Process control for laser micro spot welding using fast pyrometer systems

New electro-mechanical systems like flexible printed circuits (FPC) or 3D-MID consist of temperature sensitive substrates. This makes reflow soldering difficult or even impossible as cost reduction leads to more sensitive materials like PET. Laser micro spot welding can solve this challenge as it allows locally and temporary limited energy input. Compared to laser soldering processes the puls durations are 100 times lower, producing less heat conduction and thus damage to the substrate. Up to now the laser welding process for copper is often rejected due to poor welding reproducibility. This is caused by varying surface absorption properties. To stabilize the process we set up a high speed pyrometer to control the laser spot welding. For minimized heat conduction the pulse length was kept below 10ms. This demands a very fast and precise pyrometer measurement and a detection area of the size of the laser spot (≤ 400µm). Different strategies for power control have been investigated to obtain constant energy amount. The welding results using the process control system will be shown on copper leads.