plw7070* series and plw3535* series i2led high … led die and provides advice on handling of the...

PLW7070* Series and PLW3535* Series I2LED High Power LED Product Application Note

of 12 Document number 294351 V3

This application note describes Plessey’s MaGICTM (Manufactured on GaN-on-Si I/C)

technology LED die and provides advice on handling of the ceramic packaged devices.

Plessey’s MaGIC LED technology uses breakthrough GaN-on-Si for low cost, high

performance LEDs. The LED die are optimised for light output in the manufacturing process;

the light extraction is further enhanced with a silicone lens.

Device Structure

Plessey’s GaN technology supports a multi-junction high voltage process. This process begins

with InGaN LED layers grown on a silicon substrate as illustrated in fig. 1(a). Buffer layers are

grown to form a suitable surface on which to grow the GaN, followed by an n-GaN layer,

multiple quantum well and p-GaN final layer. To enable both electrodes to be connected on

the top surface, an additional metal layer and insulation is added. The LED wafer is then

bonded to a handle wafer and flipped (fig. 1(b)). The original substrate is removed, and the

processing completed with a top surface patterning to enhance light extraction, and top

contacts formed for both anode and cathode.

(a)

(b) (c)

Fig. 1(a) GaN growth on silicon; (b) wafer bonding and flipping; (c) wafer finishing

The surface layers are patterned to increase the light extraction. The chip is also metallised

on the back with a nickel–silver layer (fig. 2) to allow for good die adhesion and thermal

conduction to the package.


Fig.2: PLB

VH series die cross-section

Ceramic Package

Plessey’s die packaged on a ceramic header are typically finished as shown in fig. 3. The die

are mounted onto a ceramic header using an epoxy die attach. This may be replaced in

future by a silver sinter process.

Fig. 3: Section through ceramic packaged LED

The package uses aluminium nitride ceramic for good thermal performance. The ceramic is

metallised on the underside with a central thermal pad and two terminal strips. A silicone lens

is used to enhance light extraction.


3. Die Handling

The LED chips are shipped in tapes on reel and are intended to be used with standard pick-

and-place machines. In addition to enhancing the light output the silicone lens protects the

wire bonds. As the lens is soft it should not be subject to lateral nor vertical forces or damage

may ensue, either to the wire bonds, chip or even for the lens to detach from the chip. To pick

up the LEDs Plessey recommends a rubber collet that will not damage the lens.

The collet should be able to pick the LED without damaging the lens. A suggested collet

dimension is shown in fig. 4:

Fig. 4: Suggested Collet Nozzle Dimensions for 7070 package

Manual handling

When picking the die up manually, it is important not to disturb the lens in any way. The LED

should be held by the ceramic only. When using tweezers, ensure that the LED is held by the

ceramic. Contact with the lens should be avoided since this may cause damage to the bond

wires; displace the phosphor coating or spoil the optical transparency of the lens itself.


4. Soldering

Plessey’s ceramic packaged LEDs are provided with a thermal pad and two terminals that

provide the anode and cathode connections. Soldering should be performed in a reflow tool,

but can also be carried out manually using a hotplate. Unleaded solders should be used to

meet environmental regulations. Plessey uses a low temperature solder (Sn:B) to minimise

thermal stresses on the LED.

Solder paste should be applied uniformly to the PCB pads. A uniform coating can best be

applied using a stencil print (screen print). Standard SMD techniques can be used.

The solder thickness should target 50-100 microns consistent with minimum voiding. Voids in

the solder will increase the thermal resistance and degrade reliability. Voids can lead to

hotspots, which, if devleoped, allows a part of the LED to become hotter than the rest. This

could cause a local degradation of the LED leading to premature failure.

The solder reflow thermal cycle should follow the data sheet recommendations for a specific

LED.

5. Thermal considerations

High power LEDs, although now very efficient, still generate significant heat. In a white LED

the phosphor, which converts the blue light to other colours to make white light, also dissipates

energy due to quantum conversion losses. For a local phosphor in close proximity to the LED

this increases the die temperature. Thus the combination of LED and phosphor will develop a

lower overall efficacy compared to the LED alone as shown in fig. 5. An LED with 60% wall

plug efficiency (WPE), operating at 12V and 0.7A for example, could generate 5W of blue light.

