Metal treatment – Stock
Reexamination Certificate
1999-10-12
2001-01-23
Ip, Sikyin (Department: 1742)
Metal treatment
Stock
C420S560000, C420S561000
Reexamination Certificate
active
06176947
ABSTRACT:
The present invention relates to a lead-free solder alloy for use in soldering and solder interconnections. More particularly, the present invention relates to lead-free compositions containing effective amounts of tin, copper, silver, bismuth, antimony and/or indium and having a melting temperature between 175-215° C. The alloy is particularly useful in microelectronics and electronics applications.
Despite its current success in the electronics industry, Pb—Sn solder alloys face a limited future due to lead toxicity and the control or prohibition of the use of lead on a global landscape. Consequently, many initiatives on a world-wide basis have been taken to find suitable lead-free alternatives to Pb—Sn solder alloys. In the meantime, the high strength and high fatigue resistance of lead-free alloy is needed to meet the increasing level of performance in solder joints as required by the continued advancements in integrated circuit (IC) and IC package technologies.
In the hierarchy of electronics manufacturing, solder alloy is used to metallurgically join the bare chips or packaged chips onto the next level of a substrate through the formation of a desirable band of intermetallics. An instant flow and sound wetting of the solder alloy with the commonly used metallization pads such as Cu, Ag, Au, Pd, Ni and other metallic surfaces is a prerequisite to the formation of reliable solder joints under the high-speed automated manufacturing process using mild fluxes that are acceptable to electronic systems.
Surface mount technology has been a critical manufacturing technology in producing smaller, denser and faster printed circuit boards (PCB) that make modern electronics possible. The Pb—Sn eutectic solder of 63 Sn/37 Pb is most widely used in electronic assembly, particularly surface mount printed circuit boards. This solder provides another critical physical property, i.e., a moderate melting temperature, particularly below 210° C. The melting temperature of an alloy, except a eutectic composition, is often in a range specified by a liquidus and a solidus temperature. An alloy starts to soften at its solidus temperature and completes melting at its liquidus temperature. Soldering must be performed at a temperature above the liquidus temperature of the solder alloy.
A practical soldering process temperature for the surface mount manufacturing environment can be achieved at a temperature around at least 25° C. above the liquidus temperature of the solder alloy, for example, solder alloy having a liquidus temperature of 210° C. should be soldered at 235° C. as a minimum. The melting temperature of solder alloys is critical, because too high of a melting temperature will damage electronic devices and polymer-based PCB during soldering, while too low of a melting temperature will sacrifice the long-term reliability of the solder joints. For circuit board manufacturing involving typical polymer-based PCB such as FR-4, the process temperature cannot practically exceed 240° C. Therefore, a lead-free solder alloy which can replace 63 Sn/37 Pb and function in the surface mount manufacturing process must have a liquidus temperature below 215° C., preferably about 210° C.
Solder joints perform as electrical, thermal, and mechanical interconnections in an electronic system such as telecommunication, computer, avionics and automotive electronics. During the service life, solder joints are inevitably exposed to thermal stresses as the result of temperature fluctuation, power on/off, and/or harsh environmental conditions. This coupled with the mismatching thermal expansion in the interconnected materials of semiconductor, ceramic, metal, and polymer in the system, result in thermo-mechanical fatigue in solder joints. As the electronic circuitry becomes increasingly denser and the clock speed of microprocessors continues to reach ever-higher frequencies, one of the obvious effects on the design of and the material used for an electronic system is to handle the increasing heat dissipation.
In addition, the number of solder joints on each PCB continues to rise. The presence of several thousands or tens of thousands of solder joints on a PCB is not uncommon. However, any single solder joint failure results in a failed system. Consequently, requirements on the strength and fatigue resistance of solder joints are heightened. The recent developments in high pin count integrated circuit (IC) packages such as ball grid array (BGA), chip scale package (CSP), and direct-chip-attach technologies such as flip chip further demand the higher performance in fatigue resistance of solder alloys.
A number of lead-free solders have been proposed in the art. A summary of these lead-free alloys is outlined in Chapter 15 of the book “Modern Solder Technology for Competitive Electronics Manufacturing”.
U.S. Pat. No. 5,328,660 to Gonya et al for LEAD-FREE, HIGH TEMPERATURE, TIN BASED, MULTI-COMPONENT SOLDER describes a composition of 78.4 Sn 2 Ag 9.8 Bi 9.8 In. However, the fatigue resistance of this alloy is poor.
U.S. Pat. No. 5,527,628 to Anderson et al for PB-FREE SN—AG—CU TERNARY EUTECTIC SOLDER describes a composition of 93.6 Sn 4.7 Ag 1.7 Cu with the melting temperature of 217° C. The melting temperature of this alloy is still relatively high and its fatigue resistance is moderate.
U.S. Pat. No. 5,520,752 to Lucey et al for COMPOSITE SOLDERS describes a lead-free solder alloy comprising 86 to 97% Sn, 0.3 to 4.5% Ag, 0 to 9.3% Bi and 0 to 5% Cu. The fatigue resistance of the alloy is moderate or low.
U.S. Pat. No. 5,538,686 to Chen et al for ARTICLE COMPRISING a PB-FREE SOLDER HAVING IMPROVED MECHANICAL PROPERTIES describes a lead-free solder alloy with the melting temperatures from 173 to 193° C. comprising >70% Sn, 6 to 10% Zn, 3 to 10% In, <10% Bi, >5% Ag and <5% Cu. The alloys cannot wet typical substrates under electronic packaging and assembly manufacturing environment.
U.S. Pat. No. 5,580,520 to Slattery et al for LEAD-FREE ALLOY CONTAINING TIN, SILVER AND INDIUM describes a composition of 77.2 Sn 2.8 Ag 20 In with the melting temperatures from 179 to 189° C. The fatigue resistance of this alloy is low.
In summary, each of these early lead-free solders fail in at least one area to function adequately in forming reliable solder joints in the electronic packaging and assembly industry.
SUMMARY OF THE INVENTION
Accordingly, it is a primary object of this invention to provide a lead-free solder. It is an advantage of this invention to provide a lead-free solder that offers high-strength, and high fatigue resistance to withstand the increasingly adverse and harsh conditions in microelectronic and electronic applications.
It is a further advantage of this invention to provide a lead-free solder that has a moderate melting temperature range (175-210° C.) useful for mainstream electronics manufacturing.
It is a further advantage of this invention to provide a lead-free solder alloy that can readily wet common metallic substrates such as Sn, Cu, Ag, Au, Pd and Ni in microelectronic and electronic manufacturing to form sound and reliable solder joints without fluxes that are unacceptable to electronic manufacturing.
It is a further advantage of this invention to provide a lead-free solder that can adapt to the established electronic manufacturing process and infrastructure without requiring major changes in materials, processes and components.
Additional objects and advantages of the invention will be set forth in part in the description which follows and in part will be obvious from the description, or may be learned by practice of the invention. The object and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the foregoing objects and in accordance with the purposes of this invention, as embodied and broadly described herein, the solder alloys of this invention have Sn as a major constituent and effective amounts of Cu, Ag, Bi, In and Sb. The solder demonstrates compatible melting temp
Guo Zhenfeng
Hwang Jennie S.
Fay Sharpe Fagan Minnich & McKee LLP
H-Technologies Group, Incorporated
Ip Sikyin
LandOfFree
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