The widespread move to surface mount PCB assembly techniques
over the last ten years has tested the ingenuity of LED
suppliers.
LEDs are increasingly being seen as the most cost-effective
option for what just ten years ago would have been considered
higher power filament applications such as outdoor signs and car
lights. So the combined design pressures on power and size have
resulted in the adoption of thin-film semiconductor technologies
along with different package designs to improve optical power
efficiency.
But clever LED package design is not simply about supporting
current levels of 500mA, suppliers are also adding features such as
antistatic protection diodes.
When surface mounting an LED heat dissipation is the primary
design consideration for plastic leaded chip carrier (PLCC)
packaged LEDs. Forward currents of greater than 50mA are typical
and maximum current figures of over 100mA are not untypical for
higher brightness devices.
Suppliers such as Lumileds Lighting rose to the challenge with
surface mount products which are robust to temperature and humidity
extremes while supplying impressive brightness figures. The
firm’s recently redesigned Luxeon LEDs show 65lumens in white
at 350mA at an allowable junction temperature of 185°C and a
JEDEC moisture sensitivity level of 2a.
Osram Opto Semiconductors uses a proprietary surface mount
package for its 2W Golden Dragon LED range.
The lead-free device uses an indium gallium nitrite (InGaN) chip
with a low thermal resistance of 9°C/W. It is specified for
operating currents up to 500mA and product life is more than 50,000
hours, defined by 50 per cent degradation when applied under
typical operating conditions. At 500mA typical output is 40lm with
a 120 degree viewing angle.
Vishay Intertechnology has used a novel design of leadframe to
improve the heat dissipation in PLCC-3 packaged LEDs which it
claims can be driven twice as hard as PLCC-2 packaged alternatives.
The device’s thermal resistance is specified at 270k/W.
Typical luminous intensity for the TLMx320x series ranges from
50mcd to 1250mcd when the LEDs are driven with a forward current of
50mA. There is also a non-diffused lens to aid coupling to
backlights and light pipes.
Rohm specifies that its SML512WBC2W range of LEDs will support a
peak forward current of 100mA (pulse duration 30ms). Typical
brightness levels are 35mcd at a forward current of 5mA. Maximum
forward current is rated at 20mA. All of the new white LEDs will
handle a maximum reverse current of 100mA. Maximum reverse voltage
for the parts is 5V. The top-view device measures
1.6x0.8x0.55mm.
Agilent Technologies’ small ChipLEDs on an industry
standard 0603 footprint are squeezed in to a 0.5mm package height
above the board.
Like other ranges, the HSMx-C120 series uses different
semiconductor materials for different colour and output powers.
For example, AlInGaP (aluminum indium gallium phosphide) in
572nm green, 592nm amber, 605nm orange and 626nm red; AlGaAs
(aluminum gallium arsenide) 639nm red, GaP (gallium phosphide) in
572nm green, 604nm orange and 626nm red; InGaN (indium gallium
nitride) in 473nm blue and 527nm green.
Agilent Technologies is also working with Lumileds Lighting to
package its mid-power sub-1W range of LEDs in a plastic leaded chip
carrier.
Agilent has designed the PLCC-4 package to be driven at high
current above 250mW and to be compatible with both IR-solder reflow
and through-the-wave soldering.
The use of Lumileds transparent-substrate aluminum indium
gallium phosphide die puts the Envisium Power PLCC-4 products at
the top of the high output list for PLCC devices. With PLCC-4
standard package dimensions, construction of the devices allows for
a substrate made up of a moulded plastic reflector on top of a bent
lead frame. The die is attached within the reflector cavity, and
the cavity is encapsulated in epoxy.
With automotive being an important market for high and mid-range
LED suppliers have addressed the wide temperature requirements. So
LEDs are specified to operate reliably over a temperature range of
-40°C to +100°C.
In addition to power and temperature specifications which
support the latest generation of high brightness LEDs, suppliers
are also using clever package design to incorporate new
features.
As an example of the high levels of integration being achieved
in surface mount packages there is BivarOpto’s RGB (red,
green, blue) device which incorporates three individually
addressable LED dies in an industry-standard 0606 SMT device
package.
Intended for exact colour mixing in backlit keypads and LCDs,
the SMTC0606 features an internal tri-chip circuit design,
employing the use of three individually addressable LED dies. The
tri-chip RGB design is comprised of a single AlGainP and two
InGaN/SIC chip dies, featuring peak wavelengths of 635, 520 and
465nm respectively.
Absolute maximum rating of forward current drive is 125mA (Peak
If). The SMTC0606 RGB LED chip measures 1.5mm x 1.6mm x 0.6mm in
height. Units are encapsulated in a special epoxy blend to assure
proper withstanding of temperature, operation and assembly
processes.
Hampshire-based Custom Interconnect (CIL) has developed novel
LED arrays for industrial processing. Called CoolLED the arrays are
made from 300x300µm bare UV LED dies on a metallised ceramic
substrate. Each die has a cup-shaped reflector around it which the
firm claims will increase light output by 80 per cent from the same
area.
A similar technique is used to increase output in standard 5mm
LEDs and CIL has transferred this to much smaller devices.
Osram Opto Semiconductors has come up with a novel package for
infra-red LED dies. It does not need a lens to produce a beam, and
is dubbed MIDLED for moulded interconnect devices LED.“The
integrated die-cast reflector focuses most of the light and emits
it in a very narrow angle,” says the supplier, which already
has IR LEDs in its TopLED range.
The result is a radiant intensity that is claimed to be around
three times as high as that of a TopLED. At +/-20 degrees, the beam
angle of MIDLED is tighter than that of the TopLED and lens, which
emits light at an angle of +/-25 degrees.
The device is 1.6mm high and contact pads are designed so it can
be soldered beaming straight up from, or at right-angles to, the
PCB.
Osram has also integrated a separate diode in the package of its
Micro SideLEDs to provide ESD (electrostatic discharge) protection.
With an operating current of 20mA the LED achieves 800mcd.
Similarly Agilent has integrated a Zener diode to provide class 2
electrostatic protection in its white InGaN LEDs.