Thermal simulation turns to lead-free reflow

New Flo/PCB software makes it possible to design PCBs for lead-free manufacturing by predicting thermal gradients during solder reflow processing.

Flomerics has announced a new version of its Flo/PCB thermal simulation software that will help solve thermal problems in solder reflow processing associated with the use of lead-free solders.

Perhaps the greatest challenge of lead-free manufacturing is that temperature variations over the printed circuit board (PCB) during the reflow process must be maintained at much smaller levels in order to achieve the higher melt temperatures required by lead-free solders without damaging sensitive components.

The new version of Flo/PCB simulates the reflow process and predicts the temperatures at any point on the board during the entire reflow process.

This makes it possible to optimise oven settings and thermocouple attachment points prior to a physical profiling run.

It also allows board designers to investigate the effects of component layout on the solderability of the PCB assembly.

As the PCB is heated and then cooled while it passes through the reflow oven, the coldest component must be hot enough to melt the solder while all components must avoid exceeding the maximum component body temperature.

Lead-free manufacturing makes this more difficult as lead-free solders melt at a higher temperature whereas the maximum temperature that components can be exposed to without damage remains the same.

This means the temperature gradient, the difference between the maximum temperature experienced by the hottest part of the board and the maximum temperature experienced by the coldest part of the board, must be smaller than when traditional lead-based solders are used.

The temperature gradient is caused by the fact that not all components heat up at the same rate.

Small components heat up more quickly than large components, and components that are close to each other also heat up more slowly than those that are far from other components.

Components near edges and corners tend to heat up quickly.

Different package styles have different thermal response characteristics.

The conveyer speed also affects component heating rates.

The maximum body temperatures defined by IPC J-STD-020 range from 245 to 260C while full liquidity with SnAgCu alloy solders is achieved only at 235C.

The temperature gradient over the board must thus be maintained within approximately 10C.

Flomerics' new Flo/PCB software makes it possible to design PCBs for lead-free manufacturing by predicting thermal gradients during solder reflow processing.

Design engineers can evaluate the thermal gradients generated by different component layouts at an early stage in the design process when changes can be made inexpensively.

Manufacturing engineers can optimise oven settings such as conveyor speed and zonal temperatures in order to minimise thermal gradients.

The coldest and hottest points on the PCB can be identified without the need for a physical test board so that thermocouples can be attached to the appropriate points for reflow profiling.

The oven calibration starting point can be predicted to minimise the number of profiling runs required.

Back to top