Reliability and lifetime of LEDs

October 31st, 2014, Published in Articles: Vector

 

With the increasing complexity of technical equipment, modules and individual components, reliability, lifetime and the cost of exchanging and revision become increasingly important for the customer.

Fig. 1: Basis for reliability of LEDs

Fig. 1: Basis for reliability of LEDs.

The single requirement that devices should not fail is no longer sufficient for modern, powerful components or devices. More often, it is an additional requirement that they should perform their functions without failure.

However, it is only possible to make a prognosis supported by statistics and trials. Whether an individual device or component will operate without failure for a certain period of time is not known.

Nowadays, modern methods of quality management and reliability modelling are used to investigate and verify these types of question. This article provides a fundamental insight into reliability and lifetime.

Osram Opto Semiconductors associates the term “reliability” with the fulfillment of customer expectations over the expected lifetime. In other words, the LED does not fail during its lifetime under the given environmental and functional conditions.

The reliability of the products is therefore based on the chain of the materials, the manufacturing process and the function of the component (see Fig. 1). In addition, the final application must also be taken into consideration.

High reliability can only be achieved if the changing effects and inter-dependencies of the individual components are already taken into account during the development phase.

Neglecting this entirely or only focusing on one or two elements leads to a reduction in the quality of the product and, therefore, to a decrease in reliability.

Reliability of LEDs

The reliability of a semiconductor element is the property that states how reliably a function assigned to the product is fulfilled within a period of time. It is subject to a stochastic process and is described by the probability of survival R(t).

A fault or failure is indicated if the component can no longer fulfill the functionality assigned to it.

Failures and failure rates are subdivided into three phases:

  • Early failures.
  • Random or spontaneous failures.
  • Wearout period.
Fig. 2: Failure rate over time ("bathtub curve").

Fig. 2: Failure rate over time (“bathtub curve”).

The failure rate over time takes the form of a “bathtub” curve as the failure rate is especially high at the beginning and end of the product cycle (see Fig. 2). Therefore, each failure mechanism exhibits its own chronological progression and shows an individual bath-tub curve.

Many different types of definitions, analysis method and mathematical formula for reliability can be found in the literature for each of these phases. The most important definitions and methods which apply to LEDs are described here.

For the sake of simplicity, the first two phases are combined into a so-called “extrinsic reliability period”. The third phase, the wear-out period, is correspondingly designated as the “intrinsic reliability period”.

Extrinsic reliability period

Extrinsic failures (early and spontaneous failures) are generated by defective materials, deviations in the manufacturing process or by incorrect handling and operation by the customer.

More than 99% of extrinsic failures can be observed during installation of the parts in the application or in the first hours of operation. By contrast, the spontaneous failure rate for LEDs is extremely low between the early failure period and the wear-out period.

In reliability mathematics, this failure period is described by an exponential distribution, which is based on a constant failure rate over time. The average failure rate is given in failure units (FITs).

Fig. 3: LED failure rate in the extrinsic period according to Siemens Standard SN 29500.

Fig. 3: LED failure rate in the extrinsic period according to Siemens Standard SN 29500.

As a rule, an experimental determination of the middle failure rate is extremely difficult. For this reason, Osram Opto Semiconductors uses the SN 29500 standard from Siemens, which incorporates the experience of failures in the field into the typical failure rates for LEDs (see Fig. 3). In the process, no distinction is made in terms of the cause of the individual failures.

Due to continuous degradation, a failure criterion must be established to obtain a concrete evaluation of the LED failure. The point in time at which the luminous flux of the LED reaches the failure criterion is then described as the failure time or lifetime of the LED.

As a rule, the failure criterion is determined by the application. Typical values are 50% (L50) or 70% (L70), depending on the general illumination.

Intrinsic reliability period

The intrinsic reliability period describes the so-called wear-out period of the component at the end of the product cycle. It is based on increased wear and ageing of the material. This continuous change over time is generally measurable and is referred to as degradation.

For LEDs, the most significant degradation parameters are the changes in brightness or colour coordinates. Other parameters such as forward voltage generally play a subordinate role.

