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(11) | EP 0 133 817 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Membrane switches |
| (57) @ A membrane switch panel has a printed circuit base board (20) carrying bare copper
electrical circuitry (26) and contacts (26 & 28). A flexible, non-elastomer, laminar
membrane (22) overlays the board (20) and also carries a bare-copper contact (24).
An electrically insulating spacer (3) is provided between the board (20) and the membrane,
but it is provided with an opening to allow the contact (24) upon local depression
of the membrane to bridge the contacts (26 & 28) in the base board (20). The membrane (22) is adhesively sealed to the base board (20) enclosing an atmosphere of limited tarnishing ability for the contacts (20, 26 & 28) to maintain a low contact resistance and allow non-precious metals to be used for contacts. |
[0002] Previously proposed membrane switch panels include a rigid printed circuit board carrying a printed circuit having switch contacts. A single elastomer membrane covers the printed circuit board but is held spaced from the printed circuitry thereon.
[0003] The membrane, which may be rubber or silicone, is rendered electrically conductive by impregnation with an electrically conductive component. To operate a switch on the panel, the membrane is depressed in the area of a pair of contacts on the panel to bridge the contacts and so close the switch. Instead of the whole membrane being rendered electrically conductive, it may be rendered selectively electrically conductive in discrete areas corresponding to the pairs of switch contacts on the printed circuit board.
[0004] There have also been proposed membrane switch panels using non-elastomer laminate membranes of, for example, polyester or polyimide, which, because of their thinness, are flexible.
[0005] The switch contacts and conductive circuitry patterns are formed on the membranes by squeezing (squeegeeing) thixotropic electrically conductive ink paste through an appropriately stencilled mesh screen onto the membranes. The ink is then dried to leave the conductive contacts and circuitry.
[0006] In order to achieve low switch contact resistance, the ink compositions used contain precious metals such as silver, palladium and gold or combinations of such metals.
[0007] A disadvantage of such membrane switches is that they are expensive to produce because of the materials used and the need to use an additive process to form both the conductive circuitry and switch contacts on the membrane.
[0008] The present invention is based upon our surprising discovery that it is possible to achieve and maintain low contact resistance with bare copper contacts in membrane switch assemblies despite their not being fully hermetically sealed (that is, where the membrane material is incapable of providing a truly hermetic seal).
[0009] According to the invention, there is provided a membrane switch comprising a base member, a flexible laminar member sealed to the base member, and an electrically insulating separator separating at least one portion of the laminar member from a corresponding portion of the base member, said portions each carrying a respective one of a pair of mutually engageable contacts of copper having a low contact resistance, sealed in an environment of limited tarnishing ability.
[0010] According to the invention, there is further provided a membrane switch panel comprising a pair of laminar members separated by an electrically insulating spacer, at least one of the members being flexible, the two members carrying mating contacts of copper having a low contact resistance, the spacer having one or more openings each allowing access of one or more of said contacts to one or more others, and means sealing the space between the two enembers and enclosing an enviroment for the contacts which has a limited tarnishing ability.
[0011] According to the invention, there is still further provided a method of manufacturing a membrane switch comprising processing a copper foil clad laminar member by a subtractive process to form at least one electrical contact, providing a base member carrying a mating contact, said contacts having a low contact resistance, inserting a perforated electrically insulating separator between the base member and the laminar member and sealing an atmosphere of limited tarnishing ability in the space between the base member and laminar member enclosing said mating contacts.
[0012] Hembrane switch panels embodying the invention will now be described by way of example with reference to the accompanying diagrammatic drawing in which:
Figure 1 is a fragmentary section through a first one of the panels;
Figure 2 shows the panel of Figure 1 with one set of contacts held closed by an operator's finger;
Figure 3 is a fragmentary section through a second one of the panels;
Figure 4 is a fragmentary section through a third one of the panels; and
Figure 5 is a fragmentary section through a fourth one of the panels.
[0013] The switch panel in Figure 1 includes upper and lower foil clad non-elastomer membranes 2 and 4. The membranes are advantageously polyester or polyimide, while the foil cladding is advantageously of annealed electrolytic copper. The foil cladding is subjected to screen printing or photo-imaging with an etch resistant material and subsequently corrosively etched away to leave conductive patterns 5 and 6 and switch contacts 8 and 10 respectively on the upper and low membranes 2 and 4. In modification, the conductor pattern is formed by a die stamping process. The two membranes 2 and 4 are separated by an electrically insulating perforated layer 12 of polyester.
[0014] The perforations or openings in the layer 12 are large enough and so positioned that they allow mating contacts 8 and 10 on the upper and lower layers access to one another.
[0017] In operation, a marked area on the upper layer 2 is depressed by an operator to cause a contact 8 on the underside of the upper layer to engage a contact 10 on the lower layer 4 and so close the switch (see Figure 2). The resilience of the upper layer 2 will enable the two contacts 8 and 10 to separate upon release by the operator.
