The Need for Liquid Soldering Flux in Selective Soldering

SMT (surface mount technology) continues to thrive and become widely used in many applications involving PCB (printed circuit board) manufacturing, but some tasks may still require through-hole technology, especially for boards that need to be soldered. Through-hole processes may be daunting, but manufacturers have the option to use selective soldering to keep them cost-effective and precise. Prior to the use of reflow soldering, PCBs with a lot of through-hole pins were manufactured using wave soldering. Choosing the right liquid soldering flux should help prevent pins from deteriorating when they are passed through high-temperature solder waves or get in contact with solder for longer periods of time.

The choice in liquid soldering flux is typically influenced by a particular assembly’s electrical reliability requirements. PCB manufacturers must consider other factors, too, like the compatibility with the other board level components, and pin testability. To narrow down the possible choices in fluxes, consider the types of fluxes that can withstand the performance and demanding conditions of selective soldering. Liquid soldering flux is manufactured in different forms to suit every type of selective soldering process.

Liquid soldering flux products typically perform well on 1.6mm and 2.4mm PCBs, with different solder pot temperatures and solder contact times. Manufacturers typically indicate the starting point of values for the amount of preheat and flux. Process optimization experiments may be conducted according to preheat values. When used in a conventional selective soldering process, the liquid soldering flux is capable of producing acceptable IPC class III solder joints on standard PCBs that are 1.6mm or 0.062" thick.

Consider an alcohol-based flux for thicker PCBs that are around 2.4mm or 0.093" thick. Alcohol-based fluxes containing less than 6% of solids in the form of rosins and activators are ideal for thicker PCBs. Boosting the solder contact time and solder pot temperature should result in a better hole fill, unlike what preheat temperature or flux amount can produce. Just be sure that the liquid soldering flux is ideal for the PCB assembly process requirements and the design. You should also look into the throughput rate. Moreover, keep in mind that increasing the amount of preheat temperature and flux will not necessarily increase the through-hole performance.

About the Author:

Alpha Assembly Solutions inc. is a premier company engaged in the development and manufacture of high quality electronics assembly materials. Since the year of its inception, way back in 1872, the company has played its role on the frontline of innovation, bringing many first-to-market products to the fore thereby, creating a premier niche for itself in the market.

Alpha Assembly Solutions Inc. is the global leader in the development, manufacturing and sales of innovative specialty materials used in a wide range of industry segments, including electronics assembly, power electronics, die attach, LED lighting.