Lead-free solder patents

B. Liu. Lead free solder friendly thermoplastic blends and methods of manufacture thereof. US Patent 7 037 986, assigned to General Electric Company (Pittsfield, MA), May 2, 2006. 

CondUCtiVG FillGrS. Conductive fillers are used to provide the adhesive with electrical conductivity. The simplest fillers are metal particles such as gold, silver, nickel, indium, copper, chromium, and lead-free solders (SbBi) (6,7,9-11). The particles are usually spherical and range 3-15 /tm in size for ACA applications (12). Needles or whiskers are also quoted in some patents (6). 

The UK Cookson Group s, Cookson Electronics subsidiary has negotiated licences with the patent holders of various lead-free solders. These enable it to manufacture globally and supply them as a patent holder, making royalty payments on sales. The alloys concerned are Senju Sn/Ag/Cu (Senju-... 

Over the last decade, the industry has studied a wide range of alloys to replace the tin-lead alloy. The alloy selection has been based on the following considerations (Ref 12-15) toxicity, physical properties (melting temperature, surface tension and wettability, thermal and electrical conductivity), mechaiucal properties, mi-crostructural characteristics, electrochemical properties (corrosion, oxidation and dross formation, and compatibility with no-clean fluxes), manufacturability, cost, and availability. Yet another important consideration for selecting the lead-free solder alloy for commercial use is whether or not the alloy may be covered by any patents. Lead-free alloy selection, as weU as associated patent issues, have been described in detail in toe literature in Ref 16-20. 

A. Achari, M.R. Paruchuri, and D. Shangguan, Lead-Free Solder Compositions, U.S. patent 5,863,493 (1999), and European Patent 0847829... 

D. Shangguan and A. Achari, Lead-Free Solder Alloys, U.S. Patent 5,429,689 (1995), and Mexico Patent 196053... 

TABLE 22 Partial List of U.S. Patents on Lead-Free Solders... 

In addition, four and five-component lead-free solder alloys, many of them patented, will present manufacturing problems because it will be very difficult to maintain the exact alloy composition in wave soldering machines. Solder powder manufacture for these complex solder alloys will also be complicated. 

As a result of recent research activities, a large number of lead-free solders have been developed in laboratory studies, and patent applications have been filed for a variety of alloy compositions. Several of these compositions are available from commercial sources. Some examples of lead-free solders (and their melting temperature ranges) which have been considered for application in microelectronic assemblies are given in Table 2. None of these, however, are considered to be a suitable universal drop-in replacement for eutectic Sn-Pb. The most promising lead-free alloys for electronic soldering applications appear to be Sn-Ag and Sn-Ag-Cu, Sn-Cu,and Sn-Ag-Bi [23]. 

The alloy, Sn-3.4Ag. 8Bi, developed and patented by Sandia National Laboratories, was determined in the NCMS Lead-Free Solder Project to exhibit outstanding fatigue properties for surface mount applications under both accelerated thermal cycle test conditions used in the NCMS study. Sandia performed ATC testing of this alloy to 10,000 cycles for 0-100°C with 10°C/min ramp rates and 5-min dwell times [9,10]. There were no electrical failures at end of test (10,000 cycles) for 68 I/O PLCCs, 241/0 SOICs, and 1206 chip capacitors on FR4 boards, no cracks after 5000 cycles, and only minor surface cracks after 10,000 cycles. The Sn-3.4Ag-4.8Bi alloy has demonstrated considerable promise for use in surface mount applications, exhibiting greater fatigue resistance than eutectic Sn-Pb and most other lead-free alloys. It should be... 

Harrison, M.R. Vincent, J.H. Improved design life and environmentally aware manufacturing of electronic assemblies by lead free soldering. Proceedings of IMAPS Europe 99 February 1999. Han, T.H. Han, P. US Patent 5,435,857, Soldering Composition, 1994. 

M. Paruchuri and D. Shangguan, Lead-Free Electrical Solder and Method of Manufacturing. U.S. Patent 6,360,939 (2002)... 

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