A new version of its Flo/PCB thermal simulation software, designed to help
solve thermal problems in solder reflow processing associated with the use
of lead-free solders, has been unveiled by Flomerics.
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, says the company.
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 optimize 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 since lead-free solders
melt at a higher temperature while 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 245C 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, says the company.
The 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 optimize oven
settings such as conveyor speed and zonal temperatures in order to minimize
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 minimize the number of
profiling runs required, says the company.
