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A rough surface appearance is typical of SAC (SnAgCu) alloys commonly in use in Pb-free soldering applications. Among the reasons for the dull appearance is the fact that the alloy contains three different eutectic elements, each with its own melting point and solidification behavior.

Solder consists of an alloy that is a mix of two or more metals. The melting and solidification behavior will depend largely upon the formation of areas in the solder where different eutectics might solidify. This can be the case when solder contains elements such as copper and silver. With CuSn and AgSn, eutectic parts or eutectic traces can form next to the SAC eutectic during the solidification of the solder in the joint.

The different eutectics that can form in SAC alloys are Sn5Cu6 (227°C), SnAg3 (221°C) and Sn+SnAg3+Sn5Cu6 (217°C). This is true only if the total process contains only Pb-free elements. In a tin-rich alloy, tin crystals can precipitate from the alloy during cooling of the joint at 232°C. If component leads are used with SnPb plating, the lead dissolved from the plating can also introduce eutectic traces. This will lower the melting point for some parts of the solder in the joint to 183°C for SnPb eutectic, or even to 178°C for SnPbAg eutectic.

As molten solder solidifies, it will shrink approximately 4%. Most of this volume reduction will be found in those areas where the solder solidifies last. These are commonly 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 solder volume reduction can often form micro-cracks in the solder joint.

If the solder fillet moves during this process – e.g., due to pads lifting during soldering, which then move back during cooling – these micro-cracks can develop even larger cracks due to combined volume reduction and movement.

These cracks will normally be found only at the fillet surface of the joints. The solder between the copper barrel and the lead will generally make a sound connection that will give the joint its strength.

The movement of soldered parts or of solder while the solder is not yet fully solidified can at worst create cracks in solder joints, and at best merely give the solder joint a matte appearance at the surface. This phenomenon can be caused by the natural movement of the solder pad during joint formation. When multiple joints are spaced closely together (e.g., as with a connector), this solder pad movement can be considerable. It may even cause fillet tearing or lifting or pad tearing.

Such movement is caused by the differences in thermal expansion coefficients between the copper barrel that forms the plated through-hole, and the epoxy-based material located between these joints. As a result, the solder pad will be lifted in a wedge-shape from the edges of the copper barrel during contact with the solder wave and during the filling of the joints with liquid solder.

As soon as the soldered joint exits the wave, it begins to solidify. During this process, initially, more heat is transferred to the epoxy-glass board material until the solidification thermal energy is fully dissipated. Afterward, the board cools and returns to its original dimensions. Meanwhile, the wedge-like shape of the solder pad returns to a flat configuration. When this occurs, the solder is often still not completely solidified and exhibits a pasty characteristic. It is this movement that can disturb the joint surface during joint solidification and can even create cracks as a result of combined shrinkage and fillet tearing.

During solidification, the eutectic with the lowest melting point is often surrounded by already-solidified particles from the eutectics with the higher melting points. During final solidification of the solder joint, a “soup” forms with molten solder and already solidified particles that have a different grain structure than the last solidifying alloy elements. During this solidification process the solder volume will shrink; most of the reduction and contraction is found on those alloy parts in the joint that solidify last. This mix of liquids and solids solidifying at different stages, each with a different surface structure, combined with volume reduction, gives the joint a dull appearance.

Often, all these mechanisms act concurrently, but not on every group of joints at the same rate. This explains the differences in surface appearance after soldering. Since the source of the dull solder joint appearance lies in the combination of process and alloy used, the outcome should be judged to be normal. The dull or matte appearance of such solder joints should be regarded as an effect, but not a defect.

Forced cooling will help reduce the temperature of the PCB at a somewhat faster rate, but has no real effect on any of these mechanisms. Temperature measurements of the solidification behavior of soldered joints have taught us that solidification for most joints is completed within three seconds after wave departure time. Any cooling that is positioned after this has no major effect on the already solidified joint. Forced air cooling within this three-second interval will also cool the solder wave, which is an undesired effect and is not recommended. Typical values for reaching the solidification temperature using SAC alloys are 1.4 seconds, while the joint completely solidifies 3.2 seconds after wave exit.

In Pb-free soldering, matte or dull joints are normal and should be considered an effect rather than a defect. Differences in dullness or gloss between different soldered joints on a board are caused by differences in cooling behavior. This again is due to differences in the thermal layout of the individual solder joints. In a process, equal joints will normally behave equally and will therefore have the same appearance after soldering. However, joints with another layout, such as larger or smaller holes, different pad sizes, other component leads or components, might give another cooling behavior, resulting in a different joint appearance. Finally, solder composition plays a major role relative to all issues and results. Forced cooling after soldering does not remedy or prevent dull joints in lead-free wave soldering.

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