The amount of each element can have wide bearing on joint properties.

Many alloys can be used in Pb-free soldering. Most are developed for a specific process, such as reflow soldering. Apart from that, different component-lead coatings often have to be used in the assembly. Finally, the coating on the PCB affects joint formation. How can one be sure that they will work together in the assembly of a complex circuit board?

It is not easy to provide a perfect selection for all possible combinations. The point is that some element combinations in the solder or the solderable coatings will produce alloy traces in a joint that can have a relative, low melting point. These traces then can affect long-term joint reliability. This of course also depends on the requirements for the assembly.

For example, requirements are less stringent for consumer electronics than for medical equipment or automotive electronics. Another factor to consider is the solderability of the parts just before soldering. Opting for a solderable finish is no guarantee that the solderability after storage will be sufficient to create sound solder joints. One of the prerequisites for a sound soldering process is that the solderability of all parts involved must fulfill the necessary requirements. Unless this is proved, by solderability tests for example, results after soldering might be disappointing.

In principle, all combinations of solderable coatings and Pb-free solder can create a good solder joint, although some elements will produce specific effects. The extent to which these elements will affect joint properties depends on the level of these elements in the alloy. For example, the presence of indium in the solder will typically increase solder joint fatigue strength. It also creates alloy combinations with tin, however, that have a lowest melting point of around 117°C.

Bismuth is another element that, in combination with tin, will reduce the lowest melting point to about 140°C. So, if these elements are part of a solder joint, they may affect high temperature joint reliability. Although, to our knowledge, no specific data are available, the general point about the low eutectic melting points should be taken into consideration when choosing an alloy in combination with a given coating.

A gold coating may exhibit perfect solderability; however, the solder joint interface might not have a gold concentration higher than 4% after soldering. Above this percentage, the joint interface will mechanically exhibit brittle behavior. Tin coating provides good solderability when fresh. Depending on the coating thickness, this solderability will last for a prolonged time; although it may deteriorate rather quickly when copper migrates through the tin layer, which is often the case at tin layer thicknesses less than 2 µm.

To reduce the risk of whisker formation, the tin layer should have no high internal mechanical stress. A matte coating is therefore generally recommended. Silver coatings will provide good solderability when fresh. However, silver may become less solderable under the influence of sulfur, which gives the solder a brown/black surface. When soldering on thin silver plating, like those used on ceramic substrates, use of 1.5 to 2% silver in the solder is often recommended to prevent leaching. This amount will also increase solder joint fatigue strength.

We have no current data on the solderability of palladium or PdNi with respect to Pb-free solder alloys; however, we do not expect any trouble, although the solder spreading of Pb-free alloys will be less when compared to SnPb. This is the case for most solderable platings soldered with Pb-free solders.

In our Pb-free alloy tests, OSP-coated board solderability deteriorates rather quickly. This coating is best when boards are fresh from the manufacturing line and populated and soldered within a few weeks after the manufacturing date. Longer storage reduces their solderability fairly rapidly. The most probable reason for this is that the coating will polymerize in time, making it less soluble in the flux solvents.

As for the various solder alloys, their use depends on the soldering technology. SnCu and SnAg alloys will be used mainly for wave soldering, while In-containing alloys will primarily be used for reflow soldering because of their lower melting point. The indium content pushes up the cost. Nevertheless, the lower melting point and the improved fatigue strength may be a decisive factor when choosing these alloys.

The most common Pb-free alloy is SnAgCu. This family of alloys can be used for reflow and wave soldering. Currently, we have no experience soldering with SnAg, but this alloy could be used to improve the fatigue strength of the joints. Its melting point is around 220°C at 4% silver. The common SnAgCu alloys have a melting point around 217°C. The melting point of the common SnCu alloy is around 227°C. The Sn100C copper-containing alloy has about the same melting point, but creates relatively shiny joints.

Gert Schouten is a senior engineer at Vitronics Soltec (vitronicssoltec.com); gschouten@vsww.com. His column runs monthly.