Surface Mount Technology cooling
for high volumes applications
by: Cesare Capriz
Aavid Thermalloy – via XXV Aprile 32 – Cadriano (BO) ITALY
Abstract:
The automotive technology is fast moving in
integrating electronic controls in a wide range of
components, like braking system, steering system,
asset controls and so on.
For this reason the controls are more complex and
requires more power for operating, so dissipating
more power.
1.2 Heat transfer for SMT devices
The power dissipated in the chip junction is
transferred to the ambient directly from the case to
the air through convection and through radiation.
But most of if flows through the leads into the board
and from the board is dissipated into the ambient.
x
For the high volumes productions in automotive, the
PCBoards mostly use the Surface Mount
Technology and fast moving pick-and-place
machines.
Qca
Qra
T, ambient
Qjb
DIE, Tj
Due to the high ambient temperatures that we can
have in the engine area and even in the dashboard,
the task of the Thermal Engineers is more complex
from the thermal and from the reliability point of view.
The heatsinks are one of the most delicate and
difficult point in electronic design.
The paper will present a system to improve the
performances of SMT heatsinks, using solderable
aluminium, light and efficient heatsinks.
Keywords:
automation
.
SMT,
Surface
Mount,
Qba
HEAT DISSIPATED
BOARD
Qba
Figure 1: Heat paths for a standard chip
1.3 Thermal resistances of PCBs
heatsink,
1. SMT cooling
1.1 Heat transfer
In very small size heatsinks conduction is normally
not an issue due to the very short heat path.
The critical point is the small heat exchange surface
that can not be enhanced by multiple fin layers, due
to the volume limitations. So the heat dissipation
through convection is very limited.
By comparison the heat transfer through radiation is
relevant because almost all the heatsink surfaces
are also "visible surfaces" and there is no de-rating
from hidden surfaces.
For the above mentioned reasons it is important to
have the best possible radiating surfaces, with a
radiation coefficient as close as possible to the Black
Body coefficient.
With a standard PCB and a component soldered to
the recommended pad layout, the Thermal
Resistance junction to air is around 50 °C/W.
If the PCB has the ground layers, the Rth junction to
air will decrease to around 20 °C/W.
Adding the ground layers does not give a relevant
improvement, since they are parallel to the substrate
If the PCB has also the via holes, the Rth will further
decrease to around 15 °C/W.
All the above values are referred to a board that is
cooled by a normal good ventilation.
If we consider the thermal resistance for the heat to
flow till the bottom of the PCB we have the following
normal values:
While the addition of via holes is very useful because
they cross the substrate just below the heat source.
1.3 The heatsinks directly in contact with the
component
For a
standard PCB 1.5 mm thick, the Rth from the
Junction to the bottom of the substrate is around 20
°C/W
The heatsink can touch directly the metal slug to get
the best thermal results. But in this cases the
pressure you need to give to guarantee a good
thermal contact or the weight of the heatsink if this is
attached to the component, will easily damage or
break the thin leads of the component itself.
If we have a 0.8 mm thick PCB, the Rth will
decrease to 12 °C/W
The slug can be downward. In this case the heatsink
is under the PCB and touches the component
through a hole on the PCB. The heatsink can be
fairly big and often it is the support for the board.
The thermal resistance Junction to Ambient is
normally lower than 10 °C/W. The machining of the
PCB and of the heatsink to guarantee a good
thermal contact amongst heatsink and slug, make
this solution quite expensive and not suitable for high
volumes applications.
The existing heatsinks for SMT applications are
made in copper and tin plated to guarantee the best
solder conditions.
The thermal conductivity is good but for this small
size dimensions the advantage is minimal, while the
tin plating surface, a light bright grey colour, has a
very low radiation coefficient, around 0.05, that is
only 5% of the maximum Black Body radiation.
If the slug is upward, the heatsink can be glued or
clipped to the component. Since the heatsink weight
is supported by the component's lead, the heatsink
must be very light to avoid high stress under shock
and vibration conditions. According the heatsink
dimensions, the Rth j-a can range from 15 to 25
°C/W
Both the above solutions are not suitable for high
volumes automatic assembly and can not be
managed by pick-and-place machines.
Due to the fast expansion of power controls in every
electric applications, the SMT technology is also
expanding and is looking for higher heat dissipation.
Typical devices are for example PowerSO20
(Jedec Registration MO-166) and PowerSO36
packages from ST Microelectronics.
