Electrostatic discharge protection in a portable communication device

The present invention relates to a portable communication device, comprising an electronic circuit board arranged inside a housing, and a keypad, the individual keys of which are arranged, when depressed, to collapse a respective tactile dome on a domefoil interposed between the keypad and the circuit board, so as to establish electric contact with a respective contact point on the circuit board, said communication device being provided with means for protecting the circuit board from electrostatic discharge (ESD).

BACKGROUND

Portable communication devices are used in various applications to provide a means for remote communication between two parties. Common communication devices of this kind are for instance cellular telephones, cordless phones, paging devices, walkie-talkies, PDAs (Personal Digital Assistants), etc. Such devices comprise an apparatus housing, in which an electronic circuit board is arranged. The circuit board is provided with circuitry necessary for performing the operational communication tasks of the device, e.g. CPU, memory modules and radio circuits (amplifiers, local oscillators, filters, modulators, mixers, etc). Furthermore, the circuit board is often provided with LEDs (Light Emitting Diodes) or other devices for illuminating a keypad arranged in the housing.

Keypad-operated devices of the kind described above have to fulfill various requirements in order to provide a reliable communication service to the user of the device. Such requirements are for instance immunity to electrostatic discharge (ESD) as well as a sufficiently high degree of electromagnetic compatibility (EMC). When the user touches the keypad, there is an apparent risk of an electrostatic discharge between the user's fingertip and conductive elements, e.g. the electronic circuit board inside the device, and hence a keypad-operated communication device must be provided with ESD-protective means. Furthermore, the device must be shielded in a way that prevents the active parts of the communication device from interfering electromagnetically with surrounding electronic equipment. Such shielding must also protect the device from electromagnetic influences or disturbances caused by such surrounding electronic equipment.

FIG. 2

discloses a previously known keypad arrangement for a cellular telephone. A keypad

30

is made from e.g. rubber and comprises a set of individual keys

32

. Although not shown in the drawing each key

32

is labelled according to its dedicated purpose, for instance a digit “3” or a “*” sign. A domefoil

40

made from plastics or the like is interposed between the keypad

30

and an electronic circuit board

20

arranged inside the telephone housing. The domefoil

40

is provided with a plurality of tactile domes

42

, each of which is given a convex shape protruding or bulging from the domefoil

40

. One respective dome

42

is arranged beneath one respective key

32

in the keypad

30

.

The circuit board

20

is given a purely schematical appearance in FIG.

2

. The circuit board

20

comprises various electronic circuitry, such as radio transmitter/receiver circuits, amplifiers, filters, buffers, oscillators, mixers, one or several micro-processors, memory modules, etc., none of which are indicated in FIG.

2

. Furthermore, the circuit board

20

is provided with a set of contact points

22

, a respective one of which being arranged in vertical alignment with a respective pair of key

32

and dome

42

.

The functional operation of the keypad

30

is as follows. When a key

32

is depressed by a user, the back of the key will exert a force upon the underlying tactile dome

42

. The dome

42

will not be able to resist the force exerted by the key

32

but will collapse inwardly/downwardly and reach contact with the underlying contact point

22

. For this purpose the rear side of each dome

42

is made electrically conductive, for instance by providing each dome with a conductive layer, such as a silver or carbon compound. The collapsing dome establishes electric contact between a contact pair at the contact point

22

, the details of which are omitted in the drawing, since they are believed to be well-known per se to a man skilled in the art. The electric connection so established is immediately detected by a controller comprised on the circuit board.

Furthermore, the circuit board

20

is provided with illuminating means

24

realized as LEDs (Light Emitting Diodes), the purpose of which is to illuminate the keys

32

through respective openings

44

in the domefoil

40

. In

FIG. 2

one illuminating means

24

is arranged to illuminate four keys

32

surrounding the opening

44

. In addition to these openings the domefoil

40

is provided with various holes, openings or recesses

46

,

48

,

49

for various electronic components or mounting means arranged on the circuit board

20

.

In the prior art arrangement according to

FIG. 2

the rubber-based keypad sheet

32

serves as an ESD-protective means. Furthermore, the housing

12

shown in

FIG. 1

is provided on the inside thereof with a thin metallized layer functioning as a protection or a shielding against electromagnetic interference between the telephone

10

and surrounding electronic equipment. Although providing a sufficient protection against ESD as well as electromagnetic interference, the prior art arrangement according to

FIG. 2

is disadvantageous when it comes to cost considerations. Both the keypad rubber sheet

30

and the metallized layer inside the housing

10

and a non-negligable price penalty to the total cost for manufacturing the telephone

10

.

