SOLDERING COMPOSITION

The invention is directed to soldering alloys that are non-toxic, exhibit low melting temperatures, behave well in normal soldering applications such as, for example, hand soldering, wave soldering or paste reflow soldering, and impart useful long-term properties to the connections made with them.

Tin has a melting temperature of about 232° C, and is the primary ingredient ofthe inventive compositions. Tin readily wets a large number of other metals and forms a eutectic with many of these alloys. (A eutectic is a low melting temperature alloy of specific composition that changes from liquid to two or more solid phases at a precise single temperature rather than solidifying over a temperature span often referred to as a "pasty range".) While tin can be an adequate solder when used alone, it suffers from two significant drawbacks. First, its melting temperature of 232°C is too high for many soldering applications, including routine electronic circuit assembly.

The addition of other alloying ingredients can lower the melting temperature. Second, tin forms an allotropic, beta phase at reduced temperatures. This phase is non-metallic and has very low strength. The presence of certain alloying ingredients can inhibit the formation of this phase. In formulating the inventive compositions, the choice of which chemical elements should be added to tin and in what amounts was made with several factors in mind. First, low toxicity was required of all alloying elements used in the compositions. For this reason, lead, which is a standard though toxic element used for these purposes, is not used in any of the inventive compositions. Second, a substantial reduction in the melting temperature of

232°C was required. Third, high wettability of the composition on common substrate surfaces, such as copper, was important. Fourth, good melting behavior of the composition was required. For instance, it is often beneficial for a solder to exhibit a limited temperature range pasty range, thereby resembling a eutectic material. Fifth, low cost and availability of alloying elements was desirable. Sixth, a low chemical reactivity ofthe molten solder alloys with air was desirable. Seventh, a low corrosion rate was important. Finally, good mechanical properties ofthe alloy compositions were required.

The binary alloy which forms the foundation for the inventive compositions is the well known tin-silver eutectic 96.5Sn-3.5Ag with a melting temperature of about 221°C. All compositions are given in percent by weight unless otherwise indicated. Copper is then added to this binary alloy to produce a nearly eutectic tin-silver-copper ternary alloy which is one of the

inventive compositions and is the base for the remaining inventive compositions. A preferred composition of this ternary alloy is 95.8Sn-3.5Ag- 0.67Cu with a solidus temperature of about 213°C and a liquidus temperature of about 218°C. Many solder compositions are known, but most have one or more poor properties. For example, alloys containing significant fractions of antimony have poor wetting characteristics and melting temperatures that are too high for many applications. Zinc-tin eutectic solder has a favorable melting temperature of 199°C, but the zinc in the molten alloy oxidizes rapidly when contacted with air. Alloys having relatively low silver fractions have broad pasty ranges which, while suitable for many plumbing applications, are not useful in electronics applications, where a eutectic or nearly eutectic alloy is favorable. Tin-based solders having significant bismuth contents generally have poor fatigue characteristics (relative to the standard tin-lead eutectic solder). Even the tin-lead eutectic solder has drawbacks apart from its toxicity. For example, the fatigue behavior of this alloy is inferior to most of the present non-bismuth containing inventive compositions.

The inventive compositions have unusually good combinations of the most important solder properties — namely, wettability, fatigue life, cost and corrosion resistance. They also have demonstrated excellent strength

(relative to the tin-lead eutectic solder) and high resistance to electrochemical migration.

SUMMARY OF THE INVENTION A soldering composition comprising by weight about 3.1-3.5% silver, 0.5-2.7% copper and the balance tin, having a preferred composition of about

3.5% silver, 0.67% copper and 95.8% tin. A further soldering composition comprising by weight about 3.7-4.6% silver, 1.0-1.6% copper and the balance tin, having a preferred composition of about 4.5% silver, 1.5% copper and 94.0% tin. A further soldering composition comprising by weight about 3.1- 6.5% silver, 0.25-0.8% copper and the balance tin, having a preferred composition of about 5.0% silver, 0.7% copper and 94.3% tin. A further soldering composition comprising by weight about 1.5-7.0% silver, 0.4-1.4% copper, 0.5-6.0% indium and the balance tin, having a preferred composition of about 3.3% silver, 0.67% copper, 4.1% indium and 91.9% tin. A further soldering composition comprising by weight about 0.1-6.0% silver, 0.1-0.4% copper, 0.1-2.0% antimony and the balance tin, having a preferred composition of about 5.0% silver, 0.4% copper, 0.3% antimony and 94.3% tin. A further soldering composition comprising by weight about 3.0-5.2% silver,

