Thermal Considerations for the
Pentium® III processor at
550MHz Heatsinks & Airflow
in ATX chassis
May 1999
Order Number: 245184-001
Thermal Considerations for the Pentium® III processor
2
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Thermal Considerations for the Pentium® III processor
3
CONTENTS
1. INTRODUCTION.............................................................................4
1.1. Overview.......................................................................................................................4
1.2. Background...................................................................................................................4
2. EQUIPMENT UNDER TEST (EUT).................................................5
2.1. EUT Configuration......................................................................................................5
2.2. Documentation References..........................................................................................6
2.2.1. Thermal support documentation...............................................................................6
2.3. Processor setup.............................................................................................................6
2.4. Software utilities for stressing the Processor.............................................................6
3. PENTIUM® III 550MHZ PROCESSOR HEATSINK COOLING ......7
3.1. Setup..............................................................................................................................7
3.2. Equipment.....................................................................................................................7
3.3. EUT ...............................................................................................................................7
3.4. Method..........................................................................................................................8
3.5. Test results and Observations.....................................................................................9
3.5.1. Airflow tests.............................................................................................................9
3.5.2. Average Airflow for Chassis 1.................................................................................9
3.5.3. Average Airflow for Chassis 2...............................................................................10
3.5.4. Probe positions.......................................................................................................11
3.5.5. Airflow results........................................................................................................12
3.5.6. Heatsink Mechanics................................................................................................12
4. SUMMARY....................................................................................13
Thermal Considerations for the Pentium® III processor
4
1 INTRODUCTION
1.1 Overview
In various chassis the airflow will have different characteristics, these ch aracteristics could
have a possible effect on the performance of the Heatsinks used in the cooling of
Pentium® III 550MHz microprocessors. This document will look at the airflow
characteristics in two typical chassis and the effect this could have on the capacity of the
Single Edge Contact Cartridge 2 (S.E.C.C.2) Organic Land Grid Array (OLGA), Heatsink
to cool the processor. The type and specification of the heatsink and Thermal Interface
Material (TIM) used will also need to be matched to the chassis airflow combination. All
of the heatsink s tested were o f the active type, in th at a fan was fitted to the heatsink.
1.2 Background
As the power of motherboards and hardware increases, the requirement for the cooling
capacity of any given system is increasing. The demand for quieter systems and stringent
EMC regulations is causing an overall reduction in the capabilities of chassis to cope with
this increase in thermal output.
The main source of cooling in a typical ATX system is the power supply unit (PSU). The
PSU noise output has been reduced to meet noise emission requirements. The fan is the
main contributor to this noise and the most effective way of reducing fan noise is to slow
down the rotation of the fan blades, this has the effect of reducing airflow, so the PSU’s
ability to cool the ATX chassis has decreased.
Along with this reduction in PSU cooling power, the chassis have had to be designed to
meet EMC emission regulations. The criteria for meeting EMC regulations is that the
chassis should ideally be, fully enclosed with no gaps. Most chassis are not built fully
enclosed but depending on the position and size of the chassis vents, this again can reduce
the thermal management of the chassis with reduced airflow through the system.
The Pentium® III 550MHz microprocessor operates with a lower junction (Tjunction)
temperature than the previous 500MHz processors, the power density for OLGA packages
is higher, due to the smaller die size, than the previous generation of Pentium® II
processors, to meet this requirement extra cooling is required. Either the heatsink or the
chassis must supply this cooling. This document will look at the requirements of the
heatsink and airflow in 2 typical ATX chassis.
Thermal Considerations for the Pentium® III processor
5
2 EQUIPMENT UNDER TEST (EUT)
2.1 EUT Configuration
representative ATX chassis were used for the testing of the Pentium® III 550MHz microprocessor and
heatsink combinations configured as per Error! Reference source not found..
Supplier Description Model/Part Number Locatio n
Chassis 1 ATX Mini Tower N/A N/A
Chassis 2 ATX Mini Tower N/A N/A
Seventeam* (Chassis 1) ATX PSU ST-251HR Top Rear Chassis
Seasonic (Chassis 2) ATX PSU SS-235PS Top Rear Chassis
Intel (IQL2794) SE440BX-2 Motherboard 720938-208 N/A
Intel Pentium® III Processor 80525PY550512 SL242
Memory Card
Technology*64MB 100MHz DIMM PCSDRAM 374S823BTS N/A
Teac*Floppy Drive FD-235HF Top 3.5” Bay
Seagate*H/D 9Gb 1” IDE ST39140A Bottom 3.5” Bay
Creative Labs*x2 DVD ROM DVD2240E Top 5.25” Bay
Intel i740 8MB AGP Card N/A AGP Slot
Diamond*3DII Graphics Card 23150105-005 PCI Slot 3
Full Bios Revision 4S4EB2X0.86A.0017.P10
Socket Type
SL242
Add-in Card Slot Types PCI, ISA, AGP
EUT Memory 3 X 64MB Dimm
Table 0-1 System configuration
Thermal Considerations for the Pentium® III processor
6
2.2 Documentation References
2.2.1 Thermal support documentation
Supplier Reference.
