Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
2000-01-03
2002-07-09
Zimmerman, John J. (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C428S646000, C420S561000, C420S562000, C228S262610
Reexamination Certificate
active
06416883
ABSTRACT:
The present invention relates to lead-free solder compositions comprising tin, bismuth and silver. These compositions are lead-free solder compositions that are particularly useful in the soldering of electrical/electronic components to printed circuit boards.
Typical solders used in the electronics industry currently comprise a tin-lead alloy. Growing concerns over the use of lead have meant that lead-free alloys have come under consideration for the electronics industry. An article entitled “The search for lead-free solders” by W B Hampshire which appeared in Soldering and Surface Mount Technology, No 14, June 1993 provides a list of patents relating to lead-free solders. This table is as follows:
Some Lead-free Solder Patents
Number
Year
Company
Ag
Cu
Sb
Zn
Bi
Other
U.S. Pat. No. 4,879,096
1989
Oatey
0.05-3.0
0.5-6.0
0.1-3.0
Eur 0,336,575
1989
Cookson
0.01-1.5
0.02-1.5
0.08-20
0-0.1
P + 0-0.2
Re
U.S. Pat. No. 4,806,309
1989
Willard Indus
0.1-0.5
3.0-5.0
1.0-4.5
U.S. Pat. No. 4,758,407
1988
J W Harris
0-5.0
3.0-5.0
0.1-2.0 Ni
0-0.5
3.0-5.0
4.0-6.0
0-2.0 Ni
U.S. Pat. No. 4,695,428
1987
J W Harris
0.1-3.0
0.1-3.0
0.5-4.0
0.5-4.0
Eur 0,251,611
1988
Engelhard
0.05-3.0
0.7-6.0
U.S. Pat. No. 4,670,217
1987
J W Harris
0.5-2.0
0.5-4.0
0.5-4.0
UK 2,158,459A
1985
IMI Yorkshire
0.3-0.7
Jap 82 30598
1982
Aoki Metal
0.1-2.0
0.05-0.5
0.5-1.0 In
U.S. Pat. No. 1,437,641
1922
0.5-4.5
0.5-4.5
0.5-9.5
It is stated in the above article that almost exclusively the above patents were developed for plumbing applications which allow higher temperatures than for electronic applications. Such higher temperatures will easily damage the electrical components, especially semiconductor devices having no heat sink protection. Also, alloys with some pasty range are usually preferred in the making of plumbing joints, while electronics solders have much shorter pasty ranges. The authors therefore thought it doubtful that any of the above solders would substitute successfully in the electronics industry. The article looks at various eutectics of tin-x alloys but dismisses tin-bismuth alloys.
A number of other patents have been granted for lead-free solders. For example, U.S. Pat. No. 5,368,814 discloses a multi component solder alloy containing at least about 50 weight percent Bi, up to about 50 weight percent Sn (basis total Sn and Bi), and an effective amount of a physical and mechanical property enhancing third component. The third component can be Cu, In, Ag and combinations of Cu and Ag.
U.S. Pat. No. 5,320,272 discloses an electronic package having a solder connection for bonding faying surfaces formed of a tin-bismuth alloy comprising a tertiary metal. The examples show tin-bismuth alloys containing gold as the. tertiary metal. The alloy is stated to preferably contain about 48 to 68 weight percent bismuth. It is stated that tin alloys containing less than 30 weight percent or greater than 70 weight percent bismuth require relatively high reflow temperatures that tend to damage other connection components typically found in a microelectronic package. This patent discloses applying the tertiary metal to a faying surface and thereafter a deposit of tin-bismuth solder paste is applied to the film.
