02-08-2011, 04:09 PM
Innovative Printed Circuit Board Technology for Heat Removal with
Application to Power LED based Operating Room Lamps
Abstract:
Using a simple analytical model we investigate the effect of thermal vias on the heat
transfer through a printed circuit board (PCB) . We show that thermal vias can increase the heat
transfer coefficient by orders ofmagnitude. The model considers the arrangement and the geometry of
the vias, the metallization thickness and the solder fillets. The application part ofthe paper addresses
three prototypes ofhigh-power LED-based operating room lamps. The heat spreading ability ofthree
types of PCBs - two FR4-based prototypes with thermal vias and one with an insulated metallic
substrate (IMS) - is evaluated by using thermal simulation and single point temperature measurement
with miniaturized thermocouples. All the three prototypes fulfilled the requirements for the thermal
performance satisfactorily: the total thermal resistance of the FR4-based prototypes was below
1.2 K/w, and the one with the IMS even below 0.6 K/W
1. INTRODUCTION
Thermal design became a challenging task for
manufacturers of electronic assemblies. Numerous
requirements, such as mechanical, electrical and
sometimes even optical, need to be fulfilled in
addition to the thermal ones under the pressure of
costs. At an early design phase one has to decide
whether the power loss will be eliminated by natural
or by forced convection or by heat conduction into a
heat sink. To keep the junction temperature of the
power device within the specified range, the heat
should be conducted with the lowest possible thermal
resistance into the conductor layer of the circuit
carrier [1, 2]. The heat needs to be spread out
effectively to reduce the thermal resistance to the
ambient by using an increased dissipating surface
[3, 4]. This is particularly important in the case of
natural convection. The most straight forward
approach to achieve a good heat spreading effect is to
employ a single-sided PCB with a thick copper layer.
However, the need to avoid crossings and
technological limitations leads to the use of two or
even more copper layers. These are separated by a
polymer layer whose thermal conductivity is by orders
of magnitudes lower than that of the copper. Such a
composite structure could be treated as a laminate
with a strongly anisotropic thermal conductivity [5].
In a conventional low-cost design of a PCB with two
or more copper layers the heat transfer coefficient
through the PCB does not frequently allow to keep the
device operation temperature below the specified
maximum temperature: the thermal resistance of the
assembly is too high. The thermal performance of the
device can be improved significantly if all the
available copper layers are used most effectively to
achieve an optimum lateral heat spreading effect. For
this purpose the copper layers are thermally connected
parallelly by thermal vias. To obtain the necessary
increase in the heat transfer coefficient, several
hundred thermal vias might be needed in the vicinity
of the power device. Such a large number of vias
leads, however, to high fabrication costs. The
optimum number of vias can be estimated on the basis
of an appropriate thermal model.
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