Lead-free solder with low copper dissolution

Abstract

Lead-free solder compositions suitable for joining electronic devices to printed wiring boards, which comprises by weight 0.2 to 0.9% copper, 0.006 to 0.07% nickel, 0.03 to 0.08% bismuth, less than 0.5% silver, less than 0.010% phosphorus, and a balance of tin and inevitable impurities. A solder composition embodying this invention finds particular application in automated wave-soldering machines where conventional lead-free solders dissolve excessive copper from printed wiring circuitry and component terminations.

Description

CROSS REFERENCES TO RELATED APPLICATIONS [0001] This application claims priority from and is related to commonly owned U.S. Provisional Patent Application Ser. No. 60 / 761,400 filed Jan. 23, 2006, entitled: Lead-Free Solder With Low Copper Dissolution, this Provisional Patent Application incorporated by reference herein.FIELD OF THE INVENTION [0002] The present invention relates to low-cost solder compositions for bonding electronic devices or parts to printed wiring boards (PWBs). In particular, the present invention relates to solder compositions that prevent or minimize the dissolution of copper into the solder during the soldering process. BACKGROUND OF THE INVENTION [0003] Electronic assemblies are composed of a printed wiring board (PWB), sometimes also known as a printed circuit board (PCB), which is constructed of an insulating board, such as glass-epoxy or paper-epoxy, on which copper circuitry is formed on one or both sides. Electronic components, wires, or other devices wi...

AI Technical Summary

Benefits of technology

[0006] A common practice by those using automated dip or wave soldering machines, to adjust for the increasing copper percentage in the solder, is to use a second alloy with reduced copper percentage for replacement additions of solder to the pot. For example, where a solder composition of 0.7% copper in tin is the most economical lead-free solder of choice in the automated soldering machine, additions of solder to replace expended solder and dilute dissolved copper may contain only from 0% to 0.5% copper. This adjustment is not precise and can affect the quality of the soldering process and the reliability of the soldered electronic assembly. A significant advantage of the present invention is that the dissolution rate of copper into the melted solder is sufficiently low that the use of another solder alloy is not necessary.

[0007] A solder wave is formed by pumping molten solder, contained within a solder pot, up through a nozzle to provide a standing wave. Usually, only one wave is employed, but dual waves are also employed, particularly when surface mounted devices are being soldered to the bottom side of printed circuit boards. Solder cascades and solder jets also find application as wave soldering. Alternatively, the solder may be maintained in an open solder pot where an electronic assembly may be dragged across the melted solder surface to accomplish the soldering. One of the problems encountered with automated soldering processes is that the molten solder oxidizes when exposed to the oxygen in the air. The oxidized solder forms a surface oxide layer, which must be removed by a flux before the components being soldered will wet with solder. Particularly with wave soldering, the surface oxide layer is continually broken by the flow of solder in the wave. This exposes fresh solder which, in turn, is also oxidized. A mixture of oxide and solder, thus, collects within the solder pot. This mixture is known as dross, which must be removed and disposed. Dross generation adds to the cost of the process due to the lost value of the solder and the maintenance time required to remove it and repair mechanical parts of the wave soldering apparatus damaged by the abrasive action of the dross.

[0008] One method employed to minimize the formation of oxide on the solder in a wave soldering machine is to cover the surface of the melted solder with an oil. This is effective in protecting the solder from atmospheric oxygen, but the oil degrades and must be replaced periodically. Furthermore, the oils commonly used are difficult to clean off of the components being soldered and can produce a great deal of smoke at wave soldering temperatures. A solution for this problem for over three decades is to add phosphorus to the solder. U.S. Pat. No. 5,240,169 describes the use of known low dross solder containing 10 to 1000 parts per million (ppm) phosphorus.

[0009] More recently, concerns about safety and environmental pollution caused by lead (Pb) in the solder used for assembling electronic products has resulted in the development of environmentally acceptable, substitute solder compositions where the lead (Pb) has essentially been replaced with tin (Sn). There are a variety of other metals—such as silver (Ag), copper (Cu), antimony (Sb), zinc (Zn), indium (In), and bismuth (Bi)—that can be added to tin (Sn) either individually or in combination to reduce the melting temperature, improve the ductility and strength of the solder joint, and / or improve wetting to the metal surfaces being soldered.

