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 95°C and it contains at least 1 mol/litre of alkali hydroxide.
In a preferred embodiment a small quantity oftin (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 forming 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 (3-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 with thiourea as a complex former for Cu++ and/or Cu+ ions and for Sn++ions.
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 β-modification, than when an alkaline exchanging solution has been used for the pre-treatment.
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/I SnCl2 · 2H20
- 0.2 mol/I H2S04
- 0.6 mol/I thiourea.
This aqueous solution is preferably used at a temperature between 20 and 30°C, the articles being kept immersed in it for 10 minutes.
The best result as regards the quality and the adhesion 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 (85% by weight)
- 25 parts by volume of acetic acid (100% by weight)
- 20 parts by volume of nitric acid and (65% by weight)
- 0.5 parts by volume of hydrochloric acid (37% by weight).
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 an aqueous solution containing at least 0.20 mol/I of a salt of bivalent tin and at least 1 mol/I of alkalihydroxide, works with these bath-constituents only. However, it is advantageous if the solution contains also a complex-forming agent for bivalent tin-ions, such as citrate or tartrate. 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 Sn-plating, 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 autocatalytic solution. The reliability of the soldered joint is further increased by the combination of pre-treatment and addition of hypophosphite to the autocatalytic solution.
US-A-2 369 620 discloses an acid tin immersion bath containing SnCl2, H2SO4 and thiourea. With this bath only thin layers are obtained after a long time, e.g. 2.3 mg Sn/cm2 after 24 hours. The process according to US-A-2 369 620 is based on the principle of metal exchange. It is possible to continue deposition, but the increase in the amount of deposited Sn vs time is due to the fact that the obtained tin coating is not dense, but porous. In that case tin growth continues as long as parts of the copper surface are still uncoated. The solderability of these layers is poor. There is no suggestion to use such an acid immersion bath as a pre-treatment solution for an autocatalytic tin bath.
The invention will be described in greater detail with reference to the ensuing examples.
Copper plates having dimensions 3xl cm2 were subjected to the following treatments.
A dull Cu layer of 15 µm 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 (85% by weight)
- 25 parts by volume of acetic acid (100% by weight)
- 20 parts by volume of HN03 (65% by weight)
- 0.5 parts by volume of HCI (37% by weight)
- I no exchange
- II the plates were immersed at 30°C in the following aqueous solution for 10 minutes:
- 0.02 mol/I SnCl2 · 2H20
- 0.6 mol/I thiourea
- 0.2 mol/I H2SO4
- III the plates were immersed at 75°C in the following aqueous solution for 5 minutes:
- 0.02 mol/I SnCl2 · 2H20
- 0.2 mol/I NaOH
- 0.8 mol/I 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 aqueous solution:
- 0.33 mol/l SnCl2 · 2H20
- 3.85 mol/I NaOH
- 0.66 mol/I sodium citrate
and finally rinsed in demineralized water for 30 seconds. The tin-plated plates were aged by heating them in a hot air furnace at 155°C for 16 hours. The solderability was determined by means of a so-called wetting balance ("Multicore 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 than 50%.
The following results were measured:
The results hereof are:
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 H2SO4 (48% by weight) at 90°C, then rinsed with demineralized water for 10 seconds and polished in the following solution for 30 seconds:
- 55 parts by volume of H3PO4 (85% by weight)
- 25 parts by volume of acetic acid (100% by weight)
- 20 parts by volume of HN03 (65% by weight)
- 0.5 parts by volume of HCI (37% by weight).
After rinsing with demineralized water for 30 seconds one of the two exchanging methods was used: 11 the connection wires were dipped at 30°C in the following aqueous solution for 15 minutes:
- 0.02 mol/l SnCl2 · 2H20
- 0.6 mol/I thiourea
- 0.2 mol/I H2S04, or
III the connection wires were immersed at 75°C for 15 minutes in the following aqueous solution:
- 0.02 mol/I SnCl2 · 2H20
- 0.2 mol/I NaOH
- 0.8 mol/I KCN.
They were then rinsed again with demineralized water for 30 seconds and electrolessly tin-plated for 30 minutes by immersing in the following aqueous solution at 75°C:
- 0.33 mol/I SnCl2 · 2H20
- 3.85 mol/I NaOH
- 0.66 mol/I Na-citrate
and finally rinsed in demineralized water for 30 seconds. The tin-plated connection wires were aged either for 16 hours at 155°C in a hot air furnace (furnace test) or for 16 hours in steam of 100°C; RV 100% (steam test). The solderability was determined in the same manner as in Example 1.
Copper plates having dimensions of 3x1 cm2 were subjected to the following treatments. First of all they were coated by electrodeposition with a dull copper layer of 15 µm by means of an acid copper sulphate bath, rinsed in water and subjected to one of the following cleaning treatments:
I immerse at 30°C for 1 minute in
- 55 parts by volume of H3PO4 (85% by weight)
- 25 parts by volume of acetic acid (100% by weight)
- 20 parts by volume of HN03 (65% by weight)
- 0.5 parts by volume of HCI (37% by weight)
immerse for 1 minute in HCI (20% by weight) at room temperature.
II immerse at room temperature for 1 minute in
- 60 ml H2S04 (96% by weight)
- 60 ml H20
- 30 ml HN03 (65% by weight)
- 9.4 ml HCI (37% by weight)
immerse in HCI at room temperature for 1 minute.
III immerse in HN03 (38% by weight) at room temperature for 1 minute.
IV immerse in HCI (20% by weight) 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 30°C for 10 minutes in the following aqueous solution:
- 0.02 mol/I SnCl2 · 2H20
- 0.2 mol/I thiourea
- 0.2 mol/I H2SO4
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 aqueous solution:
- 0.33 mol/I SnCl2 · 2H20
- 3.85 mol/I NaOH
- 0.66 mol/I Na-citrate
- 0.90 mol/I NaH2PO2
and rinsed in demineralized water for 30 seconds.
The etching rates of the said cleaning treatments are as follows:
- 1 2.5 µm/min.
- I 11 µm/min.
- III 10 µm/min.
- IV 0 µm/min.
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:
Copper plates having dimensions 3x1 cm2 were subjected to the following treatments.
A dull copper layer of 15 µm 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 (85% by weight)
- 25 parts by volume of acetic acid (100% by weight)
- 20 parts by volume of HN03 (65% by weight)
- 0.5 parts by volume of HCI (37% by weight)
Hereafter the plates were immersed during 10 minutes at 30°C in the following aqueous solution:
- 0.02 mol/I SnCl2 · 2H20
- 0.6 mol/I thiourea
- 0.2 mol/I H2SO4.
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°C in an aqueous solution of the following composition:
- 0.33 mol/I SnCl2 · 2H20
- 2.5 mol/I NaOH
II The plates were electrolessly tin-plated during 3 hours at 75°C in an aqueous solution of the following composition:
- 0.33 mol/I SnCl2 · 2H20
- 2.5 mol/I NaOH
- 0.66 mol/I sodiumcitrate.
This solution was prepared by dissolving the Na-citrate in about half of the volume of water and adding the SnCl2 · 2H20 (soin. 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 an aqueous solution of the following composition:
- 0.42 mol/I SnCl2 · 2H20
- 2.5 mol/I NaOH
- 0.50 mol/I 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 155°C in a hot-air furnace. The solderability was determined in the same way as in Example 1.
The results are as follows: