Alloy selection, lead-free component

Ease of component removal also affects reclamation efforts. Cradle-to-grave ownership of products by their manufacturers or others in the chain of commerce requires a careful evaluation of the recycling cost of an assembly. The processing required to remove several components from an assembly can be substantially reduced by selecting components with a common termination metallurgy, so they can all be removed at a common removal temperature. The selection of a lead-free alloy for component and board finishes should be with subsequent reclamation in mind [59]. 

Because there is no single lead-free drop-in replacement for eutectic Sn-Pb, there must be a suitable marking system to indicate the particular alloy (and its melt temperature) used for assembly. This is necessary for an operator to subsequently remove a component and select a metallurgically compatible replacement component (i.e., compatible with the rework alloy, component termination finish, and original assembly alloy). Numerous investigations indicating a variety of lead-free finishes compatible with the Sn-Ag-Cu family of alloys have been conducted [64]. 

Reflow soldering involves the selective deposition of solder paste at termination pads or other features on printed wiring boards. Then, the electronic components and other devices are placed on the board, and the bond is completed by heating and melting the solder. The most common lead-free alternative alloys considered for solder paste are tin-silver (Sn-Ag), tin-silver-copper (Sn-Ag-Cu), and tin-silver-bismuth (Sn-Ag-Bi). Tin bismuth (SnBi), tin indium (Snin) and silver-fllled conductive epoxy are also considered for certain applications. 

Many metallic elements are contained in electronic assemblies either as terminations or coatings for component devices or as the electrical circuit, terminations, or coatings on PWBs. The elimination of lead in electronic products requires substitution by other metals that can provide the performance and reliable properties characteristic of traditionally used lead-bearing alloys. There are a number of metal resources that can be selected as substitutes for lead in solders and as coatings for lead-free electronic assemblies. These metals include tin (Sn), silver (Ag), copper (Cu), bismuth (Bi), antimony (Sb), gold (Au), indium (In), nickel (Ni), palladium (Pd), platinum. 

Siemens initiated lead-free solder development activities in the mid-1990s. The first Siemens demonstrators using lead-free solder included industrial PCs, automotive, and telecommunications products. These products all have double-sided SMT boards with complete lead-free solutions. The smallest component is a 0402 device and the largest is a QFP with a 0.5-mm pitch. After analyzing 162 different alloys, Siemens selected three. The melting point for the Sn-3.8Ag-0.7Cu alloy was too high therefore Siemens added bismuth or another element to lower the melting point. 

From Table 2, it is apparent that a single lead-free alloy does not exhibit the best performance in all the categories listed. As an example, among the alloys considered, Sn-8Bi-3Zn has a melting point closest to eutectic Sn-Pb however, it exhibits poor wettability. Therefore alloy selection is, as often occurs in design issues, a matter of optimizing benefits through tradeoffs. Physical characteristics such as panel size, types of moimted components, electrode finishes, temperature sensitivity of components, and so on all play a role in alloy selection. 

Vibration (high-cycle fatigue life) testing was conducted to determine (1) if there were measurable differences in the performance of the down-selected solder alloys, or (2) if the mode of failure of the Pb-free solders was distinguishably different from eutectic Sn-Pb solder in a vibration environment. Of all components vibration tested on both RTV-SM and STV assemblies, only the LCCC-44 and PLCC-84 parts experienced failures related to solder performance that were analyzed further. The PQFP-208 and PQFP-132 parts exhibited lead fractures for all solders, not just the stronger lead-free solders. The PLCC-44 components did not exhibit enough failures to be meaningfully analyzed. The 1206 discrete devices did not exhibit any failures. 

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