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April/May 2023


Selective Soldering: A Need for Innovation and Development


By Samuel J. McMaster, Andrew Cobley, John E. Graves, and Nigel Monk, Pillarhouse International S


elective soldering utilizes a nozzle to apply solder to components on the under- side of printed circuit boards (PCBs).


This nozzle can be moved to either perform dips (depositing solder to a single component) or draws (applying solder to several compo- nents in a single movement). The selective soldering methodol-


ogy thereby allows the process to be tailored to specific joints and allows multiple nozzle types to be used if required on the circuit board. Nozzles can vary by size (internal


diameter) and shape (making them suitable for different process types). This is all dictated by board design and process requirements. Selection of the nozzle type is dependent upon the product to be soldered and the desired cycle time. Hand-load selective systems


must be programmed with the param- eters for multiple solder joints. However, many inline systems are designed to be modular. This modularity allows for multiple solder stations with different condi- tions/nozzles to achieve low cycle times. Selective soldering provides many other


Figure 1: Pillarhouse inline selective soldering system.


Nozzles To ensure that controlled application of


the solder is maintained throughout the process, the solder must wet (adhere) to the nozzle. Wettability is the study of the adhe-


benefits compared to wave and hand solder- ing, such as: minimal thermal shock; lower running costs; operation under an inert envi- ronment; applicability to both low and high volume production; repeatability; and fewer operators required.


sion of liquids to solids because of the inter- action between the surface energy of the solid and the surface tension of the liquid. Surface energy (known as surface tension when refer- ring to liquids) is a result of the relative bond strength of the material and the level of unbalanced forces at the surface. Multiple methods exist to charac-


terize surface energy depending upon the components of the surface interac- tion that can be measured; however the most common is measuring the contact angle of a stationary (sessile) droplet.


When no other forces act upon a


liquid droplet (i.e., no contact with other surfaces and no air resistance due to movement), it will form a sphere as its own surface tension pulls it into that shape as it is the minimum energy shape it can be. When in con- tact with a solid, the droplet will deform and spread out. The amount of


spreading and the angle of the interface between the liquid and solid is a product of the relation between the surface tension of the liquid and the surface energy of the solid.


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