Column: Embedded design
Figure 1: 0.8mm PCB
Figure 2: 0.8mm PCB layer thicknesses
suddenly begin to exhibit a kaleidoscope of unexpected and previously never seen faults, taking cripplingly expensive man- weeks to diagnose and, fi nally, correct. Let’s pretend this isn’t my personal
experience. An 0.11mm prepreg layer isn’t actually
the thinnest I’ve ever had introduced into a PCB stack-up. Prepreg of 0.05mm or thinner is also used, and in the wrong place will cause even greater problems. I have even encountered one PCB
supplier who used multiple stacked pieces of prepreg in the outer insulating layers to produce a rough approximation of the desired uniform separation, but then lost control of its internal QA and order control processes with the second batch of boards. At this point they reverted to single prepreg layers and the unequal spacings, resulting in a major change of board stray capacitance in the middle of a very large production batch.
Addressing the challenge So, what can the poor design engineer do about this lurking horror? It comes down to careful control over
the purchasing process and diligent record keeping. Whenever a board is ordered, either the
designer and the purchaser must carefully and fully specify the inner layer stackup, or the supplier needs to provide full information and drawings. When a new supplier is brought in to supply an existing design’s boards, then it must provide product that complies with the existing internal dimensions. Variation of inner layer thicknesses
between batches – especially from the same supplier – is completely “time to call the lawyers” intolerable.
T is column on embedded design prepared by Myk Dormer continues in the next edition of Electronics World
WHAT IS THIS “PREPREG” STUFF ANYWAY?
PCB prepreg (short for “pre- impregnated”) is a fi breglass cloth that’s been saturated with a partially- cured polymer resin. It’s the glue that holds the multilayer PCB together. During lamination, heat and pressure cause the resin to fl ow, bond with adjacent copper layers and cores, and then fully cure into a solid, stable, insulating structure.
The key characteristic of prepreg is this partly cured state. Unlike solid core materials, prepreg remains pliable and tacky before lamination, after which it sets solid.
To borrow a culinary parallel: it’s the melted cheese, sticking your burger (the core) to the bun (the outer layers).
www.electronicsworld.co.uk September 2026 11
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