Lead-free material characteristics

This article discusses how lead-free material characteristics affect the process engineers responsible for implementing and optimizing the lead-free assembly process. 

Where free machining characteristics are required, this may be achieved by additions of cadmium, antimony, tin or lead (e.g. BS 4300/5). Materials for electrical use are of special composition (BS 2627, 3988), while bearings are manufactured from Al-Sn alloys. 

Fill Materials. Since the fill material is an additional fabrication material that becomes a part of the design construction, procurement documentation is required to specify a fill material type and thereby implement the via fill process. The selection and documentation of the fill material require the same consideration as the base laminate preference. This is especially critical when targeting a lead-free-compatible process. Currently, an industry-based material specification for via fill material does not exist.Therefore, specific fill-material brands may be named on the drawing, or some other form of user/supplier agreement must be established.The fabricator has preferences for the type of material used for via fill. Just as suppliers often have preferences for a specific solder mask brand, they also often prefer to use of a specific via fill material around which they have developed their principal processes. Supplier preferences can be driven by specific via fill material characteristics, such as accessibility, equipment compatibility, process supportability, plateability, and/or shelf/pot life. This may complicate source selection, or it might influence the use of a dedicated service center for the hole-fill process. The fabricator may not always know the reliability of its preferred material for a given via structure or end-use environment. 

Increased expansion causing premature PTH failures Base materials expand when exposed to soldering temperature.This expansion becomes pronounced above theTg of the resin system in the base material. The increased temperature of lead-free soldering can cause premature failure of the PTHs due to this increased expansion. See Fig. 51.3 for PTH expansion characteristics with temperature. 

A study (Ref 38) compared fatigue failure modes of lead-free solders to lead-tin solder joints, for a BGA package mounted on an or-gaiuc board. In the study, the assemblies were subjected to thermal cycling until failure, and a comparison of the failure modes for lead-free solder and lead-tin solder was made. For the case of Sn-Pb solder, the crack path lies in the bulk of the solder material near the component body (Fig. 10). For lead-free solders, however, two distinct fracture paths are observed. The first fracture path is similar to that of lead-fin solder passing through the bulk of the solder near the component side. This is shown for SAC solder (Fig. 11) and is also observed for Sn-Ag solder. The second fracture path is characteristic only of the Sn-Ag and SAC solders. This fracture path consists of very fine cracks with multiple fronts, has a shattered appearance, and is seen near both the component and the board sides (Fig. 12). 

Many North American companies are concerned that lead-free components and assemblies will not meet the reliability or functionality requirements necessary for high-end equipment supplied to banks, air traffic control systems, web-based businesses, and other mission-critical applications. Accordingly, manufacturers of aerospace and military electronics have no plans to introduce lead-free solders. There are too many reliability concerns to utilize lead-free solder materials in high-reliability equipment related to the mechanical characteristics of the materials themselves and the effects of high temperatures to process them. The effect of new and modified intermetallic compound phases within solder joints and at the interfaces is yet an additional concern, and there are many more. 

For the characterization of Langmuir films, Fulda and coworkers [75-77] used anionic and cationic core-shell particles prepared by emulsifier-free emulsion polymerization. These particles have several advantages over those used in early publications First, the particles do not contain any stabihzer or emulsifier, which is eventually desorbed upon spreading and disturbs the formation of a particle monolayer at the air-water interface. Second, the preparation is a one-step process leading directly to monodisperse particles 0.2-0.5 jim in diameter. Third, the nature of the shell can be easily varied by using different hydrophilic comonomers. In Table 1, the particles and their characteristic properties are hsted. Most of the studies were carried out using anionic particles with polystyrene as core material and polyacrylic acid in the shell. 

The zincblende (ZB), or sphalerite, structure is named after the mineral (Zn,Fe) S, and is related to the diamond structure in consisting entirely of tetrahedrally-bonded atoms. The sole difference is that, unlike diamond, the atoms each bond to four unlike atoms, with the result that the structure lacks an inversion center. This lack of an inversion center, also characteristic of the wurtzite structure (see below), means that the material may be piezoelectric, which can lead to spurious ringing in the free-induction decay (FID) when the electric fields from the rf coil excite mechanical resonances in the sample. (Such false signals can be identified by their strong temperature dependence due to thermal expansion effects, and by their lack of dependence on magnetic field strength). 

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