The new family of solders flows differently through the nozzles.

In recent years, several Pb-free experiments on selective soldering applications looked into characteristics of the new alloys. Just as in wave soldering, SnAgCu and SnCu are the most popular alternative alloys for replacing SnPb. During different Pb-free builds, it became clear that Pb-free alloys behave differently from what we have learned from SnPb processes. Compared to SnPb, for example, Pb-free alloys have different surface tensions and density, which results in different flow behavior of the solder through the nozzles. This flow should be in control; otherwise, this will result in solder defects such as bridging, or it may affect surface mount components located close to the soldering area.

Bridging. Independent experiments have demonstrated Pb-free solders tend to flow backward, in the same direction the boards are moving. This may result in bridging (Figure 1). This has been recognized in three different Pb-free applications using SN100 (SnCuNi), Sn3.0Ag0.5Cu and Sn3.8Ag0.7Cu.


Because of varying surface tension and density, the Pb-free solder’s flow makes selective wave soldering more difficult. Special measures must be taken to maintain control of the solder overflow. Nozzle technology will become more important, as it is with SnPb soldering.

To avoid bridging, other parameters may result in a more stable process, but these were not included in this experiment. As in all soldering processes, the flux and its activity at the higher solder temperatures (as well as the nitrogen atmosphere) may provide improvements, since these parameters influence oxidation and solder surface tension.

Wetting. Wetting – i.e., through-hole penetration of the Pb-free solder – is another critical issue for Pb-free selective soldering. The solder joint’s thermal solderability is important, especially for through-hole penetration. Since small nozzles are used, much heat must be transferred through this small amount of solder into the board, connector, pad and soldering area to make soldering possible. The flux has to support this process and therefore must be able to withstand temperatures of about 280˚C on the wave. In the experiment, all connectors soldered at 280˚C exhibited a good fillet solder joint.

Pb-free solder temperatures. Wetting balance tests have indicated minor differences in the soldering characteristics of Pb-free alloys; for example, SnCu exhibits poor solderability compared to SnPb at the same temperature. Nevertheless, this experiment also showed that a temperature of 280˚C was high enough for soldering connectors with SnCuNi. Operating with the temperature too high will affect not only the performance of the flux, but also may accelerate the wear of some parts that come in contact with solder that has a high flow speed. If necessary, these parts need special coatings.

In conclusion, what have we learned?

• Pb-free selective soldering is viable, but requires special attention to the flow and behavior of the solder.
  
  
• Soldering can be successful only if there is enough thermal heat in the soldering area. Small nozzles may experience problems with this heat transfer; therefore, a nozzle diameter of at least 6 mm is needed. This may affect board layout.
  
  
• High solder temperatures require strong fluxes. Material wear may increase if solder temperatures exceed 300˚C. Special coatings on critical parts must be considered.
  
  
• New nozzle technologies and material treatments are in development.

Ursula Marquez de Tino is a process and research engineer for Vitronics Soltec (vitronics.com); umarquez@vitronics-soltec.com.