Stock material or miscellaneous articles – Composite – Of metal
Patent
1996-12-26
2000-07-18
Jones, Deborah
Stock material or miscellaneous articles
Composite
Of metal
428457, 106 122, 106 123, 106 125, 106 126, 427 98, B32B 900, B05D 512
Patent
active
060904933
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention is related to the protection of copper-containing surfaces, and is more particularly concerned with the novel use of protective bismuth coatings for such surfaces and with methods of coating application. The invention is especially advantageous in connection with the protection of copper surfaces of substrates used in electronics assembly, such as printed circuit substrates.
BACKGROUND OF THE INVENTION
The use of protective coatings for copper-containing surfaces is well known. Coatings may be applied for such purposes as to prevent tarnishing, to improve/preserve solderability, to improve corrosion resistance, and to reduce contact resistance between mating parts.
Printed circuit substrates for electronics applications, for example, generally include copper connectors such as copper pads for the mounting of surface-mount type electronic devices and/or copper plated through holes for receiving the leads of pin-in-hole type electronic devices. The surfaces of such connectors are commonly provided with a lead-tin alloy coating in practice. Tin-containing coatings are problematic, however, in that they may degrade with time due to the formation of copper-tin intermetallics. These intermetallic compounds form at the boundary between the copper and tin-containing coating and grow through the coating. The inter-metallic compounds form even at room temperature. Heating, such as during soldering, serves to accelerate the process. The problem of intermetallic compound formation becomes even more significant in connection with emerging electronics manufacturing technologies, wherein printed circuit substrates are subjected to multiple heating cycles.
Organic coatings, such as imidazoles, benzimidazoles, and triazoles have been considered as possible alternatives to lead-tin coatings. However, such coatings are subject to oxidation or volatilization when exposed to the heat of electronics manufacturing processes, with a resultant loss of protection. Moreover, organic coatings fall short of hole-fill requirements after wave soldering, particularly with the use of no-clean type soldering fluxes which are seeing increasing use in the electronics industry.
SUMMARY OF THE INVENTION
In accordance with the present invention, it has been discovered that the above-discussed deficiencies of tin-containing and organic coatings can be overcome, and that superior and cost-effective surface protection can be achieved, through the use of bismuth as a protective coating material. Bismuth has been found to provide a number of significant advantages over tin-containing and organic coatings. Unlike tin-containing coatings, bismuth does not form intermetallics with copper. Thus, a bismuth coating is not subject to the degradation by intermetallics which characterizes tin-containing coatings. Bismuth is also not subject to the oxidation/volatilization problems which characterize organic coatings. Furthermore, due to its high surface energy, bismuth exhibits highly effective wetting during wave soldering, drawing molten solder up into plated through-holes by capillary force to provide excellent hole-fill. When a bismuth coating is applied by so-called immersion plating, as described hereinafter, the invention achieves a very flat coating which is ideally suited for surface mount assembly.
Thus, in accordance with one of its principal aspects, the present invention provides an article having a copper-containing surface coated with bismuth.
In accordance with another of its principal aspects, the invention provides a method of protecting a copper-containing surface, comprising the step of coating the surface with bismuth.
In accordance with another of its principal aspects, the invention provides a method of protecting a copper-containing surface, comprising the step of coating the surface with a metallic coating which does not form intermetallics with copper of the surface and which contains bismuth.
In accordance with still another of its principal aspects, the invention provides a metho
REFERENCES:
patent: 3268353 (1966-08-01), Melillo
patent: 3303029 (1967-02-01), Shipley
patent: 3615892 (1971-10-01), Heinzelman, Jr.
patent: 3615897 (1971-10-01), Banush
patent: 4014660 (1977-03-01), Schreiner et al.
patent: 5306335 (1994-04-01), Senda et al.
patent: 5554700 (1996-09-01), Schipfer et al.
Fry's Metals, Inc.
Jones Deborah
Lam Cathy F.
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