Issue link: https://iconnect007.uberflip.com/i/1190860
DECEMBER 2019 I DESIGN007 MAGAZINE 63 space trace calculation (approximately 2x to 3x trace width), but let's change from green mask to blue or white mask. Figure 3 shows the orig- inal 0.00675" traces with a 0.004" distance to the adjacent copper pour as blue mask. Figure 4 has the same 0.00675" traces with 0.004" g-sep and white mask. Let's say the drawing says, "Trace widths can be changed up to ±20% of the original trace width/space to achieve impedances." In the last scenario, with the mask color changed from green to white, we are well out of 20% to get back to 50 ohms. Figure 5 shows the results of changing to 0.00365" traces and 0.0055" g-sep from 0.00675" traces with 0.004" g-sep (linear trace/g-sep changes only), which equals great- er than 20% of trace width. 3. Change in Material Let's change from using Np-175 to P-370HR with the same two plies (1 x 106 and 1 x 1080). As you can see in Figure 6, it is fairly close to the end of the tolerance at 45.97 ohms for a 50-ohm ±10% tolerance, not factoring in any manufacturing variations, such as over-etch, under-etch, higher-than-normal plating, or lower-than-normal plating. Once again, let's change the material type and the color of the solder mask from green to white. We are out of tolerance again at 40.39 ohms with both a material change and a mask color change (Figure 7). 4. Change in the Starting Copper Weight Next, let's see what happens when we Figure 3: The same trace with a 0.004" distance to the adjacent copper pour as blue mask. Figure 4: The same 0.00675" traces with 0.004" g-sep and white mask.