SUMMARY

It is an object of the present invention to provide a portable communication device with safe and inexpensive electrostatic discharge protection A further object of some embodiments is to provide the communication device with means protecting the device from electrostatic discharge (ESD) as well as assuring electromagnetic compatibility (EMC).

The objects of the invention are achieved for a portable communication device of the kind described above by protecting the circuit board inside the communication device from electrostatic discharge by means of a domefoil formed as a non-conductive sheet providing an air-tight and/or electrostatic seal between the keypad and the circuit board

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will be described in more detail in the following, reference being made to the accompanying drawings, in which:

FIG. 1

is a schematic view of an exemplary cellular telephone, for which the invention may be applied,

FIG. 2

is a schematic perspective view of a prior art keypad arrangement in a cellular telephone, and

FIG. 3

is a schematic perspective view of a combined keypad and ESD-protective arrangement according to a preferred embodiment of the invention.

DETAILED DESCRIPTION

In

FIG. 1

there is shown an exemplary cellular telephone

10

, comprising an apparatus housing

12

, an antenna

13

, a loudspeaker

14

, a display

15

, and a microphone

16

. Furthermore, the cellular telephone

10

is provided with a keypad

30

for user communication. A user controls the operation of the cellular telephone by entering commands on the keypad

30

, i.e. by depressing an appropriate individual key

32

among the set of keys in the keypad

30

. Preferably, the keypad comprises a set of alphanumeric keys labelled “0” through “9”, as well as control keys such as “*” and “#”, an “Answer” or “YES” key, a “Hangup” or “NO” key, volume control keys, etc. From an overall point of view according to

FIG. 1

the cellular telephone

10

is well-known in all relevant aspects to a man skilled in the art, and hence no further description is made here.

The inventive solution according to

FIG. 3

proposes a less expensive way of providing the telephone with means for protection against ESD. According to the present invention the domefoil

50

, with its set of tactile key domes

52

, is formed as a sheet fully covering the entire surface between the keypad

30

and the underlying circuit board

20

. At least the front surface of the domefoil

50

is made electrically non-conductive or isolating, thereby providing an airtight and/or electrostatic seal between the keypad and the circuit board and thus functioning as an ESD-protective means. In contrast to the prior art domefoil

40

in

FIG. 2

, the inventive domefoil

50

has no open-air openings which would permit an electric spark to be transferred from a key

32

to the circuit board

20

. All previous openings, such as the openings for the illuminating means

24

, are covered with additional domes

54

of a non-conductive material isolating the circuit board

20

from the keypad

30

.

According to the preferred embodiment in

FIG. 3

the keypad

30

is realized as a keytree made from plastics or the like. Such a keytree structure is previously known per se to be a very cheap way of realizing a keypad function in a communication device. Thanks to the integrated domefoil

50

there is no longer any need for the keypad

30

to be made from rubber or having to act as ESD-protection as well as a means for user interaction.

As mentioned above the domefoil

50

is provided with additional domes

54

for the underlying illuminating means

24

(which are realized as LEDs or similar components). Obviously the additional domes

54

must be made transparent or at least semi-transparent to the light emitted from the illuminating means

24

. Preferably the additional domes

54

are made white or glossy so as to give a better illumination of the keypad

30

. In an alternative embodiment the additional domes

54

could be printed with a certain screen or dot pattern to direct the light towards the four keys

32

surrounding the dome

54

and the underlying illuminating means

24

. This pattern may exhibit lines or dots which are arranged at increasing size or density in the directions of the keys

32

.

According to one embodiment of the invention the interior of the domefoil

50

is provided with a layer of electrically conductive material, such as a silver compound. The conductive layer is connected to a grounding means so as to function as a means for protection against electromagnetic interferences thereby assuring electromagnetic compatibility. Hence, the need for a metallized apparatus housing in the prior art arrangement according to

FIG. 2

has been eliminated, and the domefoil will act as a combined means for protection against ESD as well as electromagnetic interference.

To help dissipating heat generated by the various circuits and components on the electronic circuit board

20

the domefoil

50

may be provided with air channels extending along a major part of the domefoil and reaching some of the edges of the domefoil

50

.

The present invention has been described above with reference to a few embodiments or examples, but it is to be understood that the invention shall by no means be restricted to the specific embodiments disclosed in the foregoing. On the contrary, the present invention shall only be limited by the scope of the appended patent claims.