WO2001070449A1

WO2001070449A1 - Soldering flux

Info

Publication number: WO2001070449A1
Application number: PCT/GB2001/001039
Authority: WO
Inventors: Steven James Cooper; Frank Timothy Lawrence
Original assignee: Multicore Solders Limited
Priority date: 2000-03-22
Filing date: 2001-03-09
Publication date: 2001-09-27
Also published as: GB0006989D0; AU3762001A

Abstract

A solid flux formulation comprises a natural resin, copal, optionally co-mixed with a secondary resin, and in which is dissolved one of the various activating chemicals well known to those knowledgeable in the art of soldering. This is deployed especially in cored solder wire during hand soldering. Copal resin may also be applied in other types of flux formulation: flux medium for reflow soldering and liquid flux for wave soldering.

Description

SOLDERING FLUX

The present invention relates to resin based flux formulations and to their use in soldering operations, especially in the electronics manufacturing industry.

The formation of a soldered joint demands pre-treatment of the component parts to remove oxide/tarnish and expose the underlying metal. This is achieved with a fluxing material. A widespread and popular soldering method ("hand" soldering) is to apply the solder alloy in the form of a flux cored wire. This is melted with the aid of a soldering iron, and the flux released from cores in the wire facilitates wetting of the component parts by the alloy. The type of flux used for this purpose is usually one that is solid at room temperature. This confers two benefits: there will be no tendency for the flux to seep out of the cored wire during storage; flux residue that remains on the soldered joint will be in the form of a dry and robust coating that will afford protection against subsequent exposure to adverse environmental conditions. The flux component that has traditionally been used in formulations to satisfy these needs is rosin or modified rosin.

Rosin is a natural product that occurs in the oleoresin of pine trees. World production is around one million tonnes per year, the main sources being China (40%) and the USA (25%) . Rosin has a wide variety of uses ranging from ship building to paper manufacture, adhesives to inks. Although the use of rosin as a flux to facilitate metal joining is a relatively minor consumer in terms of volume, it plays a significant role in the electronics manufacturing industry. Rosin is central to the manufacture of a wide range of consumer electronic products . It is only fairly recently that soldering culture has formally dispensed with rosin based nomenclature such as the "R", "RMA" and "RA" activity designations although, even then, they are still widely used.

In addition to having an ideal softening temperature (around 65-85°C) , rosin exhibits other physical and chemical properties that favour good soldering. It comprises a mixture of resin acids with a small amount of other (non-acidic) components. The resin acids react with metal salts which constitute the tarnish on a metal (eg copper) substrate to leave a clean metallic surface which is readily wetted by molten solder alloy. Tin and lead salts react in a similar way ensuring a correspondingly clean solder alloy surface that complements the wett'ing process. Substrate and solder alloy are maintained in this clean condition throughout the soldering process as a result of the action of resin acids which is as follows:

RCOOH + MX → RCOOM + HX where M = Sn, Pb, Cu X = oxide, hydroxide, carbonate RCOOH = resin acid

Rosin based soldering fluxes have evolved from this simple chemistry and now incorporate a range of additional components such as amine hydrohalides and carboxylic acids which enhance "activation" potential.

The following table summarises the attractive physical and chemical properties of rosin: Property Advantage Naturally acidic Removes metal oxides/salts from substrate/solder alloy, and maintains cleanliness

Shields the reaction products from the soldering process

Solubilises other flux Usually necessary for activators providing sufficient

(eg carboxylic acids, fluxing activity amine hydrohalides)

Softening point 65-85°C Ideal for the manufacture of flux cored solder wire

Forms a viscous fluid at soldering temperature, and solidifies on cooling to form a protective coating around the soldered joint

Poor water solubility Withstands post-soldering corrosion/humidity ageing

High electrical resistance Insulates adjacent conductors

Unfortunately, there are disadvantages too. Some disadvantages resulting from the chemical nature of the resin acids, can be addressed through various chemical treatments, many involving the conjugated double bond system in the especially abundant abietic type acids such as abietic acid itself:

