composition Sn-3.2Ag-3.1 Bi-5.9Sb-0.5Cu-0.16Ni-0.006Ti, exhibits a bulk microstructure consisting of a well-dispersed eutectic network of AgsSn, Bi-Sn and CueSns precipitates. Such an alloy exhibits particularly favourable mechanical properties, such as a high yield strength, tensile strength and creep rupture time.

[0012] Automotive applications, such as car doors, are subject to sudden jerks and vibrations. Electronics for such applications must therefore have high mechanical reliability or drop shock resistance. To achieve such high reliability, a fluid encapsulant (usually a liquid polymer) that fills the gap between the component and the PCB to provide a strong mechanical bond may be used. Such encapsulants are used for strengthening the joints of components and as a stress-relieving agent. These encapsulants, sometimes referred to as reinforcement materials, may be used as an underfill or a corner bond or an edge bond depending on the region of application of the polymer. These materials are applied to the PCB after it has been subjected to the reflow process and are then cured. Underfills are dispensed along edges of devices, flowing to perfectly fill the device footprint. Edgebonds provide mechanical anchors to device edges to allow increased reliability when a full underfill is not needed. Higher viscosity ensures controlled flow to prevent contact with BGA spheres. Commercial examples of encapsulant materials include ALPHA® HiTech CU31-2030 Underfill and ALPHA® HiTech CF31 -4010 Edgebond. The effect of a combination of a particular solder alloy and a particular encapsulant I reinforcement material can be difficult to predict, and may depend on, inter alia, their chemical compatibility. For example, antimony in the alloys may react adversely with one or more of the components in the encapsulant material.

[0013] The present invention aims to solve at least some of the problems associated with the prior art or to provide a commercially acceptable alternative.

[0014] Accordingly, in a first aspect, the present invention provides a solder joint at least partially encapsulated with an encapsulant, wherein the solder joint comprises a tin alloy; the encapsulant comprises silica particles dispersed in an epoxy resin; and the tin alloy comprises: from 2.8 to 4.5 wt.% silver, from 2.8 to 4 wt.% bismuth, from 1 .0 to 6.5 wt.% antimony, from 0.3 to 1 .2 wt.% copper, from 0.001 to 0.4 wt.% nickel, from 0.001 to 0.3 wt.% titanium, and the balance tin together with unavoidable impurities.

[0015] The present invention will now be further described. In the following passages different aspects of the invention 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.

[0016] The inventors have surprisingly found that the combination of such an encapsulant with such a tin alloy may result in the solder joint exhibiting particularly favourable high temperature reliability. In particular, when combined with the tin alloy and silica particles, the epoxy resin may not undergo any degradation, or may experience only limited degradation, during exposure to high temperatures. Furthermore, such high temperature reliability may be combined with particularly favourable mechanical properties of the tin alloy such as, for example, high yield strength, tensile strength and creep strength. As a result, such a solder joint may be particularly suitable for use in applications such as automotive, high power electronics and energy (including LED lighting).

[0017] Surprisingly, such a combination of high temperature reliability and favourable mechanical properties may be more pronounced in comparison to solder joints formed of other conventional tin alloys, such as Innolot or SAC305, encapsulated with the encapsulant. It is not immediately apparent why this is the case but, without being bound by theory, this may be due to a particularly favourable chemical compatibility between the tin alloy and the epoxy rein I silica particles.

[0018] The tin alloy may be capable of withstanding operational temperatures of typically at least 150 °C. The tin alloy may exhibit improved mechanical properties and high temperature creep resistance compared to SAC305 and Innolot.

[0019] The term “solder joint” as used herein may encompass a solidified mass of solder alloy connecting two or more components. A solder joint may be manufactured, for example, by placing solder alloy between opposing surfaces of the metal components, heating the solder alloy to melt the solder alloy and wet the surfaces, and then cooling the solder alloy to form a solidified joint between the two surfaces. The solder alloy may be placed between the opposing surfaces, for example, in the form of a solder paste or solder preform.

[0020] The solder joint is at least partially encapsulated with an encapsulant. The solder joint may be substantially entirely encapsulated with the encapsulant, or completely encapsulated with the encapsulant. The encapsulate is typically formed by flowing a composition containing silica particles dispersed in an epoxy resin around the solidified solder alloy, and then curing the epoxy resin, for example using heat. Such flow may occur between components connected by the solder joint, for example, due to capillary action.

[0021] The encapsulant comprises silica (SiO2) particles dispersed in an epoxy resin. By “dispersed” it is meant that the silica particles are spread throughout the epoxy resin in a substantially homogeneous manner. The silica particles may constitute the discontinuous phase, and the epoxy rein may constitute the continuous I matrix phase. The silica particles may preferably comprise fumed silica.

[0022] The silica particles may provide the solder joint with a low coefficient of thermal expansion (CTE), which may reduce thermal strain between components connected by the solder joint. The silica particles may be in the form of one or more of rods, plates and spheres, but are typically substantially in the form of spheres. The silica particles typically exhibit a mean longest dimension of from 0.5 to 10 pm, more typically from 1 to 5 pm. The mean longest dimension may be measured, for example, using a laser diffraction method. When the silica particles are in the form of spheres, the longest dimension is the diameter of the spheres. The maximum longest dimension of the silica particles, i.e. the longest dimension of the largest silica particle, is typically less than 40 pm, more typically less than 30 pm. Larger particles may hinder flow of the encapsulant material around the solder joint during manufacture.

[0023] The epoxy resin, once cured, may provide mechanical reinforcement to the joint. This may improve the mechanical performance of the solder joint and / or the high temperature reliability. The epoxy resin is typically cured epoxy resin.

[0024] The epoxy resin preferably comprises one or more of a bisphenol A-type epoxy resin (e.g. bisphenol A diglycidyl ether), a bisphenol F-type epoxy resin (e.g. bisphenol F diglycidyl ether), a bisphenol E-type epoxy resin (e.g. bisphenol E diglycidyl ether), (3',4'-epoxycyclohexane)methyl 3,4 epoxycyclohexylcarboxylate, N, N-diglycidyl-4-glycidyloxyaniline, and bis (3,4 epoxycyclohexylmethyl) adipate. The encapsulant may comprise species other than the silica particles and epoxy resin such as, for example, curing agents / accelerators, additional solid fillers, cationic initiators toughening agents, coupling agents, stabilizers and rheology modifiers.

[0025] In a preferred embodiment, the encapsulant comprises, based on the total weight of the encapsulant: from 5 to 15 wt.% bisphenol A diglycidyl ether, preferably from 8 to 12 wt.% bisphenol A diglycidyl ether; and / or from 20 to 30 wt.% (3',4'-epoxycyclohexane)methyl 3,4- epoxycyclohexylcarboxylate, preferably from 23 to 27 wt.% (3',4'- epoxycyclohexane)methyl 3,4-epoxycyclohexylcarboxylate; and / or from 0.1 to 1 wt.% fumed silica, preferably from 0.3 to 0.7 wt.% fumed silica; and / or from 5 to 15 wt.% silica particles, preferably from 8 to 13 wt.% silica particles; and / or from 1 to 5 wt.% epoxy resin containing core-shell rubber particles, preferably from 2 to 4 wt.% epoxy resin containing core-shell rubber particles; and / or from 1 to 10 wt.% latent-type curing agent, preferably from 2 to 6 wt.% latent-type curing agent; and / or from 40 to 55 wt.% anhydride-type curing agent, preferably from 45 to 50 wt.% anhydride curing agent.

