The present application relates to lead free or substantially lead free soldering and more specifically to lead free soldering of elements in a substantially oxygen free atmosphere.

Soldering has been used for many years to coat and/or join metal surfaces together. Elements such as printed boards in the electronic industry are one of the known uses of lead and tin based solders. Components are assembled to boards by solder joining, and joining surfaces are sometimes solder plated or solder coated. Circuit boards without components thereon are referred to as "unpopulated" circuit boards. Circuit boards having components assembled thereon are referred to as

"populated" circuit boards. One of the most common solder alloys used today has lead as one of the main constituents, usually in the range of 37 to 40% by weight of lead alloyed with tin. Other metals or alloys for solder coating or joining metallic surfaces have existed for special cases which do not contain lead but these are only used in a few specific cases, primarily because of the availability and numerous advantages of lead based solders which include strength, reliability and cost. For example, silver solder is available but is only used infrequently and in special circumstances because of the expense. These special solders have become known as "lead free" alloys.

With the concern today about our environment, there is a pressing need for avoiding the use of lead in solder alloys and a possible impending legislation in the United States of America to prohibit the use of lead in many industries is causing industry to seek suitable non-toxic solders. Thus, solder coating and solder joining of

electronic assemblies which presently utilizes lead solder alloys must now find a substitute lead free solder.

There are a great number of lead free solder alloys, however, most of them are expensive and they all have different properties to commercially available lead solder alloys. At the present time the costs of some of these solders make them prohibitive for use with electronic assembly production. Examples of lead free solder alloys include tin based alloys, bismuth alloy solders and indium alloy solders. These solders contain small quantities of silver, gold, antimony, copper or zinc. Discussions of such solders occur in publication SMC-WP-002 of August, 1992, entitled "Surface Mount Council White Paper, An Assessment of the Use of Lead in Electronic Assembly", pages 14 to 18. Known tin based lead free solders are disclosed in the NEPCON West '92 Technical Program Proceedings entitled "Wettability Analysis of Tin-Based Lead Free Solders" by Messrs. Vianco, Hosking and Rejent, pages 1730 and 1731.

Some of the alternate solder alloys produce excessive amounts of dross or oxide while solder coating or solder joining occurs. The presence of dross tends to inhibit soldering. Dross can cause defects in soldering including bridges or links across terminals, unfound joints, oxide inclusions, incomplete solder joining and incomplete solder coverage. Therefore, it is desirable to eliminate oxygen from the process either entirely or partially in order to limit the amount of dross. Wave soldering of tin bismuth alloys has been carried out under a liquid flux as disclosed in IBM Technical Disclosure Bulletin, Vol. 23, No. 5, October 1980.

Acceptable quality soldering of lead alloy coating or joining on elements such as populated and unpopulated

electronic circuit boards is achieved with reflow soldering or wave soldering machines operating in normal atmospheric conditions including normal air. Fluxes are applied to the surfaces to be coated or joined to ensure solder wetting. A flux is defined as a chemically active compound which, when heated, removes minor surface oxidation, minimizes oxidation of the basis metal, and promotes the formation of an intermetallic layer between solder and basis metal (IPC Standard 1989 - ANSI/IPC-T- 50D) . More recently in order to minimize dross formation during wave soldering with lead based alloys and for other reasons, wave soldering has been performed using an inert gas blanket such as a nitrogen blanket or at least in an atmosphere which is substantially oxygen free. Similarly, reflow soldering under special atmospheres has been undertaken on printed circuit boards using reduction gas mixtures or inert gases to substantially, totally or partially exclude oxygen to provide improved reflow soldering results.

In reflow soldering, solder is applied to metal surfaces in a paste form and then heated to melt the solder to solder wet the surfaces. The solder paste is applied to at least one of the surfaces to be joined and then heated to melt the solder on the surfaces and form a solder joint.

It is an aim of the present invention to provide a process utilizing lead free solders that reduces dross formation during the soldering step, and obtain the wetting times and the wetting force similar to or approximately equivalent to those for lead based solders.

It is a further aim to provide a process utilizing lead free solders that reduces drossing in wave soldering machines and reduces oxides formed during reflow soldering of circuit assemblies.

