Lead-free alloys

The major alloy of tin recovered from archaeological sites is pewter. This can be divided into those containing lead and lead-free alloys. The former could have a lead content ranging from 67 % (equivalent to plumbers solder) down to 15%. The French in Elizabethan times kept the lead of their wine goblets to below 18% as above this, the wine would become tainted As the lead and tin are insoluble in one another, they are classed as a two-phase alloy and articles could only be manufactured by casting. The lead-tree pewter was invariably an alloy of tin with a small amount of copper (0.5-7% for pewter recovered from the Maty Rose). The copper dissolved in the tin crystal structure resulted in a single-phase structure, which was considerably harder than pure tin. Hence this class of pewter could be subjected to a limited amount of mechanical working to achieve the final shape. 

Bradley, E., Handwerker, C., and Sohn, J.E. NEMI Report A single lead-free alloy is recommended. Surface Mount Technology Magazine, v.l7 1, Jan. 2003. 

The issues of defining and predicting reliability of products in this new situation have absorbed a great deal of corporate resources since the EU directive was announced. The predictive models and history were based on tin-lead alloys, and new models are needed for lead-free alloys. For this edition, we have added new material and expanded existing discussions on this subject. 

In an ideal world, the transition from SnPb to Pb-free could happen overnight. However, in reality, the transition is not likely to happen instantly instead, this is expected to occur over a period of at least 6 to 12 months. During this time, SnPb and lead-free alloys are likely to coexist on the same printed circuit board assembly (PCBA), which poses potential issues with component soldering and reliabiUty. 

Surface Finishes. Tin-lead solder alloys (e.g., 63/37) are the most popular alloy used for surface finishes on PCBs. Other surface finishes are rapidly finding their way onto the PCBs. IPC-6012 lists more than 20 different surface finishes that are now in use for PCBs. Lead-free alloys are also appearing on PCBs. It is important to understand that the composition and type of the surface finish influence solderability. The procurement documentation must state specifications for surface finish. Methods available for analyzing the alloy composition on the plated PCB include wet analysis, atomic absorption, and x-ray fluorescence (XRF). XRF is popular because of the ease of obtaining the alloy composition and thickness nondestructively. 

A key issue for manufacturers is the lead-free alloy melting point of 217°C, higher than the 183°C for lead alloys. The higher melting point wiU have significant impact on manufacturing processes and potentially on component reliability. 

A. Z. Miric, A. Grusd, Lead-free alloys. Soldering, Surf. Mount Technol. 10 (1) (1998) 19-... 

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. 

Lead-free HASL, using lead-free alloys (such as tin-copper) in place of tin-lead, is commer-... 

A. Rae and C. Handwerker, NEMTs Lead-Free Alloy Still on Target, Circuits As-sem., April 2004, p 20-25... 

IPC Solder Products Value Council White Paper, Round Robin Testing and Analysis of Lead Free Alloys Tin, Silver, Copper, http.V/leadfree. ipc. org/LeadFreeWP006. asp, 2005... 

The case of mechanical vibration is less well defined in terms of the predominance of creep versus time-independent (plastic) deformation. Although the applied stresses are often quite low, the strain rates can be sufficiently high, so that a mixture of creep and plastic deformation modes define the fatigue response of the material. This sensitivity of fatigue behavior to cyclic loading frequency has been widely studied for tin-lead (Sn-Pb) solders and is recognized as an important variable in the fatigue response of lead-free alloys (Ref 1-3). 

Creep of Lead-Free Alloys. Parameters for the deformation of lead-free solder have been published (Ref 11, 12) but are not necessarily consistent with each other. A comprehensive set... 

Figure 4 details the elastic modulus of the Sn-Ag-Cu alloy compared with that of the common eutectic solder. The moduli of both materials demonstrate strong temperature dependence. In addition, the Sn-Ag-Cu alloy is approximately twice as stiff as the eutectic Sn-Pb solder at a given temperature. As such, for displacement-controlled loadings, stresses will be higher in the lead-free alloy than those in the eutectic solder. The modeling results presented later in this chapter will confirm this expectation. 

Reflectivity depends on the solder surface appearance and therefore, the time solder alloy solidification characteristics. Good reflectivity will be observed when a tin-lead or eutectic lead-free alloy is used resulting in eutectic solidification. 

With lead-free alloys, wetting generally is less than that of Sn/Pb systems. This may leave exposed comers or edges on the pads. If full coverage is required, then a change in stencil design is needed so that the stencil aperture covers 100% of the pad. 

Printability of lead-free solder paste will not change, but its spread during reflow will, which may require tightening of the stendl-printing process. One possible issue is print accuracy, or the alignment of the printed solder paste onto the PCB pad. Because lead-free alloys do not spread or wet as well as tin/lead, any solder paste that is not accurately printed onto the PCB will stay close to where it was printed after the reflow soldering process. Figures 3 and 4 depict the same deposits before and after reflow for QFPs and passives. 

For companies planning to use existing wave-solder equipment, it is important to understand that because lead-free alloys are less dense than tin/ lead alloys, the hardware found in older wave-soldering equipment will sink to... 

Lead-free may increase the uplifting of smaller components. This is due, in part, to the reduced wetting behavior of lead-free alloys. Component placement is more important with lead-free alloys because less centering will occur during reflow, increasing the incidence of tombstones. 

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