Flat panel switches such as membrane switches are
widely used in industrial control panels. They allow a products
appearance to be designed consistent with contemporary market tastes,
while being durable, functional and environmentally sealed.
In the early days fascias used polycarbonate overlays
over the top of discrete switches mounted on PCB’s. While
overlays are still widely used, membrane switches gained popularity
as new technologies, materials, and designs all contributed to their
improved reliability and performance. The one important technical
advantage of membrane panels over PCB mounted switches is that the
PCB did not have to be positioned near the switch face.
As the buttons are not mounted on the PCB, it is
not necessary to locate a PCB adjacent to the switch, or to have
multiple cut-outs in the enclosure for PCB mounted switches. As
the PCB design is not dependant on the button location, a standardised
PCB and microprocessor can be used across a full product range,
allowing many variations in visual appearance, size and number of
Unfortunately, until recently these advantages have
only applied to panels incorporating buttons only. Other useful
features of a control panel such as encoders, LED’s and backlighting
have required PCB mounting, or mounting in the enclosure with a
wire loom and associated soldering.
It is pleasing that in recent years, a core theme in flat panel
innovation has been integration - with new flat panel switch fascias
routinely integrating LED’s, backlighting, rotary encoder,
direction control buttons offering. For designers and manufacturers,
this offers new opportunities to make the control panels reliable,
robust, user friendly.
This article will briefly outline the various characteristics
of flat panel (less than 2 mm) interface products and the solutions
INTEGRATING ENCODERS & DIRECTION CONTROLS
Encoders are used widely and provide a positive
control for many functions and data selection. They are however
typically hard mounted through rigid backing panels. This introduces
opportunity for water and dust ingress. While not necessarily expensive
in themselves, encoders can be expensive to install, requiring additional
panelwork, wiring looms and soldering.
One solution is Duraswitch’s rotary
encoder. This patented design allows an encoder to be mounted
securely within a flat panel switch of around 2.0 mm thick. This
ensures a robust and environmentally sealed construction, while
at the same time simplifying the mounting and installation - a ribbon
cable is simply plugged into the PCB in the same way as normal membrane
switch. A full range of circuit configurations can be acheived simply
by printing the appropriate circuit layout.
Using an encoder in this way offers new opportunities
for improving existing panels. For instance, it can be an effective
data entry tool for controlling graphic LCD displays. The user simply
turns an encoder designed as a switch to select a menu item, selects
using a pushbutton, and continues on through the menu system. This
means that menu selections can be done without looking at switch
panel, while ensuring that the screen is not dirtied by hands as
in a touchscreen.
Mouse or directional controls can also be used for
simplifying keypads. Using a mouse control - one single directional
device can replace a number of buttons or with appropriate firmware,
can act as a full mouse or full joystick. Both the encoder and the
mouse controller can be included in the one panel without compromising
In summary, integrating an encoder, momentary buttons,
and direction buttons in the one panel allows a wider range of user
experience, while at the same time minimising throughholes, dust
and water engress, and costs of soldering. Please take the link
to an article on integration
and encoders can be found at these links.
SOLUTIONS FOR HOSTILE ENVIRONMENTS
In hostile industrial environments, and in the public
domain, there are many situations where a flat membrane switch is
desirable, however would be vulnerable to impact through rough use
and abuse. In these environments, a conventional membrane switch
dome would be flattened and the switch short circuited.
Recent innovations have allowed switches to be more
robust. One solution is what Duraswitch call a high impact construction.
Here a layer of impact absorbing silicon rubber sits above a Duraswitch
Pushgate switch. In conventional membrane switch constructions this
would result in a severe loss of tactile feel, however when used
with a Pushgate construction, the feel remains crisp. The silicon
protects and distributes the impact across the switch face, even
allowing the switch to take a hit from a hammer. The construction
has found success with the US fuel pump industry where consumers
routinely damage switches by using the end of the nozzles to make
Conventional membrane switches, or pushgate switches
can be mounted below hard key caps for additional protection. Refer
to the attached artic
A hidden danger in humid environments is silver
migration, which affects conductive silver printed circuits such
as those used on flex circuits. The silver grows ‘dendrites’
which cause low level shorting between adjacent conductors. A knowledgable
manufacturer should be conversant with the critcal issues that effect
the formation of dendrites, but you should be aware that increasing
the supply voltage through the circuit will strongly correlate with
an increase in dendrite growth. Other considerations include track
spacing, the use of graphite shields, and the correct design of
An alternative method is to use a capacitive switch.
