In Metal basics we showed how cooling curves are used to produce a composite thermal equilibrium (or ‘phase’) diagram of the tin-lead system. Figure 1 shows the effect the relative proportions of the constituents have upon the temperature at which solidification starts and at which it is complete.
Most tin-lead alloys have a melting, or pasty range, between the temperatures at which the alloy is properly solid (solidus) and completely liquid (liquidus). While wide pasty ranges are ideal for plumbers, who may need to ‘dress’ joints, they are not satisfactory for electronic applications. At the eutectic composition of 61.9% tin: 38.1% lead:
The situation is more complex on cooling, as is shown in the following sections. Remember that, during cooling, although the phase transition takes place at the solidus temperature, the alloy will remain only partially solid until the whole latent heat of fusion has been dissipated (or, during melting, only partially liquid until the whole latent heat of fusion has been supplied). Even eutectic alloys have a pasty time!
As cooling solder reaches the solidus temperature (183ºC for the eutectic) the precipitation of crystals is not spontaneous, but requires some activation energy. This is usually supplied thermally, by the solder temperature falling below the solidus temperature. This undercooling can be as much as 8ºC.
The phenomenon can be seen in the cooling curve of Figure 2, where the temperature has been monitored as a mass of molten solder loses heat at constant rate. Note also that, as the solder solidifies, the latent heat of fusion is liberated, causing cooling to stop until the whole of the solder mass has solidified.
Source: Lea 1988
The solidified structure consists of alternate lamellae of tin and lead phases with the lamellar spacing X being related to the freezing rate R by:
X2 x R = constant
With slower cooling rates, the inter-lamellar spacing increases, and the size of colonies (regions where lamellae are orientated in the same direction) increases.
As the cooling rate increases, the number of colony nuclei formed is enhanced; at sufficiently fast cooling rates, the lamellar character is lost.
Figure 3 shows microsections of eutectic tin-lead solder both slow-cooled and fast-cooled. Differences in the microstructure have implications for the mechanical properties of the joint: slow-cooled joints are more ductile; fast-cooled joints, with a fine grain structure and closer grain boundaries, are more brittle.
Source: Frear 1991
Solid state diffusion processes continue to alter the microstructure of the cooled solder and this process is accelerated both by plastic deformation and by increasing temperature. Solder alloys operate at 70–80% of their melting point, so metallurgical changes occur in the joint over relatively short time-scales, even though diffusion happens at rates very much less than with molten solder.
The general tendency is for the microstructure to coarsen. As this happens, the interfacial area between the two phases and the free energy are reduced, and the joint becomes less brittle. A consequence is that joints do not reach their full strength until 24 hours after initial cooling.
A molten solder contains 60% tin and 40% lead.
Explain the sequence of events which occur as it cools to room temperature. How might the strength of a joint be affected by the cooling phase of a soldering process?
Through-hole interconnects are ‘over-designed’: Manko has calculated that even on single-sided boards the solder joint is stronger than the average board, and the strength of the joints on PTH (plated through-hole) boards is 8–13 times greater than is necessary. Termination problems have not been a major reliability issue!
In SM (surface mount) assemblies, however, joints are considerably weaker, because the joint area and the amount of solder used are both much smaller. Manko’s calculations and tests showed that fillets should exhibit 80% of design strength or better.
In Metal basics we graphed the relationship of the strength and rigidity of a metal with temperature. This follows a similar pattern for all metals, reducing to zero at the melting point, and reducing markedly as that temperature is approached. Unfortunately, solder is used at a temperature close to its melting point, and bulk material is not very strong.
Fortunately, the mechanical properties of a joint don’t merely reflect the strength of the bulk solder. They depend on its metallurgical microstructure, which is influenced both by the materials being connected (due to alloying or intermetallic compound formation) and by the soldering process. They also depend on the joint’s shape and size, and in particular on its thickness. It has been found for tin-lead solders that:
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Alloys of tin and lead, primarily in eutectic and near-eutectic ratios have been by far the most common materials used until very recently. Available at modest cost, these have favourable strengths, melting points and wetting characteristics. The choice of eutectic alloys, particularly in preference to cheaper materials with a higher percentage of lead, were for three reasons:
Source: Rahn 1993
Another previously popular solder was also a eutectic tin-lead alloy, but containing a small amount of silver, with the composition 62% tin : 36% lead : 2% silver. This addition has the effect of:
Source: Lea 1988
Even before the days of the RoHS Directive, which from 1 July 2006 has banned the use of lead-containing solders in most electronics assembly, a much wider range of solders was in use, depending on the melting temperature and mechanical characteristics desired. Table 1 lists some of the more frequently-used materials.
|Tin||Lead||Silver||Melting point °C||ANSI/J-STD-006||QQ-S-571E||Application|
assemblies and hybrid
High melting point lead-free
|10%||90%||275–302||Sn10A||Ball Grid Arrays|
temperature component manufacture
For materials marked Sb in the lead column,
the percentage shown includes 0.2–0.5% of antimony,
Where only a single melting point is shown, this indicates a eutectic material.
As explained in Practical Solder Paste Issues, designations given are the ‘short names’ from ANSI/J-STD-006. QQ-S-571E has been superseded by ANSI/J-STD-006, but references to it may still be found.
Most of these alternative solders:
A commonly-used solder contains 62% of tin, 36% of lead and 2% of silver. Why the tin? And why this particular ratio of tin to lead? And why the silver? And, if you wanted to buy some, what would you ask for?
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