ALPHA® Liquid Soldering Flux and the Increase in Selective Soldering

Posted 03/18/2013 by $username

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Selective soldering processes typically use higher solder temperatures and longer solder contact times than with traditional wave soldering.  This is because there is a smaller mass of molten solder in contact with the board and heat is quickly dissipated from the contact area to the rest of the board.  Higher solder temperatures and longer contact times place additional demands on the flux being used. 

Flux selection is usually influenced by the electrical reliability requirements for the type of assembly being produced as well as other characteristics such as pin testability or compatibility with other board level materials.  Once the range of possible fluxes has been narrowed by these considerations, you then need to know which fluxes are capable at performing under the demands of selective soldering.

Flux Selector Table 1
Flux Selector Table 2

These tables indicate which ALPHA® flux products perform best on 1.6mm and 2.4mm PCBs at different solder contact times and solder pot temperatures.  There is also an indication of starting values for flux amount and preheat from which, process optimisation experiments can begin.

 Here are several key points from a recent Alpha Flux Study:

·           Alpha's most popular fluxes, when used in a typical selective solder process, can produce acceptable IPC Class III solder joints on standard thickness (1.6mm / 0.062”) PCBs.

·           Alpha's alcohol based fluxes should be used for thicker (·2.4mm / 0.093”) PCB’s.

·           Use Alpha's alcohol based fluxes with less than 6% solids (activator + rosin) on thicker PCB’s.

·           Increasing solder pot temperature and solder contact time make a greater improvement on hole fill than flux amount or preheat temperature.

·           PCB design or process requirements (throughput rate) may limit solder pot temperature or contact time settings and restrict an assembler’s ability to use optimal process conditions.

·           Increasing flux amount and preheat temperature does not always increase hole-fill performance.

·           A properly structured Factorial 'Design of Experiment' is preferable for process optimization.


Your Alpha Sales Representative can assist you with any questions you may have about flux selection for your selective soldering process.

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