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A mix of liquids and solids solidifying at different stages gives joints a dull look.

Solder is an alloy mix consisting of two or more metals. Melting and solidification behavior is much dependent on the formation of areas in the solder where different eutectics might solidify. This can be the case if solder contains elements such as copper and silver. In such a case, CuSn and AgSn eutectic parts or traces can form next to the SnAgCu eutectic during the solidification of the solder in the joint.

As molten solder solidifies, it shrinks approximately 4%. Most of this volume reduction can be found in areas where the solder solidifies last. These are typically areas where traces of the lowest melting eutectic solder are found. If these traces are at the joint surface area, this mechanism can create a dull appearance. The 4% volume reduction can often also be held responsible for the formation of micro-cracks in the solder joint.

During solidification, the eutectic with the lowest melting point is often surrounded by already solidified particles from the eutectics with the higher melting points. This means that during final solidification of the solder joint, a "soup" forms of molten solder and solidified particles that have a different grain structure than the last solidifying alloy elements. During this solidification process, the solder volume shrinks by the aforementioned approximate 4%, with the main part of this volume reduction and contraction to be found on those alloy parts in the joint that solidify last. This mix of liquids and solids solidifying at different stages, each with a dissimilar surface structure, combined with volume reduction, finally gives the joint a dull appearance.

Shrink Structure Formation

When the solder alloy contains elements that can form more than one eutectic alloy, different shrinkage patterns can be formed that will give the solder joints a rough appearance. Since the cooling of a soldered joint after leaving the wave is affected by many factors – solder volume in the joint, the heat sinking effect of the parts involved, alloy composition, lead plating, and so on – the solder solidification will not be the same for all joints. This means that joints can have a different appearance at the end of the soldering process. Here's why:

Assume that a given SAC solder volume has the exact ternary eutectic composition Sn3.5Ag0.9Cu. This alloy will have a melting point of 217°C. In fact, under ideal conditions, it has only that melting point and no other melting points from the binary eutectics that also could be present in this solder volume. So, this volume of solder will solidify as one homogeneous alloy in full equilibrium because of its exact ternary eutectic composition and equal temperature. Normally, such an alloy would solidify with a smooth surface under these conditions, since the solder shrinkage is equally divided over the volume.

Next, assume that extra tin is deliberately added to this perfect ternary solder mixture. The extra tin cannot be part of the ternary eutectic because the alloy now contains too much tin. This "excess" tin, having a melting point of 232°C, will first precipitate as solidified crystals (dendrites) as the solder cools, until the remaining liquid mix has its perfect ternary eutectic composition. As this remaining liquid mix continues to solidify at 217°C, the solder shrinks by about 4%. This shrinkage originates with the remaining liquid and not from the already solidified tin dendrites. The final shrinkage will take place at the point where the joint at last reaches a temperature below 217°C. In most cases this is the part in contact with the solder wave for the longest time, typically the joint fillet at the solder side. Thus, the tin dendrite profile is primarily present at the surface of the solidified solder.

In real solder joints, the ideal ternary mix (assuming one begins with such an alloy) is mixed with metallic parts from the PCB and lead metalization. Parts of these elements dissolve into the limited amount of solder that forms the joint. These extra elements now disturb the ideal ternary eutectic. This means that the solidification of that solder mix is not at a common temperature of 217°C, but that parts of this mix may solidify at 232°, 227° or 221°C. In the event that the component leads are SnPb-plated, SnPb eutectic traces or SnPbAg eutectic traces with melting points of 183°C and 178°C, respectively, may also be found in the joint. In most cases, SAC alloys are used with a composition that deviates from the ideal eutectic composition. This might create the different eutectics that ultimately generate a rough joint surface.

Gerjan Diepstraten is a senior process engineer with Vitronics Soltec BV (vitronics-soltec.com); gdiepstraten@nl.vitronics-soltec.com. This column appears monthly.

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