High Reliability LED Chip-on-Board Assemblies
Posted 11/4/2015 by Ravi M. Bhatkal, Ph.D.
LED chip-on-board applications (COB) typically involve assembling an LED die stack directly on to a substrate such as Metal Core PCB (MCPCB), FR4 etc.
LED Chip-on-Board structures have several advantages including:
- Lower thermal resistance – since one packaging layer is eliminated
- COB designs can scale effectively based on lumen output required
- COB platforms provide higher design flexibility and can be tailored for module size and power level, especially for wide area lighting applications.
- COB designs can lower system costs.
the LED COB segment is growing rapidly, and all LED chip structures (lateral,
flip chip, and vertical) can be utilized in designing LED COB products.
of die attach materials, including solder, conductive adhesive, or sintered
materials can be used to bond the LEDs to the substrate, depending on the
specific requirements, and the tradeoffs involved.
COB designs, SAC-based die attach solder paste is a convenient form factor to
use, given the fact that existing equipment sets can be used to implement a
high throughput process, and since second reflow is not needed in COB
of the COB assembly in a given application generally depends on:
material stack selected (LED submount, solder, board material and dielectric,
and respective geometries),
manufacturing process conditions and
use environment (temperature swings and harshness of conditions)
selecting the appropriate solder alloy to be used, one needs to consider the
reliability requirements. In doing so one needs to specifically focus on the power
level, operating temperature and cycling conditions and CTE mismatch between
the LED die stack and the MCPCB, and consequently, its impact on thermal
cycle-induced creep fatigue of the solder material.
In the case
of a high power LED assembly on aluminum MCPCB, the ΔCTE between the LED and the MCPCB is
18-20, which is quite high. During
thermal cycling experienced by the COB assembly in applications such as outdoor
lighting or automotive, the high ΔCTE
causes significant strain energy build-up in the solder joint between the LED
die and the MCPCB, during the thermal cycling experienced in use. This strain energy buildup causes
micro-cracking, and eventually, failure of the joint.
Thus, for a
given LED chip structure and board material used, it is beneficial to use solder
joints with improved mechanical and thermal fatigue/creep and vibration
resistance. Alpha has accomplished this
by developing the Maxrel™ family of alloys, via a micro-structural control
approach. These advanced alloys have
been developed with special additives for improved thermal stability for high
temperature operation and higher thermal fatigue and vibration resistance. The first commercially released Maxrel alloy is
available in solder paste, preform, and wire format. Alpha’s Lumet™ P39 solder paste with Maxrel™
alloy, has been commercially implemented for high power LED assembly.
It has been
shown that the Maxrel™ alloy maintains excellent shear strength even after 1000
thermal cycles under high CTE mismatch conditions.
conclusion, creep resistance of the solder alloy used is a significant
determinant of solder joint reliability in high CTE mismatch assembly stacks
under thermal cycling conditions.
Solder joints with improved mechanical and thermal fatigue/creep and
vibration resistance and bond line uniformity, are possible by using
micro-structural control approach. Alpha
has developed the Maxrel™ alloys for improved thermal stability. Solder pastes such as Lumet™ P39 with Maxrel
alloy can be used for LED COB assemblies on MCPCB to enhance the reliability of
"To view the complete
technical paper on High Reliability LED Chip-On-Board Assemblies click here."
For further information, contact:
Vice President, Energy Technologies
Technical Marketing Manager - LED
Project Manager/Engineer - LED