WO2011023394A1

WO2011023394A1 - Solder alloy

Info

Publication number: WO2011023394A1
Application number: PCT/EP2010/005264
Authority: WO
Inventors: Olivier Hutin
Original assignee: Umicore Ag & Co. Kg
Priority date: 2009-08-29
Filing date: 2010-08-27
Publication date: 2011-03-03
Also published as: DE202009019184U1; DE102009039355A1; TW201124223A

Abstract

The present invention relates to a novel lead-free solder alloy and to the use thereof The soft solder having a low degree of leaching of copper, is forming a barrier layer, during the soldering operation, at the interface between the solder and the soldering material and thus preventing the leaching of copper.The solder alloy consists of 2.5 to 3.5% by weight silver, 0.4 to 0.6% by weight copper, 0.06 to 0.17% by weight cobalt and ad 100% by weight tin, as well as unavoidable impurities.

Description

Solder alloy

Soldering is an important technique for producing components both in series production and for workpiece prototyping.

Soldering is an economic procedure, which subjects the materials to low levels of stress, for joining metallic components by means of a metallic additive (the solder), optionally using fluxes and/or shielding gas. The melting temperature of the solder is lower than the melting temperatures of the metals which are to be joined and are wetted by the solder without being melted.

Solder materials are classified into soft solders and hard solders depending on the working temperatures thereof, solders having working temperatures of more than 450⁰C being designated as hard solders and those having working temperatures of less than 45O⁰C being designated as soft solders. By way of example, soft solders are used in electronic applications, since excessively high temperatures may damage the components used.

Highly miniaturized circuits subject to severe loading are needed not only in special applications, such as satellite and space technology, but also for technologies where failure may cause serious accidents or expensive dam- age, e.g. automotive technologies.

The detachment of just one soldered location from electronic circuits or piezo actuators in injection pumps can lead to failure of a subassembly and cause major damage. One cause of such incidents may be a lack of wetting by the solder. Another cause of such incidents is the leaching of copper from wires or conductor tracks on electrical and electronic components.

Conventional tin-lead solders are known for their outstanding wetting properties. Lead-containing solders were often used in the past. Owing to legal specifications and technical requirements, there is a need for new, lead-free solders. JP-B-3027441 describes a solder alloy consisting of more than 3% by weight and less than 5% by weight silver, 0.5 to 3% by weight copper and ad 100% by weight tin.

A disadvantage of this solder alloy is the high degree of leaching of copper when copper-containing surfaces, such as conductor tracks or wires, are soldered.

WO 2004/096484 addresses this problem with a solder alloy consisting of 10% by weight or less silver, 10% by weight or less bismuth, 10% by weight or less antimony, 3% by weight or less copper, 1% by weight or less nickel and ad 100% by weight tin. The complex composition and the high content of precious and semi-precious metals are disadvantageous. Apart from that, antimony also has a certain degree of toxicity. The object was therefore to provide a solder which is easy to produce, has a low degree of leaching of copper, comprises no or little lead and is therefore ecologically harmless.

This object is achieved by a solder alloy consisting of 2.5 to 3.5% by weight silver, 0.4 to 0.6% by weight copper, 0.06 to 0.17% by weight cobalt and ad 100% by weight tin, as well as unavoidable impurities.

The solder alloy of the invention can also comprise 2.7 to 3.3% by weight, or 2.8 to 3.1% by weight of silver.

The solder alloy of the invention can also comprise 0.45 to 0.55% by weight of copper.

The solder alloy of the invention can also comprise 0.08 to 0.14% by weight, or 0.09 to 0.13% by weight, cobalt.

The solder alloy of the invention comprises ad 100% by weight tin, i.e. in addition to the amounts of silver, copper and cobalt used, tin is used in such an amount that the amounts add up in total to 100% by weight.

In addition to the essential alloying constituents mentioned above, the solder alloy of the invention can also comprise other elements in traces or as unavoidable impurities, if these do not influence the properties of the solder alloy according to the invention. Substances of this type may be present in amounts of up to 0.01% by weight.

A solder alloy consisting of 2.9% by weight silver, 0.55% by weight copper, 0.12% by weight cobalt and ad 100% by weight tin, as well as unavoidable impurities, is particularly advantageous.

A solder alloy consisting of 3% by weight silver, 0.5% by weight copper, 0.1% by weight cobalt and ad 100% by weight tin, as well as unavoidable impurities, is very particularly advantageous.