During operation, LEDs experience a gradual decrease in luminous flux, measured in lumens. As a rule, this is accelerated by the operating current and temperature of the LED and also appears when the LED is driven within specifications.

The term “lumen maintenance” (L) is used in connection with the degradation of light in LEDs. This describes the remaining luminous flux over time, with respect to the original luminous flux of the LED.

Since ageing is based on a change in the material properties and is therefore subject to statistical processes, the lifetime values also are based on a statistical distribution.

The term “mortality” (B) describes the percentage of components which have failed. A value of B50 therefore describes the point in time at which 50% of the components fail. This value is generally specified as typical median lifetime, t50 or tml, for LEDs. A value can also be specified when 10% of the components have failed (B10 value), in addition to the median value (B50). This allows one to draw a conclusion about the width of lifetime distribution (see Fig. 5).

Fig. 4: Degradation curve.

Fig. 4: Degradation curve.

Fig. 5: Distribution of the probability of failure over the LED’s lifetime.

Fig. 5: Distribution of the probability of failure over the LED’s lifetime.

The continuous ageing process generally cannot be measured for thermo-mechanical stress on a component (e.g. temperature cycles). This means that the constant ageing process which leads to failure cannot be described by means of a characteristic measurement parameter such as light degradation during electrical operation. An extrapolation of the degradation curve to a defined failure criterion as shown in Fig. 4 is not possible here.

In this case, tests must be performed until the most abrupt failures occur to be able to make statements about the time of failure or the failure distribution. An example of this is fatigue in adhesive or bonded connections.

Reliability, lifetime: influencing factors

The reliability and lifetime of LED light sources also depend on various factors. The most important physical factors include humidity, temperature, current and voltage, mechanical forces, chemicals and light radiation (see Fig. 6).

Fig. 6: Influencing factors on reliability and lifetime.

Fig. 6: Influencing factors on reliability and lifetime.

Fig. 7: Dependence of lifetime on the junction  temperature and solder point temperature.

Fig. 7: Dependence of lifetime on the junction temperature and solder point temperature.

These can lead to total failure or influence the ageing characteristics in the long term (e.g. brightness) and therefore produce a change in reliability and lifetime.

Direct influencing factors include temperature and the resulting junction temperature Tj of the LED, for example, but the amount of current used to drive the LED is also an influencing factor. Under otherwise equal operating conditions, an increase in the ambient temperature and current produces an increase in the junction temperature. In general, however, an increase in junction temperature brings about a decrease in lifetime.

Mechanical force is another direct influencing factor. Large mechanical forces applied to the LED generally result in damage which can lead to total LED failure.

Fig. 8: Sources of influencing factors.

Fig. 8: Sources of influencing factors.

The sources of the individual factors can be found in different areas such as LED design, LED processing, customer application and the environment and can be traced back to various aspects and parameters (see Fig. 8).

If these areas are examined in detail, it becomes clear that three of the four areas can be influenced directly by the LED manufacturer or the user. The environment can ultimately not be changed and must be considered as a given in the application. For example, the source of the influencing factor, temperature, can be assigned to two areas, LED design and the customer application.

In the area of LED design, the source of the temperature influence lies with both the electrical parameters and with the transfer of heat.

Depending on the current applied (IF) and the associated voltage (UF), power dissipation is created, leading to a temperature increase in the junction of the LED. The amount of power dissipation is proportional to changes in the junction temperature.

The proportionality factor is the thermal resistance of the housing (Rth, Junction-Solderpoint) of the LED. This reflects the heat transfer characteristics of the LED.

The lower the thermal resistance, the better the LED’s thermal properties are. If heat is dissipated quickly and efficiently, the junction temperature does not increase as rapidly.

Fig. 9: The dependency of lifetime on temperature due to the influence of various Rth values (example).

Fig. 9: The dependency of lifetime on temperature due to the influence of various Rth values (example).

