Basic Knowledge of PCB Materials:

Basic Knowledge of PCB Materials:

Copper-clad laminate, also called PCB substrate.

(1) A plate-shaped material formed by filling a reinforcing material (glass fiber cloth, referred to as glass fiber cloth) with resin (pp sheet), covering one or both sides with copper foil, and hot pressing, called a copper-clad layer pressure plate (Copper Clad Laminates, [CCL]).

(2) The basic material for PCB, it is often called board.

(3) When it is used in the production of multilayer boards, it is also called core board (or core).

Copper foil

(1) The thickness of copper foil is measured in oz (ounces), and oz itself is the unit of mass. Usually, the thickness of copper foil is expressed as “thickness” by mass. The thickness of copper foil is usually defined as “oz”, 1oz copper thickness—spread one ounce of copper evenly on one square foot area. At this time, the thickness of the copper foil is called 1oz copper thickness, and its thickness is exactly 1.37mil (about 1.4mil).

The standard thickness of copper foil is 12μm (1/3oz), 18μm (Hoz), 35μm (1oz),and 70μm (2oz).

Process principle of prepreg (prepreg or pp)

pp is a sheet material made by filling treated glass fiber cloth with resin glue, and then heat treating (pre-baking) to make the resin enter the B stage. The process principle of the pressure plate is to use the prepreg from the B-stage to the C-stage conversion process to bond the various circuit layers into one.

Common performance indicators of substrate

(1) DK: The dielectric constant of the material. Only by reducing DK can a high signal propagation speed be obtained.

(2) Df: The dielectric loss angle of the material; the lower this angle, the lower the signal propagation loss is.

Note that DK is mainly related to the impedance of the signal network and also related to the capacitance between the plates, and Df is mainly related to the loss of the signal network. Factors affecting DK are:

Resin (DK of epoxy resin is between 3～4);

Glass fiber cloth (DK between 6～7);

Resin content (RC value).

(3) Tg: glass transition temperature, which has the greatest impact on vias. The glass transition temperature is the characteristic of the polymer, which refers to the resin from hard (glassy) to soft (rubbery). This is the temperature at which the shape changes.

At present, the Tg value of FR-4 board is generally 130-140, and in the manufacture of printed boards, there are several process problems that exceed this range, which will have a certain impact on the processing effect and final state of the product. Therefore, increasing Tg is a primary method to improve the heat resistance of FR-4. The Tg classification is as follows.

Ordinary Tg sheet: 130～140℃.

Medium Tg sheet: 140～150℃.

High Tg board: greater than 170°C (PCB boards with more than 8 layers must use high Tg boards).