Solution and process for increasing the solderability and corrosion resistance of metal or metal alloy surface

Field of the Disclosure

The invention relates to a solution comprising a phosphorous compound and a solderability-enhancing compound and its use in a process for increasing the solderability and corrosion resistance of a metal or metal alloy surface.

Background of the Invention

Metal coatings, particularly of tin or its alloys are frequently deposited onto surfaces of copper- and nickel-based workpieces in order to prevent the copper or nickel-based surface from oxidizing or tarnishing and to enhance solderability.

Under conditions such as elevated temperatures in air or in other oxidizing atmospheres, for example tin or other metal-coated surfaces of electronic lead frames and electrical connectors have a tendency to form oxide films during periods of shipment and storage between manufacture and assembly into electronic devices. The oxide coats discolor the surface of the tin-coated surface and impart a yellowish color which many consumers consider unacceptable. Furthermore, the oxide may degrade the contact resistance of a coated electrical terminal. A tarnish-free surface has lower electrical contact resistance and better solderability than an oxide coated surface.

Tin-based layers are also used in lead finishes for integrated circuit ("IC") fabrication. A thin layer of tin or tin alloy is applied as the final step for passive components such as capacitors and transistors.

There are many factors that determine good solderability, the three most impor- tant of which are extent of surface oxide formation (corrosion), amount of co- deposited carbon, and extent of intermetallic compound formation. Surface oxide formation is a natural occurring process because it is thermodynamically favorable. The rate of formation of the surface oxide depends on the temperature and time. The higher the temperature and the longer the exposure time, the thicker the surface oxide formed. In electroplated tin or tin alloy coatings or deposits, surface oxide formation also depends on the surface morphology of the coating or deposit. When comparing pure tin to tin alloy coatings, for example, tin alloys generally form less or thinner surface oxides when all other conditions are equal.

Generally, it is the aim to produce a tarnish-free surface, which has lower electrical contact resistance and better solderability than an oxide coated surface.

Fuchs, et al., in U.S. Pat. No. 5,853,797 disclose a method and solution for providing corrosion protection of coated electrical contact surfaces which involve exposure of such surfaces to a solution containing phosphonates, lubricants and various volatile organic solvents. Evaporation of such solvents for disposal is fraught with environmental concerns such as handling, hazard to workers, and disposal of waste into streams.

Fan, et al., in U.S. Pat. Appl. No. 2005/0268991 A1 disclose a method for enhancing corrosion resistance of a tin-based surface on a workpiece involving contacting the tin-based surface with a composition comprising a phosphonic acid compound and water to form a phosphorus-based film over the tin-based coating thereby inhibiting corrosion of the tin-based surface. Phosphonic acid containing compositions having a concentration up to about 30 vol. % of an or-

ganic solvent, and water. However, the use of organic solvents is disadvantageous since they are volatile under process conditions and often hazardous.

Lau, et al., in U.S. Pat. Appl. No. 2006/0237097 A1 disclose a method for inhibiting corrosion of metals and metal alloys by treating them with compositions containing inorganic and organic phosphoric acids like alkylphosphoric acids that prevent oxide formation on the metals and metal alloys.

EP 0 009 247 B relates to the use of alkyl monophosphonic acid containing from 8 to 10 carbon atoms in admixture with surfactants as corrosion inhibitors against metals in acid solutions. The metals are selected from the group con- sisting of aluminum and aluminum alloys, chromium-nickel-steal, ordinary steal, brass and copper.

EP 1 221 497 A2 relates to a method for inhibiting the formation of stains, especially water stains, on the exterior surfaces of aluminum alloy products. The method entails contacting the exterior surfaces of these products, particularly sheet or plate products, extrusions and/or forgings made from 5000 to 6000 Series aluminum alloys, with an organophosphonic or organophosphinic acid- derived material. Preferably, liquid forms of this material are added to an alcohol or water-based carrier solution, then sprayed, dipped, painted or rolled onto the surfaces of flat sheet or plate products to enhance their brightness.