It has surprisingly been found that merely a minor change in the silver content and the addition of cobalt as further alloying constituent significantly improve the leaching properties in relation to copper. This can be explained by the fact that, during soldering, the cobalt forms at the interface to the copper and thus forms a barrier layer of cobalt. This layer was found during investigations using wavelength dispersive X-ray analysis (WDX).

The present invention therefore also relates to a soft solder having a low degree of leaching of copper, this solder forming a barrier layer, during the soldering operation, at the interface between the solder and the soldering material and thus preventing the leaching of copper. In principle, the solder alloys according to the invention can be present in any desired form, for example as powder or shaped articles, such as wire, bars, strips, discs, films, granules, balls, stamped parts, paste or combinations thereof.

These can advantageously be used for soldering copper-containing surfaces, in particular copper-containing surfaces of conductor tracks or surfaces of piezo actuators.

The solder alloys according to the invention are particularly suitable for use in the form of solder strip in the large-scale industrial manufacture of electrical components, such as copper-containing piezo actuators. The present invention also relates to a method of soldering comprising the steps of:

- providing two workpieces to be connected,

- providing a solder alloy according to one or more of Claims 1 to 4, - applying a heat treatment to at least one of the workpieces or the solder alloy, and

- cooling the at least one of the workpieces or the solder alloy.

The invention thus also relates to solder joints containing the solder alloy of the invention. Such a solder joint comprises a first workpiece, a second workpiece and a solder alloy of the invention arranged at least partially between the first and the second workpiece. Preferably, at least one of the workpieces exhibits a surface at least partially consisting of, or being covered with copper, but it is also possible that both the first and the second workpiece contains a surface that is at least partially covered with copper or made of copper. In one specific embodiment, the first workpiece is a piezo actuator, which preferably exhibits at least one surface made of copper. The second workpiece in this embodiment comprises an electrically conductive connection. The solder alloy of the invention may be provided in any suitable form, like a strip, paste, disc wire etc. as described above. The heat treatment is suitable to melt the solder alloy and is usually a heat treatment up to about 270⁰C for a time sufficient to effect melting of the solder alloy and wetting the workpieces to be connected. The cooling step will be sufficient for the solder alloy to solidify and the solder joint to be formed. The cooling will be provided to a temperature below the melting temperature of the solder alloy of the invention.

In addition, the solder alloy can also be used in the form of finely dispersed particles by compounding with a flux and/or other constituents to produce solder pastes comprising the solder alloy according to the invention and fluxes and also, if appropriate, further constituents and auxiliaries. Further constituents may contain, but are not limited to, solvents (which may be water soluble or water insoluble), thixotropic agents, activators, tensides, plasticizers or mixtures thereof. A solder paste may e.g. comprise 30 to 90 wt.-% finely dispersed solder alloy and 70 to 10 wt.-% of at least one binder, or 75 to 98 wt.-% finely dispersed solder alloy and 25 to 2 wt. % of at least one binder, in particular 80 to 95 wt. % finely dispersed solder alloy and 20 to 5 wt.-% of at least one binder. The finely dispersed solder alloy may have, but is not limited to, particle size distributions of 10 μm to 500 μm, or 50 μm to 150 μm, or 5 μm to 50 μm, in particular 5 μm and 30 μm, wherein 55% to 70%, in particular 60% to 65%, of the particles exhibit a particle size smaller than the particle size of the maximum of the particle size distribution. The density of the finely dispersed solder alloy usually is from about 0.1 to 20 g/cm³.

A specific composition is, for example, a solder paste consisting of 30 to 90 wt. -% finely dispersed solder alloy of the invention, 0.1 to 20 wt.-% of a binder, optionally 0 to 10 wt.-% of a flux and/or 1 to 50 wt.-% of a solvent.

Suitable fluxes may comprise halogen compounds, such as ammonium chloride, zinc chloride, complex fluorides such as sodium hydrogendifluoride or potassium hydrogendifluoride, boron, boron compounds such as alkali borates or fluoroborates like sodium borate, sodium fluoroborate, potassium borate, potassium fluoroborate, phosphoric acid or its derivatives, a rosin and mixtures thereof. Suitable rosins may be, for example, tall oil rosin, hydrogenated rosin, ethoxylated amine rosin, amine rosin, methyl ester of rosin, n-oleylsarcosine and oleyl imidazoline or mixtures thereof.

