Application Guidelines
MCPCB Assembly Recommendations
A good thermal design requires an efficient heat transfer from the MCPCB to the heat sink. In order to minimize air
gaps in between the MCPCB and the heat sink, it is common practice to use thermal interface materials such as
thermal pastes, thermal pads, phase change materials and thermal epoxies. Each material has its pros and cons
depending on the design. Thermal interface materials are most efficient when the mating surfaces of the MCPCB
and the heat sink are flat and smooth. Rough and uneven surfaces may cause gaps with higher thermal resistances,
increasing the overall thermal resistance of this interface. It is critical that the thermal resistance of the interface is
low, allowing for an efficient heat transfer to the heat sink and keeping MCPCB temperatures low.
When optimizing the thermal performance, attention must also be paid to the amount of stress that is applied on
the MCPCB. Too much stress can cause the ceramic emitter to crack. To relax some of the stress, it is advisable to
use plastic washers between the screw head and the MCPCB and to follow the torque range listed below. For
applications where the heat sink temperature can be above 50oC, it is recommended to use high temperature and
rigid plastic washers, such as polycarbonate or glass-filled nylon.
LED Engin recommends the use of the following thermal interface materials:
1. Bergquist’s Gap Pad 5000S35, 0.020in thick
Part Number: Gap Pad® 5000S35 0.020in/0.508mm
Thickness: 0.020in/0.508mm
Thermal conductivity: 5 W/m-K
Continuous use max temperature: 200°C
Using M3 Screw (or #4 screw), with polycarbonate or glass-filled nylon washer (#4) the
recommended torque range is: 20 to 25 oz-in (1.25 to 1.56 lbf-in or 0.14 to 0.18 N-m)
2. 3M’s Acrylic Interface Pad 5590H
Part number: 5590H @ 0.5mm
Thickness: 0.020in/0.508mm
Thermal conductivity: 3 W/m-K
Continuous use max temperature: 100°C
Using M3 Screw (or #4 screw), with polycarbonate or glass-filled nylon washer (#4) the
recommended torque range is: 20 to 25 oz-in (1.25 to 1.56 lbf-in or 0.14 to 0.18 N-m)
Mechanical Mounting Considerations
The mounting of MCPCB assembly is a critical process step. Excessive mechanical stress build up in the MCPCB can
cause the MCPCB to warp which can lead to emitter substrate cracking and subsequent cracking of the LED dies
LED Engin recommends the following steps to avoid mechanical stress build up in the MCPCB:
o
o
Inspect MCPCB and heat sink for flatness and smoothness.
Select appropriate torque for mounting screws. Screw torque depends on the MCPCB mounting
method (thermal interface materials, screws, and washer).
o
o
Always use three M3 or #4-40 screws with #4 washers.
When fastening the three screws, it is recommended to tighten the screws in multiple small
steps. This method avoids building stress by tilting the MCPCB when one screw is tightened in a
single step.
o
o
Always use plastic washers in combinations with the three screws. This avoids high point contact
stress on the screw head to MCPCB interface, in case the screw is not seated perpendicular.
In designs with non-tapped holes using self-tapping screws, it is common practice to follow a
method of three turns tapping a hole clockwise, followed by half a turn anti-clockwise, until the
appropriate torque is reached.
COPYRIGHT © 2018 LED ENGIN. ALL RIGHTS RESERVED.
LZC-00GW00 (2.4 – 11/09/2018)
17
LED Engin | 651 River Oaks Parkway | San Jose, CA 95134 USA | ph +1 408 922 7200 | em LEDE‐Sales@osram.com | www.osram.us/ledengin
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