The present invention provides a process of soldering an element, comprising the steps of supplying solder having a lead content less than 37% by weight to surfaces on the element to be solder coated or solder joined, and blanketing the surfaces to be solder coated on the element during soldering in a substantially oxygen free atmosphere.

When soldering with lead solder alloys, it is known to conduct solder wave processes under a shield gas or reducing gas blanket in a substantially oxygen free atmosphere. Such a process is disclosed in U.S. Patent Nos. 5,044,542, 5,048,746, 5,090,651, 5,121,874 and 5,125,556, all disclosures of which are incorporated herein by reference. Reflow soldering is disclosed in U.S. Patent No. 5,069,380 which describes an inerted IR soldering system. The disclosure of this patent is also incorporated herein by reference. In another embodiment, wave soldering in a shield gas atmosphere is disclosed under a shroud or cover which substantially excludes oxygen in U.S. patent applications Serial Nos.

07/860,316, 07/961,781, and in an application entitled "Gas Shrouded Wave Improvement" filed February 4, 1993. All of the disclosures of these applications are incorporated herein by reference. The benefits of soldering in an atmosphere which has a substantially oxygen free content are known, one major benefit being the reduction of dross formation on the solder surface.

When using lead free solder alloys, or alloys which have a reduced quantity of lead therein (i.e., less than 37%), it may be necessary in at least one embodiment to apply a flux either before soldering or at the time of soldering. Fluxes improve solder wetting and are used extensively with lead base solders. They are added generally prior to soldering preferably in a substantially oxygen free atmosphere. Any flux residues

remaining after soldering are preferably not toxic, not electrically conductive and are non-corrosive.

There are a number of candidates for lead free solders. In the first place a tin based lead free solder includes tin, in some cases with a small amount of lead (trace residual impurities of 0.003% lead are presently known to be acceptable, but this may change in the future), in other cases with no lead at all. Small quantities of silver and copper are included in a tin based solder alloy. Other examples of lead free solders include tin and- bismuth, tin and indium, tin and gold, tin and antimony, and tin, copper and gold, as well as tin and zinc. In several of these solders the melting temperature may be somewhat higher than that of the commonly used types of lead tin solders but the temperature is not sufficiently high to damage the circuit boards. In one example 95% tin, 5% lead was used as a solder, in a second example 96.5%, 3.5% silver was used, and another example 95.5% tin, 4% copper and 0.5% silver were used. In a further example 42% tin and 58% bismuth was used and in a still further example 48% tin and 52% indium was used. All of these solders are known and are disclosed in the article entitled "Surface Mount Council White Paper, An Assessment of the Use of Lead in Electronic Assembly". Another candidate in a lead free solder is phosphorous included with other components.

For wave soldering, one or two solder waves are used, the soldering occurs in a tunnel or an arrangement to blanket the solder wave and elements which in one embodiment are unpopulated circuit boards or populated circuit boards, passing therethrough, pass through an atmosphere that is substantially oxygen free. The solder wave and the circuit boards passing through the solder wave are blanketed to prevent air contacting the surface of the solder and the surface or surfaces being soldered

during solder coating or solder joining. In one example nitrogen was included as the gas blanket with the oxygen content being kept down to as low as 5 ppm to 10 ppm. In another example controlling the oxygen content up to about 20% by volume provides satisfactory soldering. By utilizing nitrogen with oxygen content as high as 20% allows the production of nitrogen by utilizing a membrane technology. In a preferred embodiment oxygen is in the range of 1 ppm to 100,000 ppm (10%) in nitrogen used for blanketing the solder reservoir and solder wave as a circuit board passes therethrough. In another embodiment a carbon dioxide gas is provided as a substantially oxygen free atmosphere.

Some lead free solders, may have residual impurities of lead in the amount of up to 0.003%, but this is minimal compared to the 37% present in known types of lead solder.

In the case of reflow solder, solder paste is applied to metallic surfaces to be either coated or joined and the circuit board is heated in air or in an inert atmosphere. The inert gas is preferably nitrogen, however, other inert gases such as carbon dioxide may suffice. The inert atmosphere is substantially oxygen free to prevent oxidation occurring on the solder as the solder paste melts. In the case of reflow soldering, flux is generally included in the solder paste.

Various changes may be made to the embodiments shown herein without departing from the scope of the present invention which is limited only by the following claims.