27 Sep 2018
Aging of Potted LED Modules under Realistic Conditions
Among other things, the evaluation of methods and systems for encapsulating LED modules led to the question of the expected service life.
Information from the manufacturers of the chemical components of a potting system were imprecise and hardly meaningful.
The aim of the work is to investigate different systems and to characterize them with methods that are established in LED technology.
The central question is whether a potted LED module can achieve a lifetime under realistic conditions, which is considered to be common for LED modules. A test based on the LM-80 evaluation will show whether acceptable lifetimes are achieved and whether it seems realistic to use extrapolation such as TM-21.
For evaluation, five potting systems were selected. Standard LED modules were glued into a U-profile. The profile has been designed so that the temperature at the Tc point of the LED module settles at 95 ° C. Five sample groups were potted with the different systems. A sample group was not potted to have a reference. All LED modules were measured before treatment. With this measurement, the direct influence of the encapsulation can be measured. The measurement of all modules after completion of the encapsulation forms the value set at time t = 0.
The non-potted reference group showed the expected behaviour. Two systems did not reach the planned 10’000h, because already after 5’000h no noteworthy residual brightnesses were to be measured.
One sample group cannot be evaluated yet because the brightness values fluctuate around a mean value of 98.8% in the measuring time of 10’000 h and there is still no trend towards luminous flux loss. Here further measurements will provide information.
With two sample groups, a luminous flux decrease could be detected. One group was characterized as 6’850h L70B50. Another group was characterized by 13’250h L70B50.
It was found that the information provided by manufacturers of potting chemicals should be considered critically and that own investigations should be carried out. During the experiment, changes in chromaticity coordinate were noticed not only by the potting, but also during the measurement time. These effects are to be quantified for the evaluation.
A potting system must be able to cope with different challenges. These are among others
• Long-term stability of the IP protection
• Long-term stability of transparency
• Low chemical effects to the LEDs
• Low initial damping
• Low colour shift
• Workability (adhesion, moisture, …)