Chapter 6 of our comprehensive guide to stencils explores reflow and how to overcome faults, solder paste printing effects and screening can requirements

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Reflow diagramDuring reflow soldering the printed solder paste deposits melt and form solder joints to interconnect the component terminations to the substrate pads. There are a considerable number of solder paste manufacturers each with a range of solder pastes with subtle nuances in the formulations that require small adjustments to the reflow profile to achieve the results required.

When surface mount began, the heat sources used were infrared. Infrared ovens have improved in the last thirty years to achieve good results but today many people use forced air convection ovens to reflow the solder paste. Vapour phase technology was also used to good effect in the development of reflow soldering and is popular with lead-free process requirements. The boiling point of the chemical used is a constant and every part of the PCB assembly benefits from the same temperature.

Achieving a reflow profile that is suited to both the solder paste and the components used is critical to the success of the surface mount process. Guidance is always provided by the solder paste manufacturer and should be used as exactly that. The PCB you are processing may require small variations from the manufacturer’s suggested reflow profile to enable achievement of acceptable quality levels.

In order to adjust a profile you need to understand what’s happening on the PCB. As such, a profiler should be used with thermocouples mounted to monitor the temperature at various critical points throughout the process to ensure the temperatures achieved enable reflow of the solder paste.

Reflow profiles can be broken down into four distinct areas:

  • Pre-heating: This raises the temperature of the PCB and components without the risk of thermal shock to either.
  • Soaking: Allows the stabilisation of the temperature across the variation of different components.
  • Reflow: This occurs once the temperature is elevated above that of liquidus. Typically 40-60 seconds is spent above this temperature (time above liquidus) to ensure all the solder fillets are fully reflowed.
  • Cooling: Necessary to ensure the prevention of excessive intermetallic layers, which could cause embrittlement of the resultant solder fillets.

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