Electricity: conductors and insulators – Conduits – cables or conductors – Preformed panel circuit arrangement
Reexamination Certificate
1998-08-06
2001-11-06
Paladini, Albert W. (Department: 2841)
Electricity: conductors and insulators
Conduits, cables or conductors
Preformed panel circuit arrangement
C174S261000, C228S056300, C257S677000, C420S122000, C420S560000
Reexamination Certificate
active
06313412
ABSTRACT:
FIELD OF THE INVENTION
BACKGROUND OF THE INVENTION
In conventional assemblies of substrates and electronic components, electrical circuitries carried by the substrates have terminals for connection to electrical terminal leads of the components. For this purpose, a solder is used which has a lead constituent. These lead containing solders are known to be largely reliable for forming permanent electrically conductive solder joints between terminals and electrical conductive leads. However, lead, it is now realized, is an undesirable material to be used in industry. When used in solder, it is not a captive constituent of the solder material and is known to be environmentally unfriendly and is toxic. Conceivably it is dangerous for use both by the manufacturing worker and by the consumer. Lead presents health hazards in manufacturing, disposal, and use of assemblies, and problems arise in the safe disposal of scrap materials containing lead.
Various assemblies have been made in which leadless solder has been used. However, problems are found during manufacturing where lead is not a component of solder as resultant solder joints are found to be unreliable as separation and cracking occurs.
In Great Britain Patent 2,158,459 Tallis teaches a leadless solder and method for forming a fluid-tight, soldered joint between a pair of tubular members of copper or copper alloy in the plumbing industry, the solder comprising 0.3%-0.7% copper by weight, the balance being tin and impurities. However, considerations such as electrical conductivity, miniature component sizes and thermal cycling over wide temperature ranges are quite different in the electronics industry. For example, oxides must be completely eliminated from electronic solder joints because they affect signal transmission and weaken the metallic integrity of the joints, thus causing cracking. These cracks create capacitances at high frequencies, and thus techniques employed by Tallis are inapplicable to assembling electrical components to substrates.
In U.S. Pat. No. 4,778,733 Lubrano also teaches a low toxicity, corrosion resistant solder for use in plumbing. It comprises a tin content of 92% to 99%, a copper content of 0.7% to 6%, and a silver content of 0.05% to 3%. As silver is an expensive precious metal, its use should be avoided to keep the solder from being prohibitively expensive.
“Whiskers” are hairline cracks that form on component leads as a result of a build up of internal stresses as due to plating. When using tin, which has a melting temperature of 232° C., as a plating metal, it was found that the standard tin/lead solder having a percentage weight composition of 63%/37% respectively and a melting temperature of 183° C., never raised the temperature of the tin finishes on the components high enough to relax the stresses, thus providing a candidate for component failure.
SUMMARY OF THE INVENTION
The present invention seeks to provide a method of assembling a substrate and an electronic component which minimizes or overcomes the above lead free solder problems.
Accordingly, the present invention provides a method of soldering comprising: providing a chamber having a low oxygen atmosphere; providing an electronic component having a plurality of electrically conductive leads each of which comprises a core provided with a surface layer comprising an alloy, the surface layer having a soldering temperature; providing a solder alloy consisting of tin and copper, the tin of the solder alloy having a weight at or around 99.3% of the total weight of the solder alloy and the copper having a weight at or around 0.7% of the total weight of the solder alloy; and soldering the leads of the electronic component onto copper-based terminals in the chamber by raising the temperature of the terminals and of the surfaces of the leads at the terminal positions to the soldering temperature of the solder.
Preferably the low oxygen atmosphere has an oxygen level less than 75 ppm (parts per million), and uses nitrogen as the inert gas. In the method of assembly in a preferred and practical arrangement, each of the electrically conductive leads has a conductive body with a surface layer comprising an alloy from the list consisting of tin, nickel, tin copper, tin silver, nickel palladium, gold palladium, and silver palladium, and the conductive body necessarily remains solid, i.e., unsoftened, at the softening temperature of the coating.
The solder must be such that during rise in temperature, it comprises a pasty constituent for soldering purposes at or around a specific temperature, and will solidify to provide a solder joint immediately upon removal of heat. A eutectic solder alloy having these requirements is provided by tin having a weight percent at or around 99.3% of the total weight of the solder and with the copper being at or around 0.7% of the total weight of the solder. It is also preferable that the alloy in the coating of each electrically conductive lead has approximately the percentage weights referred to above of the constituent parts are copper and tin. Hence, the coating and the solder are in the same physical pasty state at soldering temperature. This temperature is at approximately 227° C.
A flux to be used with the above solder according to the invention must be compatible with the solder. It must also be a flux which is operable at the desired soldering temperatures which are envisaged for the invention. An organic no-clean flux is considered to be most desirable and this flux is one which should be active at the temperature which the solder alloy has the pasty constituent, i.e. around 227° C., and will successfully remove oxides of copper and tin. Such a flux suitable for this purpose is that referred to as X39 flux manufactured by Multicore Solders Inc. of Richardson, Tex.
REFERENCES:
patent: 2671844 (1954-03-01), Laubmeyer et al.
patent: 4778733 (1988-10-01), Lubrano et al.
patent: 5297008 (1994-03-01), Estes
patent: 5444293 (1995-08-01), Li
patent: 6086687 (2000-07-01), Oud et al.
patent: 2158459A (1985-11-01), None
Paul P. Conway, “Solderability testing of alternate component termination materials with lead free solder alloys”, IEEE/CPMT Int'l Electronics Manufacturing Technology Symposium, pp245-251, 1995.
Bilton David
Hamilton Murray W.
Trumble William P.
de Wilton Angela C.
Nortel Networks Limited
Paladini Albert W.
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