CH₃ COOH

Abietic acid

A propensity to oxidation, which has a darkening effect and which impairs solubility and fluxing ability, is caused by the interaction of the conjugated double bond system with atmospheric oxygen. This can be minimised in many ways: hydrogenation to fully saturate the double bond system; dehydrogenation, to increase unsaturation, leading to a more stable aromatic fused ring system; and disproportionation, in which hydrogen is transferred to yield a single double bond in one molecule, and an aromatic system in a "partner" molecule. A further reaction that can exert an inhibiting effect upon abietic type acid oxidation is polymerisation, again, involving the conjugated double bond system. The dimerised rosin which results exhibits a higher softening point by virtue of increased molecular weight, and less crystallisation tendency. As before, resistance to oxidation is enhanced by the loss of conjugation. A further reaction which exploits the conjugated double bond system is Diels Adler addition of, for example, maleic anhydride. This increases softening point, and acid functionality. Subsequent esterification, with glycerol, for example, yields a^' robust non-oxidising resin ideal as a protective coating. Finally, esterification of the acid group, with the conjugated double bond system left intact, produces liquid rosin esters that can be used as tackifiers in, for example, solder paste; these modified rosins have only low acid value .

In addition to potential shortcomings in terms of physical properties and chemistry, rosin based fluxes have long been associated with various occupational illnesses such as skin sensitization and, especially, asthma. This deleterious effect is caused by the presence of certain resin acids in rosin based flux fumes, although exposure limits have traditionally been based on the products of resin acid decomposition, mainly aldehydes measured as formaldehyde . After a recent study, the UK Health and Safety Executive (HSE) has introduced a new Maximum Exposure Limit (MEL) for the total resin acids, superseding reliance on formaldehyde as a "marker" . Modified/derivatised rosins, such as those indicated in the preceding paragraph, exhibit an equal propensity to resin acid evolution and are, therefore, treated in the same way with regard to setting exposure limits. Implementation will necessitate a full re-evaluation of COSHH assessments by users of rosin based products within the UK. This is emphasised in HSE leaflets and other recent publications. It is probable that a similar philosophy will spread to the rest of Europe. Without acceptable extraction, fumes are especially abundant during hand soldering.

In addition to hand soldering, there are two mass production soldering techniques prevalent in the electronics manufacturing industry. In reflow soldering a cream of flux medium, which includes a low volatility solvent component, and solder alloy powder is applied to a printed circuit board (PCB) prior to the application of heat. In wave soldering, the PCB is prepared by the application of liquid flux comprising "activating" chemicals in a volatile carrier solvent prior to contact with molten solder. Although rosinous fluxes may be deployed in both techniques, they pose a different exposure risk to operators because they are usually fully automated processes conducted in equipment that extracts fumes away from the work environment. However, even if exposure to fumes is minimised, there is still the possibility of skin contact (for example by handling the soldered joint) that could lead to rosiri-mediated skin sensitization.

According to one aspect of the present invention, there is provided a soldering flux composition for use in manufacturing of electrical and electronic components and in general soft soldering applications, which composition comprises copal resin and an activating agent. Typical of general soft soldering applications are engineering applications in contrast to electronics applications.

In a second aspect, this invention provides a process for the manufacture of electrical and electronic components by hand soldering, by wave soldering or by reflow soldering, in which there is carried out a step of applying copal resin as such or a flux composition containing copal resin to a surface at which soldering is to take place.

In a third aspect, this invention provides a soft soldering process in general engineering, in which there is carried out a step of applying copal resin as such or a flux composition containing copal resin to a surface at which soldering is to take place.

The copal resin can be used as sole flux vehicle or it can be accompanied by a synthetic resin component and/or a second natural resin component. The flux vehicle, as such, can act as a solid flux for a cored solder body, e.g. a wire, the copal resin being an alternative natural resin to any hitherto used that confers all of the application benefits associated with rosin. Copal resins, in general, _.however, have not been linked with adverse health and safety issues and will, therefore, not be subject to the same level of legislative control.

