Typical wave fluxes for lead-free wave soldering will have the following characteristics:
-Thermal stability at higher pot temperatures
-Stable activity at higher preheats up to 130°C.
-Sustained activity to enable longer solder contact times
-Resins that do not discolor at higher temperatures
-Ingredients that remain water washable, if applicable, after higher temperature exposure
-Higher acid value numbers and solids percentage for thicker board assembly
It has been demonstrated that liquid fluxes with higher activity and higher solids content perform better
with lead-free solders. Water washable fluxes, with flux residues developed to be cleaned in water after
soldering, contain higher levels of activators and more aggressive ingredients. They have been proven to
give the best hole-fill with lead-free including more difficult surfaces such as bare copper OSP. The
activators are potential corrosive but they remove all oxides adequately even with the longer contact
times and hotter soldering temperatures used with lead-free. Because activators in these fluxes is usually
in excess there are enough active ingredients to keep the surface oxides to a minimu.
However, new formulations of no-clean liquid fluxes, specifically designed for lead-free also work well.
These new no-cleans have enhanced activator packages designed to be thermally stable at higher
preheat and pot temperatures. They can sustain slightly longer contact times with the solder and the
activity is still present as boards emerge from the wave, reducing bridging but also promoting better holefill.
This characteristic of the flux is also called sustained activity.
The trend in wave soldering around the world is the use of VOC-free fluxes in conjunction with lead-free
solders. This offers a completely “green” wave solder operation. VOC-free fluxes do perform well with
lead-free and higher solids in the range of 4% or higher are best. Of course, spray flux applicators and
convection heaters are ideal for these water-based fluxes. With these materials proper preheat conditions
Is essential to remove moisture prior to wave entry. It is important to ask yourself the question: Is my
preheat going to be sufficient?
Board and component finish selection is another selection parameter, which can impact reliability and
production output. Component finishes containing lead or bismuth can cause fillet lifting. Most component
leads are available in pure tin format with a nickel barrier over copper. This is done to prevent tin whisker
growth. Tin is a very easy metal finish to solder to, and has a reasonable shelf life if stored under
controlled conditions.
Also the European RoHS Directive specifies a 0.1% lead maximum limit in the solder joint. Lead-bearing
terminations will increase the lead (Pb) loading in the solder pot, resulting in RoHS non-compliance
issues. Once the lead exceeds the limit of 0.1% the only way to reduce it is to dilute the solder with fresh
solder, this is costly, time consuming and hazardous.