U.S. Pat. No. 4,879,096 discloses a lead-free and antimony-free solder composition which comprises about 0.05 to about 3.0. percent by weight of silver, about 0.5 to about 6.0 percent by weight of copper, about 0.1 to 2.0 percent by weight of bismuth and about 88 percent to about 99.35 percent to be particularly useful for joining copper tubing, brass pipes and fittings to by used in plumbing. U.S. Pat. No. 4,929,423 also discloses a lead-free alloy for joining and sealing which is useful as a plumbing solder which comprises from 0.08 to 20.0 percent by weight of bismuth, from 0.02 to 1.5 percent by weight silver, from 0 to 0.10 percent by weight of phosphorus, from 0 to 0.20 percent of a rare earth mixture and the balance tin, together with incidental impurities. This alloy has a similar pasty range to the traditional tin-lead plumbing solders.
For reasons such as environmental and health reasons it would be an advantage to use a lead-free solder in electronic applications. It would also be and advantage if such a solder was able to be used with existing-machinery used for Pb/Sn alloys.
The present inventors have now discovered a new solder composition which may be used in electronic applications. according to the present invention there is a solder composition comprising:
15.0 to 30.0% bismuth,
1.0 to 3.0% silver, and otionally comprising 0 to 2.0%, preferably 0.01 to 2.0%, copper and 0 to 4.0% antimony and incidental impurities, the balance being tin.
Where the terms “comprise”, “comprises”, “comprised” or “comprising” are used in this specification they are to be interpreted as specifying the presence of the stated features, integers, steps or components referred to, but not to preclude the presence or addition of one or more other feature, integer, step, component of group thereof.
The composition of the present invention may include small amounts of, typically less than about 4.0 percent, of other alloying agents to enhance the mechanical properties. Such optional agents may include, for example, copper, antimony and/or gallium. In a preferred embodiment the solder of the present invention includes 0 to 2.0%, more preferably 0.01 to 2.0%, copper and/or 0 to 4% antimony. It is preferred that the solder of the present invention is lead-free other than any lead which is present as impurities. Commercially available bismuth may contain minor amounts of lead as bismuth is found in nature in combination with lead. It is usually in very low concentrations, in the order of a few ppm.
The solders of the present invention may be formed as a mixture of an alloy. When used as a mixture they may be applied as a paste to, for example, the surface mount of a printed circuit board by hand or automatic dispenser, using a stencil similar to silk screening or by direct imaging. The composition of the present invention may also be formed as an alloy.
Soldering is a low temperature, generally reversible, metallurgical joining process. Low temperature and reversibility are especially important in electronics applications because of the materials involved and the necessity for reworking and replacing defective components. A solder composition used in electronics fabrication must be wettable as a solder alloy and have at least on component capable of forming an electrically conductive, thermally stable, non-brittle, plastic intermetallic with the pad or land metallurgy. For these reasons the most common solder alloys have been lead-based alloys, as Sn-Pb alloys
Although alternatives have been suggested for other applications such as plumbing applications, Pb/Sn solders are most commonly used for electronic applications. Pb/Sn alloys have a low solidus temperature, they are workable and the resulting Cu/Sn intermetallics (formed at the solder/Cu contact interface) forms over a wide temperature range, good adhesion of Cu/Sn intermetallics is obtained from Pb/Sn alloys to Cu lands and pads, the process equipment is readily available and adjuncts such as resins, fluxes and solder mixes for Pb/Sn alloys are of low cost. The relatively low temperature required for processing Pb/Sn solder alloys are particularly important as is the “softness” or plasticity of lead-based solders.
The solder of the present invention has many of the above properties which make it a suitable replacement for Pb/Sn solders in electronic applications. A particular advantage of the solder of the present invention is that it may be used as a “drop in” replacement for existing Pb/Sn solders. A disadvantage with other lead-free solders which have been proposed for electronic applications is that the existing machinery used for Pb/Sn solders may not be used with the lead-free replacements. The composition of the present invention may be used as a direct substitute for Pb/Sn alloys therefore avo
Ecosolder International Pty Ltd
Meyer Jerald L.
Nath Gath M.
Nath & Associates PLLC
Zimmerman John J.
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