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.

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.

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.