Selective Soldering: Design, Process Challenges and Practical Solutions

Estimated reading time: 8 minutes

Although SMT is today’s predominant electronics assembly technology and the proportion of through-hole components continues to decline, the need for some through-hole assembly will remain for many years. As component density and board complexity increase, hand-soldering can no longer be relied upon to give acceptable or repeatable results and selective soldering techniques are growing in popularity. What is the most reliable and cost effective solution to through hole soldering in tin-lead and lead-free environments? What are the design and process challenges and what are the practical solutions?

A SMART Group workshop at the Bromsgrove, UK, premises of equipment and process materials distributor APP Electronics, set out to provide the answers, with a programme combining technical presentations, live demonstrations and hands-on sessions, introduced and moderated by Nigel Burtt, senior electronics manufacturing engineer at Renishaw and SMART Technical Committee Chairman.

Burtt launched the proceedings with a general overview of selective soldering processes, focusing on multi-point dip-nozzle soldering and single point mini-wave nozzle soldering, and described two main classes of machine, where either a robot brought the PCB assembly to a stationary soldering nozzle, or the nozzle moved to a stationary PCB assembly on a conveyor, showing examples of each type.

In principle, the objective was to mimic wave-soldering, but in localised areas. Flux was applied selectively to specific component leads, the PCB was pre-heated to drive off solvents and activate the flux, and then the component leads were brought into contact with the solder wave. A continuous flow of nitrogen around the solder nozzle was essential in selective soldering, to keep the nozzle free from dross, and could be used as a means of additional local pre-heating. The type and size of soldering nozzle was chosen to suit components and PCBs. Single point mini-wave nozzles could be used for dip soldering single leads or drag soldering multiple leads; smaller sizes gave less heat transfer and larger sizes enabled higher speeds. Burtt showed a series of video illustrations of various actual soldering operations.

Solder pot temperatures were generally higher than would be used in wave soldering, placing more constraints on the performance of the flux and potentially more local thermal stress on components and PCB materials. Additionally, there was a greater tendency for copper to be dissolved from component leads and PCB pads, especially with lead-free alloys, and Burtt recommended reference to the Good Practice Guide MAT 26, available free-of-charge on the NPL website. Layout design rules needed to take into account the additional clearances required to accommodate selective soldering, and it was recommended to remove solder mask between pads to eliminate solder balls.

Success in selective soldering is critically dependent on choosing a suitable flux, and placing it exactly where it is required. This was the essence of the first of two presentations from Wim Schouten, regional sales manager for Vitronics Soltec. He explained that although a strong flux was required to clean oxidised metal surfaces and support wetting, there were potential risks to reliability if the right formulation was not chosen.

He advised against using the same flux as used for regular wave soldering because the conditions were significantly different: in a wave soldering application all of the flux came into contact with liquid solder, the high temperature of which de-activated the flux chemistry. In selective soldering not all flux made contact with solder, and residual activators could cause reliability problems. Reliability considerations aside, selective soldering temperatures tended to be significantly higher than in regular wave soldering, so the flux might not be strong enough to work effectively and result in soldering defects. Furthermore, wave soldering flux formulations contained surfactants to promote spreading to cover the complete solder side of the PCB. In selective soldering, excessive spreading of was undesirable although the flux needed to be capable of penetrating the barrels of plated-through holes. Alcohol-based fluxes were preferred to water-based formulations because rapid solvent evaporation resulted in less spreading and higher flux solids per unit area.Page 1 of 3

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