0.4-2.7% copper, 0.4-2.6% zinc and the balance tin, having a preferred composition of about 3.6% silver, 0.67% copper, 1.1% zinc and 94.6% tin. A further soldering composition comprising by weight about 1.4-7.1% silver, 0.5- 1.3% copper, 0.2-9.0% indium, 0.4-2.7% antimony and the balance tin, having a preferred composition of about 3.3% silver, 0.66% copper, 4.2% indium, 1.3% antimony and 90.5% tin. A further soldering composition comprising by weight about OJ-10.0% silver, OJ-3.0% copper, 0.07-20.0% indium, 0.05-9.0% zinc and the balance tin, having a preferred composition of about 3.3% silver, 0.66% copper, 4.2% indium, 1.3% zinc and 90.5% tin. A further soldering composition comprising by weight about 1.5-4.5% silver, 0.3-1.4% copper, 0.1- 10.0% indium, 0.01-0.5% antimony, 0.01-3.0% zinc and the balance tin, having a preferred composition of about 3.5% silver, 0.69% copper, 0.44% indium, 0.45% antimony, 0.11% zinc and 94.8% tin. A further soldering composition comprising by weight about 0.2-7.4% silver, 0.2-1.4% copper, 0.02-8.0% indium, 0.02-10.0% bismuth and the balance tin, having a preferred composition of about 3.5% silver, 0.69% copper, 2.2% indium, 4.5% bismuth and 89.1% tin. A further soldering composition comprising by weight about 3.1-7.4% silver, 0.2-1.4% copper, 0.02-2.5% antimony, 0.02-2.4% zinc and the balance tin, having a preferred composition of about 3.5% silver, 0.69% copper, 1.4% antimony, 1.1% zinc and 93.3% tin.

Also an aspect ofthe invention is a method for soldering comprising the step of employing a solder composition ofthe invention.

DETAILED DESCRIPTION A table of the properties of the preferred compositions of the inventive solders, as well as the standard tin-lead eutectic solder, follows:

Alloy Melting Solderability Fatigue Corrosion

Range on Cu

ID

95.8Sn-3.5Ag 213°C solidus Excellent. Best of alloys Very

-0.67Cu ~218°C liquidus Very similar to shown. Good. standard alloy (12) below for the same superheating.

(2)

94.0Sn-4.5Ag 214°C solidus Excellent. Very With alloy (1), Very -1.5Cu ~215°C liquidus similar to best of all good, standard alloy alloys shown. (12) below for the same superheating.

(3)

94.3Sn-5.0Ag 214°C solidus Near excellent. Very good. Very

-0.7Cu ~216°C liquidus good.

(4)

91.9Sn-3.3Ag 211°C solidus Good but poorer Very good. Good to

-0.67Cu-4.1In ~217°C hquidus than alloys (9) very and (10) below. good.

(5)

94.3Sn-5.0Ag 214°C solidus Very good. Very good. Very

-0.4Cu-0.3Sb ~224°C hquidus good.

(6)

94.6Sn-3.6Ag 214°C solidus Shghtly worse Good. Good.

-0.67Cu-lJZn ~218°C liquidus than alloy (4) above.

(7)

90.5Sn-3.3Ag 198°C solidus Very good. Good. Good to

-0.66Cu-4.2In ~215°C hquidus Between alloys very

-1.3Sb (4) and (8). good.

(8)

90.5Sn-3.3Ag 190°C solidus Nearly as good Medium. Fair.

-0.66Cu-4.2In ~215°C hquidus as alloy (9)

-1.3Zn below.

(9)

94.8Sn-3.5Ag 214°C solidus Excellent. Best Very good. Good to

-0.69Cu-0.44In ~220°C hquidus of all alloys very

-0.45Sb-0.11Zn shown. good.

(10)

89.1Sn-3.5Ag 142°C solidus Fair to good. Fair to poor. Fair.

-0.69Cu-2.2In ~204°C hquidus About same

-4.5Bi asstandard alloy (12) below.

(11)

93.3Sn-3.5Ag 213°C solidus Shghtly worse Excellent. Good.

-0.69Cu-1.4Sb ~220°C liquidus than alloy (42)

-l.lZn above.

(12)

Standard 183°C eutectic Excellent. Fair to poor Fair to

63Sn-37Pb poor.

