International Rectifier - DirectFET Product Information

DirectFET® MOSFET: Thermal Interface Materials (TIMs)

There has been much focus on optimizing TIMs for CPUs, conversely little optimization is typically performed on TIMs in VRM/VRD applications (“A” TIM is used, not necessarily the “Best” TIM).
In 1994 the TIM resistance accounted for only 5% of the total CPU thermal resistance for the 66MHz Pentium (Advanced Packaging).
In 2000, the TIM accounted for 50% of the CPU thermal resistance for the Athlon processor.
Similarly, as power levels and the associated power densities continue to increase in VRM/VRD’s, selecting the optimum TIM is of equal importance to selecting the optimum heat sink.
There are two key factors that impact the performance of TIM’s:
- 1.Bulk thermal conductivity of the material
- 2.Surface wetting/conforming characteristics of the material

* Be aware of vendors data on H’s and R’s – these are typically application specific results which will probably vary greatly from your application

Function of TIMs

The goal of a TIM is to fill in all of the voids between the faying surfaces.

With TIM, 100% contact

No TIM, significant percentage of air gaps

Conformability vs Thermal Conductivity

As can be seen in the graph below, the Gap pad A3000 has the best thermal characteristics and the highest hardness (lowest conformability)

Thermal Interface Materials - Classifications

Material Class	Description	Benefits	Drawbacks	Bulk K	Surface Wetting	Surface Conforming	Vendors
Grease	These are the traditional TIMs filled with conductive particles of Al₂O₃, BeO, Al, Ag, etc.	Can achieve very thin bondlines <0.0005"	Difficult to apply to pre-apply to assemblies Messy Typically pumps out, effecting long-term reliability Requires controlled dispensing No electrical isolation	0.3-2W/mK Al Filled 6+	Very Good	Very Good	Hi-K Grease Shinetsu Bergquist
Gel	Grease replacement that cross-links during cure to form a gel-like substance.	Can achieve very thin bondlines <0.005” Does not pump out	Cannot be preapplied to assemblies Requires cure which can be from burn-in Messy Requires controlled dispensing	0.3-2W/mK	Very Good	Very Good	Thermoset-Lord MG Series
Adhesive	Heat cured adhesives filled with conductive particles similar to grease.	Can achieve very thin bondlines <0.005” Mechanical and thermal attach	Cannot be preapplied Typically requires process oven curing Messy Requires controlled dispensing Typically no electrical isolation	0.3-1.3W/mK	Very Good	Very Good	Dow Corning 3M
Tape	Usually pressure sensitive adhesive filled with conductive particles on a fiberglass or plastic carrier.	Mechanical and thermal attach Can be die cut and preapplied Clean, simple processing Typically electrical isolating	Typically thick bondlines with low thermal conductivity	0.7-1.5W/mK	Moderate	Poor	Bergquist 3M Dow Corning
Phase Change	A waxy type material that changes to a gel at approximately 50 C allowing it to conform to surface irregularities. Can be preapplied or supplied on a carrier.	Can achieve very thin bondlines <0.005” when preapplied Fairly clean process With carrier has electrical isolation	Typically low thermal conductivity The phase change material itself is usually very thin and does not conform to large irregularities	0.8-1.5W/mK	Very Good	Very Good for irregularities <0.002" Very Poor for large irregularities or bowing	Bergquist 3M Dow Corning
Pads	Typically thick materials 0.010”-0.250” thick. Some are very compliant with a low K and others are not very compliant with a reasonably high K.	Simple to use Can typically be re-used Can be die cut and preapplied Clean process	Typically requires moderate to high pressures to achieve reasonable performance - can be tricky to use effectively Typically do not conform well to small surface irregularities Thick bondlines	0.8-4W/mK	Poor	Very Good for large irregularities Very Poor for small irregularities	Bergquist 3M Dow Corning