Of this some 4W may become white light, with the phosphor dissipating 1W of heat. The

overall LED thus appears to offer an efficiency of only 47.5% even though the LED is providing

100 lm/W or more.


Fig. 5: Heat sources in white LED

The excess heat should be removed with a suitable heatsink. The maximum operating junction

temperature recommended is 85°C, and the package and heatsink together should be

specified to keep the temperature of the LED at or below the recommended limit. The

technology is being assessed for higher temperature operation at 135°C.


Junction temperature measurement

There is widespread interest in a commercial

system which interrogates a thermal transient and

extracts the so-called “structure function” of an

LED which provides a spectral signature of the

various time constants in an LED.

An example of the thermal resistances and

capacitances is illustrated in fig. 6. This shows that

even for the most basic case, an LED contains

several layers each with its own thermal resistance

and capacitance, and this extends up to the final

heatsink.

We have found that the transient thermal method

is problematic for LEDs because the electrical time

constant is slower than the fastest thermal time

constants, making extraction of the fine time

constants difficult.

Fig. 6 (right): Thermal resistances and

capacitances in a typical LED installation


The method we propose and use is as follows. First, the LED is mounted on a suitable heatsink

with a separate power resistor also attached as shown in fig. 7. The resistor is used to heat

the heatsink. A thermocouple is placed next to the LED. An IR camera can also be used as a

check the temperature provided that the surfaces of the LED or PCB are IR emissive. Some

surfaces like copper and gold have a very low IR emissivity but can be made more so by

covering the surface with a suitable tape, or painting with white (or black) emissive paint that

can be separately characterised.

Fig. 7 (a) Pulse test setup

Fig. 7(b) Heatsink assembly

Fig. 7: Pulse test set up (a) and heatsink (b).

Fig. 7(a) shows the basic set-up. A pulse generator is used to drive the LED in current limit

mode to full current e.g. 700mA, but isothermally. This requires the pulse width to be short,

and we recommend 1 s. The duty cycle should be long, and we recommend 1ms, giving a


1:1000 ratio. The forward voltage of the LED is measured while the device is on, using any

suitable high-speed data acquisition device including a digital oscilloscope.

The test is started by measuring the initial temperature of the LED, and with the low PWM duty

cycle applied, the heating resistor is powered and the forward voltage noted as a function of

the recorded temperature. With the LED thus very lightly powered, the position of the

thermocouple is not so critical as the LED and PCB should be at equilibrium, but to measure

the solder point temperature later, the thermocouple should be mounted right next to the LED

package as close as possible to, or touching, the LED solder point. The forward voltage is

then measured at several set points.

In the next step, the power to the heating resistor is switched off. When cool the LED can be

powered up using normal continuous operating conditions and allowed to stabilise. The

forward voltage can then be used to determine the actual temperature from the calibration

curve produced.

If using an IR camera to monitor the temperature of the LED and heatsink, it should be noted

that a black heatsink is a good approximation in most cases to a black body and may not

require any surface coating.

Thermal resistance measurements

The thermal resistance is defined as the rise in temperature between the junction and solder

point for a given power:

Rthj−sp =∆T

Pin − Pout

where T is the temperature difference between the junction and solder point, and Pin-Pout

the dissipated power, being the difference between the input or applied power and the output,

or optical power. The junction temperature is determined from the Vf:temperature results as

measured using the method described above, interpolating if necessary, and the measured

solder point temperature indicated by the thermocouple.

The LED shares some similarities with its tungsten lamp fore-runner in that the dissipated

power is not the same as the power supplied, or input. Some of the energy is converted to


light which means that the dissipated heat power is less than the supplied input power, which

is the product of the forward voltage and current. In the case of the tungsten lamp, only about

15% of the energy is converted but for LEDs, 50 to 60% of the input can be converted to light.

