Heat Sink Profile Design Using FEA Simulation for Laser

Heat Sink Profile Design Using FEA Simulation for Laser
Heat Dissipation in a CD/DVD Optical Pick-up Unit
Subramanian N.R.
Innovation (Development), Philips Optical Storage, Singapore
Yak Aik Seng
Innovation (Development), Philips Optical Storage, Singapore
Abstract:
For Optical Pickup Units (OPU), the dissipation of laser heat is a primary requirement for performance
reliability & life expectancy of the device when operated continuously at extreme service temperatures.
Heat sinks are components that enhance heat dissipation from a hot source to a relatively cooler ambient in
order to maintain the laser device within the specified service temperature for continuous operation.
Typically the heat dissipated by the laser diode anchored inside a plastic housing is fed to a copper sheet
heat sink for air convection cooling through a conductive path comprising thermal interface silicon
elastomer pad & thermal compounds. In this paper, descriptions pertain to steady-state heat transfer FEA
simulation utility in evaluating and optimising heat sink profiles for a multi-component (8 materials)
configuration. Temperature distribution has been evaluated for varying heat sink profiles & convection
area in order to limit the laser stem (case) temperature below 70°C while dissipating heat.
Introduction:
In CD/DVD mechanisms, Optical Pickup Unit (OPU) has the function of sharp focusing the laser beam
onto a transparent plastic (polycarbonate) disc containing a continuous spiral sequence of impressed pits &
lands. The laser beam reflected by the disc surface (optically modulated by the disc geometrical structure)
is directed by the OPU towards photodetectors & transformed into photocurrents. These high frequency
signals carrying the information recorded on the disc is also extracted and forwarded to the decoding
electronics (Reference 1). The OPU chassis molded out from plastics or metal die-castings serves as a
housing to anchor constituent components - a semiconductor laser, optical elements to guide the laser beam
and a photo detector to convert the incident light into electrical signals.
Laser Diode
A laser diode, as used in optical recordings, is based on the stimulated emission of photons, which takes
place in the neighborhood of the junction between two semiconductor materials. The stimulated laser
radiation is initiated when the current density in the layer exceeds the threshold value & causes photon
excitation (Reference 2). Consequently, the junction generates significant waste heat, which needs to be
transferred to the surrounding room air in order to maintain the semiconductors within their operating
temperature limits (Reference 3). This is best accomplished by attaching a heat sink to the semiconductor
casement surface thus increasing the heat transfer between the hot case and the cooling air.
Typically, in optical recording systems, the laser beam is generated by semiconductor structures &
characterized by intense, coherent and monochromatic radiation at wavelengths 790 nm & 650 nm for CD
& DVD recordings respectively. The specifications for the laser define the maximum operating current &
voltage to be 63mA & 2.3V, which gives the maximum heat dissipation from the laser to be 145mW.
Experimental measurements on the de-coupling modulator record 17mA & 5V for operating current and
voltage resulting in heat dissipation of 85mW distributed throughout the modulator surface. The operating
temperature rating for the OPU is specified to be -10°C ~+60°C and the rated temperature for the
semiconductor laser operation is specified to be -10°C ~+70°C. In the current simulation, the maximum
heat dissipations from the laser (145mW) & from the modulator (85mW) are applied as surface flux at
appropriate areas. The case temperature in the current simulation corresponds to the outer rim surface of the
laser diode. The evaluation for performance & life of the diode is considered to be successful by limiting
the temperature below 70°C at this surface when the ambient operating temperature is 60°C.
Heat Sink:
The relationship between the reliability and the operating temperature of a typical semiconductor device
shows that a reduction in the temperature corresponds to an exponential increase in reliability & life
expectancy of the device. Therefore, long life and reliable performance of a component may be achieved by
effectively controlling the device operating temperature within the limits set by the device design
engineers.
Heat sinks are devices that enhance heat dissipation from a heat-generating component to a cooler ambient,
usually air. In most situations, heat transfer across the heat source component and the contacting cool air is
the least efficient in the system as the solid-air interface represents the greatest barrier for heat dissipation.
A heat sink lowers this barrier mainly by increasing the surface area that is in direct contact with the cool
air (Reference 4). This allows more heat to be dissipated and lowers the device operating temperature. The
primary purpose of a heat sink is to maintain the device temperature below the maximum allowable
temperature specified by the device manufacturer. In most applications, the heat from micro-electronic
components needs to be dissipated by natural convection and heat transfer relies solely on the free buoyant
flow of the air surrounding the heat sink. Highly conductive material alloy of copper or aluminum sheets
that are manufactured economically either as stampings or castings are commonly used for heat sink
applications.
