Matte-finish Solder Joints after Lead-free Wave Soldering

Estimated reading time: 8 minutes

Most joints soldered using lead-free alloys exhibit a dull or frosty appearance, which differs from the smooth, bright, shiny surfaces of tin/lead solders. This article looks at several reasons for this phenomenon.

By Gerjan Diepstraten

Most joints soldered with lead-free solders exhibit a dull or frosty appearance. This differs from the smooth, bright, shiny surfaces experienced with tin/lead solders. This is typical of SAC alloys (tin/silver/copper) commonly used in lead-free soldering applications. There are a number of reasons for this occurrence. One reason is that these lead-free alloys contain three different elements, and thus three eutectics, during solder solidification. These eutectics each have their own melting point and solidification behavior.

Formation of Different Eutectic Nuclei

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

The different eutectics that can be formed in SAC alloys are Sn5Cu6 at 227°C, SnAg3 at 221°C, and Sn+SnAg3+Sn5Cu6 at 217°C. However, this is only true if the total process contains only lead-free elements. In the case of a tin-rich alloy, tin crystals can precipitate out of the alloy during cooling of the joint at 232°C. If component leads are used with a tin/lead 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 tin/lead eutectics, or to 178°C for SnPbAg eutectics.

Solder Contraction or Shrinkage While Solidifying

As the molten solder solidifies, it will shrink by about 4%. Most of this volume reduction will be found in areas where the solder solidifies last. These commonly are 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. This 4% volume reduction often can be held responsible for the formation of micro-cracks in the solder joint. If the solder fillet moves during this process due to pads lifting during soldering, for example, and moves back during cooling, these micro-cracks can develop larger cracks due to volume reduction combined with movement. These cracks will be found only at the fillet surface of the solder joints. The solder in between the copper barrel and the lead generally will make a sound connection that will strengthen the joint.

Movement of Soldered Components or Solder While Pasty

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

The differences in coefficients of thermal expansion (CTE) between the copper barrel that forms the plated thru-hole and the epoxy-based material located between these joints causes this pad movement. 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 joints with liquid solder.

As soon as the soldered joint exits the wave, it begins to solidify. Initially, during this process more heat is transferred to the epoxy/glass board material until the solidification of thermal energy is dissipated fully. Afterward, the board cools and returns to its original dimensions. During this time, the wedge-like shape of the solder pad returns to a flat configuration again. When this occurs, the solder is not solidified completely and exhibits a pasty characteristic. It is this movement that can disturb the joint surface during joint solidification and can create cracks as a result of combined shrinkage and fillet tearing. These cracks are commonly positioned parallel to the PCB surface. Occasionally, they form a completely circular crack.

Solder Joint Appearance

During solidification, the eutectic with the lowest melting point often is surrounded by already solidified particles - the eutectics with higher melting points. This means that during final solidification of the solder joint, a soup of molten solder and already solidified particles with a different grain structure is formed. During solidification, the solder volume will shrink by about 4%. Most of this volume reduction and contraction is found on alloy parts in the joint that solidifies last. This mix of liquid and solid solidifying at different stages, each with a different surface structure and combined with volume reduction, gives the joint a dull appearance.

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

Effects of Forced Cooling

Forced cooling helps reduce the temperature of the PCB at a faster rate, but has no real effect on any of these mechanisms. It can prevent further heat build-up in components from the dissipated solidification heat coming from the solder joint directly after soldering - if cooling takes place at the component side during this stage. Temperature measurements of solidification behavior of soldered joints have taught us that solidification for most joints is completed within three seconds after wave-departure time. Any cooling positioned after this has no major effect on the already solidified joint. Forced-air cooling within this three-second interval also cools the solder wave, which is undesired and not recommended. Typical values for reaching the solidification temperature using SAC alloys are 1.4 seconds, while the joint is solidified fully 3.2 seconds after exiting the wave.

Conclusion

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

Gerjan Diepstraten is the senior application engineer at Vitronics Soltec. For more information, e-mail: info@vitronics-soltec.com.

Explanation of Shrink Structure Formation

If the solder alloy contains elements that can form more than one eutectic alloy, different shrinkage patterns can be formed, giving the solder joints a rough appearance. Because factors such as solder volume in the joint, the heat-sinking effect of parts involved, alloy composition, and lead plating can affect the cooling of a soldered joint after leaving the wave, 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. Under ideal conditions, it has that melting point and no other melting points from the binary eutectics that could also be present in this solder volume. Therefore, this volume of solder will solidify as one homogeneous alloy that is in full equilibrium due to its exact ternary-eutectic composition and equal temperature. Normally, such an alloy will solidify with a smooth surface under these conditions because the solder shrinkage will be divided over the volume equally.

Next, assume that extra tin is added to this perfect ternary solder mixture deliberately. The extra tin cannot be part of the ternary eutectic because the alloy now contains too much tin. This excess tin, which has a melting point of 232°C, will 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. Final shrinkage will take place at the point where the joint reaches a temperature below 217°C. In most cases, this will be the part that was in contact with the solder wave for the longest time, commonly the joint fillet at the solder side. Thus, the tin dendrite profile is present primarily at the surface of the solidified solder.

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