GB 2 331 942 A relates to a composition comprising at least one organo phos- phonate or organo phosphonate species used for the treatment of a variety of metal surfaces to inhibit corrosion and improve adherence of coatings. The composition can include a homopolymer or copolymer of an organo phosphonate or organo phosphonate species such as vinylphosphonic acid, vinylidene- 1 ,1-diphosphonic acid or phenyl vinyl phosphonic acid.

US patent No. 3,630,790 describes a method of protecting metal surfaces from corrosion which comprises contacting the metal with an organo phosphonic, phosphonous or phosphinic acid.

-A-

Although the above mentioned methods are all directed to treatment of metal and metal alloy surfaces in order to preserve their quality regarding solderability and appearance, there is still a need for an environmentally friendly process offering improved corrosion resistance as well as whisker suppression.

Summary of the Disclosure

The present invention relates to an aqueous solution which comprises a phosphorous compound and a solderability-enhancing compound as defined herein below.

Further, the present invention relates to a process for increasing the solderabil- ity and corrosion resistance of a metal or metal alloy surfaces wherein the surface is contacted with this aqueous solution.

Brief Description of the Drawings

Fig. 1 shows the water repellent effect of a solution according to Example 2 on a lead frame in comparison to an untreated sample according to Example 1.

Fig. 2 shows a treatment sequence of plating and applying the composition according to the present invention.

Figs. 3 - 14 show the Zero Crossing Time (ZCT) and wetting force of treated and untreated samples as an indicator for solderability.

Detailed Description of the Invention

The invention is directed to a solution and a process for enhancing corrosion resistance of metal or metal alloy surfaces e.g. of nickel, copper, silver, gold, palladium, platinum, rhodium, ruthenium, indium and alloys thereof. Preferably the metal or metal alloy surface is a tin or tin alloy surface. Examples of tin alloy surfaces comprise SnPb, SnCu, SnBi, SnAg and SnAgCu.

For purposes of illustration, one such workpiece is an electronic component such as an electronic lead frame, a passive component, a bump on a wafer or is an electrical connector. Other suitable metal surfaces comprise zinc, aluminum, iron or copper or their alloy based surfaces.

The solution according to the invention is applicable to any metal surface, preferably a tin or tin alloy surface, whether part of an electronic device, engineering, functional, decorative, or otherwise. With regard to tin-based surfaces for electronic devices, the method enhances corrosion resistance and also preserves solderability of a tin or tin alloy surface during storage prior to a soldering operation involving reflow of a portion of the tin-based surface.

The solution of the present invention is also applicable for very thin metal surfaces, e.g. gold, which possess pores. Such pores can be sealed with the solutions of the present invention and prevent the oxide formation of underlying layers of e.g. nickel, copper or tin under the gold layer.

In accordance with the invention, the metal-based surface is immersed or otherwise contacted with a composition comprising a phosphorous compound, a solderability-enhancing compound and water to form a phosphorus-based film over the tin-based surface. This film inhibits corrosion of the tin-based surface, increases wettability and solderability and surprisingly also prevents whisker formation.

The aqueous solution for treating a metal surface according to the present invention comprises

(a) at least one phosphorous compound or its salt represented by the follow- ings formulas

O O

Il Il

O R1— P-O-R3 R1— O-P-O-R3

Il I I

R1— P-O-R3 O O

I I I

R2 R2 R2

I. II. III.

IV. V.

Vl.

wherein R1 , R2 and R3 are the same or different and are selected independently from the group consisting of H, a suitable counter ion like sodium or potassium, Ci-C ₂ o-alkyl, substituted or unsubstituted, linear or branched, CrC ₆ - alkaryl, linear or branched, substituted or unsubstituted and aryl, substituted or unsubstituted and wherein n is an integer ranging from 1 to 15.

