Lead-free solder processes

Luttrull, D., and Hickman, E, New Halogen-Free PCB Materials for High-Speed Applications and Lead-Free Solder Processes, Fwtore Circuits, March 2001. 

Case studies. Lead-free soldering processes have recently been developed for various advanced packages, including 0201 (0.02 in. X 0.01 in.) components, 01005 (0.01 in. X 0.005... 

The vapor phase lead-free solder process operates at a maximum temperature of 230°C, whereas many convection or IR reflow processes range from 250°C to over 300°C in some zones of the reflow process, depending on the type of PCBAs being processed. Vapor phase is compatible with typical PCB finishes, including HASL, bare copper with OSP, copper/nickel/gold, immersion tin, and immersion silver. 

The higher temperature requirements due to the changeover to lead-free solder processes required by law constituted one of the main reasons why high-performance thermoplastics are virtually the only unmodified products that can be used adequately as substrate materials. 

In principle, compounds for MID can be manufactured with a very wide variety of fillers from the groups of metallic (only for MID processes that do not require wet-chemical conductor metallization) or ceramic materials. At this time some fillers are coming into widespread use on account of their verifiably excellent thermal properties, offering considerable benefits for MID production (e.g., reduction in hotspots for lead-free soldering processes) and MID use (e.g., integrated thermal management in highly loaded MID such as LED modules). The most important of these fillers are... 

Visteon Automotive Systems has performed extensive engineering analysis on Pb-free alloys since the early 1990s. Their objective was to launch the first lead-free soldering process into volume production on a commercial automotive electronics product [36]. 

In spite of a tremendous amount of effort for the search of lead-free C4s, no industry standard has evolved as yet. Commonly cited lead-free solders are Sn-rich alloys for which electroplating processes are available. The most popular lead-free solder among them is Sn3.9AgO.6Cu with a melting temperature of 217°C. This solder is recommended by National Electronic Manufacturing Initiative (NEMI) and has been extensively studied and characterized by NIST. However, applicability of this solder as a C4 material in chips with advanced ILD is not known. 

NF260./Indium Corporation No-flow epoxy underfill designed for lead-free processes 28,000 (10 rpm) 6 mos/24 hours Lead-free solder profile 2,800... 

The thermal performance is improved without any changes to existing processing equipment, such as molding machines, dies, molds, or extruders. The compositions can also be molded into various shapes and forms, such as connectors, circuit boards, pipes, rods, films, sheets, and bearings, which render them useful in electrical applications that might result in contact with lead-free solder [86]. 

The recommended lead-free solder formulation is Sn-Ag-Cu for board assembly but there are other formulations such as Nickel-Palladium (NiPd), or Nickel-Palladium with Gold flash (NiPdAu). Passive components, to be compatible with a lower temperature Lead process (which is 215°C for 50/50 Tin/Lead formulations and 230°C for 40/60 formulations) and the higher lead-free process of up to 260°C, use pure matte Tin for their contacts. The use of lead in solder is partially based on several potential reliability issues. Pure Tin component leads have been shown to result in inter-metaUic migration in the termination of the electronic component and the growth of tin whiskers which could cause short circuits (which is why there is a exemption for military use (only) components). 

The lead-free solder adopted by mannfactnrer Texas Instrnments is a nickel-palladium-gold (NiPdAu) formulation which is claimed to be backward compatible with existing reflow soldering processes. It is also claimed to be free of the whisker-artifact problems which have been experienced when nsing snch alternatives as matted tin. A Texas Instrnments spokesman stated that the move to pnre tin would necessitate the increase of the reflow solder temperature to obtain the same reliable solder contacts. He claimed that Texas Instruments had shipped more than 30 million lead-free units which confirmed the suitability of its NiPdAu solution. 

The need to use lead-free solder in surface mount technology applications where soldering temperatures of between 220 °C and 240 °C are likely to be encountered has necessitated the use of such high temperamre polyamides as DuPont s Zytel HTN LX resins for board assembly. It is interesting to note that some Japanese companies have been using lead-free solders in their flow soldering processes since 1997. 

TAB packages are usually naturally lead-free at the component level. Assembly to the apphcation board does require lead-free solder along with a requirement for the package to be able to withstand the high temperatures associated with lead-free processing. 

The solder shock test is one of several methods to assess the thermal resistance of copper-clad laminates. It is easy to perform and represents another key test during the early assessment of a material. There are a number of different methods to choose from, which will be described in detail in Section 12.5.2. During the initial assessment of the material, it is important to choose at least one of the described test methods to make certain that the material meets the minimum requirements, especially if the material is nsed in higher-temperatnre lead-free assembly processes. Aside from solder shock testing of bare laminate material, it is also recommended that the PCB engineer consider PCB-level temperatnre shock as well as repeated reflow testing with a particnlar focns on resin-reinforcement delaminations. This will ensnre that not only the raw material bnt also the completed PCB will be able to withstand the required temperatnre regime. 

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