Solder phase diagram

And now for a real phase diagram. We have chosen the lead-tin diagram (Fig. 3.1) as our example because it is pretty straightforward and we already know a bit about it. Indeed, if you have soldered electronic components together or used soldered pipe fittings in your hot-water layout, you will already have had some direct experience of this system. 

Now for some practical examples of how phase diagrams are used. In the first, a typical design problem, we find out how solders are chosen for different uses. In the second we look at the high-technology area of microchip fabrication and study the production, by zone refining, of ultra-pure silicon. And lastly, for some light-hearted relief, we find out how bubble-free ice is made for up-market cocktails. 

Figure A 1.1 shows a phase diagram for the lead-tin system (the range of alloys obtained by mixing lead and tin, which includes soft solders). The horizontal axis is composition Xpg (at%) below and Wpg (wt%) above. The vertical axis is temperature...

Figure 1.14.6. A generic phase diagram for a mixture showing the melting behavior of a eutectic mixture such as solder.

Phase diagram for solder. The phase diagram in Figure 25.25 describes an alloy of tin (Sn) and lead (Pb) that can be melted at relatively low temperature. 

As with azeotropes, eutectics maybe ternary, quaternary, and so on, but their phase diagrams get very complex very quickly. A few important eutectics have an impact on ordinary life. Ordinary solder is a eutectic of tin and lead (63% and 37%, respectively) that melts at 183 C, whereas the melting points of tin and lead are 232 C and 327 C. Wood s metal is an alloy of bismuth, lead, tin, and cadmium (50 25 12.5 12.5) that melts at 70 C (lower than the boiling point of water ) that can be used in overhead fire sprinkler systems. NaCl and H2O make a eutectic that melts at — 21 C, which should be of some interest to communities that use salt on icy roads in the winter. (The composition of this eutectic is about 23 weight percent NaCl.) An unusual eutectic exists for cesium and potassium. In a 77 23 ratio, this eutectic melts at —48 C This eutectic would be a liquid metal at most terrestrial temperatures (and be very reactive toward water). 

Phase diagrams may be used as a guide to identify which intermetallic phase forms at the liquid solder/substrate interface. However, an equilibrium phase diagram merely indicates what phases are thermodynamically stable, given a particular composition and temperature. The kinetics of solder-substrate reactions determine the structure of the solder/substrate inter-... 

Reaction Between Ni and Sn, Eutectic Sn-Pb, and Sn-Ag. The reaction products between the Ni substrate and solders of pure Sn, eutectic Sn-Pb, and Sn-Ag can be understood using the binary Ni-Sn phase diagram, because neither Pb... 

Here is another important assumption d = X, meaning the equiaxed grain. Also, dCldX = ACIX, where AC is the Sn concentration at the Tj/Cu boundary minus that at the solder/q boundary, which is determined from the equilibrium phase diagram. Therefore,... 

In the past, the vast majority of solders have been lead-tin alloys. These materials are reliable and inexpensive and have relatively low melting temperatures. The most common lead-tin solder has a composition of 63 wt% Sn-37 wt% Pb. According to the lead-tin phase diagram, Rgure 9.8, this composition is near the eutectic and has a melting temperature of about 183°C, the lowest temperature possible with the existence of a liquid phase (at equilibrium) for the lead-tin system. This alloy is often called a eutectic lead-tin solder. 

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 microstructure of Pb-Sn solders can be described by the binary alloy phase diagram. The phase diagram is a two-dimensional construct that predicts phase development as a function of the temperature and material composition. An important caveat is that the phase diagram represents the material at equilibrium. Strictly speaking, equilibrium is achieved when the material has been cooled at an infinitely slow rate such that no further changes occur to the microstructure at the target temperature. 

The characteristics of the nonequilibrium, Pb-Sn solder microstructure are threefold. First, the compositions of the phases are not accurately represented by the phase diagram boundary lines. Second, the relative quantities of the phases are not accurately described by the lever rule. Third, the spatial distribution of phases in the microstructure are sensitive to the cooling rate. For example, in terms of individual phase compositions, excessive Sn may be retained in the Pb-rich a phase, causing a supersaturation condition. In effect, the phase boundary line between the a single-phase field and the (a -I- p) two-phase field is shifted to the right. (A more detailed discussion of approximating a nonequilibrium microstructure from the equilibrium phase diagram can be found in Refs. 5 and 7.) A consequence to the solder microstructure caused by a supersaturated Pb-rich phase is the precipitation of Sn particles in the Pb-rich phase. 

The interface reaction product formed between Pb-Sn solders and most base metals is commonly referred to as an intermetallic compound layer. Intermetallic compounds exhibit highly directional bonding similar to ceramic materials and, as such, typically have a well-defined stoichiometry, high melting temperatures, high strength, and very low ductility. Unfortunately, the binary alloy phase diagrams provide only an approximate indication of intermetallic com-... 

The binary alloy phase diagram can provide some insight into potential intermetallic compound compositions [5], For example, only single-phase fields in the phase diagram can form at the solder/base metal interface. Dual- or higher-order phase fields would not appear, as they cannot develop the concentration gradients required to support diffusion rather, the latter would exist solely as interfaces. 

A Pb-Sn solder joint is inherently unstable in that Sn in solder and metallization elements combine to lower their free energy. This is an important factor in the evolution of solder joints a driving force exists for the formation of Sn-based IMCs. The Cu-Sn, Ni-Sn, Au-Sn, and Pd-Sn phase diagrams [14] presented in Figs. 2-5, respectively, provide important examples of these IMCs and their range of stabilities. On the other hand, Pb is stable with respect to compound formation with Ni or Cu. In considering the formation of IMCs in Pb-Sn solder/metal joints, Pb is generally considered to be inert. 

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