In designing or selecting an appropriate heat sink that satisfies the required thermal and geometric criteria,
one needs to examine various design constraints imposed & the parameters available for a designer to
optimize the performance (Reference 5). Design constraints include cooling air velocity, available pressure
drop, required heat dissipation magnitude, maximum heat sink temperature, ambient air temperature,
maximum size of the heat sink, orientation with respect to gravity, appearance & cost. The optimization
parameters that a designer can explore include height, length & width dimensions, thickness,
spacing/looping, shape & profile and material. Using FEA technique the designer can evaluate these
options more effectively & gain an insight into the factors that limit and arrive at an optimal design
solution.
Procedure
OPU MODEL - Thermal Circuit
An OPU assembly model for optical recordings is shown in Figure 1. Figure 2 shows the exploded view
of the assembly constituting components with thermal conductivity values indicated appropriately. The
major components of the OPU assembly - plastics housing, laser diode, heat sink profile & the de-coupling
modulator were imported from a Pro-Engineer CAD assembly model through iges transfer to the Ansys
FEA program. Minor components - glue connects, silicon pad & heat compound interface & solder joints
were modeled in the FEA program.
Figure 1 - OPU components Assembly - FEA Model
Figure 2 - OPU components Exploded view - FEA Model
Attaching a heat sink to a semiconductor package requires that two solid surfaces be brought together into
intimate contact. Unfortunately, no matter how well-prepared, solid surfaces are never really flat or smooth
enough to permit intimate contact. All surfaces have a certain roughness due to microscopic hills and
valleys. As two such surfaces are brought together, only the hills of the surfaces come into physical contact.
The valleys are separated and form interstitial air-filled gaps. Since air is a poor conductor of heat, it should
be replaced by a more conductive material to increase the joint conductivity and thus improve heat flow
across the thermal interface. Several types of thermally conductive materials can be used to eliminate air
gaps from a thermal interface, including greases, reactive compounds, elastomers and pressure sensitive
adhesive films. All are designed to conform to surface irregularities; thereby eliminating air voids and
improving heat flow through the thermal interface.
Thermally conductive compounds are an improvement on thermal grease (paste containing conductive
ceramic fillers in silicone or hydrocarbon oils) as these compounds are converted to a cured rubber film
after application at the thermal interface. Initially, these compounds flow as freely as grease to eliminate
the air voids and reduce the thermal resistance of the interface. After the interface is formed, the
compounds cure with heat to a rubbery state and also develop secondary properties such as adhesion
(Reference 6).
Thermally conductive elastomers are silicone pads filled with conductive ceramic particles, often
reinforced with woven glass fiber or dielectric film for added strength. These elastomers are available in
thickness from about 0.1-5mm and hardness from 5 to 85 Shore A. These pads are normally used to close a
larger gap & when snapped in press-fit conditions more of the microscopic voids are filled by the elastomer
and reduce interface thermal resistance to a minimum.
The model shows the heat flow path from the laser diode to the 0.3mm thick copper alloy based heat sink.
Heat dissipated by the laser diode anchored within the plastics housing is conducted serially through the
interface compound & 0.15mm thick silicon pad. The heat dissipation from the modulator is partly
convected by their surfaces & partly transferred to the heat sink due to direct contact. The conducted heat
then spreads over the heat sink volume for convection towards a relatively cool ambient air by exterior
surfaces. Heat dissipation from the laser diode onto the housing is minimal due to the poor conductivity of
both the glue & plastic and due to the highly confined interstitial air gap.
Analysis
The 145mW maximum heat dissipation from the laser has been applied as surface heat flux over the
semiconductor chip base elements (red elements) inside the diode (Figure 3). The 85mW heat dissipation
from the modulator has been applied as a uniformly distributed flux over the outer surface external surface
elements (blue elements). Air cooling by convection is applied over all the exterior surfaces of the OPU
components except the heat flux surfaces of laser diode & modulator. This, because when both the heat flux
& convection are applied to the same surface elements, one boundary condition may supercede another
depending on the order in which they are applied. Figure 4 shows the convection applied surface elements
(red faced elements) attributed with convection parameters heat transfer coefficient & the ambient
temperature. The grey colored elements apparently seen in conjunction with other red elements correspond
to the exposed interior lateral surface elements (hidden by the heat sink) with the same convection
parameters applied to it.