The solder paste may also comprise activators, wherein the activator may comprise, but is not limited to, organic acids selected from the group con- sisting of caproic acid, phenyl-acetic acid, benzoic acid, salicylic acid, ami- nobenzoic acid, 4-n-butylbenzoic acid, 4-t-butylbenzoic acid, 3,4- dimethoxybenzoic acid, oxalic acid, succinic acid, maleic acid, malic acid, adipic acid, malonic acid and mixtures thereof. The activator may further comprises an amine, said amine may be selected from the group consisting of monoethanolamine, diethanolamine, triethanolamine, isopropanolamine or a combination thereof. In addition or instead of the flux, the solder paste may further comprise at least one binder and/or at least one solvent. The binder to be used can be, for example, an organic resin, in particular an organic resin being a synthetic hydrocarbon resin exhibiting an acid number (acid content) of 200 mg KOH (potassium hydroxide) per gram resin or less, in particular 100 mg KOH/g resin or less, more specificyally 50 mg KOH (potassium hydroxide) per gram resin or less. The softening temperature is preferably 200⁰C or less, in particular between 50⁰C and 150⁰C. In a specific embodiment, the organic resin is a saturated aliphatic hydrocarbon resin, particularly suitable for this purpose is polyisobutene, in particular with a relative molar mass from 50,000 to 500,000, in particular 60,000 to 90,000.

In one specific embodiment, the solder paste comprises as a binder a mixture of polyisobutene with a relative molar mass from 50,000 to 500,000 and paraffin with a melting-range from 40⁰C to 90⁰C, more specifically a mixture of polyisobutene with a relative molar mass from 60,000 to 90,000 and paraffin with a melting-range from 40⁰C to 60⁰C. In this case, the binder preferably consists 20 to 70 wt.-% polyisobutene and 80 to 30 wt.-% paraffin, in particular 30 to 50 wt. % polyisobutene and 70 to 50 wt. % paraffin. Suitable solvents include, but are not limited to, glycol ethers and/ or alcohols. Suitable glycol ethers are selected from the group consisting of mono-, di- or tri-propylene glycol methyl ether, mono-, di-, or tri-propylene glycol n-butyl ether, mono-, di-, or tri-ethylene glycol n-butyl ether, ethylene glycol methyl ether, tri-ethylene glycol methyl ether, di-ethylene glycol di- butyl ether, tetra-ethylene glycol di-methyl ether, or a combination thereof. Suitable alcohols are selected from the group consisting of 2-ethyl-l,3- hexanediol, n-decyl alcohol, 2-methyl-2,4- pentanediol, terpineol, isopropa- nol, ethylene glycol, propylene glycole, diethylene glycole, triethylene gly- cole, propylene glycole, hexylene glycole or mixtures thereof.

If a solvent having a high viscosity is selected, a binder might not be required, depending on the specific application. Such highly viscous solvents are for example selected from the group consisting of trimethylopropane, 1,2- octanediole, 1,8-octanediol, 2,5-dimethyl-2,5-hexanediole, isobornyl cyclohexanole or a combination thereof.

The solder paste may further comprise a plasticizer, in particular when a rosin is contained. The plasticizer can be selected from the group consisting of dimethyl phthalate, dibutyl phthalate, diisodecyl phthalate, butyl oleate, diisobutyl adipate, dioctyl adipate, dipropylene glycol dibenzoate, dioctyl sebacate and mixtures thereof. The solder paste may further comprise thixotropic agents, which can be present in amounts of from 0.01 wt.-% to 10 wt.-%, or 0.02 wt.-% to 0.1 wt. -% or 1 wt. -% to 6 wt.-%, depending on the desired properties, viscosities and the solvents and binders present. Suitable thixotropic agents may comprise fatty acids and/or ammonium salts thereof, in particular lauric acid, myristic acid, palmitic acid, stearic acid, their ammonium salts, glyceryl tris-12-hydroxy stearate, modified glyceryl tris- 12-hydroxy stearate, polyamide, stearamide and mixtures thereof.

In another embodiment of this aspect of the invention, the solder paste comprises finely dispersed solder alloy of the invention and a binder/flux mixture comprising water soluble salts of fatty acid amines having 8 to 22 carbon atoms with carboxylic acids and/or hydroxycarboxylic acids having 2 to 8 carbon atoms, which exhibit an acid number (acid content) of 250 mg KOH (potassium hydroxide) per gram or less, in particular 100 mg KOH/g or less, or from about 100 to about 200 mg KOH/g. Melting points of these binder/flux mistures are from about 25°C to about 125°C. Acetates or lactates of the fatty acid amines are specific embodiments. Examples

Production of the solder alloy

All alloying elements are used in a purity of 4N, i.e. they have an impurity content of less than 0.01% by weight. The components were weighed out in batches of 400 g and melted in a chamber furnace under an argon atmosphere at 1500⁰C. The melt was cooled and cast at a temperature of about 45O⁰C in a cold steel mould to form billets (40 mm x 10 mm x 110 mm), and then cold-rolled to form strip having a thickness of 1.5 mm.

