Solder alloys selection

Lead-Free Wave Solder Alloy Selection Reliability is Key... 

Alloy selection depends on several factors, including electrical properties, alloy melting range, wetting characteristics, resistance to oxidation, mechanical and thermomechanical properties, formation of intermetaUics, and ionic migration characteristics (26). These properties determine whether a particular solder joint can meet the mechanical, thermal, chemical, and electrical demands placed on it. 

Noble metal alloys, wires, and solders are used in dentistry because they are nontoxic, biocompatible, stress and wear resistant, noble in the oral environment, and have good casting properties. The alloys selected for use must have good fluidity and low gas absorption when molten for good reproduction in the preparation of partials, bridges, crowns, and orthodontic appliances and in filling cavities. The standards for dental alloys are set by the American Dental Association Specifications. 

Selection of the flux type depends on the solder alloy, metal finishes on the board and components, condition of the surfaces to be joined, the type of soldering process selected, required solder-joint attributes, and the intended final use of the assembly. 

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. 

These are just a few issues of concern regarding the transition to lead-free solder. Many questions still remain. This is evident in the multitude of studies, experiments, workshops and conferences available focusing on reliability, inspection, rework, alloy selection, and exemptions, as well as labeling and materials declarations. As the deadline draws near, most of the kinks should be worked out. It seems most U.S. companies are taking the EU dates seriously. No one is shirking their duties, says NEDA s Martin. No one said, No big deal, they ll slip those dates. The best advice, adds Martin, is to prepare and keep your eyes on the EU s laws as each member state completes their laws. Companies can do things in anticipation, but it all trickles down and it seems that time will tell how lead-free plays out for the entire SMT supply chain. Until then, maybe we should still keep our eyes on the price of duct tape. 

THE EU S ROMS DIRECTIVE IS DRIVING MAJOR CHANGES IN THE MANUFACTURE OF ELECTRICAL AND ELECTRONICS EQUIPMENT. THE MOST SIGNIFICANT CHANGE IS THE ELIMINATION OF LEAD FROM SOLDER JOINTS. THIS ARTICLE DETAILS THE PROCESS THAT ONE CONSUMER ELECTRONICS COMPANY USED TO MAKE THEIR LEAD-FREE ALLOY SELECTION. 

Solder joint microstructures were studied and failure mechanisms identified as component failure, design failure and alloy failure. Grains intrusion/extrusion and voiding were examined in the micro-structures and a final alloy selection was made on the basis of these results. 

The surface finishes selected were Immersion Silver, Immersion Tin, two HASL (lead-free) and four Organic Surface Protections (OSPs). Four solder alloys were planned for testing, but two were eliminated due to cost, leaving SACX and Sn/Cu/Ni. Five fluxes were evaluated, two of which were chosen because they were commonly used in other experiments. 

Pb-free alloy selection and Pb-free soldering process development. 

Melting temperatures (or temperature ranges) are important in the development and selection of these new solder alloys, information available from phase diagrams. For example, a portion of the tin-rich side of the silver-tin phase diagram is presented in Figure 9.10. Here, it may be noted that a entectic exists at 96.5 wt% Sn and 221°C these are indeed the composition and melting temperatnre, respectively, of the 96.5 Sn-3.5 Ag solder in Table 9.1. 

The soldering and curing temperatures of the connection mediums differ. The composition of the solder alloy is the main factor determining the temperatures that have to be achieved in the soldering process to ensure that the solder melts in the peak phase and wets the contacts. Consequently, when selecting a connection medium or, to be more precise, a solder alloy for MID, it is very important to take into consideration the maximum temperature capability or the temperature resistance of the plastic. 

IV. LEAD-FREE SOLDERS AND SELECTION CONSIDERATIONS A. Solder Alloys... 

The next phase will address eliminating lead from board finishes—the protective coatings applied to termination pads on printed wiring boards to protect metal conductors from degradation (e.g., oxidation, corrosion) and remain solder-wettable. Finishes are applied in a number of ways, including dipping into a molten metal bath (e.g., tin, solder), electroless plating, etc. Alternative finishes must, of course, be compatible with the lead-free alloy selected in Step 1. 

In most cases, when pure liquid Sn is alloyed with an element having lower surface tension than Sn, the surface tension of the resulting mixture is reduced and the converse is also true. A selection of references for solder alloy surface tension is given in Table 5. 

When utilizing a lead-free solder in a wave process, the machine and process must be modified to ensure the alloy selected is compatible with the internal parts of a wave machine. Lead-free solders are not drop in replacements , for eutectic Sn-Pb solder. Issues such as lead contamination, flux chemistry compatibility, dross removal equipment, and dissolution of coatings on the surfaces of a wave machines internal parts into the solder alloy must be considered. 

Given the wide selection of lead-free solder alloys with higher processing temperatures than eutectic Sn-Pb solder, a choice required a solder that had equivalent or better mechanical properties, adequate availability, and one with suitable soldering characteristics. The focus was... 

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