WO1999004048A1

WO1999004048A1 - Tin-bismuth based lead-free solders

Info

Publication number: WO1999004048A1
Application number: PCT/US1998/014183
Authority: WO
Inventors: Seach Hwee Goh
Original assignee: Litton Systems, Inc.
Priority date: 1997-07-17
Filing date: 1998-07-13
Publication date: 1999-01-28

Abstract

This invention relates to a lead antimony free solder composition. The alloy components, based on the total weight of alloy, fall in the following ranges: Ag 2.0 - 5.0 % by weight, Cu 0.3 - 2.0 % by weight, Bi 0.5 - 7.0 % by weight, Sn 90.0 - 93.5 % by weight. The alloys fall in two groups: high silver and low bismuth which have sharp melting points (5 °C or less difference between liquidus and solidus) and low silver high bismuth which have narrow melting points (20 °C or less).

Description

TIN-BISMUTH BASED LEAD-FREE SOLDERS This invention relates to lead-free and antimony free solder compositions that are meltable at temperatures sufficiently low for manufacturing electronic components and high enough for two-sided reflow, and that are suitable for wave soldering, the manufacture of wire solder and the manufacture of solder powder for use in making solder paste that is suitable for reflow soldering.

Lead-based solders have been widely used in the electronics industry. Particularly in electronic assembly manufacturing of printed circuit boards and components therefor, solders containing tin and lead have provided highly reliable connections in both automated and manual soldering on printed circuit boards. For example, tin-lead alloys of 60% tin and 40% lead or 63% tin and 37% lead have historically been used for most electronic soldering operations. These alloys were selected and indeed preferred because of their low melting temperatures, mechanical strength, low relative cost, superior wetting characteristics and electrical conductivity.

However, such use of tin-lead solders is becoming increasingly impermissible because of their potential toxic effects on workers and their potential contamination of the environment. Long-term exposure to lead can greatly affect the health of workers, and even small amounts of this metal can affect the neurological development of fetuses in pregnant workers.

Moreover, it is also true that many components and printed circuit boards are easily damaged by exposure to high temperatures during manufacture or assembly. It is consequently important that the melting range of a lead-free and antimony-free solder is low enough to minimize damage to such components and circuit boards while being high enough for two-sided reflow.

These lead-free and antimony-free solder compositions are acceptable for the electronics industry, are viable substitutes for tin-lead solder alloys, have higher tensile strength than either 63Sn37Pb or 96.5Sn3.5Ag solders, have sufficient ductility for drawing wires, have comparable wetting characteristics to 63Sn37Pb solder, and have a melting range that is high enough to allow for two-sided reflow but still lower than the melting temperature of 96.5Sn3.5Ag solder, whereby the peak reflow temperature need not be high and the width of the melting range selectively helps to spread the solder during reflow.

The alloy components of the present invention, based on the total weight of alloy, fall in the following ranges: Ag 2.0 - 5.0%

Cu 0.3 - 2.0%

Bi 0.5 - 7.0%

Sn 90.0 - 93.5% with Bi content preferably being 6.0% or less.

Although Bi content in the low silver content alloys of the present invention can be tolerated at elevated levels up to 7.0%, at this level increased coarsening and brittleness occurs. Bi content above 7.0% by weight, based on total alloy, adversely impacts on solder joint strength and ductility of the alloy and should be avoided.

The alloys of the present invention are denoted as tin-bismuth based because the alloy was developed from a tin-bismuth matrix.

In its one aspect, the present invention provides Sn-Bi matrix with Ag and Cu additives that provides a sharp melting point that is, a less than 5°C difference between the solidus and liquidus temperatures. A sharp melting point is highly desirable and indeed critical when soldering with automated equipment such as conveyorized wave or reflow soldering machines. In these processes, a mushy melting point (plastic range) causes granular cracking (cracks along the grain structure of the solder) . A sharp melting point is highly desirable during wave or reflow soldering because of the potential for movement during solidification of solder. The alloys that possess this property have the following formula, wherein the component content is designated by weight %, based on total alloy weight:

Ag 4.0 - 5.0% Cu 1.3 - 1.9%

Bi 0.8 - 1.7%

Sn 91.5 - 93.5% It is believed that, in accordance with the present invention, a sharp melting point can be achieved by maintaining bismuth content at a level that does not substantially exceed the amount of Bi that is soluble in Sn at room temperature. It has been observed that with the sharp melting point alloys of the present invention there is not always a direct correlation between the Sn-Bi ratio and the sharpness of the melting point. It is believed that this effect is at least in part a result of diminishing the ratio of tin to bismuth by intermetallic tin-copper and tin-silver formation.

