Lead-free solder wettability

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. 

In terms of printability, tack, slump, and solder balling, there is no clear and consistent difference between the Sn-Pb and lead-free solder pastes (Ref 59-60), because these performances depend on the solder paste formulation, not directly on the solder alloy. Very clear and consistent differences have been observed, however, in wettability between the tin-lead and lead-free solder pastes. In general, the wettability of lead-free solder paste is not as good as the tin-lead solder paste. For example, lead-free solder paste exhibits very limited spreading on OSP during reflow, and exposed corners after reflow are quite common, unless overprint or round corner pads are used. The difference in wettability between OSP and ENIG surface finishes, which is already evident for the tin-lead solder, becomes even more pronounced for lead-free solders. This has been observed with a variety of solder paste and flux formulations from a number of vendors. 

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. 

It may be noted that the tin-lead solder is shiny with excellent wetting. The only lead-free solder joint that comes close to this feature is Sn96.5 Ag3.5, a eutectic solder. All lead-free solders show appearance of a graininess and lack of good wettability. 

Many of the lead-free solder systems of interest consist of some minor additions of third or fourth elements to binary alloy systems to achieve a desired benefit such as enhanced wettability or reliability, etc. 

The wettability and solderability of lead-free solder candidates must be of sufficient adequacy to form solder joints whose reliability is not degraded relative to eutectic Sn-Pb joints due to wettability-related issues. 

Vianco, P. Hosking, F. Rejent Wettability analysis of tin-based, lead-free solders. Proc. NEPCON WEST (Des Plains, IL) 2000, 3017. 

Melton, C. The effect of reflow process variables on the wettability of lead-free solders. J. Metals 1993, 45 (7), 33-35. 

FIG. 39 Comparison of the wettability of several lead-free solder baths and the eutectic Sn-Pb benchmark solder for various chip components and finishes. The lead-free alloys each show varying degrees of wettability with the various component electrode finishes, whereas the eutectic Sn-Pb solder completely wet for each test condition. (Courtesy of Panasonic.)... 

Using a test vehicle which included QFPs with pitches ranging from 0.3 to 0.5 mm, and CSPs with pitches ranging from 0.4 to 0.6 mm, a study was conducted to understand the manufacturing impact of three no-clean, lead-free solders from the Sn-Ag-Cu family compared to a standard no-clean eutectic Sn-Pb solder paste. The evaluation focused on printability, solder paste pot-life, wettability, reflow process window, and inspection [33]. 

Sn-Ag-Cu (composition unknown) Compared with eight other promising lead-free solders in wettability, fatigue resistance, and T Sn-Ag-Cu was claimed as the best 45... 

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. 

The next critical component for this material was the development of the flux and activator system for the paste. This proved to be a key factor in the success of this project. As the wettability of lead-free alloys is lower than eutectic Sn-Pb, a suitable flux system was critical to the successful introduction of the MD s conversion to lead-free. In addition to the flux system having the properties to promote wetting, the requirements for screen printing, stencil life, tackiness, and so on were also important when evaluating a solder paste system. Flux technology is discussed in... 

The JWES group evaluated various test methods, and tested a range of solder alloys. Measurements of melting point, tensile strength, wettability, and strength of leaded solder joints and chip joints were performed as part of a Pb-free solder evaluation, including the Sn-Ag-Cu, Sn-Ag-Bi, and Sn-Ag-Bi-In alloy systems. The main results for the alloys are summarized in Table 21. 

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