Melt temperature lead-free solder

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. 

The lead-free solders are mostly based on Sn-containing binary and ternary alloys. Among them, the Sn-Ag system is one of the earliest commercially available lead-free solders and has been recommended for general-purpose use as a substitute for Sn-Pb eutectic solder. Addition of nano-particles of second phase helps in improving thermo-mechanical properties such as melting temperature, mechanical strength, mechanical fatigue resistance, creep resistance and solder-joint reliability. 

In the temperature range from 230 to 280 °C, which is relevant for lead-free soldering, the crystalline regions in thermoplastics will melt. In non-crosslinked plastic films, there are no sufficient bonds left to hold the material together. The crystallites in crosslinked thermoplastics also melt, but the chemical crosslinking bonds hold the macromolecule together in its amorphous zones so that they can keep their shape - even at low stiffness levels. The crosslinked film remains sufficiently strong and stable short-term up to approx. 300 °C [720]. 

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. 

Other lead-free solder alloys which have been developed and evaluated by fee industry include Sn-Ag, Sn-Ag-Bi, Sn-Bi-In, and other ternary and quaternary alloys (Ref 24-33). The use of alloys containing indium (In) may be limited due to fee scarcity of indium and its high cost. The eutectic Sn-Ag alloy (melting temperature 221 °C, or 429 °F) was qualified for automotive applications in fee 1990s on ceramic thick film substrates and PWBs (Ref 3,12-15,34- 5). The microstructural characteristics of the Sn-Ag-Cu system under different reflow conditions and... 

For high temperature applications, the eutectic Au-20Sn alloy (melting temperature 278 °C, or 532 °F) has been used for die attach, especially for optoelectroiucs packaging. Tin-antimony (melting temperature 235 °C, or 455 °F) is another example of a high temperature lead-free solder alloy. For low temperature applications, alternatives include Sn-Zn, Sn-Zn-Bi, Sn-Bi-ln, and Sn-Bi-Ag (Ref 46 8). 

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... 

Rework for lead-free solders has been found to be more difficult, because the lead-free solder alloys typically do not wet or wick as easily as the Sn-Pb solder due to their difference in wettability. This can be easily seen with QFP packages. In spite of these differences, successful rework methods (both manual and semi-automatic) have been developed (Ref 74-75) with lead-free solders (Sn-Ag-Cu, or Sn-Ag), for many different types of components. Most of the rework equipment for tin-lead can still be used for lead-free solder. For area array packages, it is helpful to use a rework system with split vision and temperature profiling features. The soldering parameters must be adjusted to accommodate the higher melting temperature and reduced wettability of the lead-free solder. The other precautions for tin-lead rework (such as board baking) still apply to lead-free rework. 

Directive 2002/95/EC Restriction of the Use of Certain Hazardous Substances in Electrical and Electronic Equipment (RoHS) requires the components to be free of toxic substances, with only a few exceptions. Only lead-free solder can be used. Lead-free solders generally have higher melting points, so soldering operations have to be hotter and the components correspondingly temperature-resistant. 

Solders are typically classified as either soft or hard. Some confusion often results from this convention because some hard solders are really braze materials if the melting temperature criterion is applied. Soft solders typically consist of alloys containing lead and tin, but also often contain indium (In), bismuth (Bi), antimony (Sb), or silver (Ag). In practice, most soft alloys melt at temperatures lower than 450°C, usually between 180°C and 300°C. High-tin solders, typical of lead-free solders, tend to be stiffer, harder, and less ductile compared to high-lead solders. Hard solders often contain metals such as Au, Zn, Al, and Si. 

The compositions of some lead-free solder alloys and their corresponding melt temperatures are listed in Tables 14- Tables 16, covering melt temperatures between about 180°C and 200°C. Some higher-temperature compositions are hsted in Table 17, but are only of utility for niche applications owing to such issues as cost (Au n) and health concerns (Sb-containing alloys). 

TABLE 14 Some Lead-Free Solder Alloys with a Melting Point or Liquidus Temperature Below 150°C... 

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