Lead-free soldering Components

Joints between metals, including those in electronic components, have traditionally used SnPb solders. However, in the European Union, new environmental legislation aims to phase out this use of lead by 2006 or 2007 a move to lead-free solders is also being made in Japan and the US. Eutectic SnPb solder exhibits many desirable properties... 

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). 

Victrex is best known for its semi-crystalline polymer PEEK, which is insoluble in all common solvents and can be used at temperatures of up to 300 °C. PEEK S 2005 total sales volume was 1,972 tonnes a 9.4% increase from the 2004 figure of 1,802 tonnes. Victrex USA has reformulated its PEEK products in order for them to be used for lead-free soldering of components. As a consequence PEEK is challenging use of PPS in such applications. 

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. 

COMPONENT-TO-PWB RELIABILITY ESTIMATING SOLDER JOINT RELIABILITY AND THE IMPACT OF LEAD-FREE SOLDERS... 

The peak reflow temperatnre of lead-free solders is higher than that of tin-lead solders. Assembling lead-free components with Pb/Sn paste wonld involve limiting the peak reflow temperatnre to that of Pb/Sn components ( 220°C). Consequently, the lead-free solders may not fnlly melt during reflow, resulting in unpredictable long-term solder joint reliability. 

Darveaux, Robert, Shear Deformation of Lead Free Solder Joints, Electronic Components and Technology Conference, 2005, pp. 882-893. 

Termination Metallurgy. Traditionally, most components have leads (QFP, SOT, etc.), balls (BGA, CSP), or end terminations (chip capaci-tors/resistors, EGA, QFN, etc.), plated or coated with the tin-lead alloy, to provide solderability. Although the tin-lead plating provides adequate solderability with lead-free solder as well, it will not meet the environmental legislation requirements. Some tin-lead-plated components may be used with the lead-free solder during the transition the impact of lead in lead-free solder joints has been discussed previously. 

A number of lead-free component termination finishes have been evaluated (Ref 13, 55) and used over the years. For passive components (such as chip capacitors and resistors), matte Sn plating has been used for many years with the tin-lead solder, and can be used with lead-free solder as well. For leaded components (e.g., quad flat pack or QFP), plating of matte tin or tin alloys may be used with lead-free solders (forward compatible). The tin whisker concern will be discussed in a later section. Nickel-lead has been used with the tin-lead solder for many years, and Ni-Pd-Au is currently an alternative for leaded components for lead-free soldering Ni-Pd typically does not provide as good wett-ahifity as tin. Area array packages with SAC halls are available and work well with the SAC solder. 

Reflow Process. The key parameter for the reflow profile is the peak temperature. Adequate reflow temperature is needed for the solder to melt, flow and wet, interact with the copper on the pad and the component termination, and form sound intermetallic bond when cooled and solidified. Typically, 30 °C (55 °F) superheat (above the melting temperature) is desired. Eor lead-free soldering, because of concerns about the thermal stability of the components, efforts are needed to minimize the soldering temperature. For SAC alloy with the eutectic temperature at 217 °C (422 °F), the minimum reflow... 

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... 

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