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18 PCB007 MAGAZINE I SEPTEMBER 2025 D R I V I N G I N N OVAT I O N F i g u re 9 : C ro s s - s e ct i o n of b a c k- d r i l l i n g w i t h o u t s l i ve r a n d g o o d st u b re s u l t . ▼ Direct Imaging Machine (Outer Layer and Solder Mask) For high-resolution imaging of outer layers and sol- der mask, a warped panel can cause severe focus and resolution issues. Modern DI machines must be able to handle this and should integrate multi- ple functions to compensate for warpage: • Vacuum table: A powerful vacuum holds the panel down to a flat surface • Clamping (better from both sides): For par- ticularly warped or flexible panels, this feature provides additional, rigid support • Contactless autofocus: Our most advanced solution uses a contactless autofocus system with an individual Z-axis for each print head. This allows the machine to dynamically mea- sure the topography of the panel's surface in real-time and adjust the focus of each imaging head, ensuring a perfect, high-resolution expo- sure even on an uneven or warped surface Laser Drilling Machine While laser machines have vacuum tables and optional clamping, if the distance between the panel's top layer and second layer varies across the PCB, it can create a significant challenge for precise drilling. Even if the laser machine's parameters remain the same, a different thickness between the top and the fol- lowing layer can lead to a change in the laser- drilled hole's wall form. Figure 12 shows how this affects the wall form (more and less trapezoidal). Laser machines can be equipped with an inte- grated touch probe to check the PCB's thickness at key points. In severe warpage cases, the touch probe can create a full topographical "map" of the panel. The machine's control system uses this data to adjust the Z-axis as it moves across the panel, ensuring consistent via drilling relative to the top surface. However, the process engineer's exper- tise is the most critical factor. Their knowledge of material properties, laser parameters, and machine functions is what ultimately enables the machine to produce high-quality vias consistently. Positioning in X and Y An X-Y alignment error can cause a sliver defect, where the larger back-drill bit partially misses the plated hole. To tackle that, we created drilling machines with individual tables and CCD cameras at each station for the per-station alignment solu- tions, which help during the back-drilling stage. Positioning in Z The primary warpage challenge is in the Z-axis. Panel warpage, non-ideal inner layers, and inherent Z-axis tolerances can lead to a catastrophic scenario where the back-drilling penetrates a layer that should remain untouched. Schmoll offers special machine and process functions and proprietary Z-axis control that actively compensates for these effects, maintaining a precise stub length while preventing unintended layer cuts. F i g u re 1 0 : S c h e m e of t h e Z- a x i s of e a c h p r i nt h e a d . ▼