Issue link: https://iconnect007.uberflip.com/i/1472851
28 DESIGN007 MAGAZINE I JULY 2022 regarding transmission line modelling. e H1 dimension represents the core thickness, and the H2 dimension is that from the top of the core to the foil. You can see H2 is denot- ing prepreg as the trapezoid is pressed into the Er2 region. By how much? Well, if you work from a microsection, the dimensions will be clear to see. But when modelling with a solver before the board is built, you must make a pre- diction as to how much the T1 will impact the H2 dimension. ink about it: If there is a lot of copper on the signal layer, most of the T1 will need to be added to the pressed thickness of the prepreg; if the routing density is low, then far less. is is where a stackup tool comes in handy, as a good stackup tool will virtually press the pre- pregs and consider the Cu density on the sig- nal layer to calculate the optimum value for H2 to feed into the solver. is is the point I was stressing earlier: Commercial PCB transmis- sion line field solvers must possess a variety of tools and capabilities over and above the core solver engine to feed it with good mechanical data to solve. As you look at Figure 2, consider the ques- tion posed by Figure 1. By using Speedstack to pre-process the material data, the pressed height of H2 and the impact of T1 have been calculated to feed into the solver engine. e le-hand side of the image shows the raw pre- preg thicknesses. e signal trace (shown in blue) has a 5-mil core below and two sheets of 3-mil prepreg above. Another core is placed above the two prepregs at the top extent of the highlighted area. Now look at the structure on the right-hand side of the picture. is is an offset stripline located with the signal on the blue layer on the LHS. As you would expect, H1 = 5 mil—the lower core thickness—but H2 does not equal 6 mil (3 mil plus 3 mil prepreg). H is calculated as 6.28 mils, the combined thickness of the structure when the two sheets of prepreg are pressed into the copper distributed on the signal layer. Taking care of this type of pre-processing is Figure 2: Auto virtual pressing and calculation of H2.