METHOD OF AUTOCATALYTICALLY TIN-PLATING ARTICLES OF COPPER OR A COPPER ALLOY

Abstract: Improvement of an electroless tin-plating method of ar­ ticles of copper or of a copper alloy with a strongly alkaline solution which contains at least 0.20 mol/litre of bivalent tin ions and at least 1 mol/litre of alkalihydroxide at a tempera­ ture between 60 and 95 °C. The improvement consists of a pre-treatment with an acid solution of a bivalent tin salt and a complex former for Cu⁺⁺ and/or Sn⁺⁺ ions with which a layer of copper at the surface is exchanged for tin. The com­ plex former preferably is thiourea.

Description: Method of auto catalytically tin-plating articles of copper or a copper alloy. The invention relates to a method of autocatalytically tin-plating articles of copper or of a copper alloy. GB-PS 2 039 534 discloses an electrolessly operating tin-plating solution which consists of a strongly alkaline aqueous solution containing at least 0.20 mol/litre of bivalent tin ions. The operating temperature of said solution is between 60 and 950C and it contains at least 1 mol/litre of alkali hydroxide. In a preferred embodiment a small quantity of tin (IV) ions and/or a strong reduction agent, for example hypophosphite or a borazane, is or are added to the solution. As a result of this an improvement of the quality of the deposit and the deposition rate is achieved. However, the soldering properties of tin deposited electrolessly directly on a copper surface are insufficient in practice without additions. Even with an addition of hypophosphite, which for practical reasons is rather to be avoided, the desired soldering properties are often not obtained. In an article by A. Molenaar and J.J.C. Coumans in Surface Technology 16, 265-275 (1982) a method is described according to which a surface of copper or a copper alloy is subjected to a pre-treatment with a tin exchanging bath. This pre-treatment is carried out by means of an alkaline solution with cyanide ion as a complex forning agent for Cu+ and/or Cu++ ions and for Sn++ ions. In the experiments which have led to the present invention, however, it has been found that satisfactory results are not obtained in all respects with this method. The exchanged layer which is obtained from the alkaline solution has the structure of 5 -tin, the same modification which is also obtained from the autocatalytic solution. In spite of the fact that the same modification is deposited, often no optimum soldering properties are obtained. According to the invention it has been found that considerably better soldering properties are obtained if the pre-treatment for the electroless deposition of the tin is carried out with an acid reacting exchanging solution. In spite of the fact that the exchanged layer also comprises the alloy Cu6Sn5, better soldering properties are nevertheless obtained after growing said layer by means of the autocatalytic method which deposits tin in the form of the 13- modioication, than when an alkaline exchanging solution has been used for the pretreatment. These improved results cannot be well explained; they are ascribed by Applicants to the fact that adsorption of tin-II-complex having disproportionating properties is promoted by the exchanging reaction in acid medium with thiourea as a complex former for Cu++ and/or Cu+ ions and for Sn++ ions. An advantage of the use of the acid exchanging solution which preferably comprises thiourea as a complex former, as compared with the alkaline solution with cyanide, is that this solution is not poisonous. Various acids may be used in the exchanging solution, for example, hydrochloric acid, sulphuric acid or citric acid, but the very best results are obtained by means of a sulphuric acid solution. One embodiment has the following composition: 0.02 mol/l SnCl2.2H20 0.2 mol/l H2S04 0.6 mol/l thiourea. This solution is preferably used at a temperature between 20 and 300C, the articles being kept immersed in it for 10 