Indium Blog

SMT Components "Blowing-Off" PCB

Recently I was at a customer who reported that their SMT components were “blowing off” their PCB. In most of the instances the component was still on the PCB but completely off pad.

Further investigation showed that the solder paste print for the component was well defined and that the component, after pick-and-place, was not skewed but placed correctly within the paste deposit.

Inspection of the oven revealed there were no obstructions on the conveyor and that the conveyor did not vibrate or shake excessively during reflow. The static pressure (air flow within the oven) was set at a low pressure.

The reflow profile used was a ramp to peak type profile, peak and time above liquidus was well within paste spec limits, but the initial (first zone) was set at a low temperature, ~70°C.  After changing the first zone to 100°C the issue was resolved.

Solder paste flux chemistries are unique, performing a number of functions including printability and retention of the shape of the stencil aperture they are printed through. These flux chemistries include ingredients that you don’t find in a simple liquid flux. Because of this the reflow profile plays a very important roll. In the first heating zones the solvent in the flux chemistry needs to evaporate increasing the tack of the solder paste (ability of the paste to hold onto the component). At 70°C the solvent does not evaporate quickly enough, a minimum of 100-110°C is recommended.

Note that setting the first zone too high (>130°C) can cause solder defects such as solder balling and solder beading