[0018] It is possible to achieve contact resistances of an ohm or less (typically 0.2 to 0.3 ohms) using a construction according to the present invention, and despite the membrane being slightly porous and allowing some change of the atmosphere within the switch assembly, the rate of change is slow, and low contact resistance can be maintained for an appreciable design life, and hence the need to use precious metal contacts is avoided.
[0019] Care needs to be exercised during manufacture, however, to ensure that the air trapped between the layers 2 and 4 is dry (preferably less than about 55 percent relative humidity at 20°C) and free from significant contamination (preferably the concentration of sulphur dioxide and oxides of nitrogen should each be less than one part per million, particulate contaminants should also be minimised).
[0020] Instead of air, the space between the two sets of membranes may be filled with dry nitrogen or any other generally inert atmosphere.
[0021] The effects of poor sealing and hence of poor control of the atmosphere within a switch assembly can be judged by comparing contact resistances of satisfactory switches according to the invention, and switches which are otherwise identical but have defective seals. In a series of tests, contact resistance was measured using a conventional four pole measurement technique under two conditions, both typical of the kinds of applications where membrane switches are generally used:-
(a) with a maximum applied potential of 200 millivolts and a current of 50 microamps: correctly sealed switches had contact resistances in the range 93 to 270 milliohms, and poorly sealed switches had contact resistances rising to greater than 4000 ohms:
(b) with a maximum applied potential of 5 Volts and a current of 2 millianps: correctly sealed switches had contact resistances in the range 92 to 179 milliohms, and poorly sealed switches had contact reistances rising to greater than 2500 ohms.
[0022] The above resistance values were obtained with a force of 3 Newtons applied to the switch (the tested switches having a contact gap 5 x 10-3 inches (0.127 x 10 m)). Contact resistance is to some extent dependent upon the force applied to the switch, 3 Newtons being typical of the force needed to achieve switching in a large number of different switch designs. Clearly, if a particular switch design requires a larger force to trigger switching a corresponding increase in the force applied during resistance measurements should be used if the results are to be comparable.
[0023] The present invention can thus provide membrane switches having low contact resistances which are preferably less than 10 ohms more preferably less than 1 ohm and can be made lower than 0.3 ohm.
[0024] Those skilled in the art will appreciate that the contact resistances quoted above (for correctly sealed switches) are low enough to enable the switches to be used to switch logic levels; an application which would conventionally be reserved for switches having precious metal contacts.
[0025] Known membrane switches have contact resistances of as much as several hundred ohms (with carbon loaded contacts), with anything less than about a hundred ohms (less than 100 ohms is attainable with silver/palladium inks; contact resistances of 1 ohm or less only being achievable with very heavily loaded silver inks) being considered to be a low contact resistance. In this specification the term low contact resistance is taken to mean a contact resistance of less than 100 ohms when measured using a conventional four pole measurement technique with a maximum applied potential of 200 millivolts and a current of 50 microamps.
[0026] The switch panel shown in Figure 3 has a rigid printed circuit board 20 carrying printed circuitry including pairs of contacts 26 and 28. A flexible membrane 22 of polyester overlies the printed circuit board 20 and carries on its underside contacts 24. A perforated layer 30 of electrically insulating material separates the membrane 22 from the board. The contacts 24, 26 and 28 are all of copper and formed by an etching or other subtractive process.
[0027] In operation, when the portion of membrane 22 in the vicinity of the contact 24 is depressed by an operator, the contact 24 bridges the contacts 26 and 28 on the printed circuit board 20 and so closes the switch.
[0028] As with the panel of Figure 1, the environment between the board 20 and the layer 22 is such that the contacts are not significantly subjected to tarnishing.
[0030] In Figure 4 both the printed circuit board 20 and the flexible layer 22 carry electrical circuitry 40 as well as contacts 42.
[0032] In Figure 5 the flexible layer 22 carries both contacts 50 and electrical circuitry 52, while the printed circuit board carries only contacts 54.
[0033] It will be appreciated that while the contacts in the Figures are shown as being raised above the adjoining electrical circuitry, they can be flush with the circuitry. Also, while the insulating spacer layer is shown as being enclosed between the upper and lower layers, it can protrude from them.
[0034] The use of copper to provide the electrical circuitry and the contacts enables fused (molten) solder to be applied as a low cost contact finish, a protective conductor finish, and/or for jointing purposes.
[0035] An advantage of the above described panels stems from the appreciation that with a sealed environment, low cost copper contacts can be used where before it was thought that only precious metal contacts could be. This enables the use of copper foil clad laminates and allows traditional subtractive printed circuit techniques to be used for fabricating conductors and contact patterns on non-elastomer membrane switch panels. Instead of a subtractive process, a die stamping process can be used.
[0036] A further advantage of the above described panels is that they can readily be given protection against the well known problem of static discharges. In many environments where equipment utilising membrane switches is used, static electricity is both generated and carried by the equipment user. When the equipment user touches a membrane switch, the static charge which he is carrying can be discharged via the switch (particularly if the contact gaps within the switch are small) damaging circuitry associated with the switch. Switches according to the invention can readily be given appreciable protection against this damaging problem by providing the upper layer (or whichever of the layers is to be the outermost or touched surface) with an additional layer of copper foil on the outside of the assembly, ie the outer layer is made from a flexible membrane having a copper foil layer on each side. The extra copper layer then being connected to earth.
