Laser Reflection as a Simple Prospect Tool for Nondestructive Quality Control of Charged Lapping Plates

In the present work, a simple laser/detector-based system was assembled and mounted in situ at the production line of the Hitachi hard-disk recording-head facilities in Mexico. The system was set to scan the surface of rotating lapping plates charged during time windows of varying lengths. The specular-reflection component was measured as a function of angular distance along the plate surface. The optical system showed enough sensitivity to follow the incorporation of abrasive into the plate surface. Moreover, two different charging stages were identified. Relevant topographical information could also be extracted. For example, the data distribution skewness was used to identify major surface defects, such as scratches. The reflection-signal spatial pattern was matched to that of the lapping plate topographer (LPT) surface profile by means of a coherence-spectrum analysis. The system assembly is relatively straightforward and it occupies little space. Thus, it could become a compact multitask substitute or complement of larger equipment at the manufacturing line. Note that even when the data analysis performed has led to promising results, the technique still needs to be fine-tuned in order to increase its precision and reliability.