Each of the properties described in the above table was measured using conventional means. Melting temperature was measured using a Dupont Model 2100 thermogravimetric analyzer containing a differential scanning calorimetry (or "DSC") cell commercially available from Thermal Analysis Instruments of New Castle, Delaware. The solder sample to be measured was placed in one of two locations within the DSC cell. A dummy specimen was placed in the other locations in the DSC cell. The temperature of the apparatus was then raised at a specified rate, and the difference in thermocouple voltages from the two specimens was monitored. The melting process was detected through changes in this voltage signal. For example, the melting point of pure elements and eutectic alloys can be detected by a sharp drop in this signal. Solderability was measured using a conventional wetting balance method. Summarizing this method, liquid flux was applied to a standard test strip (of copper, in this case). The test strip was then fastened to a Multicore Universal Solderability Test (or "MUST") device commercially available from

Multicore Solders of Richardson, Texas. A molten bath of the solder to be tested was then placed in a solder pot contained in the device and brought to a predetermined temperature. An automatic test cycle ofthe device then began by raising the solder pot until electrical contact was made with the test strip, at which point the pot was raised an additional predetermined amount. The apparent weight of the test strip was then measured as the solder lifted and then wet up on the test strip. The rate of wetting and the maximum weight of the solder applied to the strip indicate solderability.

Fatigue was measured using a test electronic circuit board containing multiple leads. The solder to be measured was applied to the board to form one or more continuous circuits (referred to as "daisy chains") connecting the leads. As strain is applied to the board, the solder will accumulate fatigue until the circuit is broken. Different methods used to apply strain to the board involved an isothermal bending test, in which the board was bent in different directions at high speed (approximately two cycles per minute) and uniform temperature, and a thermal cycling test, in which the board was more slowly cycled through hot and cold temperatures (approximately 80° C to -30° C) repeatedly.

Corrosion was tested using a conventional process in which the solder sample to be measured was formed into an electrode. Both this electrode and a standard calomel electrode were placed into a 0.04% ammonium chloride solution, and the potential between these two electrodes was then measured. A more positive potential for the electrode being tested indicates a more corrosion resistant solder sample. Referring to the table shown above, alloys (l)-(3) generally contain a higher silver content than most existing tin-silver-based solders. This somewhat higher silver content results in a more nearly eutectic solder, desirable in electronic applications.

The addition of indium in alloy (4) improves the wetting behavior and lowers the melting temperature, with little impairment of fatigue life or corrosion resistance. The addition of copper also lowers the melting temperature and helps strengthen the alloy.

The antimony addition to alloy (5) suppresses the undesirable beta tin phase referred to above. A higher silver content and the elimination of antimony and nickel distinguish alloy (6) from existing solders. The removal of antimony softens the alloy but not to a significant degree, while the removal of nickel results in a

better behaved alloy that is easier to manufacture. The higher silver content, as mentioned above, results in a more nearly eutectic alloy.

The addition of indium in alloy (7) reduces the melting temperature and, because the indium and antimony levels are relatively low, improves the fatigue characteristics.

The combination of indium and zinc in alloy (8) lowers the melting temperature. Also, the addition of zinc, in place of indium, lowers the cost of the alloy.

The addition of indium in alloy (9) improves the wetting behavior and, with the antimony and zinc additions, all in limited amounts, improves fatigue life.

The addition of bismuth and indiu together in alloy (10) reduces the melting temperature.

The higher silver content of alloy (11) distinguishes it from existing solders. As stated above, this higher silver content results in a more nearly eutectic solder that is desirable in electronic applications.

Each of the inventive compositions can be used in all the major modes of usage for electronic soldering (for example, hand soldering, wave soldering and paste reflow soldering). Each of the inventive compositions can be made easily by melting pure tin and adding the remaining alloying elements. For quantities of up to about one kilogram, this can be done in a ceramic crucible or in borosilicate glass labware. The resulting compositions can be used as melted for wave soldering. For use in hand soldering, the resulting compositions generally are extruded to form a wire which can contain flux, if desired.

Conventional reflow soldering requires the placement of the solder components into prints of solder paste which are then heated, usually in a belt oven. For this use, the solder generally is made into a powder and blended with a suitable flux and other vehicle materials. Solder powders can be made using a variety of known techniques. One such technique involves atomizing molten solder with a burst of pressurized nitrogen, collecting the powders, separating into the desired size fraction, remelting the other size fractions, and repeating the process.