Yet another aspect of the present invention resides in providing an alloy having a narrow melting/solidification range (less than 20°C) which can be produced with low silver content, in the range of 2.0-3.0% Ag.

This alloy can be used at significant savings for those soldering operations where a sharp melting point is not critical. The alloys that possess the narrow melting/solidification range fall within the following formula:

4.0 - 7.0% Bi, 2.0 - 3.0% Ag , 0.3 - 0.7% Cu and 90.0 - 92.5% Sn, wherein the % is weight percent based on total weight of alloy.

Unexpectedly, in these alloys of lower copper and silver content, increasing Bi appears to reduce rather than increase brittleness. An increase in brittleness that would be expected as a result of increasing bismuth content was not observed in the alloy specifically exemplified in Table 4. The tested narrow melting/solidification range alloy of example 2 which has a lower silver and copper content than the sharp melting alloys of the present invention has displayed a higher tensile strength and ductility than the higher silver and copper content alloys and, therefore, is most suitable for applications where increased tensile strength and ductility are required; that is, where an alloy is required to interfill large spaces between metal surfaces. The solder alloy compositions of the present invention may be prepared as solid round wire in diameters of from 0.020 to about 0.250 inch. The solder alloy compositions may also be prepared as wire cored with rosin, organic, or inorganic fluxes. Alternatively, the solder alloy compositions can be prepared in shapes and sizes to meet special requirements. For example, the solder alloy composition is easily manufactured as solder balls according to the disclosure of U.S. Patent 5,088,639. The solder alloy composition may further be prepared as pastes or creams by mixing a powder form of the solder with a suitable flux or carrier by techniques well known in the art, such as those disclosed in U.S. Patent 5,334,261. The solder alloy composition may further be prepared and used in the form of foils, sheets, or ribbons in various thicknesses or weights.

The solder alloy compositions of the present invention provide joints of components on a printed circuit board that are high in tensile strength, ductile, shiny and based on thorough wetting and spreading during reflow at melting.

The narrow melting point temperature range of the high-bismuth and lower silver solder alloy compositions and the sharp melting point temperature range of the high silver solder alloy compositions, specifically exemplified herein, provide liquidus temperatures which are essentially equal. These temperatures, coupled with the acceptable spreading and wetting characteristics of both the described solder alloy compositions falling under the general formula:

Ag 2.0 - 5.0% Cu 0.3 - 2.0%

Bi 0.5 - 7.0%

Sn 90.0 - 93.5% with Bi content preferably being 6.0% or less, show that the solder alloy compositions of the present invention are usable with existing mass and hand soldering equipment without damaging most printed circuit boards or electronic components, although, as noted before, the sharp melting point range alloys are preferred.

A sharp melting point or limited difference between the solidus and liquidus temperatures, that is, a melting point temperature range of less than about 5°C and preferably about 3°C, is achieved in accordance with the present invention at liquidus temperatures below that of Sn-Ag eutectic. The foregoing features coupled with acceptable spreading and wetting characteristics, shows that the sharp melting point solder alloy compositions of the present invention can be advantageously used with existing mass and hand soldering equipment.

The superior tensile strengths and their acceptable elongation also indicate that the solder alloy compositions of the present invention may be successfully substituted for the known tin-lead alloys currently used for electronics assembly and printed circuit board manufacture.

The solder alloy compositions of the present invention, especially the sharp melting point temperature range alloys, are well suited for many different applications. The solder alloy compositions may be employed in the coating of circuit boards and printed circuit board manufacture by use of "hot-air leveling" or "roll-tinning." These processes improve solderability on the circuit board.

Components may be precoated by hot solder dipping to help preserve solderability.

In addition, the solder alloy compositions may be used in the assembly of electronic components on printed circuit boards produced using wave reflow soldering.

Turning first to the high silver content alloys that provide lead-free solder alloy compositions that melt sharply or within a limited temperature range that is high enough to allow for two- sided reflow but still lower than the melting temperature of 96.5Sn/3.5Ag solder, whereby the peak reflow temperature need not be high, such alloys may be utilized in the form of pastes, containing alloy in powdered form that consist essentially, based on weight of total solder powder, of 4.0 - 5.0% Ag , 1.3 - 1.9% Cu , 0.8 - 1.7% Bi, and 91.5 - 93.5% Sn.

