How to Control Your Controlled Impedance
Hello again, I’m Amit, the PCB guy. Today I’d like to discuss controlled impedance.
You can get controlled impedance in two ways: first is to specify the dielectric thicknesses that you’re looking for on your fabrication drawing and not specify any controlled impedance at all. Second, which I think is the better way, is to specify the layers in which you’d like your impedance lines on and the target ohms. Standard tolerance is plus or minus 10% ohms. If tighter tolerance is needed, we can deliver plus or minus 5% ohms. Tighter tolerance would require a different game plan ahead of time. Example fabrication notes can be found in our controlled impedance design guide.
Remember, we also plan based on the press-out thicknesses that we expect from the prepreg. This is based on the amount of resin in the prepreg and the amount of copper area and thickness of copper on the opposing layers. We get the copper area from the data you send us to create a final model. So, we don’t follow the thickness on the data sheet, we follow our press-out thicknesses in our modeling, which varies slightly from design to design. That’s why after an initial stack-up and target line widths, we come back to you for approval on small adjustments to the trace widths and spacing.
There is a difference between the core and the prepreg. The core dielectric thickness doesn’t really vary after lamination because the copper is on the outside. If you use prepreg for the dielectric thickness, the height does vary based on the copper area and height of the copper and the glass cloth styles that we’ve chosen at the time of planning the build. All of our lamination cycles have computer controlled profiles to achieve consistency because this is what happens during lamination.
Some customers, however, choose to go with a core construction VS a foil construction. The core construction is when we use an already cured core of material for the outer layer and the next layer in, so that the dielectric thickness doesn’t change after lamination.
This construction takes away the variation in dielectric height. However, this type of construction or stack-up isn’t always possible, like in HDI designs, for instance.
All manufacturers have a restriction on the height of copper based on the spacing requirements. Our table is here:
This means that if you’re doing HDI or have trace and space less than three mils, you should pay attention to the copper weight that is possible at the time of modeling. If you have a blind via on that layer, then the manufacturer has to consider the aspect ratio of the blind via which should be .75:1 in order to ensure good plating in the blind via. So, for modeling impedance on HDI designs, the thickness of the dielectric is controlled by the aspect ratio of the micro via. And the thickness is a big part of modeling.
More copper weight equals more variation in the shape of the trace. Sometimes, you can even get undercut which has to be factored into the impedance modeling. So what you get when you buy from a more expensive PCB manufacture is better tolerances between they’re investing in controlling their processes better which means better performing circuit boards for you. So it’s not just the base materials, it’s also how the board is processed which impacts the final performance.
How do we ensure that you get what you’re asking for? We perform a cross section of the panel and measure the relevant thickness. Here’s a cross section of a plated through-hole. This ensures that there’s good connection to the inner layers and that copper wrap is present and even throughout the hole.
For Class 3 boards, there are different requirements that also include CAF and etch-back.
So now that we know we have the key measurements manufactured properly, we use a TDR machine to test that the ohms are accurate and that ultimately the model that we created in the beginning was accurate.
You will see the TDR report in the documentation that goes with the order.
If your board is fairly complex with lots of impedance on multiple layers, then it is customary for us to build an initial stack-up model for you. If your requirements are not so strenuous, you can also use our free online stack-up and controlled impedance tool. It’s a free 2-D field solver and will get you where you need to be for trace and space requirements and overall board thickness limitations.
Thanks again for tuning in! Until next time…