minutes. The best result as regards the quality and the adliesion of the tin deposit is obtained when the surface of copper or the copper alloy is previously cleaned. This cleaning may be done mechanically or chemically. The chemical method may be a cleaning, polishing or etching method but the surface is preferably treated by means of a chemical polishing solution. One embodiment has the composition: 55 parts by volume of phosphoric acid 25 parts by volume of acetic acid 20 parts by volume of nitric acid and 0.5 parts by volume of hydrochloric acid. Herein the articles are kept immersed at room temperature for 30-60 seconds. The electroless tinplating solution, the treatment in which follows the exchange-pretreatment, comprises a solution containing at least 0.20 mol/l of a salt of bivalent tin and at least 1 mol/l of alkalihydroxide, works with these bath-constituents only. However, it is advantageous if the solution contains also a complexforming agent for bivalent tin-ions, such as citrate or tartxare.The presence of tartrate is to be preferred as higher tin-concentrations are then attainable. The presence of citrate or tartrate moreover produces a tin-deposit of further improved solderability. By giving an additional treatment in a solution of about pH = 6 (4-8) without further constituents after the exchange-treatment and prior to the electroless Snplating, also the solderability and the structure of the deposited tin are favourably influenced. In certain cases it may be advantageous to nevertheless add hypophosphite to the auto catalytic solution. The reliability of the soldered joint is further increased by the combination of pre-treatment and addition of hypophosphite to the autocatalytic solution. The invention will be described in greater detail with reference to the ensueing examples. Example 1: 2 Copper plates having dimenions 3 x 1 cm were subjected to the following treatments. A dull Cu layer of 151um was electro-deposited on said plates by means of an acid copper sulphate bath. They were then rinsed in water and polished in the solution of the following composition for 1 minute: 55 parts by volume of H3P04 25 parts by volume of acetic acid 20 parts by volume of H"03 0.5 parts by volume of HCl and rinsed again in demineralized water for 30 seconds. One of the methods below was then used: I no exchange II the plates were immersed at 30 C in the following solution for 10 minutes: C.02 m SnCl2.2H20 0.6 m thiourea 0.2 m H2S04 III the plates were immersed at 750C in the following solution for 5 minutes: 0.02 m SnCl2. 2H20 0.2 m NaOH 0.8 m KCN They were rinsed in demineralized water for 30 seconds. The plates were electrolessly tin-plated at 75 C for 3 hours in the following composition: 0*33 m Sncl2.2H2o 3.85 m NaOH o.66 m sodium citrate and finally rinsed in demineralized water or 30 seconds. The tin-plated plates were aged by heating them in a hot air furnace at 1550C for 16 hours. The solderability was determined by means of a so-called wetting balance (11Multicore Solders") as described inter alia in Circuit World 10, NO 3, PP. 4-7, 1984. The forces occurring upon providing a sample in a soldering bath in accordance with time are measured by the recording apparatus coupled to said balance. The plates were immersed edge-wise while using a slightly activated flux in a liquid soldering bath. In the first instance an upward force was exerted on the plates which decreases when the surface layer of the solder has flattened along the plate. The time expiring inbetween is indicated by t1. The wetting then causes a downward force. The fraction of said force which is measured after 3 seconds with respect to an ideally wetted plate is indicated by F3/FmaX. This method of measuring is described in the IEC standard sheet 68-2-20. The value of t1 must in practice be smaller than 1 sec. and F3/FmaX must be larger 3 max than 50%. The following results were measured: t1(sec) F3/Fm() I > 4 0 )4 0 II o.6 63 0.6 64 0.6 69 0.8 59 III ) 4 0 1. 