Figure 3 - Heat Dissipation from Laser & Modulator (Surface Flux)
Figure 4 - Convection coefficient applied to air exposed surfaces
Estimating the heat transfer coefficient (h) of the convecting air is a difficult task to designers. The heat
transfer coefficient is affected by many parameters, which have been defined differently by various
investigators (Reference 7). For natural convection using a heat sink, determining the convection
coefficient h is influenced by parameters - temperature gradient, air velocity, profile geometry, flow
confinement. Since the OPU assembly is being evaluated for satisfactory performance at 60°C maximum
operating temperature similar to a natural convection oven, the airflow inside the chamber is largely due to
heat dissipation induced density variations & bouyancy effects. Hence, we shall consider the velocity of
flow to be slightly higher than zero velocity around the heat sink areas although the flow is very confined
around the laser diode & interface components encapsulated by the plastic housing. From the relationship
chart for air (Reference 8) between the heat transfer coefficient, temperature gradient & air velocity, the h
value for air in the current simulation is approximately 15 W/m2-°C for temperature difference of 10°C
between the heat sink and surrounding air at flow velocity slightly greater than 0 ms-1.
Steady-state heat transfer analyses were conducted on the OPU model with differing heat sink
configurations and convecting areas. These design options were evaluated in order to achieve case
temperature to lie around 70°C as required by the application specifications.
Analysis Results & Discussion
Figure 5 shows the temperature distribution on the initial heat sink configuration. The case temperature for
the 5cm2 profile area, 79°C-80°C is far higher than the desired 70°C. This inadequacy can be attributed to
insufficient convecting areas and to constrained profile layout of the heat sink by the housing geometric
construction. Due to the uneven profile, the heat spread emanating from the laser source is not transferred
by adequate convection and depends more on the initial conduction through the heat sink followed by
lateral convection. Heat dissipation is also affected by the high thermal gradient (3°C) across the interfacial
thermal compound and silicon pad.
Figure 5 - Temperature Distribution (Convection area of heat sink: 5 cm2)
An increased area (11.7cm2) profile model is shown in Figure 6. The profile has been looped so as to
enhance area available for convection & therefore more heat dissipation occurs. Another aspect is the
incomplete contact between the modulator and heat sink has been attempted in this model (Figure 7) to
check whether the heat sink can be fully utilized for convecting away more heat dissipation from the laser
diode. The temperature distribution plot shows that the modulator is not effectively cooled. The bye-passed
modulator has a maximum temperature of 82°C due to low convection & inherent poor thermal
conductivity. However, the model is quite effective in reducing the laser case temperature to 73°C-74°C.
Figure 6 - Temperature Distribution (Looped Heat sink/Modulator By-pass)
Figure 7 - Temperature Distribution (Convection area of heat sink: 11.7 cm2)
Another approach to improve the heat dissipation has been considered in the model shown in figure 8. As
the laser diode casement area is significantly smaller than the heat sink profile, there is an additional
thermal resistance, called the spreading resistance. This spread resistance could typically be 5-30% of the
total heat sink resistance and accounts for additional temperature rise caused by the smaller heat source
(Reference 9). To minimize this heat spread resistance from the laser casement unit; downward extrusion
of the heat sink profile towards the other end of the housing has been considered (Figure 8). This
downward profiling augments the total cross-section area to 14cm2. The temperature distribution plot
shows a more even heat spread & the case temperature 70°C-71°C lies close to the desired temperature.
The improvement could be attributed to the uniform heat spread in all directions. In addition, the reduced
thermal resistance in the heat sink & the extra area contributing additional convection aid the improvement.
Thermal gradient across the conductive path has been lowered to 1°C.
Figure 8 - Temperature Distribution (Convection area of heat sink: 14 cm2)
Inferences can be made from the effect of heat sink convection area on the effectiveness of heat dissipation.
Figure 9 shows the graph on the case temperature variation with convection area. It is seen that for a
convection area of 15 cm2 the case temperature can be lower than the laser device operating specification of
70°C while transferring 230 mW heat energy. Also shown in the graph is the variation of heat transfer
efficiency for the OPU model with heat sink convection area. The heat transfer efficiency is calculated as
the ratio between heat energy dissipated by the system to the input power based on the temperature
difference between the heat sink surface & the surrounding ambient air at 60°C. It is seen that with
increasing convection area the heat transfer efficiency is significantly improved. Transfer efficiency of
90% is achieved with 14 cm2, which is very high compared to that of 55% for an area of 5 cm2.
Figure 9 - Influence of Convection area on Case Temperature / Heat Transfer Efficiency
Conclusion
In this effort, an example of heat sink design for an OPU model has been illustrated to show utility of FEA
techniques for evaluating heat transfer requirements in cooling of electronic components. The analyses
indicate the possible solution for a moderately complex heat transfer problem. Increasing convection area
& altering profile layouts show marked improvements in heat energy dissipation. Designers can explore
differing profile configurations, material options, thickness variations; geometry options using finite
element approach to achieve desired optimal solutions. Using FEA technique the designer can evaluate
these options more effectively & gain an insight into the factors that limit and arrive at an optimal design
solution.
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