Micrometer-fine throughs in circuit boards must be drilled very precisely. A new measurement method should make it possible to regulate the laser drilling process of microvias in the future.
Fraunhofer IPM (measuring technique supplier), 3D-Micromac AG (laser microprocessing specialist), and Andus Elektronik GmbH (German printed circuit board producer) have worked together to develop an optical process to inspect the quality of microvia holes by ultra-short pulse laser (UKP). The new development allows for inspection to occur in process, with the drilling process also now able to be regulated automatically.
UKP lasers have become increasingly popular in structuring printed circuit boards. This is because these particular lasers can place holes with minimal diameters in demanding materials between vias. However, due to the fluctuation of thickness and homogeneity of the insulation, UKP lasers require additional regulation, especially considering that these lasers cannot select materials.
During the production of printed circuit boards, which are highly demanding, micrometre-fine through contacts (or microvias) are drilled into multilayer circuit boards. Laser drills handle this process using high throughput and the precision of a few micrometres. There is now a measurement method to test the quality assurance of these holes, based on laser-induced plasma spectroscopy.
The connections between the layers of printed circuit boards are drilled using lasers, with the microvias having diameters of less than 200 micrometres. This process must be completed quickly, cost-effectively, with flexibility, and with a particular focus on precision. Ensuring that this process is completed correctly can lead to an increase in the predicted lifespan of the product the circuit board is used.
Before the creation of this new quality assurance process, any inspections occurred downstream. Typically, destructive processes like cross-grinding or electrical functional testing will occur sporadically, which was found to be insufficient for safety-critical applications.
Researchers involved in the HILaM project relied on laser-induced plasma spectroscopy (LIBS) to analyse and regulate the ablation process. The sensor is integrated into the system with the ablation laser and “records the material-specific plasma emission that occurs during material removal.” From this, the identity of which materials have been removed can be deduced. For example, if the removal of copper has been confirmed, then the insulation layer has been removed to a satisfactory level. The algorithm that the inspection process uses will then decide that the laser within the borehole is aligned to the next position.
This new inspection process aims to make 100% inline quality control a reality.