[0026] In a preferred embodiment, the encapsulant comprises, based on the total weight of the encapsulant: from 1 to 10 wt.% bisphenol A diglycidyl ether, preferably from 3 to 7 wt.% bisphenol A diglycidyl ether; and / or from 10 to 20 wt.% (3',4'-epoxycyclohexane)methyl 3,4- epoxycyclohexylcarboxylate, preferably from 13 to 17 wt.% (3', 4'- epoxycyclohexane)methyl 3,4-epoxycyclohexylcarboxylate; and / or from 0.05 to 0.3 wt.% imidazole-based curing agent, preferably from 0.08 to 0.12 wt.% imidazole-based curing agent; and / or from 0.1 to 2 wt.% 3-glycidyl-oxypropyltrimethoxy-silane, preferably from 0.6 to 1.2 wt.% 3-glycidyl-oxypropyltrimethoxy-silane; and / or from 15 to 25 wt.% anhydride-type curing agent, preferably from 18 to 22 wt.% anhydride-type curing agent; and / or from 50 to 70 wt.% silica particles, preferably from 55 to 62 wt.% silica particles; and / or from 0.5 to 2 wt.% latent-type curing agent, preferably from 0.8 to 1 .2 wt.% latent-type curing agent.

[0027] In a preferred embodiment, the encapsulant comprises, based on the total weight of the encapsulant: from 40 to 60 wt.% of a mixture of bisphenol A diglycidyl ether and bisphenol F diglycidyl ether, preferably from 50 to55 wt.% of a mixture of bisphenol A diglycidyl ether and bisphenol F diglycidyl ether; and / or from 5 to 15 wt.% N , N-diglycidyl-4-glycidyloxyaniline, preferably from 8 to 12 wt.% N, N-diglycidyl-4-glycidyloxyaniline; and / or from 1 to 10 wt.% imidazole-based curing agent, preferably from 3 to 07 wt.% imidazole-based curing agent; and / or from 0.5 to 1 .5 wt.% dicyandiamide, preferably from 0.8 to 1 .2 wt.% dicyandiamide; and / or from 0.5 to 2 wt.% latent-type curing agent, preferably from 0.8 to 1 .2 wt.% latent-type curing agent; and / or from 0.5 to 1 .5 wt.% epoxy resin containing core-shell rubber particles, preferably from 0.8 to 1 .2 wt.% epoxy resin containing core-shell rubber particles; and / or from 5 to 15 wt.% silica particles, preferably from 8 to 12 wt.% silica particles.

[0028] In a preferred embodiment, the encapsulant comprises, based on the total weight of the encapsulant: from 5 to 15 wt.% bisphenol A diglycidyl ether, preferably from 8 to 12 wt.% bisphenol A diglycidyl ether; and / or from 25 to 35 wt.% (3',4'-epoxycyclohexane)methyl 3,4- epoxycyclohexylcarboxylate, preferably from 28 to 32 wt.% (3',4'- epoxycyclohexane)methyl 3,4-epoxycyclohexylcarboxylate; and / or from 35 to 55 wt.% anhydride-type curing agent, preferably from 38 to 44 wt.% anhydride curing agent; and / or from 1 to 10 wt.% latent-type curing agent, preferably from 2 to 6 wt.% latent-type curing agent; and / or from 1 to 3 wt.% stabilizer, preferably from 1 .5 to 2.5 wt.% stabilizer; and / or from 1 to 5 wt.% epoxy resin containing core-shell rubber particles, preferably from 2 to 4 wt.% epoxy resin containing core-shell rubber particles; and / or from 5 to 15 wt.% silica particles, preferably from 8 to 12 wt.% silica particles.

[0029] In a preferred embodiment, the encapsulant comprises, based on the total weight of the encapsulant: from 30 to 40 wt.% bisphenol A diglycidyl ether, preferably from 32 to 36 wt.% bisphenol A diglycidyl ether; and / or from 5 to 15 wt.% N , N-diglycidyl-4-glycidyloxyaniline, preferably from 8 to 12 wt.% N, N-diglycidyl-4-glycidyloxyaniline; and / or from 0.5 to 1 .5 wt.% stabilizer, preferably from 0.8 to 1 .2 wt.% stabilizer; and / or from 0.1 to 1 wt.% epoxy resin containing core-shell rubber particles, preferably from 0.3 to 0.7 wt.% epoxy resin containing core-shell rubber particles; and / or from 0.1 to 1 wt.% coupling agent, preferably from 0.3 to 0.7 wt.% coupling agent; and / or from 1 to 5 wt.% dicyandiamide, preferably from 2 to 4 wt.% dicyandiamide; and / or from 0.1 to 1 wt.% latent-type curing agent, preferably from 0.3 to 0.7 wt.% latent-type curing agent; and / or from 40 to 60 wt.% silica particles, preferably from 48 to 52 wt.% silica particles; and from 0.1 to 1 wt.% 4,4’-diamino diphenyl sulfone, preferably from 0.3 to 0.7 wt.% 4, 4’ -diamino diphenyl sulfone.

[0030] In a preferred embodiment, the encapsulant comprises, based on the total weight of the encapsulant: from 1 to 10 wt.% bisphenol A diglycidyl ether, preferably from 3 to 7 wt.% bisphenol A diglycidyl ether; and / or from 1 to 10 wt.% bisphenol E diglycidyl ether, preferably from 3 to 7 wt.% bisphenol E diglycidyl ether; and / or from 25 to 35 wt.% (3',4'-epoxycyclohexane)methyl 3,4- epoxycyclohexylcarboxylate, preferably from 28 to 32 wt.% (3', 4'- epoxycyclohexane)methyl 3,4-epoxycyclohexylcarboxylate; and / or from 0.1 to 1 wt.% cationic initiator, preferably from 0.3 to 0.7 wt.% cationic initiator; and / or from 50 to 70 wt.% silica particles, preferably from 55 to 63 wt.% silica particles; and / or from 0.5 to 1 .5 wt.% fumed silica, preferably from 0.8 to 1.2 wt.% fumed silica.

[0031] In a preferred embodiment, the encapsulant comprises, based on the total weight of the encapsulant: from 10 to 20 wt.% bisphenol A diglycidyl ether, preferably from 13 to 17 wt.% bisphenol A diglycidyl ether; and / or from 20 to 30 wt.% bis (3,4 epoxycyclohexylmethyl) adipate, preferably from 23 to 27 wt.% bis (3,4 epoxycyclohexylmethyl) adipate; and / or from 0.1 to 1 wt.% cationic initiator, preferably from 0.3 to 0.7 wt.% cationic initiator; and / or from 50 to 65 wt.% silica particles, preferably from 55 to 60 wt.% silica particles; and / or from 1 to 3 wt.% fumed silica, preferably from 1 .8 to 2.2 wt.% fumed silica.