The leaching properties are determined by analysis of the solder microstruc- ture after the soldering operation. The enrichment of the solder with copper is triggered by the leaching properties. An investigation of the copper element distribution above the diffusion zone in the solder makes it possible to quantitatively assess the amount of copper which has diffused in from the copper substrate, and this was carried out by wavelength dispersive X-ray analysis (WDX).

Example 1

A solder alloy was produced, as described above, from 3% by weight silver, 0.5% by weight copper, 0.1% by weight cobalt and ad 100% by weight tin.

Comparative example 2

A solder alloy was produced, as described above, from 3% by weight silver,

0.5% by weight copper and ad 100% by weight tin.

For comparison, Figure 1 shows the distribution of the Cu content (determined by WDX) in the solder region above the diffusion zone after soldering at 270⁰C peak temperature and a retention time of 60 seconds. This is a worst-case scenario in terms of the copper diffusion since the thermal load- ing is higher than in many applications. Even under such challenging conditions, the solder according to the example of the invention

(SnAg3Cu0.5Co0.1) absorbed less copper than the comparative example (SnAg3Cu0.5). The amounts of copper are given in % by weight in the measuring field, each bar indicating an average value from 4 measuring fields in each case. Figure 1 shows the surprising result that merely a minor change in the silver content and the addition of cobalt as further alloying constituent significantly improve the leaching properties in relation to copper. The results are summarized in Table 1 below.

Figure 2 shows the cobalt distribution at the interface between solder and copper substrate, as determined by WDX. The structure of the cobalt layer, which prevents the copper from migrating into the solder, can clearly be seen. Table 1 :

Example 3

Solder-paste composition:

90.0 wt. -% solder powder made of the alloy of Example 1, oxygen content:

300 ppm), 6.0 wt. -% paraffin, melting-range 42-44°C, 4.0 wt.-% polyiso- butene, molar mass 60,000 (Oppanol(R) B12, BASF AG)

Viscosity (70⁰C, Brookfield RVT): 50 Pa. s.

Example 4

Solder composition:

89.0 wt. -% solder powder made of the alloy of Example 1 (oxygen content: 300 ppm), 7.5 wt.-% hard paraffin, melting-point 90⁰C, 3.5 wt.-% polyiso- butene, molar mass 85,000 (Oppanol(R) B15, BASF AG)

Viscosity (70⁰C, Brookfield RVT): 35 Pa. s.

Claims

Patent Claims

1. A soft solder having a low degree of leaching of copper, this solder forming a barrier layer, during the soldering operation, at the interface between the solder and the soldering material and thus preventing the leaching of copper.

2. Solder alloy according to Claim 1 consisting of 2.5 to 3.5% by weight silver, 0.4 to 0.6% by weight copper, 0.06 to 0.17% by weight cobalt and ad 100% by weight tin, as well as unavoidable impurities.

3. Solder alloy according to Claim 2 comprising 2.7 to 3.3% by weight silver.

4. Solder alloy according to Claim 2 or 3 comprising 0.45 to 0.55% by weight copper.

5. Solder alloy according to one or more of Claims 1 to 4 comprising 0.08 to 0.14% by weight cobalt.

6. Soldered joint comprising a solder alloy according to one or more of Claims 1 to 5.

7. Solder paste comprising particles of a solder alloy according to one or more of Claims 1 to 5 and at least one further constituent being selected from the group of fluxes, solvents, activators, binders, thixotropic agents, tensides, plasticizers and mixtures thereof.

8. Shaped article of a solder alloy according to any of claims 1 to 5.

9. Shaped article of claim 8 being a solder strip.

10. Use of a solder alloy according to one or more of Claims 1 to 5, of a solder strip according to Claim 9 or of a solder paste according to claim 7 for soldering copper-containing surfaces.

11. Use according to Claim 10, the copper-containing surfaces being conductor tracks or surfaces of piezo actuators.

12. Method of soldering comprising the steps of:

- providing two workpieces to be connected,

- providing a solder alloy according to one or more of Claims 1 to 5,

- applying a heat treatment to at least one of the workpieces or the solder alloy, and

- cooling the at least one of the workpieces or the solder alloy.