Intel Application note, AP-586 Pentium® II Processor thermal design guidelines. June 1997
Pentium® II Processor at 233, 266, 300 & 333MHz. June 1997
Pentium® II Processor at 350, 400 & 450MHz. August 1998
Pentium® II Processor specification update February 1999.
Intel® Pentium® III Processor at 450, 500 and 550MHz. May 1999
P/N 244452-002
Intel® Pentium® III Processor specification update May 1999. P/N 244453-003
Table 0-2 Support documentation.
2.3 Processor setup
Processor tested at 550/100MHz CPU/FSB speed.
No secondary fans were fitted in these chassis.
A standard fan was fitted in the PSU’s. The PSU fans were extracting air fro m the Chassis.
2.4 Software utilities for stressing the Processor
Hi power stress software was utilized for these tests. The software was de signed to run the
processor core and the BSRAM L2 cache near to their respective maximum achievable
power levels.
Thermal Considerations for the Pentium® III processor
7
3 PENTIUM® III 550MHZ PROCESSOR HEATSINK COOLING
3.1 Setup
Thermocouples or the Maxim 1617 are attached to the specified components (see section
3.4) and the EUT is placed in a Thermal Chamber. During all thermal test runs thermal
grease (Thermalcote II, Thermalloy Inc.) or a specified thermal interface material (TIM)
was present between the EUT and it’s Heatsink.
3.2 Equipment
The accuracy of the type “K” thermocouples used during this testing is +2.5/-0
C.
The accuracy of the Maxim 1617/Thermal diode is +/-3.0
C. With an off set of 4 . 8
C.
Supplier Description Model/Part Number Serial Number
Thermotron Thermal Chamber (walk in) WP-499-THCM2-705 23065
Thermotron Thermal Chamber S-8SLE 24207
Cambridge Accusense Airflow monitor ATM-24
Cambridge Accusense Airflow probe CAFS-220-5M
Testo Testo air volume flow tunnel N/A
Testo Testo digital anemometer. 0560.4900
Testo Testo probe. 0635.1549
Maxim Thermal diode monitor MAX1617EV
Table 0-3 Thermal equipment.
3.3 EUT
See section 2
Thermal Considerations for the Pentium® III processor
8
3.4 Method
Measurements were taken directly from the Tplate/Tcase/Tjunc or chassis of the EUT. The Tjunc
measurement is made via the thermal diode in the processor core and the Maxim 1617-evaluation
kit, the Tplate/Tcase measurements are made with type “K” thermocouples. The EUT was tested in a
thermal chamber for 2 hours at a temperature of 35°C @ 35% Humidity, or until the EUT has
reached thermal equilibrium.
KEY:
Tplate = Temperature measured at the point of contact between the metal plate on the processor and
the heatsink attached.
Tcase = Temperature measured at the point of contact between the case of the processor core or the
case of the component under test and any heatsink attached to the component.
Tjunc = Temperatur e m easured by a d io d e b uilt into the pr ocessor silicon.
Thermal Considerations for the Pentium® III processor
9
3.5 Test results and Observations
3.5.1 Airflow tests
All airflow measurements are in Linear Feet / Minute (LFM). Please refer to Section 3.5.4 for
probe placement.
3.5.2 Average Airflow for Chassis 1
0
20
40
60
80
100
120
060 120 180 240 300 360 420 480 540
Time (Seconds)
Airflow (LFM)
Series1
Series2
Air Flow Probe Position in Chassis. Av. LFM
Series 1 (Probe 1) Next to the PSU. 79.18
Series 2 (Probe 2) On the nearest Dimm to the processor 5mm above the Dimm facing the processor. 92.82
Thermal Considerations for the Pentium® III processor
10
3.5.3 Average Airflow for Chassis 2
0
20
40
60
80
100
120
140
0 60 120 180 240 300 360 420 480 540
Tim e
(
Seconds
)
Airflow (LFM)
Series1
Series2
Air Flow Probe Position in Chassis. Av. LFM
Series 1 (Probe 1) Next to the PSU. 115.46
Series 2 (Probe 2) On the nearest Dimm to th e processor 5mm above the Dimm facing the processor. 103.76
Thermal Considerations for the Pentium® III processor
11
3.5.4 Probe positions
Figure 0-1 Chassis 1
Tintake
Probe 1
Probe 2
Figure 0-2 Chassis 2
Probe 1 = series 1
Probe 2 = series 2
NOTE.
