The demand for smaller boards means designers need to pack more functionality into a smaller form factor. A multilayer stack-up is one of the first things you can do to achieve these goals. Let’s take a look at the materials that make up the different layers of a PCB stack-up.
Take a look at this cross-section of a rigid-flex board:
Can you count the number of layers? For the uninitiated, the cross section of a multilayer board can cause some confusion, but it’s actually pretty simple once you understand the basics behind what constitutes a layer in a multi-board design.
Whether you’re looking to build a rigid, flex, rigid-flex, or inlay board, the fundamentals of a stackup remain the same:
Stack the layers of the board symmetrically above and below a middle dielectric layer that’s sandwiched between two copper planes called a core.
Additional layers should be added in pairs to maintain the symmetry of the board.
You can laminate multiple layers together using a dielectric layer called prepreg as required to meet a target thickness requirement.
In this case, the rigid-flex board has four layers. Here they are from top to bottom:
The first layer is likely a signal layer consisting of soldermask, and .5 oz copper traces on FR4 substrate.
The second and third layers are .5 oz copper ground or power planes.
The fourth layer is likely another signal layer consisting of soldermask, copper traces, and FR4.
Since this is a rigid-flex board, it also has two flex layers consisting of coverlay, coverlay adhesive, and a flexible polyimide core. Did you figure out the secret to counting the layers in a multi-layer board? Simply count the number of conductive layers in a board.
Now let’s take a closer look at the materials found in a typical stack-up.
Soldermask, as its name suggests, protects the conductive traces on the topmost and bottommost layers of the board from solder and other conductive materials. It’s the thin polymer layer coating the outermost layers of your board that gives it its iconic green color.
Copper foil is the most common conductive layer found within PCBs. It is used to form the traces on a board or the power and ground planes within the stack. Thin copper is used for fine pitch traces, allowing for more routes per layer. Thick copper for higher current carrying capacity.
FR4 is a non-conductive substrate that provides PCBs with mechanical strength. The substrate is a glass reinforced epoxy laminate material made up of fiberglass cloth and flame retardant epoxy resin binder.
Prepreg is similar toFR4 but is pre-impregnated with a pre-dried but not hardened resin that can flow and stick when heated during the lamination process. It is used to glue the different layers together during the laminating process.
Coverlay/adhesive is used in flex and rigid-flex board designs to bind flex layers together in a board. The coverlay is typically polyimide while the adhesive can be either an epoxy or acrylic based flexible adhesive. It encapsulates and protects components in a flexible board playing a similar role to the soldermask.