A sharp melting point or limited spread, that is, a melting point temperature range of less than about 5°C and preferably about 3°C is achieved in accordance with the present invention at liquidus temperatures below that of Sn-Ag eutectic. The foregoing features coupled with acceptable spreading and wetting characteristics, show that the alloys of the present invention are readily usable with existing mass and hand soldering equipment without damaging most printed circuit boards or electronic components.

Since the composition of the present invention contains copper and the embodiments provide for a spread of copper content, minor increases in the copper content do not readily affect performance of the composition. In addition, this new composition will not absorb copper as quickly as prior art tin-lead solders. As a result, this new alloy can remain functional much longer than prior art tin-lead alloys, thereby reducing overall solder consumption drastically and also reducing capital outlay by manufacturers.

In surface mount assembly or wave soldering of components to printed circuit boards, the present alloy solder composition can employ the same soldering fluxes, slightly higher temperatures, and process parameters similar to prior art tin-lead solders now currently in use. As noted by reference to the examples, compositions very close to a eutectic alloy which exhibit physical characteristics important to high speed, low-defect soldering can be achieved within the ranges specified herein. Since the solder alloy is less easily contaminated than tin-lead alloys, an increased usable life of the solder bath results. Further, solder joints formed by wave soldering yield high joint strengths and excellent electrical conductivity with even distribution of intermetallics throughout the solder joint.

The alloys of the present invention find particular application as the solder powder component of solder pastes and in wave soldering.

As noted above, the low silver/high bismuth content lead-free solder alloy compositions also may be utilized in the form of pastes. They consist essentially, based on weight of total solder powder, of 4.0 - 7.0% Bi, 2.0 - 3.0% Ag, 0.3 - 0.7% Cu, and 90.5 - 92.5% Sn as balance.

The low silver solder powder preferably consists essentially of 5.5% Bi, 2.5% Ag, 0.5% Cu, an 91.5% Sn as balance.

The solder compositions of the present invention are also suitable as solid round wire in diameters of from 0.020 to about 0.250 inch. The solder compositions may also be prepared as wire cored with rosin, organic, or inorganic fluxes. Alternatively, the solder compositions can be prepared in shapes and sizes to meet special requirements. The solder compositions may further be prepared and used in the form of solder spheres, foils, sheets, or ribbons in various thicknesses or weights.

The Sn-Bi based alloy of the present invention differs from the high silver content ternary alloys of the prior art in that the copper and silver additives introduced in the alloy of the present invention provide copper and silver content at low levels in the modified tin-bismuth binary solder system of the present invention. The low silver/high bismuth alloy of the present invention has a markedly superior tensile strength as compared to binary tin/ lead and tin/silver alloys of the prior art and a narrow "mushy" zone of less than 20°C.

EXAMPLES OF THE PREFERRED EMBODIMENTS EXAMPLE 1

The high silver content, lead-free solder composition of this invention, consisting essentially of tin, copper, silver, and bismuth, is exemplified by the following alloy examples, on a weight basis:

92.5% tin, 4.5% silver, 1.5% copper, and 1.5% bismuth, and

92.7% tin, 4.6% silver, 1.7% copper, and 1.0% bismuth.

This composition does not contain lead, which has been well documented to be toxic. This alloy also displays superior properties when compared to conventional 63 Sn/37 Pb and 96.5 Sn/ 5.3 Ag solders, as demonstrated by the following test data:

Table 1 - Tensile Properties ALLOY TENSILE STRENGTH, MPa %ELONGATION

92.5Sn4.5Agl.5Cul.5Bi 78.5 13.6

92.7Sn4.6Agl.7Cul.0Bi 70.8 12.6 63Sn37Pb 42.0 30.0

96.5Sn3.5Ag 41.6 30.9

Table 2 - Spreading and Wetting Properties

ALLOY SPREAD WETTING PROPERTIES RATE, % Time to zero Time to 2/3 F ax . uN force, s Fmax,s 92.5Sn4.5Agl.5Cul.5Bi 79.5 0.670 1.168 425.0 92.7Sn4.6Agl.7Cul.0Bi 78.0 1.666 2.580 356.7 63Sn37Pb 91.0 0.740 1.308 387.1

96.5Sn3.5Ag 76.2 -

Table 3 - Melting Range ALLOY MELTING RANGE. "C

92.5Sn4.5Agl.5Cul.5Bi 213 -216

92.7Sn4.6Agl.7Cul.0Bi 215.5

63Sn37Pb 183

96.5Sn3.5Ag 221

Tensile testing was done according to ASTM-E8M-94 , using specimens of 25 mm gauge length and strain rate of 0.4 mm/mm/min. The specimens were cast from molten solder.

