A domed contact element for a keypad assembly

The present invention relates to a domed contact element for use in a keypad assembly of an item of electronic apparatus particularly, but not exclusively, a mobile phone.

Figures 1(a) and 1(b) show cross-sectional views of one of a plurality of key assemblies making up a typical keypad assembly.

Referring to Figure 1(a), the key assembly, generally designated 10, comprises a body portion 12 defining an aperture 14 and a key 16 mounted in register with the aperture. The key 16 is mounted relative to the body portion 12 by means of a skirt region 18 which is flexible and permits the movement of the key 16 into and out of the aperture 14, when it is depressed, but naturally biases it to occupy the position shown in Figure 1(a). On their upper surfaces, the key 16, the body portion 12 and the skirt region 18 are painted; the upper surface of the key 16 includes an indicia region 19 which is painted so as to bear an indicia serving to indicate the function of the key 16, for example, an alphanumeric character or other symbol. The key 16 includes a base 20 from which a depending projection or pip 22 centrally projects. The pip 22 is cylindrical and has an exposed end 24. The key 16, including the pip 22, the body portion 12 and the skirt region 18 are made from a single piece of silicon rubber. The key assembly 10 further comprises a contact element 30 having a resiliently distortable dome 32, with an apex 33, which is snappable between bias positions. The contact element 30 is mounted on an underlying circuit board 40, as shown in Figure 1(a), such that a small spacing exists between the exposed end 24 and the outside surface of the dome 32. The inside surface of the dome 32 is conductive. On the circuit board 40, in the region enclosed by the dome 32, a pair of spaced electrical terminals 36 are provided.

The dome 32 is distortable so as to snap from a first natural-bias position in Figure 1(a), in which the electrical terminals 36 are not electrically connected to each other to a second distorted position, as shown in Figure 1(b), in which the inside of the dome 32 provides electrical connection between the electrical terminals 36.

In use, the user depresses the key 16 causing it to travel into the aperture 14 and thus the exposed end 24 of the pip 22 to bear against the dome 32. The continued travel of the pip 22 causes the continued distortion of the dome 32, until it reaches a condition at which it snaps into the second position shown in Figure 1(b). The making of the electrical connection between the electrical terminals 36 enables external circuitry (not shown) to register the depression of the key 16. When the key 16 is released, the resilience of the dome 32 propels the key 16 upwardly and the dome 32 resumes its first position as shown in Figure 1(a). The resilience of the skirt region 18, then causes the key 16 to re-adopt its position in Figure 1(a).

It is known to provide the contact elements of a known keypad assembly in the form of a structure comprising a layer of adhesive material to which a plurality of individual metal domes are adhered. US Patent No. 4,343,973 discloses such a structure. The individual metal domes are pre-placed at the appropriate position on the layer such that during assembly of the keypad assembly the structure can be fixed in position as a unitary member.

In one aspect, the present invention provides a domed contact element suitable for a keypad assembly, comprising a domed contact element for a keypad assembly comprising a substrate having the dome formed therein, the dome being depressible towards the substrate so as to move from a first natural-bias position to a second position characterised in that there is provided a soldering site on the substrate by which the contact element can be soldered to a circuit board; and a port positioned to collect solder flowing from the soldering site.

In another aspect, the present invention provides a domed contact element suitable for a keypad assembly, comprising domed contact element for a keypad assembly comprising a substrate having the dome formed therein, the dome being depressible towards the substrate so as to move from a first natural-bias position to a second position characterised in that there is provided a soldering site on the substrate by which the contact element can be soldered to a circuit board; and a guarding formation projecting from the substrate to divert solder flowing from the soldering site.

In the construction of an item of electronic apparatus with a keypad assembly, the present invention enables the contact elements of the keypad assembly to be individually placed and soldered in position to a circuit board of the apparatus as an integral part of the manufacturing process which places and solders the other components of the apparatus to the circuit board. The present invention thus provides a contact element which is very production friendly.

Preferably, the contact element includes a pick-up site on the substrate by which a pick-and-place machine can vacuum pick-up the contact element.

Exemplary embodiments of the invention are herein described with reference to the accompanying drawings, in which:

• Figures 1(a) and 1(b) show cross-sectional views of a key assembly;
• Figure 2 shows a plan view of a first contact element in accordance with the present invention;
• Figure 3 shows a side view of the first contact element of Figure 2;
• Figure 4 shows a view from below the first contact element of Figure 2;
• Figure 5 shows a further plan view of the first contact element of Figure 2;
• Figure 6 shows a plurality of the first contact elements in a typical arrangement; and
• Figure 7 shows a plan view of a second contact element in accordance with the present invention.

Referring to Figures 2 and 3, a contact element in accordance with the invention is generally designated 50. It comprises an elongate, planar substrate 52 having a central longitudinal axis indicated by the line A-A¹ and a dome 54 upstanding at opposite ends of a first face thereof. A curved hole 56 formed in the substrate 52 adjacent each dome 54 defines an intermediate section 58 of the substrate. On the first face of the intermediate section 58 of the substrate, a flat pick-up site 60 is provided (See Figure 5). On the second face of the intermediate section 58 of the substrate, a soldering site 62 is provided (See Figure 4). The first contact element 50 is punched and formed from one piece of metal.

The contact element 50 is functionally identical to the contact element 30 in Figures 1(a) and 1(b) and is arranged, in use, in the same way.

When assembling an item of electronic apparatus, the contact elements 50 can be mounted to a printed circuit board (PCB) as an integral part of the reflow soldering process which mounts the other components of the apparatus to the PCB. This process is well known in itself and involves, in general terms, taking a PCB having the appropriate conductive tracks and solder paste areas already applied thereto; using a pick-and-place machine to position the components correctly on the PCB; and heating up the PCB and components to melt the solder paste in order to physically and electrically connect the components to the PCB in a permanent fashion. The contact element 50 is picked up by its pick-up site using the suction nozzle of the pick-and-place machine and then placed in position on the circuit board with its soldering site 60 in contact with a solder paste area on the PCB. In this way, the contact element 50 is handled like any other component to be mounted to the PCB. On applying heat, the solder paste area melts and the substrate 52 becomes adhered to the PCB via its soldering site 60. The holes 56 provide a port to inhibit the seepage of solder into the dome area which could adversely affect the action of the domes 54. Any melted solder flowing towards the domer area collects in the area defined by holes 56 rather than progress into the dome area. In this way, the holes 56 serve as a collection duct for melted solder.

In other embodiments of the invention, the ports need not pass completely through the substrate 52 but may take the form of a recess formed in the second face thereof.

In other embodiments, in addition to or instead of a port, a guarding formation projects a small distance from the second face of the substrate 52. The guarding formation is positioned between the soldering site 60 and the dome area and has an arcuate shape like that of a hole 56 when viewed in Figure 4. The guarding formation or ridge provides a barrier which diverts the flow of solder away from the dome area, preferably into collecting ducts or ports.

Figure 6 shows an array of contact elements 50 arranged how they typically might be on a PCB.

Figure 7 shows an alternative contact element 70 consisting of only one dome 54 and two soldering sites 62a, 62b. Like parts have been given the same reference numerals. The pick-up site can be any suitable location on the substrate 52. The contact element 70 is also functionally identical to the contact element 30 in Figures 1(a) and 1(b) and is arranged, in use, in the same way.