[0032] In a preferred embodiment, the encapsulant comprises, based on the total weight of the encapsulant: from 15 to 25 wt.% bis (3,4 epoxycyclohexylmethyl) adipate, preferably from 18 to 22 wt.% bis (3,4 epoxycyclohexylmethyl) adipate; and / or from 5 to 15 wt.% multifunctional epoxy resin, preferably from 8 to 12 wt.% multifunctional epoxy resin; and / or from 0.5 to 1 .5 wt.% cationic initiator, preferably from 0.8 to 1 .2 wt.% cationic initiator; and / or from 60 to 75 wt.% silica particles, preferably from 65 to 70 wt.% silica particles; and / or from 1 to 2 wt.% fumed silica, preferably from 1 .3 to 1 .7 wt.% fumed silica.

[0033] For the avoidance of doubt, where both silica particles and fumed silica are contained in the above preferred embodiment encapsulants, the wt.% of silica particles does not include the wt.% of fumed silica.

[0034] The tin alloy comprises from 2.8 to 4.5 wt.% silver. Preferably, the tin alloy comprises from 3 to 4 wt.% silver, more preferably from 3.1 to 3.5 wt.% silver, even more preferably from 3.15 to 3.3 wt.% silver. In a preferred embodiment, the tin alloy comprises about 3.2 wt.% silver. The presence of silver in the specified amount may serve to improve mechanical properties, for example strength, through the formation of network-like intermetallic compounds such as, for example, AgsSn. In addition, the presence of silver may improve wetting and spread. Higher levels of silver, in particular levels higher than 4.5 wt.% silver, may increase the liquidus temperature and larger precipitates of AgsSn formed in the solder matrix act as sites of crack initiation and subsequent failure. Lower contents of silver may not form enough AgsSn precipitates that may be helpful in improving the strength. The tin alloy comprises from 2.8 to 4 wt.% bismuth. Preferably, the tin alloy comprises from 3 to 4 wt.% bismuth, more preferably from 3 to 3.5 wt.% bismuth, even more preferably from 3.1 to 3.3 wt.% bismuth. In a preferred embodiment, the tin alloy comprises about 3.1 wt.% bismuth. The presence of bismuth in the specified amount may serve to improve mechanical properties through solid solution strengthening. Bismuth may also act to improve creep resistance. Bismuth may also improve wetting and spread. However, bismuth addition in excess of the specified amount may result in precipitation of bismuth in tin resulting in a more brittle alloy.

[0035] The tin alloy comprises from 1 .0 to 6.5 wt.% antimony. Preferably, the tin alloy comprises from 2 to 6 wt.% antimony, more preferably from 3 to 6 wt.% antimony, even more preferably from 3.1 to 6 wt.% antimony, still even more preferably from 3.2 to 6 wt.% antimony, still even more preferably from 5.7 to 6 wt.% antimony. In a preferred embodiment, the tin alloy comprises about 5.9 wt.% antimony. The presence of antimony in the specified amount may serve to improve mechanical properties through solid solution strengthening. Antimony may also act to improve creep resistance and thermal fatigue resistance. Antimony may also increase the liquidus temperature of the alloy. Antimony addition lower than the specified range may not have the required improvement of mechanical strength and thermal fatigue resistance. Antimony additions higher than the specified range may increase the liquidus temperature such that the prescribed reflow temperature also increases. Reflow temperatures higher than 260 °C can lead to various issues during soldering, such as damaging printed circuit boards and components.

[0036] The tin alloy comprises from 0.3 to 1 .2 wt.% copper. Preferably, the tin alloy comprises from 0.4 to 0.8 wt.% copper, more preferably from 0.4 to 0.6 wt.% copper, even more preferably from 0.45 to 0.55 wt.% copper. The presence of copper in the specified amount may serve to improve mechanical properties, for example strength, through the formation of Cu-Sn intermetallic compounds. Copper addition in the specified range results in the optimum quantity of intermetallic compound precipitates required for strengthening the alloy.

[0037] The tin alloy comprises from 0.001 to 0.4 wt.% nickel. Preferably, the tin alloy comprises from 0.01 to 0.3 wt.% nickel, more preferably from 0.1 to 0.2 wt.% nickel, even more preferably from 0.13 to 0.19 wt.% nickel. The presence of nickel in the specified amount may serve to improve mechanical properties through the formation of intermetallic compounds with tin and copper, which can result in precipitation strengthening. In addition, the presence of nickel may act to reduce the copper dissolution rate. Nickel may also increase thermal reliability by decreasing IMC growth at the substrate / solder interface.

[0038] The tin alloy comprises from 0.001 to 0.3 wt.% titanium. Preferably, the tin alloy comprises from 0.005 to 0.2 wt.% titanium, more preferably from 0.005 to 0.05 wt.% titanium, even more preferably from 0.005 to 0.01 wt.% titanium, still even more preferably from 0.005 to 0.008 wt.% titanium. The presence of titanium in the amounts specified may improve one or more of strength, solid state interfacial reactions and thermo-mechanical reliability.

[0039] In a preferred embodiment, the tin alloy consists of from 2.8 to 3.2 wt.% silver, from 2.8 to 3.2 wt.% bismuth, from 5.5 to 6.5 wt.% antimony, from 0.3 to 0.8 wt.% copper, from 0.08 to 0.2 wt.% nickel, 0.005 to 0.02 wt.% of titanium, and the balance tin together with unavoidable impurities. Such an alloy may exhibit a particularly favourable combination of favourable mechanical properties, favourable solderability, superior high temperature creep properties and superior thermo-mechanical properties and fatigue life, such as those evaluated in thermal cycling or thermal shock tests covering a wide temperature range and long dwell times.

[0040] In a preferred embodiment, the tin alloy consists of 2.9 to 3.2 wt.% silver, 2.9 to 3.2 wt.% bismuth, 5 to 7 wt.% antimony, 0.3 to 0.7 wt.% copper, 0.09 to 0.18 wt.% nickel, 0.002 to 0.01 wt.% titanium, and the balance tin together with unavoidable impurities. Such an alloy may exhibit a particularly favourable combination of favourable mechanical properties, favourable solderability, superior high temperature creep properties and superior thermo-mechanical properties and fatigue life, such as those evaluated in thermal cycling or thermal shock tests covering a wide temperature range and long dwell times.

[0041] In a particularly preferred embodiment, the tin alloy consists of: about 3.2 wt.% silver, about 3.1 wt.% bismuth, about 5.9 wt.% antimony, about 0.5 wt.% copper, about 0.16 wt.% nickel, about 0.006 wt.% titanium, and the balance tin together with unavoidable impurities. Such an alloy may exhibit a particularly favourable combination of favourable mechanical properties, favourable solderability, superior high temperature creep properties and superior thermo-mechanical properties and fatigue life, such as those evaluated in thermal cycling or thermal shock tests covering a wide temperature range and long dwell times.

