LED circuit boards

As part of a research project at IKV, LED circuit boards used in the front headlamps of cars were provided with a silicone lens using an LSR injection moulding process. Despite constant process parameters, it was found that, with some batches, the silicone component frequently became detached in the area of the LED chip, as a result of which the desired light diffusion was impaired. In addition, cloudy streaks were occasionally observed in the silicone lens. Tests were carried out to detect the cause of the problem.

• Macroscopic and microscopic documentation of the faults
• Examination of affected areas with a laser confocal microscope
• Field emission scanning electron microscopic examination (FESEM) of cross-sections through the lenses affected by the damage, in combination with energy-dispersive X-ray spectroscopy (EDX)

Foreign particles were detected in the affected areas, both on the silicone lens and on the chips. In the immediate vicinity of the LED chips, a fluid-like contamination was observed. Cloudy streaks in the silicone lens evidently originated on the LED board.

An examination of other LED chips under the light microscope also revealed contamination on new parts as a consequence of inadequate transport protection. The streak formation probably occurs during the injection moulding process and is presumably attributable to an interaction between the silicone material and the encapsulation material of the LED chips.

The EDX examination revealed the use of different materials in the area of the LED chips. The detachments were found to come from a batch in which components of alternative materials were used. The non-defective batch was found to contain, among other things, germanium and silicon, while the batch with the defects contained silicon and magnesium.
With the aid of these tests, the causes of the defects were remedied immediately.

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