Solderability is a relatively recent concern in electronics soldering. Until fairly recently, most component leads were plated with tin or tin/lead. Soldering is the process that creates intermetallic bonds with metal surfaces that do not melt during application of the joining material (solder). However, tin and tin/lead melt at soldering temperatures and the solder merely mixes with the melted plating. This is not soldering; it is “reflow” and very easy compared to true soldering.
Reflow is Simple
In reflow, there is no need to remove oxides before applying solder; the oxides, being lighter than pure metal, float on the combination of liquid plating metal and liquid solder. Flux, also being lighter than liquid metal, also floats on the melted metal, where it can easily contact the oxides and break them down. In reflow, the flux simply makes the final connection shiny and cosmetically pleasing.
Most beliefs about soldering originated during this era of reflow. One such belief, which has disastrous consequences today, says that liquid flux should not be used during hand soldering. The belief is that the flux contained in wire solder is sufficient to do the job. While that can be true for reflow, relying exclusively on the flux in solder results in incomplete wetting during soldering.
Soldering is Not So Simple
The ban on lead in electronics changed our business profoundly by eliminating tin/lead component surfaces. Meanwhile, tin plating has become increasingly uncommon because of concerns about tin whiskers with pure tin. The risk of whisker shorts is quite real for multileaded surface mount parts such as I.C.s.
The new component surfaces are not tin or tin/lead; they are metals with higher melting temperatures that do not reflow during application of the solder. In other words, these are metals that are soldered, not reflowed. And the surfaces must be thoroughly deoxidized before solder is applied. This will not happen if the flux is contained in the wire solder; the flux in the solder cannot be released until the solder melts. The melted solder forms a barrier between the flux and surface metal, preventing complete deoxidation and causing incomplete wetting.
Liquid Flux is Essential
The only way to ensure that flux reaches the surface oxides before solder melts is by applying liquid flux first. And more than just a trace amount of flux is required. Flux acid (the part that removes oxides) is neutralized during the deoxidation chemical reaction. Trace amounts of flux will be neutralized before the part has been completely deoxidized. In soldering, flux is more than our friend – it is essential. Yet, every few days, industry “experts” write strongly-worded instructions that use of liquid flux is a sin. There is even a widely-used “The Seven Sins of Hand Soldering” video that says “The best way of reducing the use of excessive flux is to only use the flux contained in the solder wire.” (The video is sold by the trade association that publishes standards such as J-STD-001 and A-610. They really should know better.)
Unfortunately, there’s much more to the flux business than randomly picking an off-the-shelf flux. We’ll look at the science of flux selection next time.