Column: Embedded design


The PCB is a component, too By Myk Dormer, RF Engineer and Director, Smallwireless Limited I


can guarantee that there are eyes rolling already in readership-land, right now: “Oh look, another conceited RF engineer is going to wax lyrical about striplines and interdigital fi lters, and all manner


of other microwave black magic that no ‘ordinary’ engineer will ever actually use”. However, this essay is not about that


subject. Nor am I going to extol or decry any particular PCB substrate material for its loss tangent or electromagnetic stability – or try to sell you a marginally better one. T is is not about the geometry of your


layout at all. I am concerned with the third dimension: T e thickness of your board and the structures that make up that vital millimetre or so between one side and the other.


About the humble PCB How a PCB is constructed is no mystery to even a student engineer. It’s a sandwich. Two or more layers of thin copper foil are glued to and separated by layers of resin-impregnated glass fi bre, and fi nished off with outer layers. T ere is a solder- friendly and tarnish-resistant surface fi nish, then a polymer “solder resist” and, fi nally, perhaps an “ident” or “silk screen” layer. Drill a bunch of holes of various sizes, and plate all or some of them with more metal to interconnect conductors on various copper layers, and you’re pretty much there. So, how can this ever go wrong? As we know, the devil is oſt en in the


detail. A two-layer board is unlikely to throw up manufacturing issues, but a four-layer example is a very diff erent case. Four- and higher-layer boards have a


If the circuit makes use of controlled impedance


tracks such as RF striplines, then the associated


calculations will be


radically thrown off , and the resulting performance can get very unpredictable


particular vulnerability. T ink how they are assembled: In eff ect, a thinner two-layer board “core” is manufactured, then two further layers of insulating “prepreg” (see Box on the next page) are added, before the outer copper layers (etc.) are sandwiched on, resulting in a copper, insulator, copper, insulator, copper, insulator, copper stack, and so on; see Figure 1. In this typical 0.8mm nominal thickness PCB example, the insulators are about 0.2mm thick, leaving a nice predictable uniform spacing between the conducting layers. Sadly, it’s not always like this, as some PCB suppliers take a shortcut. T ey only


10 September 2026 www.electronicsworld.co.uk


stock a single thickness of prepreg (usually about 0.1mm), which they use for every multilayer board they make. So, if you order a four-layer 0.8mm board, the core thickness ends up at almost 0.4mm, and the outer insulating layers are barely 0.1mm, which can really change the characteristics of the PCB. We are looking at outer layer pairs that are now only separated by a very thin insulator (Figure 2), whilst the inner pair layers are considerably further apart, which is not good. T e eff ects on even relatively low frequency circuits can be little short of catastrophic. T e thinner insulating layer greatly


increases stray capacitances on the one hand (de-tuning low-capacitance, high-impedance circuits like VCO and resonant fi lter, and increasing crosstalk and coupling between tracks on adjacent layers), while reducing the RF impedance of grounding holes (or vias). If the circuit makes use of controlled impedance tracks such as RF striplines, then the associated calculations will be radically thrown off , and the resulting performance can get very unpredictable. If every board was the same, from


prototype right through to production, then this eff ect would be inherently designed out. Although, again, at RF those thin outer layers can also introduce unexpected reactive losses or temperature driſt eff ects, as a capacitor made of FR4 prepreg is not a high-performance part. However, the real chaos comes when


one PCB supplier is used in prototyping and early production stages, and another then supplies these thin-prepreg boards. We then start to see established designs that were fully characterised and approved


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