[0042] The alloy will typically comprise at least 70 wt.% tin, more typically at least 80 wt.% tin, still more typically at least 84 wt.% tin.

[0043] It will be appreciated that the alloys described herein may contain unavoidable impurities, although, in total, these are unlikely to exceed 1 wt.% of the composition. Preferably, the solder alloys contain unavoidable impurities in an amount of not more than 0.5 wt.% of the composition, more preferably not more than 0.3 wt.% of the composition, still more preferably not more than 0.1 wt.% of the composition, still more preferably not more than 0.05 wt.% of the composition, and most preferably not more than 0.02 wt.% of the composition. As will be appreciated, tin alloys are typically manufactured using Sn99%. Accordingly, in reality, the unavoidable impurities are unlikely to exceed 1 wt.% of the composition. The typical impurities of Sn99% are unlikely to have a material effect on the alloys in such an amount.

[0044] The tin alloys described herein may consist of the recited elements. Alternatively, the solder alloys described herein may consist essentially of the recited elements. It will therefore be appreciated that in addition to those elements that are mandatory (i.e. tin, silver, bismuth, antimony, copper, nickel and titanium) other non-specified elements may be present in the composition provided that the essential characteristics of the composition are not materially affected by their presence.

[0045] The tin alloys may be manufactured by mixing the corresponding pure elements, or by mixing premade alloys, in any form factor, and by using any manufacturing method, if their final compositions are covered by the specification described herein.

[0046] The encapsulant is preferably in the form of an underfill or an edgebond. Underfills and edgebonds may be particularly suitable for improving the high temperature reliability of the solder joint.

[0047] The ratio by weight of epoxy resin to silica particles in the encapsulant is preferably from 1 :1 to 1 :3 or from 1 :2 to 1 :5. Such ratios may result in the solder joint exhibiting particularly favourable mechanical properties and / or high temperature reliability, and / or may result in reduced thermal strain between components connected by the solder joint.

[0048] The encapsulant preferably comprises from 5 to 70 wt.% silica particles, based on the total weight of the encapsulant, more preferably from 10 to 65 wt.%. In a preferred embodiment, the encapsulant comprises from 5 to 15 wt.% silica particles, based on the total weight of the encapsulant. In a preferred embodiment, the encapsulant comprises from 60 to 70 wt.% silica particles, based on the total weight of the encapsulant.

[0049] The encapsulant preferably further comprises a curing accelerator dispersed in the epoxy resin. During manufacture of the joint, the presence of the curing accelerator may expedite curing, thereby potentially avoiding exposing connected components to long high-temperature curing steps, which may result in damage to the components. In addition, this may result in a faster, lower energy intensive manufacturing method. The curing accelerator preferably comprises an anhydride curing accelerator and / or a latent curing accelerator.

[0050] The ratio by weight of epoxy resin to curing accelerator in the encapsulant is preferably from 1 :0.0001 to 1 :0.3 or from 1 :0.0001 to 1 :10. Such ratios may result in particularly favourable curing rates during manufacture.

[0051] The silica preferably comprises spherical silica, i.e. the silica particles preferably comprise spherical silica particles. Spherical particles may result in smoother flow.

[0052] In a preferred embodiment, the encapsulant consists of, or consists essentially of, the silica particles, the epoxy resin and optionally the curing accelerator. The substantial absence of other species may result in particularly favourable mechanical properties and / or high temperature reliability.

[0053] The solder joint is preferably in the form of a ball grid array (BGA). The effects of the present invention may be particularly pronounced when the solder joint is in the form of a ball grid array.

[0054] The tin alloy preferably has a solidus temperature of at least 205 °C, preferably at least 208 °C. This may increase the high temperature reliability of the solder joint and may enable the solder joint to be used in a device used at high operating temperatures, for example operating temperatures of 150 °C or more.

[0055] The tin alloys preferably has a liquidus temperature of 235 °C or less, preferably 233 °C or less. This may enable the solder joint to be manufactured using reflow temperatures low enough to reduce the occurrence of damage to components connected by the solder joint. The tin alloy is preferably lead-free. By “lead-free” it means that no lead is added intentionally. Thus, the lead content is zero or at no more than accidental impurity levels. This may be beneficial from environmental and health perspectives.

[0056] In a further aspect, the present invention provides an electronic device comprising an electronic component connected to a printed circuit board by a solder joint, the solder joint being the solder joint as described herein.

[0057] In a further aspect, the present invention provides use of the solder joint as described herein to improve the thermal cycling characteristics of an electronic device.

[0058] The present invention will now be further described with reference to the following drawings, in which:

[0059] Figure 1 shows a schematic of a solder joint according to the present invention.

[0060] Figure 2 shows a schematic of a solder joint according to the present invention.

[0061] Referring to Figure 1 , there is shown a solder joint (shown generally at 1 ) between a printed circuit board 2 and an electronic component 3. The solder joint is formed of balls of solder alloy 4 in the form of a ball grid array. The solder joint is encapsulated with an encapsulant 5, in the form of an underfill.

[0062] Referring to Figure 2, there is shown a solder joint (shown generally at 1 ) between a printed circuit board 2 and an electronic component 3. The solder joint is formed of balls of solder alloy 4 in the form of a ball grid array. The solder joint is encapsulated with an encapsulant 5, in the form of an edgebond.

[0063] The present invention will now be described further with reference to the following non-limiting examples. Several encapsulant materials were prepared, with the compositions set out below (the label “UF” referring to an underfill material, and the label “EB” referring to an edgebond material):

[0064] UF1 :

[0065] RM Content

[0066] B PA epoxy resin 10

[0067] (3’,4’-

[0068] Epoxycyclohexane)methyl

[0069] 3,4- epoxycyclohexylcarboxylate 25

[0070] Fumed silica Q 5

[0071] Silica10 5core shell rubber type epoxy 3

[0072] Latent-type curing agent 4

[0073] Anhydride curing agent 47

[0074] 100

[0075] UF2:

[0076] RM Content

[0077] BPA epoxy resin 5

[0078] (3',4'- Epoxycyclohexane)methyl 3,4- epoxycyclohexylcarboxylate 15 Imidazole-based curing agent 0.1 3-glycidyl- oxypropyltrimethoxy-silane 0.9 Anhydride curing agent 20

[0079] Silica 58

[0080] Latent-type curing agent 1

[0081] 100

[0082] UF3: RM Content

[0083] BPA / BPF type epoxy resin 52

[0084] N, N-Diglycidyl-4- glycidyloxyaniline p-tert-Butyl phenylglycidylether 20

[0085] Imidazole-based curing agent 5 dicyandiamide 1

[0086] Latent-type curing agent 1 core shell rubber type epoxy 1

[0087] Silica 10

[0088] 100.0

[0089] UF4:

[0090] RM Content

[0091] B PA epoxy resin 10.00

[0092] (3', 4'-

[0093] Epoxycyclohexane)methyl

[0094] 3,4” epoxycyclohexylcarboxylate Anhydride curing agent 36.00

[0095] Anhydride curing agent 5.00

[0096] Latent-type curing agent 4.00 stabilizer 2.00 core shell rubber type „nnepoxy silica 10.00