Common Thermal Interface Materials Cost Trades

Mfr.	Material	Type	Thickness (mils)	Pressure Required	Dispense/ Apply	Clips/ Screws	Can be applied to heat sink?	Mechanical Placement?
Dow Corning	1-4173	1 part heat-cured adhesive	#	Yes	Dispense	Clips	N	Y
Dow Corning	SE 4451	2 part heat-cured adhesive	#	Yes	Dispense	Clips	N	Y
Dow Corning	3-6652	2 part heat-cured adhesive	#	Yes	Dispense	Clips	N	Y
Dow Corning	TP-1500 Pad	Tacky - Phase Change at 52°C	10	>5psi, 20psi typ.	Apply	Clips	Y	Y
Bergquist	Gap Pad 3000	conformable filled polymer sheet	15	>10psi	Apply	Clips/ Screws	N	Y
Bergquist	Gap Pad 2000	conformable filled polymer sheet	10	>10psi	Apply	Clips/ Screws	N	Y
Bergquist	Hi Flow 300	Phase Change at 55°C	2.4	>10psi	Apply	Clips	Y	*
Bergquist	Hi Flow 625	Phase Change at 65°C	5	>10psi	Apply	Clips	Y	*
Bergquist	Hi Flow 818	Phase Change at 65°C	5.5	>10psi	Apply	Clips	Y	*
Bergquist	Sil Pad 800	Conformable silicone elastomer	5	>10 higher better	Apply	Clips/ Screws	Y	*
Bergquist	Sil Pad 900	Conformable silicone elastomer	9	>10 higher better	Apply	Clips/ Screws	Y	*
Bergquist	Sil Pad A1500	Conformable silicone elastomer	10	10-50psi	Apply	Clips/ Screws	Y	*
Bergquist	Sil Pad A2000	Conformable silicone elastomer	10	10-50psi	Apply	Clips/ Screws	Y	*
Bergquist	Bond Ply 100	Pressure sensitive adhesive tape	5	>10psi	Apply	Clips	Y	Y
Bergquist	Bond Ply 100	Pressure sensitive adhesive tape	11	>10psi	Apply	Clips	Y	Y
Thermoset (Lord)	Gelease MG-120	Thermal grease/gel	#	5-7lbf	Dispense	Clips	N	Y
Shin Etsu	X-23-7783D	Thermal grease	##	-	Dispense	Clips	N	Y
Shin Etsu	X-23-7762	Thermal grease	##	-	Dispense	Clips	N	Y

(1) contains 5mil beads/ filler - Electrical isolation is dependent on application
* Require adhesive for pick and place +10% cost adder
# Assumed 30mil high dispensed area with 8x 25mil high medium-can directfets included
## Assume 6mil bond line on 8x medium-cap directfets only

Selecting the optimum material for a VRM/VRD application involves several factors:

Electrical isolation (DirectFET MOSFETs, and flange mounted D-PAKs, etc.)
Attachment to a PCB (back-side cooling VRM’s) - very difficult to achieve a high required pressure without deforming the PCB.
Cost vs. performance, $0.04 to $0.53 for a 4-Phase DirectFET VRM 1U, but how to predict performance?

TIM Case Study #1 - Pad vs. Adhesive

Proper selection of the optimal “class” of TIM enabled a significant improvement in performance The higher performing TIM was an adhesive with a lower thermal conductivity than the competing interface pad
Performance improvement gain was based on area of contact and thickness


	"Play Audio

TIM Case Study #2 - Optimized Pad

Testing of interface materials in the application is critical

Initial first order material selection can be based on the data sheet, however to optimize performance, empirical testing must be performed

Vin = 12V, Vout=1.35V
4 phase, 500KHz
6604+6618 per phase
VRM10 heat sink
Imax based on Tcase=100 °C

"Play Audio

Case Study #3 - Incorrect Material

Customer implemented a thin, high thermal conductivity pad because it had the best “metrics”
In the application it only makes contact with <20% of the area – resulting in non-repeatable poor performance
When the screws are tightened, the board flexes and comes out of contact with the PCB
Should use thicker and softer pad or a conformable material like Gel

IR’s proprietary DirectFET® technology is covered by US Patent 6,624,522 and other US and foreign pending patent applications.

Discovery Center
Selector Guide
Thermal Calculations

DirectFET MOSFET Overview
Applications
Engineering
FAQs
Design Support

Introduction
High Frequency Operation
Increasing Power Density
Reworking Process
Board Layout
Heatsinking
Mechanical Features
Board Mount Guidelines
Reliability

Heatsinking Fundamentals
Select the Correct Heatsink
Heatsinking Fasten
Miniature Heatsinks
Fin Performance Analysis
TIMs


International Sites: \| Chinese 简体中文 \| Korean 한국어 \| Japanese 日本語 \|		About International Rectifier \| Contact Us \| Privacy
©1995-2008 International Rectifier