[0037] The present invention makes it possible for membrane switch panels to be made cheaply and easily using conventional printed circuit board technology, without the need for the more expensive elastomers and conductive polymers or inks. Using this approach, it is also possible to make membrane switch panels from just two pieces (or even from one piece) by the simple expedient of folding a single membrane which carries both the upper and the lower conductors and contacts (the one piece switch panel has its perforated layer 12 made integrally with the upper and lower layers, so there are two folds). Care should be taken if adopting this folded construction to ensure that an effective seal is achieved adjacent to the fold(s). Generally the membrane material will exhibit some 'memory' effects, and unless a sufficiently strong bond is created adjacent to the fold, the membrane parts will separate and destroy the seal, with consequent loss of low contact resistance.
[0038] Unlike the copper tracks on typical printed circuit boards, which are routinely given some protective finish, there is generally no need to provide any form of protective finish to the conductors within the switch, as it has been found that they do not tarnish if the seal is effective. However, if it is desired to provide some other finish, the copper, unlike conductive irks, can be tinned or electroplated to provide a suitable finish. The use of solid metallic conductors also enables the switches to handle generally greater powers than equivalent switches using conductive inks.
1. A membrane switch comprising a base member, a flexible laminar member sealed to
the base member, and an electrically insulating separator separating at least one
portion of the laminar member from a corresponding portion of the base member, said
portions each carrying a respective one of a pair of mutually engageable contacts
of copper having a low contact resistance, sealed in an environment of limited tarnishing
ability.
2. A switch, according to claim 1, wherein said contact resistance is less than 10
ohms.
3. A switch, according to claim 1, wherein said contact resistance is less than 1
ohm.
4. A switch according to claim 1 wherein said contact resistance is less that 0.3
ohm.
5 A switch, according to any preceding claim, wherein said laminar member and said
base member are both flexible non-elastomer laminar members.
6. A switch, according to claim 5, wherein said base member is adhesively secured
to a rigid support.
7. A switch, according to any one of claims 1 to 4, wherein the base member comprises
a printed circuit board.
8. A switch, according to any one of the preceding claims, wherein said environment
of limited tarnishing ability is nitrogen.
9. A switch according to any one of claims 1 to 7 wherein said environment of limited
tarnishing ability comprises air wherein the concentrations of sulphur dioxide and
oxides of nitrogen are each less than one part per million.
10. A switch according to any one of claims 1 to 7 wherein said envirorment of limited
tarnishing ability comprises air having relative humidity at 20°C of less than 55
per cent.
11. A switch, according to any preceding claim, wherein one of said base member and
said laminar member carries electrical circuitry connected to the electrical contacts
it carries.
12. A switch, according to any preceding claim, wherein at least one of said laminar
member and separator is of polyester.
13. A membrane switch panel comprising a pair of laminar members separated by an electrically
insulating spacer, at least one of the members being flexible, the two members carrying
mating contacts of copper having a low contact resistance, the spacer having one or
more openings each allowing access of one or more of said contacts to one or more
others, and means sealing the space between the two members and enclosing an environment
for the contacts which has a limited tarnishing ability.
14. A panel, according to claim 13, wherein at least one of said two members carries
electrically conductive circuitry.
15. A panel, according to claim 13 or 14, wherein one of said members carries a metallic
layer on a surface remote from the space between the two members.
16. A panel, according to any one of claims 13 to 15, wherein said pair of laminar
members comprise a single folded flexible membrane.
17. A panel, according to any one of claims 13 to 16, wherein said contact resistance
is less than 10ohms.
18. A panel, according to any one of claims 13 to 16, wherein said contact resistance
is less that lohm.
19. A panel according to any one of claims 13 to 16 wherein said contact resistance
is less than 0.3 ohm.
20. A panel according to any one of claims 13 to 19 wherein said environment of limited
tarnishing ability is nitrogen.
21. A panel according to any one of claims 13 to 19 wherein said environment of limited
tarnishing ability comprises air wherein the concentrations of sulphure dioxide and
oxides of nitrogen are each less than one part per million.
22. A panel according to any one of claims 13 to 19 wherein said environment of limited
tarnishing ability comprises air having a relative humidity at 200C of less than 55 percent.
23. A method of manufacturing a membrane switch comprising processing a copper foil
clad laminar member by a subtractive process to form at least one electrical contact,
providing a base member carrying a mating contact, said contacts having a low contact
resistance, inserting a perforated electrically insulating separator between the base
member and the laminar member, and sealing an atmosphere of limited tarnishing ability
in the space between the base member and laminar member enclosing said mating contacts.
24. A method of manufacturing a membrane switch comprising processing a copper foil
clad laminar member by a die stamping process to form at least one electrical contact,
providing a base member carrying a mating contact, said contacts having a low contact
resistance, inserting a perforated electrically insulating separator between the base
member and the laminar member, and sealing an atmosphere of limited tarnishing ability
in the space between the base member and laminar member enclosing said mating contacts.