[0097] 100.00

[0098] UF5:

[0099] RM Content

[0100] BPA epoxy resin 34.0

[0101] N, N-Diglycidyl-4-10 0glycidyloxyaniline stabilizer 1.0 core shell rubber type epoxy coupling agent 0.5 dicyandiamide 3.0

[0102] Latent-type curing agent 0.50 silica 50.0

[0103] DDS 0.5

[0104] 100.0

[0105] EB1 :

[0106] RM Content

[0107] BPE type epoxy resin

[0108] 5

[0109] BPA type epoxy resin (3',4'-

[0110] Epoxycyclohexane)methyl

[0111] 3,4- epoxycyclohexylcarboxylate 30

[0112] Cationic initiator 0.5

[0113] Silica 50

[0114] Silica 8.5

[0115] Fumed silica 1

[0116] 100

[0117] EB2:

[0118] RM Content

[0119] BPA type epoxy resin

[0120] 15

[0121] Bis (3,4

[0122] Epoxycyclohexylmethyl)

[0123] Adipate 25

[0124] Cationic initiator

[0125] 0.5

[0126] Silica 57.5

[0127] Fumed silica 2

[0128] 100

[0129] EB3:

[0130] RM Content

[0131] Bis (3,4

[0132] Epoxycyclohexylmethyl)

[0133] Adipate 20 mutifuctional epoxy resin 10 Cationic initiator 1

[0134] Silica 52.5

[0135] Silica 15

[0136] Fumed silica 1.5

[0137] 100

[0138] The properties of such encapsulant materials are set out in tables 1 and 2 below:

[0139] Table 2: Properties of the Edgebonds

[0140] The coefficient of thermal expansion (CTE) and glass transition temperature (Tg) are critical factors for thermo-mechanical reliability, specifically concerning thermal cycling characteristic life (see Tables 3 and 4 below). A lower CTE combined with a higher Tg effectively reduces the stresses generated during alternating cooling and heating cycles. It has been observed that UF2 and EB1 exhibit low CTE and high Tg, both of which contribute significantly to the extraordinary thermal cycling life of joints.

[0141] In the context of drop shock, the advantageous processability of the underfills facilitates the complete filling of the intervening spaces between the solder joints, thereby enhancing the mechanical strength of these joints. The compositions UF1 through UF5 exhibit the following characteristics. UF1 demonstrates rapid flowability due to low viscosity and enables infiltration at ambient temperature. It possesses a high glass transition temperature (Tg), providing excellent thermal shock resistance. UF2 is capable of ambient temperature infiltration and features a high Tg and low coefficient of thermal expansion (CTE), thereby achieving outstanding thermal shock reliability. UF3 exhibits excellent flowability due to low viscosity, facilitating infiltration at ambient temperature and possessing superior surface insulation resistance (SIR), achieving performance that meets the 004A standard. UF4, characterized by outstanding flowability at low viscosity, allows for ambient temperature infiltration and is suitable for rework processes. Finally, UF5 displays a high Tg and low CTE, contributing to enhanced reliability.

[0142] The compositions of EB1 through EB3 exhibit a rapid curing rate, excellent heat resistance (TCT reliability), low water absorption, a high glass transition temperature (Tg), and a low coefficient of thermal expansion (CTE). Furthermore, these compositions demonstrate superior dispensing workability, including jetting workability, and allow for rework at temperatures below 200 °C in EB2 and EB3. The flowability can be adjusted to minimize solder ball damage in EB2 and EB3. The EB3 presents excellent visual inspection characteristics in fluorescent black color, while EB2 achieves flame retardancy with UL VO. Finally, these compositions offer outstanding storage stability.

[0143] The thermal cycling behavior of solder joints formed of various tin alloys with and without encapsulation was investigated. The tests were conducted as per the IPC 9701 A standard. Characteristic life in thermal cycling is given at 63.2 % of cumulative failures of the BGA228 packages when their electrical resistance is monitored in-situ using a high-speed data logger. A failure, as per the IPC 9701 A standard, is defined when there is an increase of 20% in the measured electrical resistance for five consecutive readings. The results are set out in tables 3 and 4 below:

[0144] Table 3: Electrical resistance failure results in thermal cycling for BGA228 (conditions: -40 to 150 °C, with 20 minutes dwell time at each temperature)

[0145] As can be seen from Table 3 above, of the three alloys, Innolot has the most favourable thermal cycling behavior without encapsulation. However, surprisingly, when edgebond EB1 is applied, the joint formed of Sn-3.2Ag-3.1 Bi-5.9Sb-0.5Cu- 0.16Ni-0.006Ti shows the greatest improvement and the best absolute high temperature reliability. In combination with the superior mechanical properties of Sn-3.2Ag-3.1 Bi-5.9Sb-0.5Cu-0.16Ni-0.006Ti in comparison to SAC305 and Innolot, such a joint may be particularly suitable for use in a device with a high operating temperature. Table 4: Electrical resistance failure results in thermal cycling for BGA228 (conditions: -40 to 125 °C, with 20 minutes dwell time at each temperature)

[0146] As can be seen from Table 4 above, joints formed of Innolot and Sn-3.2Ag-3.1 Bi- 5.9Sb-0.5Cu-0.16Ni-0.006Ti have fairly similar thermal cycling behavior without encapsulation. However, surprisingly, when underfill UF3 is applied, the joint formed of Sn-3.2Ag-3.1 Bi-5.9Sb-0.5Cu-0.16Ni-0.006Ti shows the greatest improvement and the best absolute high temperature reliability. In combination with the superior mechanical properties of Sn-3.2Ag-3.1 Bi-5.9Sb-0.5Cu-0.16Ni-0.006Ti in comparison to SAC305 and Innolot, such a joint may be particularly suitable for use in a device with a high operating temperature.

[0147] The mechanical reliability of various solder joints formed of Sn-3.2Ag-3.1 Bi-5.9Sb- 0.5Cu-0.16Ni-0.006Ti alloy was evaluated using the drop shock test. This test was performed at 1500 G corresponding to a drop height of 25-26 cm on similar boards as that for thermal cycling but with components placed only at the four corner positions. A total of 28 components were tested for each combination. The failure criterion was 1 V lesser than the initial applied 5 V for the time duration of 0.5 msec for 4 consecutive drops. In case of very long survival of components, the test was terminated at ~10000 drops. The results of the drop shock test are summarized in table 5.

[0148] Table 5: Effect of underfills and edgebonds on drop shock performance of Sn- 3.2Ag-3.1 Bi-5.9Sb-0.5Cu-0.16Ni-0.006Ti alloy joints As can been seen from Table 5 above, an at least 200% increase in drop shock characteristic life is observed. Unlike the underfills, the edgebonds do not encapsulate the solder interconnect, resulting in lower drop shock characteristic life than the former. UF3 has the highest adhesion strength to the solder flux residue resulting in the maximum improvement of drop shock. Thus, the combination of high reliability alloys with suitable encapsulants as underfills or edgebonds result in ultra-high reliability solder joints for extreme work environments.