As an example, two components with differing Rth values (2,5 and 8 K/W) are examined at the same solder point temperature TS = 90 °C and the same operating conditions (current)
(see Fig. 9).

The junction temperature of the component with low thermal resistance only increases to ~104 °C. By contrast, however, the component with the higher thermal resistance exhibits a junction temperature of >135°C. As mentioned previously, the lifetime of an LED is reduced with an increase in the junction temperature.

At the same solder point temperature, the component with the lower Rth achieves a longer lifetime than the component with the higher Rth.

In addition to an increased lifetime, lower thermal resistance offers an additional advantage: at the same solder temperature, a component with a low Rth achieves higher light output. The reason for this is the decrease in efficiency of the LED with an increase in junction temperature.

The LED manufacturer can already take into consideration the factors which influence lifetime and reliability in the development phase (see Fig. 9).

The effects of these factors can be reduced through the following measures:

  • Robust design.
  • Optimal thermal management.
  • Stable and optimised production processes to minimise the risk of spontaneous failure.
  • Customer support including LED designs in the customer application.

In the area of customer applications, the influencing factor of temperature can be traced back to heat dissipation. Here, the layout and material of the circuit board play an important role.

Categorised under “thermal management”, which includes the selection of appropriate circuit board material (e.g. FR4, IMS), the layout of LEDs, component density, additional cooling etc., the user can also specifically target his application to accommodate the influencing factors.

The following measures can be taken:

  • Optimal thermal board management.
  • Optimal design for efficient use of the LED.
  • Handling the LED according to specifications.
  • Considering the strengths and weaknesses of LEDs

Insufficient thermal management directly leads to a reduction of the reliability and lifetime of the LED. High overall or system reliability can only be achieved by considering all areas and all influences.

Validating and confirming reliability, lifetime

All LED packages and chip families from Osram Opto Semiconductors undergo a number of tests for validation and confirmation of reliability and lifetime. The selection of tests, test conditions and duration occurs by means of an internal qualification specification based on JEDED, MIL and IEC standards.

Table 1: Example reliability test matrix.

Test

Conditions Duration Stress factors
Resistance to soldering heat (RTSH) JESD22-A113

Convection soldering

260°C/10 sec

3 runs

Temperature, chemicals,

mechanical forces

Resistance to soldering heat (RTSH) JESD22-A106

Wave soldering

260°C/10 sec

3 runs Temperature, chemicals,mechanical forces
Temperature & humidity bias (T&HB) JESD22-A101

T = 85°C

R.H. = 85%

IF = 5 mA/10 mA

1000 h Temperature, humidity
Temperature cycle (TC) JESD22-A104

-40°C/+100°C

15 min at extreme temps.

300/500/1000 cycles Mechanical forces
Power temperature cycling (PTC) JESD22-A105

-40/+85°C

IF = [max derating]

ton/off = 5 min

1000 h Temperature, current, mechanical forces
Steady state life test (SSLT) JESD22-A108

T = 25°C

IF = [max derating]

1000 h Temperature, current
Steady state life test (SSLT) JESD22-A108

T = 85°C

IF = [max derating]

1000 h Temperature, current
Pulsed life test (PLT) JESD22-A108

T = 25°C

IF = [max derating]

1000 h Temperature, current
ESD JESD22-A114

Human body model

2000 V

1 pulse per polarity direction Voltage

Table 1 shows the list of tests typically performed and the test conditions, test duration and the stress factors involved.

The test conditions and test duration can be set based on the company’s internal qualification specification and the requirements profile.

The mechanical stability of an LED is checked by means of a solder heat resistance test as well as powered and unpowered temperature cycle tests. Here, the cycle count and the temperature difference serve as measures of stability. These types of tests are also drawn upon to evaluate the failure rate.

Contact Ryan Hunt, Osram Opto Semiconductors, Tel 079 525-1779, r.hunt@osram.co.za

Related Articles

  • To automate thermal image processing
  • Digital farming’s inroads into South African agriculture
  • Versatile and flexible LED strip lights
  • Diverse product range for mining
  • Do passive RFID tags need hazardous area certification?