With appropriate decoders, a capacitive switch panel can take the
form of either a flex or PCB circuit, and can be mounted protected
from harm behind glass or polycarbonate. With a circuit construction
similar to a conventional membrane, the capacitive switch has the
benefit of allowing a much larger key contact area.
INTEGRATING SWITCH AND PCB
While the paragraphs above have focuses on the benefits
of flex circuit, there are also many situations where integrating
the switch and the PCB can have a number of benefits. In complex
flat panel switches, it can be difficult to take to the tail out
of an area densely populated by buttons. This is because the tail
is made of the same layer of material as the bottom circuit.
In these switches it can be practical to replace the bottom conductor
(usually a flex circuit) with a thin PCB (say 0.8 mm) This also
gives the opportunity for other components to be mounted on the
reverse side of the switch. This could for instance, include the
microprocessor, resistors, and LED’s and other components.
Replacing the bottom flex conductor with a PCB also
allows the construction of complex and tight circuit matrix without
the risk of silver migration. A further advantage is that the PCB
can act as the rigid mounting panel with standoffs for LCD mounting
Backlighting has always been an issue in flat panel
switching panels as the domes or silver pads have obstructed lighting
from the rear. Electroluminescent lighting has been well accepted
as a form of backlighting in switches as it is thin, and cold, and
can be mounted directly below the overlay. Now that EL has been
well accepted, users are more concerned with how the backlighting
layer affects the tactile feel of the switches. Adding a layer to
the switch ‘deadens’ the tactile feel. The Duraswitch
Pushgate does offer a better alternative in these situations as
the tactile feel is not as affected by heavy overlays.
It is worth noting that EL does have a definite
life (typically half-life is quoted at 3-5000 hours) and may not
be suitable for high humidity, or intrinsically safe environments.
EL operates on an AC voltage so will require an inverter (such as
Indicator lighting is routinely undertaken using
LED’s surface mounted on the flex circuit. Given that the
typical thickness of a membrane switch is 1.00, and a Duraswitch
at 2.00 mm, LED’s have had to be embossed at additional cost
for both tooling and manufacture. With LED packages now being as
thin as 0.6 mm, embedding LED’s often be undertaken without
embossing. LED’s would typically be positioned to the side
of the button, although the Pushgate allows an LED to be mounted
in the centre of the button. Resistors can be mounted if required
RFI & ESD SHIELDING
RFI and ESD shielding layers can be easily integrated
within flat panel switches. In its simplest form, a printed silver
conductive RFI grid can be included on the upper side of the circuit
layer of a tactile switch, while it could also be include as a separate
layer. Earthing of the RFI can be acheived using a separate pin
on the connector, or terminated using a screw contact on a flap
extending beyond the outside dimension of the switch.
If a printed silver grid does not provide the required
amount of protection, specialist material layers are available to
integrate between the upper layers of the switch. It is worth noting
that the window area of a control panel is often vulnerable to rfi,
so a transparent conductive window shield is required.
When allowing for electrostatic discharge, the discharge
typically extends across the surface of the switch and to any conductive
surfaces on the edge. It is therefore important that the designer
ensure that there are no conductive tracks running close to the
edge of the switch.
In summary, recent innovations in technology have
allowed flat panel switches to far exceed their initial capability.
In all situations, but giving your manufacturer the fullest information
regarding the environment where the switch will be used, as well
as the intended use, will allow the manufacture to make informed
decisions as to an appropriate design. The purpose of this article
was to illuminate other options that are available to improve the
overall function and application of the switch