Into the thick of it
We’re used to talking about the layers of a PCB in terms of routing layers: top, bottom, internal-1, and so on. But there are many other “layers” in between, above, and below: cores, prepreg, plating, finish, soldermask, silkscreen, etc. that contribute to the final thickness of a board. A two-layer board is straightforward because it has a laminate core (usually FR-4) with two copper sheets on its top and bottom. It is sourced ready for production and its thickness is largely known before we even start manufacturing. As we add internal copper layers, however, things get more complicated.
To help our discussion let’s first define three different thicknesses:
- Nominal: this is the discrete thickness we start out with such as 1.0mm, 1.55mm, 3.2mm, etc. We can think of this as a label used for referring to a collection of different buildup thicknesses that are ‘around’ this value.
- Calculated: this is a prediction of the thickness that is based on adding all the expected individual raw materials layers that compose the buildup. A calculation may factor in how they expand or contract during the manufacturing process. This figure includes top-laminate to bottom-laminate and the top and bottom copper, but does not include plating, finish, soldermask, and silkscreen.
- Measured: this is the measured thickness of the manufactured board from top-laminate to bottom-laminate plus the raw material thickness of the top and bottom copper. This value does not include plating, finish, soldermask, and silkscreen.
Naively we may think that since we know the thicknesses of the raw materials we can easily predict the thickness of the finished board with confidence, or even control the final thickness better so that it’s closer to nominal thicknesses. That’s actually quite hard to do. Firstly, the thickness tolerances expected from the material suppliers is about 10% (so a 1.55mm board can be between 1.4mm and 1.7mm). Secondly, the actual thickness depends on the design! A sparse copper internal layer will ‘accept’ much more prepreg resin as it flows during pressing, compared to when there’s a mostly solid copper internal layer. Also, the thickness of the surface copper sheet may be known but the thickness and distribution of the plating and finish will be different depending on the copper distribution; this is the reason why the material thickness of the copper is added to the measured thickness (as defined above) instead of being included together with the rest of the buildup when the measurement is done.