JLCPCB 4 layer pcb stack mentions a copper core as the image attached. Is this a error or the layer 2 and 3 are wittingly in short?
This image makes no sense...
Taken from: https://cart.jlcpcb.com/impedance?_ga=2.35895136.1315427483.1592868937-1547839306.1592059153
The ‘close parentheses’ is in the wrong place, which why this is confusing.
It should say “1.1 mm (with copper) core”, which is exactly the total of the two 17.5um copper layers and the 1.065mm FR-4 core.
It may help to visualize how PCB stackups are produced
Internal Copper layers are actually provided to your PCB manufacturer as raw material in the form of unatached sheets of copper, they are etched and cut before being glued onto a fiberglass substrate (known as " prepreg layer ") and these are glued together to create a stackup.
Copper layers come in several standard "weights" which dictate the thickness, which is on the order of 0.5-2 thousandths of an inch.
Prepreg comes in a variety of standard thicknesses , typically on the order of 1-20 thousandths of an inch.
To create rigidity, a solid "core" of thicker fiberglass is also used, with several , slightly thicker , standard thicknesses. This material may have copper pre attached for e.g. 2 layer applications which resembles proto PCB material. It is more or less "fat prepreg"
Typically a core is 20-60 thousandths of an inch. And it forms the bulk of the material of a PCB.
A 2 layer board is all core.
A 4 layer board can either use 2 cores and one prepreg but typically 2 prepreg layers and one core. It is somewhat easier to produce in this arrangement. . This results in close spacing between layer 1-2 and layer 3-4 vs close spacing between Layer 2-3 (2 cores)
A good MFG allows you the freedom to select your stackup, maybe for a premium if it requires special steps or "nonstandard" material for them.
There are sometimes reasons, dictated by application, to pick one vs another but it is normal to have such small spacing between layers in a multi layer board.
The core is just that - the core of the PCB. It's not made out of conductive material. The total width of core and inner layers is shown for your reference only.
