Applications Engineering Notes

Failure Mode Analysis (FMA) Report on ESD Damaged White LEDs

Background:

On April 2003, one of our customers reported that they had experienced over 100 failures in their recently received first batch of 100K white LED lamps. The customer reported that the white LED lamps had instantaneously flashed ON after being connected together during product assembly and then did not work anymore thereafter. Immediately it was suspected that Electro Static Discharge (ESD) damage was causing such failures. As measures were put in place in their production facility to reduce all possible sources of ESD damage (proper grounding of all equipment and personnel), a dramatic drop in the failure rate/quantity while running production was experienced (from over 166 pieces per 18,000 to 1 per 900).

Brite-LED received 166 �bad� LEDs, which were sent by the customer for test and evaluation to confirm and determine the actual cause(s) of their failure and thus the reason for this report.

Evaluation and Testing:

1. Each of the 166 LEDs were tested individually with an HP Lab Bench DC Power supply, by supplying a varying current. All of the LEDs came ON. However, this happened at different forward current (I_F) levels.

2. A few came on at or around just 1mA, a large group came ON at around 15mA, other group came ON only after the applied forward current reached 50mA, and a few came ON after the applied forward current reached 150mA.

3. On the LEDs that required the very high currents to turn ON, the forward voltage increased very slowly at first as the applied forward current was increased, but once they reached the turn ON point, the current suddenly dropped and the voltage settled to the corresponding expected V_F levels.

4. Once all the LEDs that required very high currents to turn ON got �Started�, they came ON in subsequent trials at much lower currents (8mA to 15mA). As if they got �semi-fixed�.

5. A randomly picked group of 30 LEDs was inspected under a microscope to look for any evidence of bond wire or die damage. None was found. The wire bonds were intact. This finding revealed good quality LED packaging given that the customer had bent outwardly the leads of all LEDs extremely close (<1mm) to the lower side of the epoxy lens, which could have caused wire bond breakage.

Fig 1. Some LEDs come ON at I_F=1mA

6. A forward current was slowly applied to some LEDs while the die was closely observed to find the turn ON point and how the die behaved. It was found that in the LEDs with a high current turn ON point, only half or a quarter of the die would come ON at first. (See attached picture sequence below).

Close-up Pictures of LED that came ON at 1mA

Fig 2. LED is OFF (No current applied) Fig 3. LED just starts to turn ON (I_F≈0.7mA; V_F=2.47V)

Close-up picture of a more severely damaged LED that comes ON at 29mA

Fig 4. Only a corner of the LED die comes ON at first. (I_F=29mA; V_F=2.42V).

8. Another LED, similar to the one in figure 4, did not initially come ON until a 170mA forward current was applied (@ V_F=3.48V), then suddenly the V_F dropped to 2.71V and the forward current dropped to 16mA. After that, the LED came ON at 15mA on subsequent trials.

Conclusions of Findings:

1. As initially suspected, Electro Static Discharge (ESD) caused the damage to the LEDs.

2. This conclusion is based on the all of the compounded clear evidence and behavior of the LEDs.

3. ESD damage can range from catastrophic failure to various degrees of non-so-evident damage.

4. ESD damage to an electronic device, such as an LED, may lead to intermittent behavior or premature device failure.

5. ESD damaged devices can appear dim, dead, shorted, or with low V_F or V_RAll of the devices tested show one or all of these symptoms

6. Light Emitting Diodes are static sensitive devices. InGaN on Sapphire die (used in these white LEDs) is generally considered and thus rated as an ESD Class 1 device (can survive 20V bias with Machine Model Testing, and 130V from Human Body Model Testing). Static charges of 30K volts are not uncommon and can be generated quite easily.

7. ESD caused internal damage in the LED die by creating shunt current paths, which was evidenced by the low V_F and high I_F.

8. When the applied Forward current was high enough, new paths were created, thus �fixing� the LED. However, although this might be considered a way of �fixing� the LED, in reality the LED is not completely �healed�. It�s efficiency and life is permanently affected, both most likely decreasing.

Recommendations:

An ESD Safety and Control Program must be put effectively in place. Generally such program includes provisions for Charge Protection; Grounding; Shielding; Neutralization; Use of Anti-Static Materials, Work Stations and other Materials and Equipment, and last but most important, Education and Training.

The primary method of ESD Control is to properly ground (or bring to the same potential) all conductors that come in contact or are in near proximity to the electrostatic sensitive devices (the LEDs in this case). These conductors include humans, tools, mats, other electronic devices, boards, connectors, packaging, etc. ESD sensitive devices should always be placed inside enclosed antistatic shielding bags and containers during all routine handling such as inventory storage, transportation and WIP.

An evidence of the impact of all of the above listed measures is the dramatic decrease in the failure rate and quantity of LEDs after ESD prevention and control measures were put in place and/or increased in the customer�s production line area.