1 47 In the same manner a test was carried out starting from copper plates without a dull electroplated copper layer. Exchange II (acid exchange with thiourea) and III (alkaline exchange with cyanide) were used. The results hereof are: ti F3/Fm(*) II 0.8 71 1.0 46 III ) 6 0 > 6 0 Example 2: The connection wires of glow discharge lamps consisting of copper-clad wire having a diameter of 3 and 4 millimetres were subjected to the following treatment. First of all they were immersed for 10 seconds in H2S04 (48) at 900C, then rinsed with demineralized water for 10 seconds and polished in the following solution for 30 seconds: 55 parts by volume of H3P04 25 parts by volume of acetic acid 20 parts by volume of HN03 0.5 parts by volume of HCl. After rinsing with demineralized water for 30 seconds one of the two exchanging methods was used: II the connection wires were dipped at 300 C in the following solution for 15 minutes: 0.02 m SnCl2.2H20 m m thiourea 0.2 m H2S04, or III the connection wires were immersed at 750C for 15 minutes in 0.02 m SnCl2.2H20 0.2 m NaOH 0.8 m KCN. They were then rinsed again with demineralized water for 30 seconds and electrolessly tin-plated for 30 minutes by immersing in the following solution at 75 C: 0.33 m SnCl2.2H20 3.85 m NaOH 0.66 m Na-citrate and finally rinsed in demineralized water for 30 seconds. The tin-plated connection wires were aged either for 16 hours at 1550C in a hot air furnace (furnace test) or for 16 hours in steam of 1000C; RV 100* (steam test). The solderability was determined in the same manner as in example 1. furnace test steam test diameter coniiection wires (mm) t1 (sec F3/Fm (%) t1(sec) F3/Fm(%) II 3 0.3 75 0.4 92 3 0.4 75 0.3 100 3 0.3 92 0.3 100 3 0.3 92 0.4 75 4 0.6 47 0.4 82 4 0.4 47 0.4 68 4 0.3 90 0.4 90 4 0.3 74 0.3 68 III 3 3 0 3.5 0 3 2 8 ;6 0 3 3 0 > 6 0 3 4 0 5.5 4 3.5 0 > 6 0 4 O.5 67 > 6 0 4 4 0 3 6 0 4 2 8 )6 0 Example 3: 2 Copper plates having dimensions of 3x1 cmê were subjected to the following treatments. First of all they were coated by elec-rodeposition with a dull copper layer of 15/um by means of an acid copper sulphate bath, rinsed in water and subjected to one of the following cleaning treatments: I immerse at 30 0C for 1 minute in 55 parts by volume of H3P04 25 parts by volume of acetic acid 20 parts by volume of HN03 0.5 parts by volume of HCl immerse for 1 minute in HCl 1:1 at room temperature. II immerse at room temperature for 1 minute in 60 ml H2S04 60 ml H20 30 ml HN03 9.4 ml HCl immerse in HCl at room temperature for 1 minute. III immerse in HNO 1:1 at room temperature for 1 minute. 3 IV immerse in HCl 1:1 at room temperature for 1 minute. After each of these treatments the plates were rinsed in demineralized water for 30 seconds and then the plates were immersed at 300C for 10 minutes in: 0.02 m SnCl2.2H20 0.2 m thiourea 0.2 m H2S04 and again rinsed in demineralized water for 30 seconds. The plates were finally tin-plated electrolessly at 75 C for 3 hours by immersing in the following solution: 0.33 m SnCl2.2H20 3.85 m NaOH 0.66 m Na-citrate 0.90 m NaH2P02 and rinsed in demineralized water for 30 seconds. The etching rates of the said cleaning treatments are as follows: I 2.5um/min. II 11um/min. Il 10 /um/min. IV O /um/min. 0 After tin-plating, the plates were aged at 155 C in a hot- air furnace for 16 hours. The solderability was determined in the same manner as in example 1. The solderability was evaluated as follows: t1 (sec) F3/Fm(ffi) I 0.8 57 0.7 46 II o.6 67 0.6 72 III 0.5 78 0.5 75 IV 0.8 58 0.6 64 Example 4: 2 Copper plates having dimensions 3 x 1 cm were subjected to the following treatments. A dull copper layer of 15/um was electrodeposited on them by means of an acid coppersulphate bath. They were subsequently rinsed in water and during one minute polished in the solution of the following composition: 55 parts by volume of H3P04 25 parts by volume of acetic acid 20 parts by volume of HN03 0.5 