(b) at least one solderability-enhancing compound or its salt represented by the following formula

R2 R6

R3 R5

VII.

wherein m, n, o and p are integers ranging from 0 to 200 and are the same or different and m+n+o+p is at least 2. Preferably m+n+o+p ranges from 4 to to 100, more preferably from 10 to 50.

wherein R1 and R7 are the same or different and are selected independently from the group consisting of H, a suitable counter ion like sodium or potassium, Ci-C ₂ o-alkyl. substituted or unsubstituted, linear or branched, C-ι-C ₆ -alkaryl, linear or branched, allyl, aryl, sulfate, phosphate, halide and sulfonate and wherein each of the R2, R3, R5 and R6 groups may be the same or different and are selected independently from the group consisting of H, CrC ₆ -alkyl, linear or branched, substituted or unsubstituted and wherein R4 is selected from the group consisting of Ci-C ₁₂ -alkylene, linear or branched, substituted or unsubstituted, arylene 1 ,2-, 1 ,3- and 1 ,4-substituted, naphthylene, 1 ,3-, 1 ,4- 1 ,5- 1 ,6- and 1 ,8-substituted, higher annulated arylene, cylcloalkylene, -O-(CH2(CH ₂ ) _n OR1 , wherein R1 has the meaning defined above, and moieties represented by the following formulas

VIII.

wherein the substitution independently is 1 ,2-, 1 ,3- or 1 ,4 for each ring and wherein q and r are the same or different and range independently from 0 to 10

and R8 and R9 are selected independently from the group consisting of H and Ci-C ₆ -alkyl, linear or branched..

Substituted alkyl, alkaryl and aryl groups described herein are hydrocarbyl moieties which are substituted with at least one atom other than carbon and hydrogen, including moieties in which a carbon chain atom is substituted with a hetero atom such as nitrogen, oxygen, silicon, phosphorous, boron, sulfur, or a halogen atom. The hydrocarbyl moieties may be substituted with one or more of the following substituents: halogen, heterocyclo, alkoxy, alkenoxy, alkynoxy, aryloxy, hydroxy, protected hydroxy, hydroxycarbonyl, keto, acyl, acyloxy, nitro, amino, amido, nitro, phosphono, cyano, thiol, ketals, acetals, esters and ethers.

Preferred are aqueous solutions, wherein R1 and R7 of the solderability- enhancing compound VII. are selected independently from the group consisting of H, methyl, sodium, potassium, halide, sulfate, phosphate and sulfonate.

Preferred are aqueous solutions, wherein R2, R3, R5 and R6 of the solderabil- ity-enhancing compound VII. are selected independently from the group consisting of H, methyl, ethyl, n-propyl and isopropyl.

Preferred are aqueous solutions wherein R4 of the solderability-enhancing compound VII. is selected from the group represented by the following formulas

IX. X.

and wherein R8 and R9 are selected from the group consisting of H, methyl, ethyl, n-propyl and isopropyl.

Solderability-enhancing compounds according to formula VII. having the following formulas are particularly preferred.

and wherein n = 1 - 20, preferably 3 - 8.

and wherein n = 1 - 20, preferably 2 - 10.

HO- - CH 2 — CH 2— CH2 - CH 2- ■ OH

wherein n = 1 - 20, preferably 2 - 7.

Even more preferred is the combination of above mentioned solderability en- hancing compounds with phosphorous compounds according to formula II.

The aqueous compositions according to the present invention usually have a pH of 1 - 8, preferably of 2 - 5. In order to ensure a constant pH value during operation preferably a buffer system is applied to the solution. Suitable buffer systems comprise formic acid / formiate, tartaric acid / tartrate, citric acid / cit- rate, acetic acid / acetate and oxalic acid / oxalate. Preferably, the sodium or potassium salt of the aforementioned acid salts are used. Besides the mentioned acids and corresponding salts, all buffer systems can be applied which result in a pH value of the aqueous compositions of 1 - 8, preferably of 2 - 5.

The buffer concentration is in the range of 5 - 200 g/l for the acid and of 1 - 200 g/l for its corresponding salt.

The at least one phosphor compound a) represented by the formulas I. - Vl. of the aqueous solutions is preferably used in an amount of 0.0001 to 0.05 mol/l, more preferably 0.001 to 0.01 mol/l.

The at least one solderability-enhancing compound (b) represented by the formula VII. is generally used in an amount of 0.0001 to 0.1 mol/l, preferably 0.001 to 0.005 mol/l.

Optionally, the solution may additionally contain an anti-foaming agent which is commercially available.

The aqueous compositions according to the present invention are usually applied during a process sequence as shown in Figure 2.

Plating preferably with tin or a tin alloy is by standard methods known in the art and for example described in Metal Finishing, Guidebook & Directory, 2006, p. 266 - 277. Generally, metal plating can be both by electrolytic as well as elec- troless plating methods.

