Economical packaging for EMI shields on PCB

Details Economical packaging for EMI shields on PCB Economical packaging for EMI shields on PCB

1999-08-13

2002-10-08

Ballato, Josie (Department: 2836)

Electrical transmission or interconnection systems

Anti-induction or coupling to other systems

Magnetic or electrostatic field control

C361S818000

Reexamination Certificate

active

06462436

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates generally to protective enclosures for electronics and electronic circuits.
2. Description of the Prior Art
Electronic circuitry assemblies, printed circuits boards, and substrates containing circuitry and electronic components mounted thereon, often require electromagnetic interference (EMI) shields to limit the likelihood of signal interferences from electromagnetic waves, such as those caused by radio-frequency (RF) signals.
It is known that the operation and performance of sensitive electronic components, such as integrated circuits (ICs), can be affected by the presence of interfering electromagnetic signals. Certain electronic components and devices are known to emit electromagnetic signals during their operation. In particular, on a circuit board, components emitting EMI signals can detrimentally affect the performance, reliability and even operability of other electronic components on the same board. Three essential elements must be present for an EMI situation to exist, including: an electrical noise (EMI) source, a coupling path, and a victim receptor. The noise source emission can be either a conducted voltage or current, or an electric or magnetic field propagated through space. It is known that certain equipment and systems can serve as both EMI sources and receptors. A coupling path may exist between signal sources and receptors and can be divided into two basic groups: radiation or field coupling by electromagnetic wave propagation through space or materials (hereinafter known as “air coupling”), and coupling via conducted paths through which current can flow (hereinafter known as “board coupling”). Additionally, and herein after, the use of the term “board coupling” includes the transmission of EMI across a circuit as well as electromagnetic wave propagation through the circuit board or substrate material and the term “air coupling” includes the transmission of EMI through the air due to electric field and/or magnetic field emanations.
To minimize the presence of interfering signals with sensitive electronic components and the effects of air coupling and board coupling, whether the interfering source is on the same assembly or apart from the receptor device being a sensitive electronic, the use of EMI shielding is often employed.
EMI signals may occur and interfere with electronic components due to sharing of conductors with EMI sources, emanation of electric fields, emanation of magnetic fields and electromagnetic radiation. EMI shielding causes an electromagnetic wave propagating through space to be absorbed or reflected when the wave contacts the shield wall. EMI signals are first reflected off a shield wall usually due to the material of the shield. EMI signals, also known as energy, which are not completely reflected may then be absorbed by the shield wall such that only residual energy is able to emerge from the opposing side of the shield wall. The emergent residual energy is the resulting EMI. Therefore, the effectiveness of the shielding is determined by the shield's reflectance and absorbance characteristics, which are dependent on the shield material and the shield interface with the substrate and circuit ground.
EMI shields are often installed over or in proximity to sensitive electronic components on a circuit board to inhibit interference from propagating to a receptor or to prevent EMI signals from being emitted by an emitting source. An EMI shield may be of varying in shape and size in relation to the sensitivity of the electronics and the material used in construction of the EMI shield. Generally, EMI shields range in size from one square inch to over two square feet.
It is known that an EMI shield is typically comprised of a metal sheet, a casting, or other conductive material such as a mesh or paint which is formed into a shape in relation to both the components and the space available on the circuit board. An EMI shield is usually precisely placed on a circuit board at a prescribed location and is attempted to be grounded, usually with a circuit ground. The EMI shield is typically installed by securing the shield to the circuit board. Often, compression fittings or screws are used to secure the shield in place. However, in securing the shield, often shield edges move slightly from the prescribed location, thereby affecting shield performance. It is known that the precision placement of the shield in contact with the substrate and a ground is critical to effectively isolate the circuit from interference and effectively ground errant electromagnetic waves.
Problems frequently arise due to movement of the shield during securing and also due to gapping along the interface between the shield edge and circuit board. In particular, the surfaces of the standard boards and shield edges are not uniformly planar and these nonuniformities cause inconsistencies in contacts due to spaces or gaps resulting at the interface between a board and a mounted shield edge. Similar problems also arise when shield edges are improperly positioned by as little as 0.0001 inches from the predetermined location. Unfortunately, these problems usually cause EMI coupling, such as interferences due to common impedance (e.g. board coupling) and electric and magnetic fields (e.g. air coupling), resulting in EMI signals interfering with sensitive components.
Prior art solutions, in attempting to resolve these problems, have maintained the use of a shield edge having a surface which is finished flat, as conventional belief is that a flat edge limits the movement of the edge in relation the edge's position on the substrate. However, the prior art's use of a flat edge surface does not overcome problems resulting from movement, gapping or the effects of nonplanar surfaces. A flat edge surface does not substantially prevent EMI effects, and in particular, does not substantially limit EMI effects caused by air coupling. Consequently, the prior art has been less than satisfactory.
In
FIG. 1
, prior art EMI shields and substrates are depicted.
FIGS. 1A through 1C
depict shields which are to be secured onto a substrate at precise locations using a securing means, such as a plurality of screws (not pictured). Each of these prior art figures necessitates that the shield edge be precisely aligned with the predetermined location on the substrate, as previously discussed.
FIG. 1A
depicts a conductive gasket
10
to be placed on a substrate
20
and a casted EMI shield
30
having a cut-out
40
fitted to receive the gasket
10
. The gasket
10
is sandwiched into the cut-out
40
and the resulting shield
30
and gasket
10
are compressed onto the substrate
20
and secured by a securing means.
FIG. 1B
depicts a gasket
50
, which forms in place on the planar contact surface
60
of the shield
65
. When compressed into place, the gasket
50
also forms to take the shape of the surface
75
of the substrate
70
. The gasket
50
is comprised of a deforming material, which alters its shape when compressed under pressure or at elevated temperature. A machine (not pictured) is typically used to form the gasket
50
in place along the edge
60
of the shield
65
. The shield
65
and gasket
50
are then compressed into place on the surface
75
of the substrate
70
and secured by a securing means.
FIG. 1C
depicts a non-contacting interface
80
between a substrate
90
and an edge
95
of a shield
99
.
Each of the FIGS.,
1
A through
1
C, shows a cross-sectional view when the substrate, the grounding means and the shield are each planar with respect to one another. In situations where these elements are not planar, the interface contact is interrupted or is inconsistent and the EMI reflectivity and absorbtivity of the shield at the interface is generally unsatisfactory. Additionally, each of the prior art depictions is generally ineffective in substantially preventing EMI due to board coupling and air coupling effects, since gaps at interfaces result in substantially reduced shielding perf

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