[0149] The invention will now be further described with reference to the following numbered clauses:

[0150] 1 . A solder joint at least partially encapsulated with an encapsulant, wherein the solder joint comprises a tin alloy; and the encapsulant comprises silica particles dispersed in an epoxy resin.

[0151] 2. The solder joint of clause 1 , wherein the encapsulant is in the form of an underfill or an edgebond.

[0152] 3. The solder joint of clause 1 or clause 2, wherein the ratio by weight of epoxy resin to silica particles in the encapsulant is from 1 :1 to 1 :3 or from 1 :2 to 1 :5.

[0153] 4. The solder joint of any preceding clause, wherein the encapsulant further comprises a curing accelerator dispersed in the epoxy resin.

[0154] 5. The solder joint of clause 4, wherein the ratio by weight of epoxy resin to curing accelerator in the encapsulant is from 1 :0.0001 to 1 :0.3 or from 1 :0.0001 to 1 :10.

[0155] 6. The solder joint of any preceding clause, wherein the silica comprises spherical silica. 7. The solder joint of any preceding clause, wherein the encapsulant consists of, or consists essentially of, the silica particles, the epoxy resin and optionally the curing accelerator.

[0156] 8. The solder joint of any preceding clause, wherein the solder joint is in the form of a ball grid array (BGA).

[0157] 9. The solder joint of any preceding clause, wherein the tin alloy has: a solidus temperature of at least 205 °C, preferably at least 208 °C; and / or a liquidus temperature of 235 °C or less, preferably 230 °C or less.

[0158] 10. The solder joint of any preceding clause, wherein the tin alloy is lead-free.

[0159] 11 . The solder joint of any preceding clause, wherein the tin alloy is a lead- free solder alloy comprising: from 2.7 to 5 wt.% silver; from 0.01 to 4.8 wt.% bismuth; from 0.01 to 2 wt.% copper; from 0.01 to 0.5 wt.% nickel (or from 0.008 to 0.5 wt.% nickel); one or more of: up to 6.8 wt.% antimony (or up to 7 wt.% antimony); up to 6 wt.% indium, up to 0.5 wt.% titanium, up to 0.5 wt.% germanium, up to 0.5 wt.% rare earths, up to 0.5 wt.% cobalt, up to 5.0 wt.% aluminium, up to 5.0 wt.% silicon, up to 0.5 wt.% manganese, up to 0.5 wt.% chromium, up to 0.5 wt.% iron, up to 0.5 wt.% phosphorus, up to 0.5 wt.% gold, up to 1 wt.% gallium, up to 0.5 wt.% tellurium, up to 0.5 wt.% selenium, up to 0.5 wt.% calcium, up to 0.5 wt.% vanadium, up to 0.5 wt.% molybdenum, up to 0.5 wt.% platinum, and up to 0.5 wt.% magnesium; and the balance tin together with any unavoidable impurities.

[0160] 12. The solder joint of clause 11 , wherein the tin alloy comprises from 2.9 to 4.5 wt.% silver, preferably from 2.9 to 3.8 wt.% silver.

[0161] 13. The solder joint of clause 11 or clause 12, wherein the tin alloy comprises from 1 .0 to 4.2 wt.% bismuth, preferably from 2.0 to 4.2 wt.% bismuth, more preferably from 2.5 to 4.1 wt.% bismuth, even more preferably from 2.9 to 4.1 wt.% bismuth.

[0162] 14. The solder joint of any of clauses 11 to 13, wherein the tin alloy comprises from 0.4 to 0.8 wt.% copper.

[0163] 15. The solder joint of any of clauses 11 to 14, wherein the tin alloy comprises from 0.001 to 0.38 wt.% nickel, preferably from 0.01 to 0.28 wt.% nickel, more preferably from 0.02 to 0.18 wt.% nickel.

[0164] 16. The solder joint of clause 11 , wherein the tin alloy is a lead-free solder alloy comprising: from 3.6 to 3.9 wt.% silver, from 2.8 to 3.2 wt.% bismuth, from 0.5 to 0.8 wt.% copper, from 0.008 to 0.02 wt.% nickel, from 0.002 to 0.01 wt.% titanium, and the balance tin together with unavoidable impurities.

[0165] 17. The solder joint of clause 11 , wherein the tin alloy is a lead-free solder alloy comprising: from 3.5 to 3.9 wt.% silver, from 2.9 to 3.2 wt.% bismuth, from 1 to 2 wt.% antimony, from 0.5 to 0.8 wt.% copper, from 0.08 to 0.2 wt.% nickel, and the balance tin together with unavoidable impurities.

[0166] 18. The solder joint of clause 11 , wherein the tin alloy is a lead-free solder alloy comprising: from 2.9 to 3.2 wt.% silver, from 2.9 to 3.2 wt.% bismuth, from 5 to 7 wt.% antimony, from 0.3 to 0.7 wt.% copper, from 0.09 to 0.18 wt.% nickel, from 0.002 to 0.01 wt.% titanium, and the balance tin together with unavoidable impurities.

[0167] 19. The solder joint of clause 11 , wherein the tin alloy is a lead-free solder alloy comprising: from 3.4 to 3.7 wt.% silver, from 2.8 to 3.2 wt.% bismuth, from 0.5 to 0.8 wt.% copper, from 3.8 to 4.2 wt.% antimony, from 0.03 to 0.08 wt.% nickel, from 0.003 to 0.008 wt.% germanium, from 0.01 to 0.04 wt.% silicon, and the balance tin together with unavoidable impurities.

[0168] 20. The solder joint of clause 11 , wherein the tin alloy is a lead-free solder alloy comprising: from 3.3 to 3.7 wt.% silver, from 3.8 to 4.5 wt.% bismuth, from 0.5 to 0.8 wt.% copper, from 5 to 6 wt.% antimony, from 0.05 to 0.1 wt.% nickel, from 0.004 to 0.01 wt.% germanium, and the balance tin together with unavoidable impurities.

[0169] 21 . An electronic device comprising an electronic component connected to a printed circuit board by a solder joint, the solder joint being the solder joint of any preceding clause.

[0170] 22. Use of the solder joint of any of clauses 1 to 20 to improve the thermal cycling characteristics of an electronic device.

[0171] 23. The solder joint of any of claim 1 to 10, wherein the tin alloy is a lead-free solder alloy comprising: from 2.5 to 5 wt.% silver; from 0.01 to 5 wt.% bismuth; from 1 to 7 wt.% antimony; from 0.01 to 2 wt.% copper; one or more of: up to 6 wt.% indium, up to 0.5 wt.% titanium, up to 0.5 wt.% germanium, up to 0.5 wt.% rare earths, up to 0.5 wt.% cobalt, up to 5.0 wt.% aluminium, up to 5.0 wt.% silicon, up to 0.5 wt.% manganese, up to 0.5 wt.% chromium, up to 0.5 wt.% iron, up to 0.5 wt.% phosphorus, up to 0.5 wt.% gold, up to 1 wt.% gallium, up to 0.5 wt.% tellurium, up to 0.5 wt.% selenium, up to 0.5 wt.% calcium, up to 0.5 wt.% vanadium, up to 0.5 wt.% molybdenum, up to 0.5 wt.% platinum, and up to 0.5 wt.% magnesium; optionally up to 0.5 wt.% nickel; and the balance tin together with any unavoidable impurities.

