LEAD-FREE
FREQUENTLY ASKED QUESTIONS

SOLDERING FLUX

Will the fluxes used for tin-lead alloys work with lead-free solder?

Generally the same fluxes will work for lead-free solder. One factor is that the organic flux ingredients begin to decompose and deteriorate above 200°C and more rapidly as the temperature heads up to 250°C.

The water-soluble fluxes are more active and more heat-stable than the no-clean fluxes that are designed to self-destruct with the heat of soldering. The VOC-free no-clean fluxes can withstand the heat better than the alcohol-based no-clean fluxes.

Heat stability is more evident with solder paste. The water-soluble types are more heat-stable, and the no-clean types for tin-lead solders do not work well with lead-free alloys because of the increased reflow temperature, i.e., 240°C for lead-free compared to 210°C for tin-lead.

What chemistry changes are required for the fluxes designed for lead-free assembly?

Since lead-free alloys have a higher surface tension a good activity flux is required. The formulations change required are new activator systems, which can sustain higher preheat and soldering temperatures. The flux systems will also require new wetting and gelling agents able to sustain these higher temperatures. If new chemicals aren't added to fluxes a higher incidence of defects would occur.

In reflow soldering new resins and gelling agents are used to give the solder paste added hot slump resistance. This property gives the solder paste a lesser likelihood of creating mid-chip balling, solder balls and bridges.

In liquid flux formulations new activators are used to give the liquid flux sustained activity after emerging from the wave solder, reducing icicling and bridges. The activators are also more thermally stable to take the higher pot temperatures.

What is the best pre-heat configuration for a Pb-free compatible wave-soldering machine using VOC-free flux?

For VOC-free fluxes or water-based fluxes the pre-heat temperature has to reach 100?C (measured on top side of the assembly) for at least 20-30 seconds.

Ideal pre-heat configuration is:

• First zone: calrod, a medium wave heat source that allows the flux (read water) to evaporate without being moved. Any air turbulence in the first zone should be avoided since the liquid flux will be blown under components or between board and carriers.
• Second zone: forced convection. Here the air turbulence will allow the moisture to evaporate, dry the board and activate the flux. The second zone should also take the assembly over the 100?C level
• Third zone: could be any type of pre-heat system. The third zone will maintain the temperature over 100?C without going to warm. It will optimize the activation of the flux, add some heat into the assembly in order to minimize the ?T when entering the wave.