Copal (also known as Manila) resins are the exudations from Agathis trees of the botanical family Araucariaceae . They are obtained by tapping of the tree in which the bark is cut off horizontally down to the wood. The wood underneath is then cleaned to provide a surface upon which the resin collects. After every collection of resin a thin slice of bark is cut off to provide a fresh opening for the resin to flow along. Production is predominately in Indonesia, Papua and the Philippines. Like rosin, copal resins are naturally acidic, having an acid value of around 120- 140 mg KOH g^"1, and they are soluble in a wide range of organic solvents, especially alcohols. Their softening point lies in the range 80-135°C. They have many applications, for example in spirit varnishes for wood, insulation and textile impregnation, mirror backing, book binders' varnish, gasket and linoleum cements, overprint varnish for labels and cartons, and metal primers. In the form of aqueous dispersions, copal resins are used in inks, for paper sizing and for stiffening fibre- and paper-board. When mixed with other natural resins and waxes they can be used as floor and motor polishes. They feature in pressure and optical adhesives, and in dentistry as modelling compounds and cavity varnishes. Significantly, with a view to the impact of fume exposure, copal resins are used in fireworks, both as combustible matter and binder (see Chemistry in Industry, 20^th December 1999) , and they have been burned as incense for thousands of years .

Specific components of copal resin are believed to be:

Mancopalinic (C₈H₁₂0₂) acid a complex, monobasic acid containing two double bonds.

Mancopalenie (C₁₀H₁₄O₂) acid a complex, monobasic acid containing three double bonds.

Mancopalolic (C₁₀H₁₆O₂) acid a complex, monobasic acid containing one double bond.

Mancopalresene (C₂₀H₃₂O) an inert substance possibly formed by the reactions between the above acids .

Recent research indicates the presence also of a dibasic acid, and that one of the monobasic acids contains a hydroxyl group .

The same formulation philosophy can be^' used with copal resin as with rosin or modified rosin which it replaces insofar as other forms of fluxing materials are concerned, e.g. flux medium for reflow soldering, in which case it will be applied in the form of a paste containing solder alloy powder, and liquid flux for wave soldering. In both cases, formulation is favoured by good solubility (similar to that exhibited by rosin and derivatised rosin) in flux solvents such as C₂_₅ alkanols. Thus copal resin can be used in solder creams and liquid fluxes for reflow soldering and wave soldering respectively. Moreover, copal resin that has been modified using the various techniques that have traditionally been applied to improve the physical and chemical properties of rosin in each of the three aforementioned types of fluxing material can also be used.

Usually soldering flux compositions embodying this invention will contain an activating agent. This may be a .carboxylic acid, usually a long chain (16-20) monocarboxylic acid such as stearic of palmitic acid, or a dicarboxylic acid, preferably in the range of C₂- C₁₀, or even a polycarboxylic acid. In addition to, or as an alternative to, an acid activator, there may be employed an amine hydrohalide activator such as triethylamine hydrochloride . Activating agent (s) may constitute from 0.1 to 90%, preferably from 0.1 to 50% and most preferably 0.1 to 25% by weight of the total flux composition.

It is also sometimes beneficial to incorporate a second resinous component to modify physical and/or chemical properties such as melting point and melt viscosity. This may be a natural or synthetic resin. Polyamide resin is particularly attractive. Such second resinous component may be present at a concentration of 0-90% by weight of the total flux composition, preferably 20- 80%.

When copal resin is not to be employed alone, it may also be comprised by a flux composition which may be liquid, containing a solvent which is preferably a C₂_₅ -alkanol, a solid flux, or a flux medium. The copal resin may constitute from 0.01 to 99% by weight of a flux composition depending on whether it is liquid and/or whether any activating agent is present. With liquid flux compositions, the copal resin will typically be present in an amount of from 0.01 to 80% by weight, preferably 0.01 to 40% by weight.

