L
EDmodule or luminaire reliability and lifetime
are about more than simply extrapolating the
LED chip testing data and applying it to an
LED luminaire.
Many people mistakenly think that LED lifetime
is measured only in lumen depreciation till the
moment at which the initial lumens fall below 70
or 80 %. This, however, is just one aspect of LED
lifetime. The failure mode that causes the most
impact during the lifecycle of the luminaire is cata-
strophic failure.
The IEC PAS 62717 standard brings clarity to
the initial assessment of these two parameters.
Reliability is not just about being able to produce
high quality products on a one-off basis, but about
being able to do so consistently over time. Control
processes should be set up to ensure that manu-
facturing quality remains high over time, and not
only in the first batches or laboratory conditions.To
guarantee high quality, long lasting products that
meet stated specifications, a lighting manufacturer
should have a multifaceted approach to quality,
including development, supplier and manufactur-
ing quality.
When designing and manufacturing LED light
engines with relatively long life times and associ-
ated service liabilities, there is an increased need
for transparent and well controlled failure rates.
Besides the LED driver, the LED light engine may
become one of the most critical components in the
chain, when manufacturing a luminaire. The basis
of reliable LED modules lies in the robustness of
the design, but controlling the LED manufacturing
processes is as crucial.
What critical factors should be taken into ac-
count for manufacturing quality? What could go
wrong and what impact would this have? Statisti-
cal Process Control is commonly used in state of
the art manufacturing environments. For lighting
purposes, the parameters controlled in this way
should not be limited to straightforward electrical
measurements. Light performance aspects such as
flux, SDCM, CRI and CCT are also crucial repetitive
quality parameters and not necessarily part of the
expertise of a PCB assembler. Besides this there
are some particular attribute parameters that are
important to control.
Soldering
Soldering is a special field of expertise that forms a
particular source of concern in the reliability of LED
systems. Although it has been around for decen-
nia in manufacturing environments, it represents
a new challenge when used for LED lighting, an
application with high thermal stress and large size
components.The thermal mechanical stress, result-
ing from switching from high temperatures in the
‘on’ state to low temperatures in the ‘off ’ state,
may induce cracks in a solder with too many voids,
when present in a critical place. Furthermore, voids
may negatively influence the thermal dissipation of
heat, all leading to short life.
LED lighting manufacturers need to make
informed choices about soldering in their manu-
facturing processes or specifications. Should an
investment be made in an expensive manufactur-
ing set-up, to comply with Electrostatic Discharge
(ESD) requirements, controlling the percentage of
voids to 25%, as suggested by some LED chip sup-
pliers, or should 100% full checks be introduced?
Can accelerated life tests and models be depended
on to allow for more, but controlled voids, depen-
dent on the mission profiles of the products?These
are decisions that companies need to make on an
The importance of
manufacturing excellence
for quality and reliability
by Henk Rotman, Philips Lighting Commercial Marketer: Large Scale Projects
This article highlights some crucial aspects relating to manufacturing and catastrophic failures.
LiD
08-09/14
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