A further handicap is given by the weight, more than
3 times the equivalent heatsink in aluminium, and
the material cost.
The high weight comports both a weight increase in
the final equipment and a reduced speed for the
pick-and-place assembling machines.
2. The inventive step
2.1 Description
The best solution from the thermal point of view is to
use aluminium black anodised for the heatsink body.
The increase of thermal resistance due to the lower
aluminium conductivity is minimal and around 2-3 %
of the total thermal resistance.
They are expected to dissipate very high wattages
compared to package size; the possible range of
applications may exceed 20W in the future but the
current requirements must be limited to approx. 5-6
W dissipation because the current D2 Pak SMT
heatsinks can not give a thermal performance better
than 16 °C/W
1.4 Current SMT heatsinks
In the SMT heatsinks the heat generated from the
device is conducted down into the PCB through the
component's lead and spreads from the Cu drain
pad.
From the drain pads the heat is conducted into the
sink that dissipates the heat into the ambient.
The improvement of the radiation coefficient is
impressive, reaching 0.90, that is 90% of the
maximum Black Body radiation
The problem we have to solve is that black anodised
aluminium can not be soldered to the PCB.
The inventive step has been to combine the
technology expertise in automatically assembling the
tin plated solderable tags (X-feed) with the
technology of manufacturing pre-black anodised or
pre-black painted aluminium coils
The X-feed technology has been tested under the
most severe shock and vibration conditions for
automotive and traction applications and used for
more than 10 years in high volumes mass
production.
2.2 Advantages
-
-
-
and adapting it to the peculiar SMT heatsinks.
-
The solderable tin plated tags have been
transformed in "Skis" that are fixed mechanically to
the heatsink body by self-riveting or staking.
-
Material cost reduction from copper to aluminium,
including PCB design reducing the needs forvia
holes and ground layers
Increased thermal performance, thanks to the
improved radiation of black finish
Material weight reduction, that aids fast feed of
pick & place machines, aids the vacuum pick up
and so reducing the manufacturing operations
time
Fast and low cost single press stroke to
manufacture each heatsink
Improves Infra-red reflow due to the black finish
that improve the heat absorbing, getting a better
cycle and quality
Readily recyclable heatsink, by virtue of
mechanical assembly
Possible customisation of overall size and
footprint design
Available in volume also packed in Tape & Reel
format
There are various design possibilities:
Base ski - stable & best conduction
Edge ski - limited space
L ski - stable, sink space limited
3. Performance comparison
3.1 Simulation and Test Description
We have run the CFD predictions in 3 basic
environments:
- Vented Plastic Enclosure; vents modelled as 75%
open area (like a grill)
- Sealed Aluminium Box; 20*80*80mm (height is
above PCB, applies to both)
- No Enclosure; Open Domain 20*100*100mm
PCB properties:
2
Copper drain pad = 0.1*13.7*21.2 mm on 80 mm
PCB, horizontally oriented
Dissipating approx. 0.7W @ 30 °C above ambient
with enclosures, in thermal contact with the box
Modelling
2 ΔT levels with same max case
temperature of 115 °C:
- Typical electronics environment, 35 °C ambient,
80 °C ΔT
- Typical automotive environment, 85 °C ambient,
30 °C ΔT
4. Conclusion
The heatsink family called "Slalom" is worldwide
Patented by Aavid Thermalloy.
The "Slalom" aluminium heatsink performs at least
20% better in both CFD thermal simulations and in
Laboratory tests, compared to a traditional copper
heatsink tin plated, with a lower weight and a lower
cost.
At the page bottom there are the results of the
thermal simulations as above described.
5. Glossary
The actual performance is dependant on real
application, however from the CFD analysis:
- The heatsink operates best at higher ΔT rise
above ambient temperatures, because there will
be higher radiation heat loss from black surface
- The optimum environment is a reasonable open
domain around the sink or a vented box; not a
close sealed enclosure
SMT: Surface Mount Technology
PCB: Printed Circuit Board
Qjc: Heat transferred from the Junction to the Case
Qjb: Heat transferred from the Junction to the PCBoard
Qca: Heat dissipated through Convection to the Ambient
Qra: Heat dissipated through Radiation to the Ambient
Qba: Heat dissipated from the Board to the Ambient
X-feed: Technology for automatically assembling small
components on a heatsink, feeding them from a coil
perpendicular to the main body coil
CFD: Computational FluoDynamic