parts by volume of HCl and rinsed again in demineralized water during 30 seconds. Hereafter the plates were immersed during 10 minutes at 30 C in 0.02 m SnCl2.2H20 m m thiourea 0.2 m H2S04 After rinsing with demineralized water during 30 seconds one of the following three tinplating methods were applied. I The plates were electrolessly tin-plated during 3 hrs at 75 0C in a solution of the following composition: 0.33 m SnCl2.2H20 2.5 m NaOH II The plates were electrolessly tin-plated during 3 hours at 75 C in a solution of the following composition: 0.33 m SnCl2.2H20 2.5 m NaCH 0.66 m sodiumcitrate. This solution was prepared by dissolving the Na-citrate in about half of the volume of water and adding the SnCl2.2H20 (soln.1), dissolving NaOH in about half of the volume of water (soln 2) and combine solutions 1 and 2 while stirring vigorously. III The plates were electrolessly tin-plated at 75 C during 3 hours in a solution of the following composition: 0.42 m SnCl2.2H20 2.5 m NaOH 0.50 m Na-K-tartrate This solution was prepared by dissolving the Na-K-tartrate in about half of the volume of water, dissolving the NaOH in the remaining volume of water, combining the two solutions and adding thereto the SnCl2.2H20. Finally, the plates were rinsed in demineralized water during 30 seconds. The tin-plated samples were aged during 16 hours at 1550 C in a hot-air furnace. The solderability was determined in the same way as in Example 1. The results are as follows: t1(sec) F3/Fm(*) I 1.4 36 1.9 25 0.6 52 1.6 37 II 0.9 60 1.0 45 0.6 57 1.0 39 0.6 63 0.8 50 0.6 75 0.6 70 0.7 41 1.0 52 III 0.6 79 0.5 77 0.8 71 0.5 78 0.5 77 0.5 83 0.5 85 0.6 85 0.6 89 0.5 83.

Claims: 1. A method of autocatalytically tin-plating articles of copper of a copper alloy in which the ar- ticles are first of all subjected to a treatment with a solution of a bivalent tin salt and a complex former for Cu++ and/or Cu+ ions and for Sn++ ions, with which a layer of copper is exchanged for tin after which the article is immersed in a strongly alkaline solution which contains at least 0.20 mol/litre of bivalent tion ions and at least 1 mol/litre of alkalihydroxide at a temperature between 60 and 95 C, characterized in that the pre-treatment solution is an acid solution of a bivalent tin salt. 2. A method as claimed in Claim 1, characterized in that the pre-treating solution comprises thiourea as a complex former. 3. A method as claimed in Claim 1 or 2, characterized in that the pre-treatment is carried out in a sulphuric acid-containing solution. 4. A method as claimed in any of the Claims 1 to 3, characterized in that the pre-treatment is carried out in a solution which contains per@li@re: 0.07 mol SnCl2.2H20 0.2 mol H2S04 and 0.6 mol thiourea. 5. A method as claimed in any of the Claims 1 to 4, characterized in that the articles of copper or a copper alloy are cleaned mechanically or chemically before they are exposed to the exchanging solution. 6. A method as claimed in Claim 5, characterized in that a polishing solution is used of the following composition: 55 parts by volume of phosphoric acid 25 parts by volume of acetic acid 20 parts by volume of nitric acid, and 0.5 parts by volume of hydrochloric acid. 7. A method as claimed in any-one of Claims 1 to 6, characterized in that the strongly alkaline, bivalent tin-ions containing solution also contains a complex-forming agent for bivalent tin-ions, such as citrate or, preferably, tartrate. 8. A method as claimed in any of the Claims 1 to 7, characterized in that the strongly alkaline bivalent tin ions-containing solution also comprises a soluble hypophosphite. 9. Articles of copper or a copper alloy at least a part of the surface of which has been tin-plated according to any of the preceding Claims.

Inventors: COUMANS, JACOBUS JOHANNES CHRETIEN, MEENDERINK, BRIGITTA CHRISTINA MARIA, MOLENAAR, ARIAN
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1985
Main classification: C23C18
IPC codes: C23C18