The airflow measured by the probes below 30 LFM can be discounted as the accuracy of
the probes is not guaranteed.
T
i
ntake
Thermal Considerations for the Pentium® III processor
12
3.5.5 Airflow results
From the diagrams shown in the Airflow tests Section 3.5.1 it can be seen that the Average
Airflow for Chassis 1 is 79 LFM for probe 1 and 92 LFM for probe 2, whereas the Average
Airflow for Chassis 2 is 115 LFM and 103 LFM respectively. These averages are based on the
airflow for all 5 of the heatsinks tested. This effectively highlights the effect of using the same
heatsink in differing chassis. Both of the chassis had a similar internal layout but as shown in
Table 2-1 System configuration, they used different PSU’s.
For all heatsinks tested chassis 2 produced the lower Tjunction readings this reflects the higher
average airflow through this chassis.
Average Tjunction for all 5 Heatsinks
Chassis 1
88ºC
Chassis 2
79ºC
The shape and design of the heatsink will have a marked effect on the airflow pattern seen. For
example in chassis 1, one heatsink design, horizontal finned with ducting, gave 140 LFM for
probe 1 and 84 LFM for probe 2. Where a vertical finned under the fan with 45º fin’s either side,
heatsink gave 21 LFM and 124 LFM. Similar results were seen for chassis 2.
3.5.6 Heatsink Mechanics
The design of the heatsink will play an important part in the overall thermal solution of the
system. There are 2 basic heatsink shapes used for the Pentium® III Processor, Horizontal finned
and Vertical finned. Also there are various thermal interface materials (TIM) used by the different
heatsink manufacturers.
In separate tests it was found that for low profile systems (micro (µ) NLX etc.) the horizontal finned
heatsink out performs the vertical. For ATX chassis as used for these tests neither horizontal of
vertical finned heatsinks showed any discernable advantage over the other. The main difference
noted was the interaction of the heatsink fan exhaust with the chassis airflow and the effect on the
processor Tjunction temperature. See Section 3.5.5.
As an extreme example of the problems caused by using a poorly specified heatsink with very low
thermal conductance TIM, a test was carried out in chassis 1. The active heatsink used was a
Pentium® II heatsink designed to fit on the Tplate of the Pentium® II processor. An interposer was
designed to adapt this heatsink to the Pentium® III OLGA package. When assembled the processor
core was touching the TIM of the interposer the interposer was touching the TIM of the active part
of the heatsink.
From cold it only required 34 seconds for the processor Tjunction temperature to reach 127ºC, the
limit of the Maxim test equipment in use. The Tjunction temperature maximum specification is 80ºC.
This heatsink is a real, currently available heatsink.
The type of TIM used by the heatsink manufacturer is a very important part of the heatsink
specification, in the tests carried out a difference of 15ºC was noted on one heatsink, if the pressure
Thermal Considerations for the Pentium® III processor
13
of the retention mechanism was increased. For this processor the pressure of the TIM to the
processor core was insufficient for maximum heat transfer. In general the graphite type materials
require a higher clamping force than th e white tape materials. Caution should be used here that the
clamping force does not exceed the specification for the Pentium® III Processor. In a separate test
using chassis 1, a difference of 6ºC in Tjunction temperature was noted for the same heatsink using
TIM from different manufactures.
The clip arrangement for the heatsink should be matched to the motherboard the heatsink will be
used with, as the retention mechanism for some heatsinks could foul components behind the SL242
processo r slot.
4 SUMMARY
The choice of heatsink should be matched to the chassis. If using a µNLX or similar type chassis then
horizontal finned heatsinks are more suitable. For larger chassis then either horizontal of vertical heatsinks
could be used.
For all chassis the type of heatsink and TIM used is crucial to the overall performance of the thermal
solution. The fan exhaust from the heatsink will interact with th e chassis airflow or visa v ersa. The result of
this interaction could be to the benefit of the cooling solution but equally it could do the opposite and
degrade the solution.
The only certain way to match a heatsink to the chassis is to carry out experiments with a range of heatsinks
either from the same manufacturer or from different manufacturers. Alternatively if only 1 heatsink solution
is available then test this so lution with a range of different TIM’s.
Care should be taken when choosing the heatsink to ensure the retention mechanism will not make contact
with any components around the processor and that the clamping force exerted by the mechanism will not
exceed the maximum allowable for the processor core.
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