HANDLING LEDs


LED failures may be attributed to certain irregularities either at the manufacturer’s end or at the user’s end. This write up elaborates on the causes of failure of LEDs and the various precautions that can be taken to avoid them.

Causes for LED failure:

1)       Cratering: A crack develops under the ball bond metallization zone. If you pull the bond wire (not possible in encapsulated LEDs), a chip pulls out, leaving a 'crater', hence the name.

Symptoms: If you apply pressure to the ball (sometimes just by pressing on the top of the LED), light output momentarily increases (or perhaps, is restored, in the case of total failure). Decapsulate and test bond-chip integrity. If you check the VI characteristics (Voltage- Current) with a curve tracer, you may see 'breakover' characteristic if you apply sufficiently high bias voltage.

Causes (singly or in combination): a) incorrect ball bonding parameters such as too much pressure, bad capillary, contaminated pad etc. b) tension on bond wire, related either to incorrect looping, vibration or shock c) power density of input pulses exceeds device capabilities.

2)       Die attach migration shunts junction and/or reduces optical transmission: This is more likely to happen with LEDs that use silver-filled epoxide die attach materials, as opposed to eutectic (solder) attachment, but it can occur in either case. The silver can creep up the side of the die, eventually shorting it out.

Symptoms: Can be seen using appropriate visual inspection techniques. Usually shows up looking like a parallel resistance on curve tracer VI characteristics (Voltage- Current), i.e., current starts to flow at low bias voltage, whereas the healthy junction shows little current flow until the bias is near the threshold for the material, usually between 1.1 and 1.8 Volts for LEDs.

Causes: The manufacturer is using too much die attach material if this happens. However, the problem is aggravated by high temperatures and pulse energy levels.

Cures: Get vendor to control process properly. Reduce drive levels and/or temperature.

Following are some precautions to be taken during manufacture and use of LEDs:

 (1) Lead Forming

-  At least 3mm from the base of the epoxy bulb should be kept when forming leads.

-          Do not use the base of the lead frame as a fulcrum during lead forming.

-          Lead forming should be done before soldering.

-  Because the stress at the base may damage the characteristics or it may break the LEDs. Do not apply any bending stress to the base of the lead.

-When mounting the LEDs onto a PCB the holes on the circuit board should be exactly aligned with the leads of the LEDs. Stress at the leads should be avoided during mounting of the LEDs on to the PCB as this causes damage to the epoxy resin leading to degradation of the LEDs.

 

(2) Storage

-          The LEDs should be stored at 30 C or less and 70% RH or less after being shipped. Storage life is about 3 months.

If the LEDs are to be stored for more then 3 months, they can be stored for a year in a sealed container with nitrogen atmosphere and moisture absorbent material.

-        Rapid transitions in ambient temperature, especially, in high
humidity environments where condensation can occur should be avoided.

(3) Static Electricity

-        Static electricity or surge voltage damages the LEDs.

It is recommended that a wristband or an anti-electrostatic glove be used when handling LEDs.

-        All devices, equipment and machinery must be properly grounded.

It is recommended that measures be taken to avoid surge voltage to the equipment that mounts LEDs.

-  Damaged LEDs exhibit some unusual characteristics such as increase in current leak, decrease in forward voltage, LEDs not lighting at low current etc.

     Criteria: VF>2.0V at IF=0.5mA

(4) Heat Generation

- Thermal design of the end product is of most importance. Heat generation of the ED is to be considered while designing the system.

-          The thermal resistance of the circuit board, density of LEDs and other components on the board affects the co-efficient of temperature increase per input electric power. Heat generation must be lowered and should be well maintained within the limits specified.

-The operating current should be decided after considering the ambient maximum temperature of the LEDs.

 (5) Cleaning

-  It is recommended that isopropyl alcohol be used as a solvent for cleaning the LEDs. In case of other solvents, it needs to be confirmed whether the solvents will dissolve the resin or not. Freon solvents should not be used to clean the LEDs because of worldwide regulations.

-  Do not clean LEDs using ultrasonic vibrations. When it is absolutely necessary, the influence of ultrasonic cleaning on the LEDs depends on factors such as ultrasonic power and assembly condition.

  Before cleaning, a pre-test should be done to check whether any damage would occur to the        LEDs.

 

(6) Safety Guidelines for the human eye

- In 1993, the International Electric Committee (IEC) issued a standard con­cerning laser product safety (IEC 825-1). Since then this standard has been applied for diffused light sources (LEDs) as well as lasers.  In 1998 IEC 60825-1 Edition 1.1 evaluated the magnitude of the light source.

In 2001 1C 60825-1 Amendment 2 converted the laser class into 7 classes for end products.

Components are excluded from this system. Products which contain visible LEDs are now classified as class 1. Products containing UV LEDs can be classified as class 2 in cases where viewing angles are narrow, optical ma­nipulation intensifies the light, and/or the energy emitted is high. For these systems it is recommended to avoid long term exposure. It is also recommended to follow the ICE regulations regarding the safety and labeling of products.

(7) Soldering Condition for LED lamps

   i. Epoxy resin of LED

Epoxy resin of the LED needs to be cured within some temperature range for sufficient time. In absence of this, epoxy will not harden allowing moisture penetration. This may lead to cracking of the epoxy inside the LED, hence reducing the LED lamp life.

ii.Temperature of  Glass (Tg Point)

To ensure that the epoxy resin is hard enough, factors such as temperature profile of the oven, exact time for curing and selection of epoxy resin are important. After that we need to measure the Tg point for checking the right consistency of the epoxy resin. Tg point can range between 125-135 centi-grade. If Tg is lower than 125 degree, the epoxy will be soft. Temperatures higher than 135 degree make the epoxy too hard leading to its cracking.

iii. For Automatic Soldering

(a) All soldering equipment should check the temperature of the PCB before soldering.

 (b) If the pre heat temperature is not below the Tg point, the LED will be soft soldered and the bonding gold wire will break.

 (c) The soldered PCB with LED should not be subjected to shock immediately after soldering because the epoxy is still soft inside the LED. Shock may cause the gold wire to break. The best way to harden the epoxy is to have a cooling fan which will cool gradually.

 

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