Testing of the quality of solder joints through the analysis of their thermal behaviour with an interferometric laser probe

摘要

AbstractWe present an optical contactless method for testing the quality of solder joints in surface mounted components by measuring their thermal dynamic behaviour. We detect surface normal displacements induced by Joule heating with a high resolution interferometnc laser probe. This probing method, based upon a homodyne Michelson interferometer, is an interesting tool for investigating the mechanisms of heat deposition and flow inside electronic devices. It allows the precise time evolution of the surface normal displacement to be measured (the laser probe has subnanometric resolution).This new approach of thermal behaviour laser testing is based upon the analysis of the diffusion of heat produced by the Joule effect in the structure (lead, solder and copper strip) from short current pulses and will influence heat diffusion. Solder joint failures (intermetallics, hidden voids, etc. ) behave as a thermal barrier. The optical test consists of measuring the dynamics of the solder joint expansion, and the variations from a standard response (good quality solder joints) will reveal defects. Important variations have been observed in solder joints that have undergone thermal cycling ageing tests.We have also investigated the thermal response of joints on IMS (insulated metallic substrate) and epoxy substrates. They show a very different time response.