Printed circuit board with electrostatic discharge damage prevention

A printed circuit board for preventing electrostatic discharge damage includes several electronic components thereon. The printed circuit board defines a number of through holes therein. The printed circuit board includes a signal layer. The signal layer defines a first copper foil and second copper foil thereon. The first copper foils are disposed around the corresponding through holes and connect with the through holes. The second copper foils are disposed around the first copper foils and extend to two adjacent edges of the printed circuit board. The first copper foil and the second copper foil have a number of saw teeth. A gap between the first copper foil and the second foil is in the range from 0.1-0.125 mm.

CROSS-REFERENCE TO RELATED APPLICATION

Related subject matter is disclosed in co-pending U.S. patent application No. 11/295,944, filed on Dec 6, 2005 and entitled “PRINTED CIRCUIT BOARD WITH INSULATIVE AREA FOR ELECTROSTATIC DISCHARGE DAMAGE PREVENTION”, which is assigned to the same assignee as that of the present application.

BACKGROUND

1. Field of the Invention

The present invention relates to a printed circuit board for preventing electrostatic discharge damage, and more particularly to a printed circuit board achieving this without the need for added materials.

2. General Background

In general, electronic products including printed circuit boards (PCBs) are susceptible to damage by electrostatic discharge during manufacturing, assembly, transportation and utilization. Therefore electrostatic discharge protection is necessary for electronic devices, especially printed circuit boards. The trend toward miniaturization of electronic components on printed circuit boards has lead to a reduction in compression and current surge resistance of electronic components. Therefore electrostatic fields and electrostatic currents from electrostatic discharges are more likely to damage precision electronic components on printed circuit boards. Damage resulting from an electrostatic discharge may impair the functioning of entire electronic devices.

Referring toFIG. 3andFIG. 4, a conventional four-layer printed circuit board includes a signal layer11, a power layer15, a ground layer17, and a plurality of electronic components21. The printed circuit board defines a plurality of through holes31therein, and each through hole31has an electrical connection with the ground layer17through an inner wall of the through hole31. Typically, the through holes31have metal coatings on the inner walls thereof. The electronic components21on the printed circuit board are connected to the ground layer17through pins and solder balls, so that the electronic components21can be grounded. The ground layer17can also provide EMI (Electro Magnetic Interference) shielding for the printed circuit board.

During the manufacture and assembly of printed circuit boards, electrical charges on the surface of an operator's body may be released through electrostatic discharge on contact with the printed circuit boards. The body of the operator commonly contacts an edge of the printed circuit board; therefore in general, the electrostatic discharge takes place on the edge of the printed circuit board. Because electrostatic current often discharges to the ground through a route which has a shortest distance and minimal impedance, the electrostatic current flows to the through holes31from the edge of the printed circuit board, and in doing so passes through one or more of the electronic components21on the printed circuit board. The electrostatic current may thereby damage the electronic components21.

There are many means of preventing damage from electrostatic discharge on a printed circuit board. A common means is to design a protective circuit on the printed circuit board. However, in designing a protective circuit, each component's function and position on the printed circuit board should be considered. Additionally, a protective circuit requires additional electronic components and layout spaces on the printed circuit board. Furthermore, this kind of design is complex and increases the cost of manufacturing.

What is needed is a printed circuit board which can prevent damage due to electrostatic discharge without costly additions.

SUMMARY

An exemplary printed circuit board for preventing electrostatic discharge damage includes several electronic components thereon. The printed circuit board defines a plurality of through holes therein, and includes a signal layer. The signal layer defines a plurality of first copper foils and second copper foils thereon. Each first copper foil is disposed around a corresponding through hole and connects with an inner wall of the through hole. Each second copper foil is disposed around a corresponding first copper foil, and extends to two adjacent edges of the printed circuit board. The first copper foil and the second copper foil have a plurality of saw teeth facing each other across a gap therebetween. The gap between the first copper foil and the second foil is in the range from 0.1-0.125 mm.

When electrostatic discharge takes place due to an operator's touching of the printed circuit board, the electrostatic current is released to the saw teeth of the first copper foil and then grounded through the through hole. Thus the electronic components on the printed circuit board are protected from electrostatic discharge.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Referring toFIG. 1andFIG. 2, a printed circuit board50in accordance with a preferred embodiment of the present invention is shown. The printed circuit board50includes a signal layer51on one main side thereof. The printed circuit board50defines four through holes81in four corners thereof, respectively. A plurality of electronic components60is installed on a central region of the printed circuit board50. A first copper foil83and a second copper foil87, together defining a low-impedance protective area, are arranged on each corner of the signal layer51. The first copper foil83is disposed around a corresponding through hole81, and connects with an inner wall of the through hole81. Typically, the through holes81have metal coatings on the inner wall thereof. The first copper foil83has a plurality of tips85on an outer edge thereof. In the illustrated embodiment, the tips85are continuous series of outer saw teeth. The second copper foil87lies around the first copper foil83without contacting it, and extends to two adjacent edges of the printed circuit board50. The second copper foil87has a plurality of tips89on an inner edge thereof, the tips89facing the tips85of the first copper foil83across a gap therebetween. In the illustrated embodiment, the tips89are continuous series of inner saw teeth. The gap between the first copper foil83and the second copper foil87is in the range from 0.1-0.125 mm.

When an operator touches the printed circuit board50, electrostatic discharge may occur at the second copper foil87because the second copper foil87extends to the edges of the printed circuit board50and has a very low impedance in comparison with the components60. If electrostatic discharge occurs, according to the point discharge principle, electrostatic charges accumulate at the tips89. When a threshold amount of electrostatic charges accumulate at the tips89, the electrostatic charges are released to the tips85of the first copper foil83and then released to ground via the through holes81. Thus, the electronic components60on the printed circuit board50are protected from damage due to electrostatic discharge. The printed circuit board50can prevent damage due to electrostatic discharge without costly additions. The principles of the present invention can be applied to a four-layer printed circuit board and also a six-layer printed circuit board.