Spread rate tests were done on copper according to JIS Z 31907-1986, using Kester #135 flux, at 250"C, with approximately 0.3 g of solder.

Wetting tests were done according to MIL-STD-883D, using Kester #135 flux, at 250°C. Copper coupons measuring 20 mm x 40 mm were used. "Time to zero force" is the time taken, after a Cu coupon was dipped into the solder bath, for the interfacial force between the solder and the Cu coupon to reach zero. The smaller the wetting time, the better the solder. Fmax is the maximum wetting force of the solder. The higher the Fmax, the better the wetting properties.

EXAMPLE 2

The low silver, lead-free solder composition of this invention, consisting essentially of tin, copper, silver, and bismuth, is exemplified by the following alloy example, on a weight basis:

91.5% tin, 0.5% copper, 2.5% silver, and 5.5% bismuth. This alloy does not contain lead, which has been well documented to be toxic. This alloy also displays superior properties when compared to conventional 63 Sn/37 Pb and 96.5 Sn/5.3 Ag solders, as demonstrated by the following test data:

Table 4 - Tensile Properties ALLOY TENSILE STRENGTH, MPa %ELONGATION

91.5Sn0.5Cu2.5Ag5.5Bi 90.0 16.4

63Sn37Pb 42.0 30.0

96.5Sn3.5Ag 41.6 30.9

Table 5 - Spreading and Wetting Properties ALLOY SPREAD WETTING PROPERTIES

RATE, % Time to zero Time to 2/3 Fmax, uN force, s Fmax, s 91.5Sn0.5Cu2.5Ag5.5Bi 79.4 0.754 1.058 335.4 63Sn37Pb 91.0 0.740 1.308 387.1

96.5Sn3.5Ag 76.2

Table 7 - Melting Range ALLOY MELTING RANGE, °C

91.5Sn0.5Cu2.5Ag5.5Bi 198-215

63Sn37Pb 183

96.5Sn3.5Ag 221

Tensile testing was done according to ASTM-E8M-94, using specimens of 25 mm gauge length and strain rate of 0.4 mm/mm/min. The specimens were cast from molten solder.

Spread rate tests were done on copper according to JIS Z 31907-1986, using Kester #135 flux, at 250°C, with approximately 0.3 g of solder.

Wetting tests were done according to MIL-STD-883D, using Kester #135 flux, at 250"C. Copper coupons measuring 20 mm x 40 mm were used. "Time to zero force" is the time taken, after a Cu coupon was dipped into the solder bath, for the interfacial force between the solder and the Cu coupon to reach zero. The smaller the wetting time, the better the solder. Fmax is the maximum wetting force of the solder. The higher the Fmax, the better the wetting properties.

From Table 4, it can be seen that the new alloy has more than twice the tensile strength of either of the prior art lead-free, binary alloys. Nevertheless, it has sufficient ductility for the drawing operation required to make wires.

Table 5 shows that the new alloy also has comparable wetting characteristics when compared to 63 Sn/37 Pb.

Table 6 shows that the melting range of the new alloy is high enough to allow for two-sided reflow, but it is still lower than the temperature of 96.5 Sn/3.5 Ag . This means that the peak reflow temperature need not be as high. The wide melting range is also beneficial in spreading the solder during reflow.

As will be appreciated by those skilled in the art, the alloys of the present invention may include additives in amounts not exceeding 1% to improve wettability, enhance fatigue strength and/or refine solder joint grain size.

Although the present invention has been described with reference to a preferred embodiment, workers skilled in the art will recognize that changes may be made in form and detail without departing from the principles and scope of the invention.

Claims

WHAT IS CLAIMED IS:

1. A solder alloy composition, based on weight of total solder, consisting essentially of:

Ag 2.0% - 5.0% by weight Cu 0.3% - 2.0% by weight Bi 0.5% - 7.0% by weight Sn 90.0 %- 93.5% by weight.