[0172] 24. The solder joint of clause 23, comprising from 2.8 to 4.5 wt.% silver, preferably from 3 to 4 wt.% silver.

[0173] 25. The solder joint of clause 23 or clause 24, comprising from 1.0 to 4.0 wt.% bismuth, preferably from 2.0 to 4.0 wt.% bismuth, more preferably from 2.5 to 4 wt.% bismuth, even more preferably from 2.8 to 4 wt.% bismuth, still even more preferably from 3 to 4 wt.% bismuth.

[0174] 26. The solder joint of any of clauses 23 to 25, comprising from 1 .0 to 6.5 wt.% antimony, preferably from 2 to 6 wt.% antimony, more preferably from 3 to 6 wt.% antimony, even more preferably from 3.1 to 6 wt.% antimony, still even more preferably from 3.2 to 6 wt.% antimony.

[0175] 27. The solder joint of any of clauses 23 to 26, comprising from 0.3 to 1 .2 wt.% copper, and preferably from 0.4 to 0.8 wt.% copper. 28. The solder joint of any of clauses 23 to 27, comprising from 0.001 to 0.4 wt.% nickel, preferably from 0.01 to 0.3 wt.% nickel, more preferably from 0.02 to 0.2 wt.% nickel.

[0176] 29. The solder joint of any of clauses 23 to 28, comprising from 0.001 to 5.5 wt.% indium, preferably from 0.02 to 4 wt.% indium, more preferably from 0.5 to 3 wt.% indium.

[0177] 30. The solder joint of any of clauses 23 to 29, comprising from 0.001 to 0.3 wt.% titanium, preferably from 0.005 to 0.2 wt.% titanium, more preferably from 0.007 to 0.05 wt.% titanium.

[0178] 31 . The solder joint of any of clauses 23 to 30, comprising from 0.001 to 0.3 wt.% germanium, preferably from 0.001 to 0.1 wt.% germanium, more preferably from 0.001 to 0.02 wt.% germanium.

[0179] 32. The solder joint of any of clauses 23 to 31 , comprising from 0.002 to 0.3 wt.% rare earths, preferably from 0.003 to 0.05 wt.% rare earths.

[0180] 33. The solder joint of any of clauses 23 to 32, comprising from 0.01 to 0.2 wt.% cobalt, preferably from 0.01 to 0.2 wt.% cobalt, more preferably from 0.02 to 0.1 wt.% cobalt.

[0181] 34. The solder joint of any of clauses 23 to 33, comprising from 0.001 to 3 wt.%, preferably from 0.005 to 2 wt.% of aluminum, more preferably from 0.01 to 1 .5 wt.% aluminum, even more preferably from 0.015 to 1 wt.% aluminum, still more preferably from 0.02 to 0.08 wt.% aluminum.

[0182] 35. The solder joint of any of clauses 23 to 34, comprising from 0.001 to 3 wt.% silicon, preferably from 0.005 to 2 wt.% of silicon, more preferably from 0.01 to 1.5 wt.% silicon, even more preferably from 0.015 to 1 wt.% silicon, still more preferably from 0.02 to 0.08 wt.% silicon.

[0183] 36. The solder joint of any of clauses 23 to 35, comprising one or more of: from 0.001 to 0.5 wt. % chromium, from 0.01 to 0.5 wt.% of iron, from 0.001 to 0.5 wt.% of phosphorus, from 0.001 to 0.5 wt.% of gold, from 0.2 to 0.8 wt.% of gallium, from 0.001 to 0.5 wt.% of tellurium, from 0.001 to 0.5 wt.% of selenium, from 0.001 to 0.5 wt.% of calcium, from 0.001 to 0.5 wt.% of vanadium, from 0.001 to 0.5 wt.% of molybdenum, from 0.001 to 0.5 wt.% of platinum, and from 0.001 to 0.5 wt.% of magnesium.

[0184] 37. The solder joint of any of clauses 23 to 36, comprising from one to three elements, preferably one or two elements, more preferably two elements selected from nickel, titanium, germanium, indium, manganese, rare earths, cobalt, aluminium, silicon, chromium, iron, phosphorus, gold, gallium, tellurium, selenium, calcium, vanadium, molybdenum, platinum and magnesium, preferably selected from nickel, titanium, germanium, indium, manganese, rare earths, cobalt, silicon, iron and gallium.

[0185] 38. The solder joint of any of clauses 23 to 37, comprising nickel and one of titanium, germanium, indium, manganese, rare earths, cobalt, aluminium, silicon, chromium, iron, phosphorus, gold, gallium, tellurium, selenium, calcium, vanadium, molybdenum, platinum and magnesium, preferably selected from nickel, titanium, germanium, indium, manganese, rare earths, cobalt, silicon, iron and gallium. 39. The solder joint of any of clauses 23 to 38, wherein the wt.% of antimony is greater than the wt.% of bismuth.

[0186] 40. The solder joint of any of clauses 23 to 39, wherein the sum of the wt.% of antimony and the wt.% of bismuth is greater than or equal to 6.5.

[0187] 41 . The solder joint of any of clauses 23 to 40 consisting of: from 2.5 to 4 wt.% silver; from 2.8 to 4.2 wt.% bismuth; from 3.2 to 6.2 wt.% antimony; from 0.4 to 0.8 wt.% copper; from 0.04 to 0.18 wt.% nickel; one of: from 0.007 to 0.05 wt.% titanium, from 0.001 to 0.02 wt.% germanium, and from 0.005 to 0.01 wt.% manganese; and the balance tin together with any unavoidable impurities, wherein: the wt.% of antimony is greater than the wt.% of bismuth, and the sum of the wt.% of antimony and the wt.% of bismuth is greater than or equal to 6.5.

[0188] 42. The solder alloy of any of clauses 1 to 10, wherein the solder alloy comprises: from 3 to 5 wt.% silver; from 0.01 to 0.2 wt.% bismuth; from 4 to 6 wt.% antimony; from 0.3 to 1 wt.% copper; one or more of: up to 6 wt.% indium, up to 0.5 wt.% titanium, up to 0.5 wt.% germanium, up to 0.5 wt.% rare earths, up to 0.5 wt.% cobalt, up to 5.0 wt.% aluminium, up to 5.0 wt.% silicon, up to 0.5 wt.% manganese, up to 0.5 wt.% chromium, up to 0.5 wt.% iron, up to 0.5 wt.% phosphorus, up to 0.5 wt.% gold, up to 1 wt.% gallium, up to 0.5 wt.% tellurium, up to 0.5 wt.% selenium, up to 0.5 wt.% calcium, up to 0.5 wt.% vanadium, up to 0.5 wt.% molybdenum, up to 0.5 wt.% platinum, up to 0.5 wt.% magnesium;and the balance tin together with any unavoidable impurities.