This has caused some confusion as to the power to be used in the thermal resistance

calculation. One simple approach is to assume that the dissipated power is Vf x If, which

ignores the radiated light (Pout=0). This leads to an apparent, or “electrical” thermal resistance

which is about half the actual thermal resistance. This will, though, predict the correct junction

temperature, for nominal conditions, when multiplied by the applied input power. However,

there are some circumstances which require the real thermal resistance to be considered.

The light output from an LED is dependent on several factors. LEDs exhibit an efficiency drop

with increasing current, once beyond a low current threshold; an efficiency drop with

increasing temperature, and also whether there is a phosphor present in contact or not, as the

phosphor can generate heat due to conversion losses. If an “effective” thermal resistance is

calculated on the basis of a specific set of conditions, this may change if the conditions change.

The real, or “thermal” thermal resistance is determined by measuring the optical output power

from an LED under a given condition. This output power is then subtracted from the input

power to give the dissipated power, thus giving the real thermal resistance. However, to utilise

the “thermal” thermal resistance requires knowing the efficiency of an LED. Not only will this

vary with conditions, but also with lifetime. Therefore, in addition to providing a real thermal

resistance figure, a model of the light output which varies with current, temperature and time

is needed. Even this may be inadequate if, for example, a light fitting scatters or reflects some

of the light back onto the LED which will be reabsorbed, converted into heat, reducing the

effective efficiency.

As many factors need to be considered in order to use the real thermal resistance, the

datasheet figures are provided as guidelines to the junction temperature, and are correct when

multiplied by the electrical input power for nominal operating conditions. Any actual luminaire

design should be designed considering the real thermal resistance, but the junction

temperature will depend on the effective efficiency which may be affected not only by the LED

operating conditions of current, temperature and time, but also the efficiency of the luminaire.

A suitable guideline for the output power covering most of the stated conditions at worst case

(end-of-life, high temperatures and currents for example) is to assume 25% output. This is just

for thermal calculations as opposed to actual light output.


6. ESD protection

Plessey’s ceramic packaged LEDs are supplied with an ESD protection diode which is

connected in reverse parallel with the LED internally. Fig. 8 illustrates the electrical circuit.

Fig. 8: ESD protection

For this reason, it is not possible to operate the LEDs in reverse bias. Circuits employing these

LEDs should be designed not to drive the LEDs in the reverse direction.


Legal Notice

Product information provided by Plessey Semiconductors Limited (“Plessey”) in this document is

believed to be correct and accurate. Plessey reserves the right to change/correct the specifications and

other data or information relating to products without notice but Plessey accepts no liability for errors

that may appear in this document, howsoever occurring, or liability arising from the use or application

of any information or data provided herein. Neither the supply of such information, nor the purchase or

use of products conveys any licence or permission under patent, copyright, trademark or other

intellectual property right of Plessey or third parties.

Products sold by Plessey are subject to its standard Terms and Conditions of Sale that are available

on request. No warranty is given that products do not infringe the intellectual property rights of third

parties, and furthermore, the use of products in certain ways or in combination with Plessey, or non-

Plessey furnished equipments/components may infringe intellectual property rights of Plessey.

The purpose of this document is to provide information only and it may not be used, applied or

reproduced (in whole or in part) for any purpose nor be taken as a representation relating to the products

in question. No warranty or guarantee express or implied is made concerning the capability,

performance or suitability of any product, and information concerning possible applications or methods

of use is provided for guidance only and not as a recommendation. The user is solely responsible for

determining the performance and suitability of the product in any application and checking that any

specification or data it seeks to rely on has not been superseded.

Products are intended for normal commercial applications. For applications requiring unusual

environmental requirements, extended temperature range, or high reliability capability (e.g. military, or

medical applications), special processing/testing/conditions of sale may be available on application to

Plessey.

Contact

Customer Enquiries/Sales

+44 1752 693000 | [email protected]

www.plesseysemi.com

Plessey Semiconductors Ltd | Plymouth

Tamerton Road, Roborough

Plymouth, Devon

PL6 7BQ United Kingdom

P: +44 1752 693000

F: +44 1752 693700

http://www.plesseysemi.com/

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