Tin or tin alloy plating is preferred, but also zinc, aluminium, iron or copper plating may be employed.

For example, a typical tin plating bath may include one or more water soluble tin salts such as tin sulfate, tin alkyl sulfonates such as tin sulfonate, tin alkanol sulfonic acid, and tin halides such as tin chloride and tin fluoroborate. The bath also includes electrolytes such as sulfuric acid, alkyl sulfonates, alkanol sulfonates and halide salts to provide an electro-conductive matrix. Surfactants also may be included as well as other conventional additives to provide a desir- able tin layer. The amounts of the components are conventional and are well known in the art and may be obtained from the literature.

The numbers in the following paragraphs refer to the treatment sequence as shown in Figure 2.

The plating step 1. often is in an acidic plating bath followed by a rinse step 2. with demineralised water and neutralisation step 3. applying an alkaline solution in case the plating bath is acidic and vice versa. Optionally, the plated surface is again rinsed with demineralised water and then treated in step 5. with a solution according to the present invention.

Preferably, the substrate is immersed into the solution. Alternatively, spraying or dipping is also possible.

The treatment time can vary between 1 s and 10 minutes, preferably the contact time is at least 5 s and does not exceed 60 s. Generally, the solutions are at temperatures of 15°C to 60 ⁰ C, preferably from 2O ⁰ C to 40 ⁰ C.

The metals and metal alloys may then be optionally rinsed with water to remove any excess oxide inhibiting composition.

A post bake step may follow the treatment and rinse step by heating the substrate to a temperature between 100 and 200 ⁰ C and a time of 30 and 120 minutes. This step is particularly preferred if the substrate is a lead frame. The post baking further reduces the whisker formation.

A reflow process may follow the treatment and rinse step. Any suitable reflow process may be used. Reflow may be by heating by vapor phase reflow, laser reflow, plasma, oven melting, and passing an electrical current through the metals and metal alloys, or by any other method of heating the metals and metal alloys above their melting temperature.

The solderability of surfaces treated with solutions according to the present in- vention have been tested in accordance with the International Standard IEC 68- 2-20 (edition 1979). For this method as component terminations three multi rectangular leads (width: 0.62 mm, thickness 0.62 mm) are being used. After ap-

plying the liquid flux to the lead termination and mounting the component in the holder, the leads are suspended from a sensitive balance. The termination is brought into contact with the cleaned surface of a solder bath and immersed to the prescribed depth.

The resultant forces of buoyancy and surface tension acting upon the immersed termination are detected by a transducer and converted to a signal which is continuously monitored as a function of time, and recorded on a high speed chart recorder or displayed on a computer screen. Experimental details can be found in the Standard.

All solderability tests were made on MENICSO ST 50 (Metronelec). The following parameters were used:

Alloy: SnAgCu Temperature: 245°C

Density: 7,2 mg/mm ³ Immersion time: 10 s

Sensitivity: 2,5

Immersion depth: 3 mm

Immersion speed: 21 mm/s

The flux applied is non-activated R-type (Rosin) from Multicore based on 25% colophonium in isopropanol and in accordance with the Standard.

The tests are performed after 8 and 16 hours. Additionally to the test described in the Standard IEC 68-2-20 the leads are treated in a pressure cooker at a temperature of 105°C at high humidity (100%) and a pressure of 1.192 atm for 8, 16 and 24 hours to simulate very harsh aging conditions.

The present invention is further illustrated by the following examples.

General: The substrate to be plated is a lead as shown in Figure 3, (width: 0.62 mm, thickness 0.62 mm), having a copper surface.

The copper surface is plated with tin using a commercially available immersion tin plating bath (Stannopure HSM-HT, Atotech Deutschland GmbH) consisting of tin methanesulfonic acid (70 g/l tin), 200 g/l methanesulfonic acid, wetting agent and grain refiner. The bath temperature is 40 ⁰ C ₁ the current density 10 A/dm ² , the plating time 2.0 min and the coat thickness (Sn) 10 μm.

Example 1 (comparative)

10 multi rectangular leads as described before are plated with tin according to above mentioned Stannopure HSM-HT process.

