Issue link: https://iconnect007.uberflip.com/i/1129312
JUNE 2019 I PCB007 MAGAZINE 75 shifting of ground and power. And at the same time, we keep the CAF bridge in place. Data Size One difference between using micro-ma- chining and the traditional PCB data sets can be the number of lines of machine-code/G- code required. For some designs, the amount of programming code can easily exceed the memory capacity of the current generation CNC machines. To reduce the number of lines of CNC code required, one could always write a specific machine code that defines a slot us - ing a sequence of numbers for slot length and depth. Naturally, this makes the process more complex in that data files need to be cut up with correspondingly increased potential for error. Micro-machining Experiences Depth control is one of the challenges in set- ting up the VeCS-2 slots process. Dielectric tol- erances are a critical parameter when setting the depth. Dielectric measurement across the panel needs to be performed to set the correct depth. The layer below the slot needs to have isolation thick enough to accommodate the tol- erance of the Z-axis on the CNC machine. This tolerance needs to be in the region of ±0.02 mm. It should be noted also that the type of pressure foot as well as resetting the Z-axis counter through contact with the surface are important in achieving good depth tolerances. Later in this series, I will introduce layer de- tection. This technique prevents what would otherwise be a complex, time-consuming, and somewhat inaccurate depth-setting process. Similar techniques will also be used for back drilling and depth-control drilling. The development of the techniques for cross- routing took some time. For example, forming the cross route initially had problems with bit wander (Figure 5). Note that a prerequisite to the routing process is to fill the slot with tra- ditional via filling materials. This embeds the vertical trace and keeps it protected from the routing process. With the assistance of HPTec, we addressed the bit wander by switching to an alternative bit (Figure 6). We saw a major improvement using these alternative bits, and after adjusting the parameters, we achieved straight and well- positioned cross routes (Figures 7-9) with little to no burring. Additionally, we achieved verti- cal trace widths of 0.1 mm. Figure 4: Critical dimensions at and around the bottom of the slot. Figure 5: Cross route results using a standard bit. Figure 7: Clean cross routes using alternative bits and parameters. Figure 6: Alternative bit from HPTec.