Printed circuit board

A PCB includes a number of insulation layers, a number of circuit layers, a signal-interfering component, and a signal-sensitive component. The circuit layers and the insulation layers are stacked alternately. The circuit layers include at least two first circuit layers, a second circuit layer, and a ground layer. The ground layer has a first side and a second side facing away the first side. The first circuit layers are positioned near the first side and include an outmost first circuit layer and at least one inner first circuit layer positioned between the outmost first circuit layer and the ground layer. The second circuit layer is positioned near the second side. The signal-interfering component is positioned on the outmost first circuit layer. The signal-sensitive component is positioned on the second circuit layer. Each inner first circuit layer defines a copper-remove area corresponding to an orthogonal projection of the signal-interfering component thereon.

BACKGROUND

1. Technical Field

The present disclosure relates to a printed circuit board (PCB).

2. Description of Related Art

A pulse width modulation (PWM) circuitry can be formed on a PCB for providing pulse signals. The PWM circuitry requires high-speed alternating current during operation to generate powerful electromagnetic waves. It is not uncommon for a signal-sensitive source, such as voltage reference circuitry to be included and arranged close to the PWM circuitry and thus be susceptible to electromagnetic interference from the PCB.

Therefore, it is desirable to provide a PCB that can overcome the limitations described.

DETAILED DESCRIPTION

Referring toFIG. 1, a PCB100, according to an exemplary embodiment, includes six circuit layers, five insulation layers101, a signal-interfering component102, and a signal-sensitive component103. The circuit layers and the insulation layers101are stacked alternately and one insulation layer101is laid between every two adjacent circuit layers. The six circuit layers are four first circuit layers10,20,30,40, a second circuit layer50, and a ground layer60. The ground layer60includes a first side and a second side facing away the first side. The four first circuit layers10,20,30,40are disposed on the first side in this order from a far end to a near end. The second circuit layer50is disposed on the second side.

The signal-interfering component102(such as a PWM circuitry) includes a first portion and a second portion electrically connected thereto. The first portion includes a number of electrical elements (such as capacitance, resistance, inductance) positioned on the first circuit layer10. The second portion includes a number of signal traces layered on the first circuit layer20. The signal-sensitive component103such as a voltage reference circuitry is positioned on the second circuit layer50and away from an orthogonal projection of the signal-interfering component102on the second circuit layer50. In other embodiments, the signal-interfering component102also can be at least one high-speed signal transmission line. The signal-sensitive component103also can be a universal serial bus (USB) circuitry or a video graphics array (VGA) circuitry.

The first circuit layers30and40host a number of signal transmission lines (not shown) away from orthogonal projections of the signal-interfering component102on the first circuit layers30and40. The first circuit layers30and40define a first copper-removed area31and a second copper-removed area41corresponding to the orthogonal projections of the signal-interfering component102on the first circuit layers30and40. The first circuit layer10and the ground layer60cooperatively form a first parallel plate condenser therebetween. According to a formula

C=K⁢⁢ξ⁢⁢Sd,
wherein C is the capacitance of the first parallel plate condenser, K is a constant, ζ is a dielectric constant, S is the area of the facing surfaces of the two polar plates of the condenser, d is the distance between the two polar plates, when d increases, then C will decrease. Also considering a formula of the capacitive reactance of the parallel plate condenser:

XC=1w⁢⁢C,
wherein XCis the capacitive reactance of the parallel plate condenser, w is the frequency of an alternating current ran through the parallel plate condenser. When C decreases, then XCincreases. Therefore, the electromagnetic wave generated by the signal-interfering component102is blocked to be transported to the ground layer60, and accordingly the electromagnetic waves are blocked to be transported to the signal-sensitive component103. Thus, the interference of the signal-interfering component102to the signal-sensitive component103is reduced or even eliminated.

The second circuit layer50defines a third copper-removed area51corresponding to an orthogonal projection of the signal-interfering component102on the second circuit layer50. Because the propagation speed of electromagnetic waves is highest in the air, the electromagnetic waves are transported thereby and not by a second parallel plate condenser formed by the ground layer60and the second circuit layer50. Accordingly, influence of the electromagnetic waves on the signal-sensitive component103is minimal.

Referring toFIG. 2, the differential voltage of the PCB is about 11.2659 mV. That is to say, the voltage of the signal-sensitive component103is formed in a relativity stable range, and interference of the signal-interfering component102to the signal-sensitive component103is low.

It can be understood, because in the present embodiment, the signal-interfering component102is a PWM circuitry which generates considerable heat during operation, if the PWM circuitry is only disposed on a first circuit layer10, the heat will not be dissipated quickly and can collect on the first circuit layer10, causing damage thereto and influencing other elements on the first circuit layer10. If the signal-interfering component102generates a little heat, it can be only disposed on a first circuit layer10.