2. The solder alloy composition according to claim 1, wherein the Bi content is 6.0% by weight or less.

3. A solder alloy composition according to claim 2, wherein said alloy consists essentially of 4.0 - 5.0% Ag, 1.3 - 1.9% Cu, 0.8 - 1.7% Bi, and 91.5 - 93.5% Sn.

4. A solder alloy composition according to claim 3, wherein said alloy consists essentially of 4.6% Ag, 1.7% Cu, 1.0% Bi, and 92.7% Sn.

5. A solder alloy composition according to claim 3, wherein said alloy consists essentially of 4.5% Ag, 1.5% Cu, 1.5% Bi, and 92.5% Sn.

6. The solder alloy of claims 1, 2, 3, 4 or 5, wherein said alloy is a powder in the form of a solder paste that is useful in electronic assembly.

7. A solder joint comprising a lead-free and antimony-free solder solidified in contact with an electrical conductor, wherein said solder composition consists essentially of 4.0% - 5.0% Ag , 1.3 - 1.9% Cu, 0.8 - 1.7% Bi, and 91.5 - 93.5% Sn.

8. A method of surface mounting an assembly of electronic components onto a printed circuit board using wave soldering by passing the substrate to be soldered in contact with a solder wave wherein the solder consists essentially, based on weight of total solder in said wave, of 4.0 - 5.0% Ag, 1.3 - 1.9% Cu, 0.8 - 1.7% Bi and 91.5 - 93.5% Sn.

9. In a method of surface mounting an assembly of electronic components onto a printed circuit using reflow soldering and a solder paste, the improvement which comprises using the powdered solder in said solder paste, a solder powder consisting essentially of, based on weight of total powder in said paste, 4.0 - 5.0% Ag, 1.3 - 1.9% Cu, 0.8 - 1.7% Bi, and 91.5 - 93.5% Sn.

10. The method of claims 8 or 9 , wherein said solder consists essentially of 4.6% Ag, 1.7% Cu, 1.0% Bi, and 92.7% Sn.

11. The method of claims 8 or 9 , wherein said solder consists essentially of 4.5% Ag, 1.5% Cu, 1.5% Bi, And 92.5% Sn.

12. A solder alloy composition according to claim 1, wherein said alloy consists essentially of 4.0 - 7.0% Bi , 2.0 - 3.0% Ag, 0.3 - 0.7% Cu, and 90.0 - 92.5% Sn.

13. The solder alloy composition according to claim 12, wherein the Bi content is 6.0% or less.

14. A solder alloy composition according to claim 12, wherein said powder consists essentially of 5.5% Bi, 2.5% Ag, 0.5% Cu, and 91.5% Sn.

15. The solder alloy of claims 2, 3, 4, 5, 12, 13 or 14, wherein said alloy is formed into a solder wire, bar ribbon or sphere that is useful in electronic assembly.

16. The solder alloy of claim 2, wherein said wire has a core of flux.

17. The solder alloy of claims 7 or 12, wherein said alloy is a powder in the form of a solder paste that is useful in electronic assembly.

18. A method of surface mounting an assembly of electronic components onto a printed circuit board using wave soldering by passing the substrate to be soldered in contact with a solder wave wherein the solder consists essentially of, based on weight of total solder powder in said wave, 4.0 - 7.0% Bi, 2.0 - 3.0% Ag, 0.3 - 0.7% Cu, and 90.0 - 92.5% Sn.

19. The method of claim 18, wherein said solder consists essentially of 5.5% Bi, 2.5% Ag, 0.5% Cu, and 91.5% Sn.

20. A solder joint comprising a lead-free and antimony-free solder solidified in contact with an electrical conductor, wherein said solder consists essentially of 4.0% - 7.0% Bi, 2.0 - 3.0% Ag, 0.3 - 0.7% Cu, and 90.0 - 92.5% Sn, having a solidus temperature of 198┬░C and a liquidus temperature not exceeding 17 ┬░C thereabove.

21. In a method of surface mounting an assembly of electronic components on to a printed circuit using reflow soldering and a solder paste, the improvement which comprises using as the powdered solder in said solder paste, a solder powder consisting essentially of, based on weight of total powder in said paste, 4.0 - 7.0% Bi, 2.0 - 3.0% Ag, 0.3 - 0.7% Cu and 90.0 - 92.5% Sn.