[0189] 43. The solder alloy of clause 23, wherein the alloy consists of from 2.8 to 3.2 wt.% silver, from 2.8 to 3.2 wt.% bismuth, from 4.5 to 5.5 wt.% antimony, from 0.3 to 0.8 wt.% copper, from 0.08 to 0.2 wt.% nickel, 0.001 to 0.01 wt.% of germanium, and the balance tin together with unavoidable impurities.

[0190] 44. The solder alloy of clause 23, wherein the alloy consists of from 2.8 to 3.2 wt.% silver, from 2.8 to 3.2 wt.% bismuth, from 5.5 to 6.5 wt.% antimony, from 0.3 to 0.8 wt.% copper, from 0.08 to 0.2 wt.% nickel, 0.005 to 0.02 wt.% of titanium, and the balance tin together with unavoidable impurities.

[0191] 45. The solder alloy of clause 23, wherein the alloy consists of from 3.1 to 3.7 wt.% silver, from 3 to 3.5 wt.% bismuth, from 3 to 3.8 wt.% antimony, from 0.4 to 0.9 wt.% copper, from 0.01 to 0.9 wt.% nickel, from 0.001 to 0.01 wt.% of germanium, and the balance tin together with unavoidable impurities.

[0192] 46. The solder alloy of clause 23, wherein the alloy consists of from 3.2 to 3.9 wt.% silver, from 3.5 to 4.5 wt.% bismuth, from 5.5 to 6.5 wt.% antimony, from 0.3 to 0.9 wt.% copper, from 0.05 to 0.12 wt.% nickel, 0.001 to 0.01 wt.% of manganese, and the balance tin together with unavoidable impurities. (Alloy 4)

[0193] 47. The solder alloy of clause 23, wherein the alloy consists of from 3.5 to 4.2 wt.% silver, from 0.01 to 0.1 wt.% bismuth, from 5 to 6 wt.% antimony, from 0.4 to 0.9 wt.% copper, from 0.001 to 0.01 wt.% of germanium, from 0.2 to 0.8 wt.%b indium, from 0.02 to 0.08 wt.% cobalt and the balance tin together with unavoidable impurities. The foregoing detailed description has been provided by way of explanation and illustration, and is not intended to limit the scope of the appended claims. Many variations in the presently preferred embodiments illustrated herein will be apparent to one of ordinary skill in the art, and remain within the scope of the appended claims and their equivalents.

Claims

CLAIMS:1 . A solder joint at least partially encapsulated with an encapsulant, wherein the solder joint comprises a tin alloy; the encapsulant comprises silica particles dispersed in an epoxy resin; and the tin alloy comprises: from 2.8 to 4.5 wt.% silver, from 2.8 to 4 wt.% bismuth, from 1 .0 to 6.5 wt.% antimony, from 0.3 to 1 .2 wt.% copper, from 0.001 to 0.4 wt.% nickel, from 0.001 to 0.3 wt.% titanium, and the balance tin together with unavoidable impurities.

2. The solder joint of claim 1 , wherein the tin alloy comprises from 3 to 4 wt.% silver, preferably from 3.1 to 3.5 wt.% silver, more preferably from 3.15 to 3.3 wt.% silver.

3. The solder joint of claim or claim 2, wherein the tin alloy comprises from 3 to 4 wt.% bismuth, preferably from 3 to 3.5 wt.% bismuth, more preferably from 3.1 to 3.3 wt.% bismuth.

4. The solder joint of any of claims 1 to 3, wherein the tin alloy comprises from 2 to 6 wt.% antimony, preferably from 3 to 6 wt.% antimony, more preferably from 3.1 to 6 wt.% antimony, even more preferably from 3.2 to 6 wt.% antimony, still even more preferably from 5.7 to 6 wt.% antimony.

5. The solder joint of any of claims 1 to 4, wherein the tin alloy comprises from 0.4 to 0.8 wt.% copper, preferably from 0.4 to 0.6 wt.% copper, more preferably from 0.45 to 0.55 wt.% copper.

6. The solder joint of any of claims 1 to 5, wherein the tin alloy comprises from 0.01 to 0.3 wt.% nickel, preferably from 0.1 to 0.2 wt.% nickel, more preferably from 0.13 to 0.19 wt.% nickel.

7. The solder joint of any of claims 1 to 6, wherein the tin alloy comprises from 0.005 to 0.2 wt.% titanium, preferably from 0.005 to 0.05 wt.% titanium, more preferably from 0.005 to 0.01 wt.% titanium, even more preferably from 0.005 to 0.008 wt.% titanium.

8. The solder joint of any of claims 1 to 7, wherein the tin alloy consists of from 2.8 to 3.2 wt.% silver, from 2.8 to 3.2 wt.% bismuth, from 5.5 to 6.5 wt.% antimony, from 0.3 to 0.8 wt.% copper, from 0.08 to 0.2 wt.% nickel, 0.005 to 0.02 wt.% of titanium, and the balance tin together with unavoidable impurities.

9. The solder joint of any of claim 1 , wherein the tin alloy consists of 2.9 to 3.2 wt.% silver, 2.9 to 3.2 wt.% bismuth, 5 to 7 wt.% antimony, 0.3 to 0.7 wt.% copper, 0.09 to 0.18 wt.% nickel, 0.002 to 0.01 wt.% titanium, and the balance tin together with unavoidable impurities.

10. The solder joint of claim 1 , wherein the tin alloy comprises: about 3.2 wt.% silver, about 3.1 wt.% bismuth, about 5.9 wt.% antimony, about 0.5 wt.% copper, about 0.16 wt.% nickel, about 0.006 wt.% titanium, and the balance tin together with unavoidable impurities.11 . The solder joint of any preceding claim, wherein the encapsulant is in the form of an underfill or an edgebond.

12. The solder joint of any preceding claim, wherein the ratio by weight of epoxy resin to silica particles in the encapsulant is from 1 :1 to 1 :3 or from 1 :2 to 1 :5.

13. The solder joint of any preceding claim, wherein the encapsulant further comprises a curing accelerator dispersed in the epoxy resin.

14. The solder joint of claim 13, wherein the ratio by weight of epoxy resin to curing accelerator in the encapsulant is from 1 :0.0001 to 1 :0.3 or from 1 :0.0001 to 1 :10.

15. The solder joint of any preceding claim, wherein the silica comprises spherical silica.

16. The solder joint of any preceding claim, wherein the encapsulant consists of, or consists essentially of, the silica particles, the epoxy resin and optionally the curing accelerator.

17. The solder joint of any preceding claim, wherein the solder joint is in the form of a ball grid array (BGA).

18. The solder joint of any preceding claim, wherein the tin alloy: has a solidus temperature of at least 205 °C, preferably at least 208 °C; and / or has a liquidus temperature of 235 °C or less, preferably 233 °C or less; and / or is lead-free.

19. An electronic device comprising an electronic component connected to a printed circuit board by a solder joint, the solder joint being the solder joint of any preceding claim.

20. Use of the solder joint of any of claims 1 to 18 to improve the thermal cycling characteristics of an electronic device.

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