When you choose controlled impedance on the board, it is best to specify the layers in which you would like your impedance lines on and the target ohms. The standard tolerance is +/- 10% ohms. If a tighter tolerance is needed, we can deliver +/- 5% ohms. A tighter tolerance would require a different game plan ahead of time. Example of fabrication notes can be found in our controlled impedance design guide.
If you create impedance traces with slightly different widths, we are able to locate them easily. For instance, instead of a 4-mil width, make it 4.1-mil thick so we can identify which traces need impedance requirements quickly, and minimize mistakes.
Using the dielectric value of the materials, you naturally get groups that are competing with each other at the dielectric value. FR4 is to the far left and the Teflon and Durand materials are to the right. You should have an idea which material best suits your application and choose the one that is the easiest for your manufacturer to process. Always inquire as to how much experience a manufacturer has with that particular material. You can access another presentation on choosing the right PCB material.
Core Vs Foil Construction
You can see the difference between the core and the prepreg. The core dielectric thickness does not really vary after lamination because the copper is on the outside. If you use prepreg for the dielectric thickness, the height varies based on the copper area, the height of the copper, and the glass styles that we have chosen at the time of planning the build. All of our lamination cycles have computer controlled profiles to achieve consistency.
Selecting PCB Materials
The impedance of traces is also defined by the PCB materials used on the board. Impedance of materials and the expected impedance based on certain parameters is called controlled dielectric. If you like math, you can take the controlled dielectric approach to control the impedance you need. Once you make your calculations, you can specify the dielectric space required between the copper layers in your fabrication drawing notes, and then lay out your traces with the right trace and spacing.
Another thing to ask: Is the material you need available in the thickness that you require? Common prepreg glass styles are 106, 1080, etc. the thickness of the generic glass styles are given in the chart. The thickness of the material is found on the material data sheets. However, we calculate the final press-out thicknesses that we expect from the prepreg, which depends on the amount of resin in the prepreg, the amount of the copper area percentage, and the thickness of the adjoining copper layers.
So, we don’t follow the thickness on the data sheet, we follow our own press-out thickness in our modeling, which varies slightly from design to design.
That’s why after an initial stack-up, we come back to you for approval on small adjustments to the trace widths and spacing. And if we can’t meet the impedance requirement that you are looking for with the material you have selected, we will suggest an alternative material.
Copper Height and Plating
The height of the copper plays a role in the modeling and the question is: How does the manufacturer end up with that specific copper weight or thickness? If it is on an internal layer, we mainly print and etch, which means that we begin with the same height of copper that we finish with. If it is a plated layer, or if the lines are narrow, 3-mil trace and space or below, for instance, we may start with a thinner copper foil and plate up to achieve the final copper weight. This is important because any variation in the plating can have an effect on the impedance results.
The hole wall and the surface get plated at the same time. So the copper thickness laid into the hole also ends up on the surface of the board. The Class 2 requirements are for 8/10 to go in the hole, so we add more than that – about an ounce. One ounce translates to an approximate ounce and a half of copper on the outer layer.
There is also a way to plate the hole but not the complete surface of the board. This is called button or dot plating: you have to design for this where the pad size can handle a wrap of copper plated in the hole as well.
Customers with high-speed requirements will sometimes request this process. Without plated copper on the surface, you can eliminate plating variation and get a smoother surface which is important because of the skin effect.