When we design a PCB we think of its final thickness in discrete values: 0.2mm, 0.8mm, 1.55mm, 2.4mm, and others. But the reality is that we will almost never get a PCB back with those exact thicknesses. Here I will discuss why that is, the many factors that affect boards’ final thickness, the tolerances we might expect, where and how we measure, and how to plan your project accordingly.
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.
Calculating the thickness
When we choose a 6-layer 1.5mm board the available buildups can range from 1.47mm to 1.82mm in thickness ― notice the figures in the bottom right of the images ― mostly influenced by thickness of the copper foils (12µm vs 70µm).
The calculated thickness depends on our design choices. Let’s take for example a “1.55mm” 6-layer board from Eurocircuits. In the ‘Buildup Editor’ we can see that over 20 possible buildups are available thicknesses that range from 1.47mm and 1.82mm! This makes sense: if we choose thicker copper foils, the overall board thickness will increase. Let’s choose the 1.82mm buildup, which is already about 17% thicker than 1.55mm. Now we need to apply the PCB supplier’s tolerance which is normally 10% across the industry, including at Eurocircuits. This gives us a thickness range of 1.64mm to 2mm.
But that’s not all. Recall that the final values above do not include plating, soldermask, and silkscreen. Together those may add up to 0.125mm on each side of the board, so we need to add 0.25mm to our worst-case thickest board. This means that depending on our finish and what’s on the surface of where we’d measure the board may be over 2.2mm thick!
The example above demonstrates three primary things. Firstly, that the nominal board thickness is merely a label for a collection of possible board thicknesses. Secondly, that we may get a board back that can be significantly different from the nominal label we started out with. And finally, the thickness depends on what features exist where we measure.
Can we have better control?
Since cores and prepreg come in many different thicknesses, a reasonable question is why not use the thicknesses of materials that make the calculated thickness as close to nominal as possible? There are two main reasons why this isn’t practical.
Firstly, thinner cores and prepregs ― 0.1mm or thinner ― are brittle and much harder to work with reliably. Also, thin prepreg sheets do not adhere very well to copper and laminate and using only one sheet can cause increased occurrences of delamination (at Eurocircuits we almost always use two prepreg sheets).
Secondly, cores and prepreg sheets have a limited shelf-life ― in the order of months ― so keeping stock of many thicknesses can be costly and wasteful. Keeping a small amount of ‘standard’ most common thicknesses that can compose the majority of desired buildups is both cost-effective and ensures availability since they can be sourced from multiple suppliers.
Board thickness at Eurocircuits
Let’s summarise the important points:
- Nominal board thickness is often just a label.
- Calculated board thickness is the addition of the nominal thicknesses of each layer that composes the buildup (laminates plus copper). The measured thickness is the actual laminate-to-laminate thickness with the addition of the raw external copper thickness.
- Board thicknesses do not include plating, finishes, soldermask, and silkscreen.
- Like Eurocircuits, most manufacturers will guarantee a 10% tolerance from the calculated thickness.
And most importantly: If board thickness is a crucial dimension in your product, never expect to receive an exact thickness, and make sure that there is sufficient allowance that considers all worst-case tolerances. Still, in cases when there is a rigid thickness requirement, work with us early on to make sure that this is achieved reliably.
Our powerful Visualiser provides tools for defining the board thickness where we must pay attention to the calculated board thickness. This is primarily done in the Buildup Editor tool.