Introduction of Grounding Method in Production of Multilayer PCB

Introduction of Grounding Method in Production of Multilayer PCB

Circuit board manufacturing, circuit board design, PCBA processing manufacturers will explain the grounding methods in the production of multilayer PCB

According to the rule of thumb, PCBs usually use 4-layer boards in high-density and high-frequency situations, which is 20 DB better than 2-layer boards in terms of EMC. Under the condition of four layers of boards, a complete ground plane and a complete power plane can often be used. Under this condition, only the ground wires of circuits divided into several groups need to be connected to the ground plane, and the working noise needs to be specially treated.

There are many ways to connect the ground wire of each circuit to the ground plane, including:

Single point and multi-point grounding mode

① Single point grounding: the ground wire of all circuits is connected to the same point on the ground plane, which is divided into series single point grounding and parallel single point grounding.

② Multipoint grounding: the ground wires of all circuits are grounded nearby, and the ground wires are short enough for high-frequency grounding.

③ Mixed grounding: single point grounding and multi-point grounding are used together.

In low frequency, small power and the same power layer, single point grounding is the most appropriate, usually used in analog circuits; The star type connection is generally adopted here to reduce the influence of possible series impedance. High frequency digital circuits need to be grounded in parallel. In this case, the grounding hole can be used for simple processing, as shown in the left half of the figure; Generally, all modules will use two kinds of grounding methods comprehensively, and use the mixed grounding method to complete the connection between the circuit ground wire and the ground plane.

Mixed grounding mode

If the whole plane is not selected as the common ground wire, for example, when the module itself has two ground wires, the ground plane needs to be divided, which often interacts with the power plane. Pay attention to the following principles:

(1) Align all planes to avoid overlap between irrelevant power planes and ground planes, otherwise, all ground planes will be divided into invalid planes and interfere with each other;

(2) In the case of high frequency, the parasitic capacitance between layers through the circuit board will produce coupling;

(3) The signal lines between ground planes (such as digital ground plane and analog ground plane) are connected by ground bridges, and the nearest return path is configured through the nearest through-hole.

(4) Avoid unnecessary radiation caused by high-frequency routing such as time clock line near the isolated ground.

(5) The ring area formed by the signal line and its loop shall be as small as possible, which is also called the loop minimum rule; The smaller the ring area is, the less the external radiation is, and the smaller the external interference is received. During ground plane segmentation and signal routing, the distribution of ground plane and important signal routing shall be considered to prevent problems caused by ground plane slotting.

The connection methods between the ground are summarized here.

① Common wiring connection of circuit board between ground: this method can ensure reliable low impedance conduction between two ground wires, but it is only limited to the grounding method between middle and low frequency signal circuits.

② Ground to ground large resistance connection: the characteristic of large resistance is that once there is a pressure difference between the two ends of the resistance, a very weak conduction current will be generated. After the charge on the ground wire is discharged, the pressure difference between the two ends will eventually be zero.

③ Ground to ground capacitive connection: the characteristics of capacitance are DC cut-off and AC conduction, which are used in floating ground system.

④ Earth to earth magnetic bead connection: the magnetic bead is equivalent to a resistance that changes with frequency, and it shows the resistance characteristics. It is used between ground and ground for weak signals with fast and small current fluctuations.

⑤ Ground to ground inductance connection: inductance has the characteristics of restraining circuit state changes, and can cut peak and fill valley. It is usually applied between two ground and ground with large current fluctuations.

⑥ Small resistance connection between ground: small resistance adds a damping to prevent the overshoot of rapid change of ground current; When the current changes, the rising edge of the impulse current becomes slower. PCB manufacturers, PCB designers and PCBA manufacturers will introduce the grounding methods in the production of multilayer PCB boards.