[0010] Some examples are described by the patents referenced in Table 1 below: TABLE 1(Prior Art) Values Given in Weight PercentU.S. Pat.No.SnAgCuSbZnInBiNiPOther1,239,195Balance0.5-1  0.5-1  ———————1,437,64185-950.5-4.50.5-4.5—0.5-9.5—————4,193,530Balance————0.1-0.5  0.1-0.5———4,670,21790.0-98.50.5-2  —0.5-4.00.5-4.0—————4,758,40792.5-96.90.1-0.53.0-5.0————0.1-2.0——4,879,096  88-99.350.05-3  0.5-6  ———0.1-3———4,929,423Balance0.01-1.5 0.02-1.5 ———0.08-20—0.10Rare Earth 0-0.25,094,81395.680.08-0.162.8-3.5—0.2-0.5——0.08-0.16——5,352,40793-981.5-3.50.2-2.00.2-2.0——————5,817,194Balance≦10≦3————0.5-5.00.05-1.5—5,837,191Balance0.05-0.6 0.05-0.6 0.75-2  ———0.05-0.6 ——5,863,49391.5-96.52.0-5  0-2————0.1-2  ——5,980,82281.4-99.60.1-5.00.1-5.50.1-3.0——  0.1-5.0—0.001-0.01Ge 0.01-0.15,985,212>75.0—0.01-9.5 ——  0-6.0———Ga 0.01-5.06,139,979Balance—0.7-2*  3.0-5.0*———0.01-0.5 —*at least one6,179,935Balance >0-4.0 <0-2.0———— >0-1.0—Ge >0-16,180,055Balance—0.1-2  ————0.002-1   —Ge 0-16,296,722Balance—0.1-2  ————0.002-1   —Ga 0.001-16,365,097Balance ≦4≦2———≦21≦0.2 —Ge <0.16,440,360Balance—0.8————0.9—6,488,888Balance 0.1-3.5*0.1-3* — 7-10——0.01-1  0.001-1  *at least one6,649,127Balance—0.3-3  —0.5-10 —0.5-8——Ge 0.005-0.056,660,226>88.00.5-9  0.5-2  ——————Co 0.1-2.06,702,176Balance1.0-4.00.2-1.3——————Co 0.02-0.066,843,86288.5-93.23.5-4.50.3-1.0——2.0-6.0——0.01—

[0011] There are specific problems with the above listed solder alloy compositions that make them not as desirable for soldering of electronic assemblies as the tin-lead solder compositions they are intended to replace.

Problems solved by technology

Copper is particularly susceptible to dissolving in lead-free solders because the lead-free solders melt at higher temperatures than tin-lead solder and consist of more than 95% tin.

A very small increase in copper from 0.7% to 1.0% will raise the liquidus melting point of the tin-copper solder alloy to 239° C., while an increase in copper to only 1.5% results in a liquidus melting point of 260° C. One solution would be to increase the solder temperature to accommodate the increased amount of copper, but this would not be acceptable because of the damage caused by excessive heating of the electronic devices being soldered.

This adjustment is not precise and can affect the quality of the soldering process and the reliability of the soldered electronic assembly.

One of the problems encountered with automated soldering processes is that the molten solder oxidizes when exposed to the oxygen in the air.

This exposes fresh solder which, in turn, is also oxidized.

Dross generation adds to the cost of the process due to the lost value of the solder and the maintenance time required to remove it and repair mechanical parts of the wave soldering apparatus damaged by the abrasive action of the dross.

Furthermore, the oils commonly used are difficult to clean off of the components being soldered and can produce a great deal of smoke at wave soldering temperatures.

There are specific problems with the above listed solder alloy compositions that make them not as desirable for soldering of electronic assemblies as the tin-lead solder compositions they are intended to replace.

Tin without the addition of other metals is unacceptable for soldering electronic assemblies for several reasons.

First, soldering temperatures required for using tin (Sn) with its high melting point (232° C.) may damage electronic components.

Second, the wetting ability is poor because of the high surface tension of the melted tin.

Third, the tensile strength and ductility are unacceptably low because of the course grain structure of the solidified metal.

Nevertheless, the ability of these solders to wet or bond to copper is impaired by the high surface tension and slow wetting speed of tin-silver solders on copper.

Also, silver is too costly for large scale manufacturing of electronic products, and the appearance of the completed solder joints is dull or frosty, unlike the acceptable appearance of tin-lead solder.

Tin-copper solders, such as the SnCu0.7 eutectic composition, melt at an acceptable temperature (227° C.) for hand or automated soldering, but the surface tension of the solder alloy is still high compared to the conventional tin-lead solder, resulting in inferior wetting ability compared to that of the tin-lead solder.

Tin-antimony solder, such as the well-known composition SnSbO5, provides solder joints with acceptable tensile strength, but the melting temperature (232-240° C.) is higher than alloys of tin and copper or silver, and therefore too high of a melting temperature for heat-sensitive electronic components.

Also, antimony seriously reduces the ability of the solder to spread on a copper surface because of the formation of a copper-antimony intermediate phase between the copper and the solder alloy.

U.S. Pat. No. 1,437,641 describes a well-known tin-zinc solder alloy with acceptable melting temperature, but unacceptably rapid oxidation and corrosion problems.

However, for automated applications, such as wave soldering, tin alloys containing zinc (Zn) are subject to very rapid oxidation while being pumped to generate a standing wave of solder, resulting in a large production of dross, i.e., a mixture of metal oxides and metal particles that float on the surface of the melted solder.

There is also much concern about the potential galvanic corrosion of solder joints made with solders containing zinc.

However, for soldering electronic components to a printed circuit board, solder alloys that contain more than about 2% to 5% bismuth (Bi) are incompatible with lead (Pb) that may be contained on the electronic component terminations, resulting in potential cracked solder joints.

Tin alloys containing indium (In) have the same high cost problem as those with silver, even though indium additions are able to improve the wetting ability of the solder.

The use of germanium in price competitive solder compositions is precluded because of the very high cost of germanium.