After plating the leads are treated according to the process sequence as shown in Figure 2 and described above, except that step 5 is omitted.

The alkaline dip (step 4) is carried out in a solution containing 10 g/l potassium phosphate, K ₃ PO ₄ , at room temperature for 15 s. Post baking is at 150 ⁰ C for 1 hour.

The solderability test results according to IEC 68-2-20 after 8 and 16 hours as well as the test in a pressure cooker at a temperature of 105 ⁰ C at high humidity (100%) and a pressure of 1.192 atm for 8, 16 and 24 hours to simulate very harsh aging conditions are shown in Table 1 and in the figures 3 - 8. The graphs represent an average value obtained from ten measurements for each test.

Example 2

10 multi rectangular leads as described before are plated with tin according to above mentioned Stannopure HSM-HT process.

The alkaline dip (step 4) is carried out in a solution containing 10 g/l potassium phosphate, K ₃ PO ₄ , at room temperature for 15 s. The inventive composition

according to step 5 of Figure is an aqueous solution containing 5 g/l α,α',α"- 1 ,2,3,-Propanetriyltris[ω-hyclroxypoly(oxy-1 ,2-ethandiyl)], CAS No. 31694-55-0

and 1 g/l n-octylphosphonic acid at room temperature for 15 s.

Post baking is at 15O ⁰ C for 1 hour.

After plating the leads are treated according to the process sequence as shown in Figure 2.

The solderability test results according to IEC 68-2-20 after 8 and 16 hours as well as the test in a pressure cooker at a temperature of 105 ⁰ C at high humidity (100%) and a pressure of 1.192 atm for 8, 16 and 24 hours to simulate very harsh aging conditions are shown in Table 1 and in the figures 9 - 13. The graphs represent an average value obtained from ten measurements for each test.

Generally, the lower the Zero Crossing Time, the better is the wetting and there- fore the solderability of metal surfaces. For industrial purposes Zero Crossing Times of more than 3 seconds are not considered acceptable for solderable surfaces. The resulting Wetting Force value also is an indicator for the properties of a solderable surface. Generally, the higher the Wetting Force the better the solderability of a surface. Ideally, the Wetting Force remains constant for all test performed. A Wetting Force of 0 indicates that no wetting occurs.

The results of the tests are shown in Table 1.

Table 1 :

Zero Crossing Time (ZCR) and Wetting Force values in mN/mm for the steam test performed on tin surfaces according to Examples 1 (comparative) and Example 2

As can be seen from the Table, tin surfaces treated with a composition of the present invention (Example 2) show Zero Crossing Times of less than 1.5 seconds after all tests. Also the Wetting Force remains almost the same and constantly high even after severe treatment conditions. The Wetting Force is measured after 5 seconds to be able to compare the forces. Particularly the pressure cooker test for 24 hours represents very harsh conditions. According to comparative Example 1 no treatment of the tin plated surface is performed. The Zero Crossing Time exceeds the critical value of 3 seconds in the pressure cooker even with the lowest time already (4.0 s). The Wetting Force drops to less than half of the original value after 16 hours of steam treatment already and is almost zero after steam cooker treatment.

From the results the effect on solderability by the treatment with compositions of the present invention becomes apparent. Compared to the prior art it also becomes apparent that the compounds can be used in concentrations much lower

than described. WO 2005/121405 A1 according to Example 1 disclosed the use of 10 g/l n-octylphosphonic acid instead of 1 g/l herein.

Additionally, the discoloration of samples prepared according to Examples 1 and 2 was observed. After 4.000 hours at 55 ⁰ C and 85% humidity, the three leads treated according to Example 1 show yellow discoloration, while the leads of Example 2 retain the silver colour of the freshly plated tin surface.

Figure 1 shows the water repellent effect of a solution according to Example 2 on a lead frame compared to an untreated sample. The contact angle theta of water on freshly plated matt tin deposits, non treated and treated with the com- position according to the present invention had been determined by Drop Shape Analysis (DSA) with a Krϋss DSA10 Mk2. The contact angle for the lead treated according to example 1 of 124° indicates an excellent wettability. In contrast, leads of Example 1 show almost no wettability.