The following example formulations demonstrate the ability of copal resin to act as a base for soldering fluxes .

Example 1

Initial tests were conducted with copal resin as supplied (i.e. with no added activators)

Copal resin 100%

Example 2 An acid activated solid flux was prepared according to the following formulation by melting the components in a beaker on a hot plate and stirring.

Copal resin 85.3% Adipic acid 12.7%

Stearic acid 2.0%

Example 3

A halide and acid activated solid flux was. prepared according to the following formulation by melting the components in a beaker on a hot plate and stirring:

Copal resin 94%

Stearic acid 4% Triethylamine hydrochloride 2%

Example 4

An acid activated solid flux which contains a second (non-rosinous) resin component was prepared according to the following formulation by melting the components in a beaker on a hot plate and stirring: Copal resin 40%

Adipic acid 10%

Stearic acid 10%

Polyamide resin 40%

The efficacy of all four formulations was assessed by using a soldering iron to melt solder wire (63:37 Sn:Pb) onto a copper substrate upon which each had been pre-deposited. In all cases the solder alloy wetted the substrate, forming a robust union. The result with Example 1 confirms fluxing action with copal resin. Examples 2 to 4 indicate a formulating capability similar to that achievable with traditional rosin based technology. Thus, the solder fluxes were suitable for use in soft soldering in general engineering as well as in the manufacture of electrical or electronic components.

Claims

1. A soldering flux composition for use in manufacturing of electrical and electronic components and in general soldering applications, which composition comprises copal resin. and an activating agent .

2. The soldering flux composition in accordance with claim 1, of which the copal resin constitutes from 0.01 to 99% by weight.

3. The soldering flux of claim 2, of which the copal resin constitutes from 0.01 to 80% by weight.

4. The soldering flux of claim 3, of which the copal resin constitutes from 0.01 to 40% by weight.

5. The soldering flux composition in accordance with claim 1 or 2, which additionally contains a natural or synthetic second resin component .

6. The soldering flux composition in accordance with claim 5, wherein up to 90%, preferably from 20 to 80%, total composition of said soldering flux is constituted by the second resin component .

7. The soldering flux composition in accordance with claim 5 or 6, wherein said second resin component is a polyamide resin.

8. The soldering flux composition in accordance with any preceding claim, wherein said activating agent comprises at least one mono-, di- or poly-carboxylic acid and/or an amine hydrohalide .

9. The soldering flux composition in accordance with claim 1 or 8, wherein from 0.1 to 90% by weight of the total composition of said soldering flux is constituted by activating agent (s), subject to the weight percentages of copal resin, activating agent and second resin component not exceeding 100%.

10. The soldering flux composition in accordance with claim 9, wherein from 0.1 to 25% by weight of the total composition of said soldering flux is constituted by activating agent (s), subject to the weight percentages of copal resin, activating agent and second resin component not exceeding 100%.

11. The soldering flux composition in accordance with any preceding claim, in which said flux additionally includes a solvent system.

12. The soldering flux composition in accordance with claim 10, wherein the solvent system comprises at least one alkanol having 2 to 5 carbon atoms .

13. The soldering flux composition in accordance with ^' any of claims 1 to 10, which constitutes a -core of a cored solder body.

14. The soldering flux composition in accordance with any of claims 1 to 11, in the form of a paste in which it is admixed with solder alloy powder.

15. A process for the manufacture of electrical and electronic components by hand soldering, by wave soldering or by reflow soldering, in which there is carried out a step of applying copal resin as such or a flux composition containing copal resin to a surface at which soldering is to take place.

16. A soft soldering process in general engineering, in which there is carried out a step of applying copal resin as such or a flux composition containing copal resin to a surface at which soldering is to take place.

17. A process in accordance with. claim 15 or 16, wherein there is applied a flux according to any one of claims 1 to 14.