9882, 9885, 9890 Tech Bulletin

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April, 1999
3
Technical Bulletin
System Design and Performance for 3MTM
Thermally Conductive Tapes 9882, 9885, 9890
Notes on System Design
and Performance
This publication is intended to aid the user in optimizing the design and performance
of systems using 3Mª Thermally Conductive Tapes 9882, 9885, and 9890.
Note: this information presented should be considered representative or typical and
should not be used for specification purposes. The user is responsible for evaluating
the tape under actual conditions of use and with the substrates intended for the userÕs
application, to determine whether the tape is suitable for a particular use and method
of application.
The information of this bulletin is organized in three sections:
I. Thermal and Mechanical Performance Optimization
¥ Thermal impedance of the assembly
¥ Mechanical performance
¥ Minimizing entrapped air
II. Performance at High and Low Temperatures
¥ Short term exposure
¥ Long term exposure
¥ Limiting factors
¥ Heat and humidity
¥ Temperature cycling
III. Bond Building / Rework
¥ The bond building effect
¥ Bond building on ceramic
¥ Bond building on anodized aluminum
¥ Rework procedures
Additional information on these topics or others not covered may be requested by
calling the toll free number on the last page.
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Thermal Impedance of the Assemblv Adhesive Contact Area
Technical Bulletin
System Design and Performance for 3MTM Thermally Conductive Tapes 9882, 9885, 9890
I. Thermal/Mechanical
Performance
Optimization
Optimized thermal and mechanical performance of systems assembled with 3Mª
Thermally Conductive Adhesive Transfer Tapes 9882, 9885 and 9890 depends,
among other things, on an balance of the material properties of the adhesive and the
substrates and the use of assembly conditions appropriate to the parts.
Thermal Impedance of the Assembly
In an ideal case, substrates are perfectly flat and the bond is made without air
entrapment:
Adhesive
Ideal, Flat Substrates
Adhesive
Non-Flat Substrates
In this case, thermal impedance of the bond area, Rtot (¡C/W), would be equal to the
adhesive thermal resistance (¡C/in2/W) divided by the bond area, Atot (in2). (Thermal
resistance values typical of the 3Mª Thermally Conductive Tapes can be found in
the Data Page.) To optimize thermal performance one would simply use the thinnest
adhesive available.
Substrates often will have several thousands of an inch (mils) runout of flatness of
their surfaces and using the thinnest adhesive may lead to air gaps in the bond line of
the assembly as represented in the edge-on view below. When both substrates are
rigid materials this effect is especially likely to occur.
Now the adhesive no longer covers the entire area, Atot, rather it covers a contact area
smaller than Atot and air fills the rest of the area. Calculation of total thermal
impedance Rtot in the case of parallel adhesive and air thermal impedances is much
more complicated, but suffice to say the thermal impedance of the assembly is higher
because of air pockets. (Please see the 3M Technical Bulletin ÒHeat Flow Calculation
for 3M Thermally Conductive Tapes [9882, 9885, 9890]Ó for a method to determine
the total thermal impedance.) Choice of an adhesive layer much thicker than the
flatness runout can help reduce the air, as portrayed the bond area cross-sections
Bond Area Using 2 mil Adhesive
(30% Contact, 70% Entrapped Air) Bond Area Using 10 mil Adhesive
(85% Contact, 15% Entrapped Air)
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Mechanical Performance Minimizing Entragperl Air High Temperatures g> 85°C Short Term Minutes
Technical Bulletin
System Design and Performance for 3MTM Thermally Conductive Tapes 9882, 9885, 9890
I. Thermal/Mechanical
Performance
Optimization
(continued)
Depending on the thickness of the air layer, the reduction of entrapped air by using a
thicker adhesive may be enough to compensate for the increased thermal resistance
of the thicker adhesive layer. Optimized thermal performance may be found in a
trade-off between entrapped air and adhesive thickness. As a rule of thumb, the
flatness runout of both parts should be one half the thickness of the adhesive layer or
less.
Mechanical Performance
Entrapped air is detrimental to mechanical performance as well as thermal
performance; shear strength, for example, is proportional to the actual adhesive
contact area. Maximizing the contact area of the bond by minimizing entrapped air
can be beneficial for mechanical reliability during assembly as well as long term
reliability of the finished product.
In addition, incomplete contact over the bond area can affect the long term reliability
of the adhesive at high temperatures (e.g. ³ 125¡C). As discussed in Section II, an air
gap that extends to the edge of the parts, as pictured above left, will facilitate
oxidation of the adhesive at high temperatures and reduce the long term reliability of
the assembly.
Minimizing Entrapped Air
One way to determine if parts or assembly methods are likely to entrap air is to bond
the substrates individually to a glass plate with the adhesive, and then inspect the bond
area through the opposite side of the glass. Methods to reduce entrapped air include A)
cleaning parts to remove particulate contamination, B) choosing an adhesive with
thickness significantly greater than the flatness runout of the parts, C) using sufficient
assembly force to compress the adhesive and fill the gaps, D) applying force
sequentially to corners or edges rather than just to the center of the part, and E) heating
the adhesive and parts during bonding, which softens the adhesive and facilitates
II. High/Low
Temperature
Performance
Some performance considerations for use of the 3Mª Thermally Conductive Tapes
9882, 9885, and 9890 under extremes of high and low temperatures are presented
below. Data pertain to temperature extremes seen by the bonded assembly, rather
than a roll or face of the adhesive. At any temperature the bond strength is dependent
on the area bonded, which in turn requires flat, clean surfaces and assembly
conditions (temperatures, pressures, dwell times) optimized by the user to the parts
and equipment. Thermal properties have not been directly measured as a function of
environmental condition, though it is believed that wetting of the surfaces by the
adhesive and strong adhesion levels are conditions that contribute to stable thermal
properties.
High Temperatures (> 85¡C)
Short Term (Minutes)
Testing has shown excellent stability of the product in assemblies subject to
temperatures up to 260¡C (such as solder - reflow process) for several minutes.
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Adhesive Contact Area
Technical Bulletin
System Design and Performance for 3MTM Thermally Conductive Tapes 9882, 9885, 9890
II. High/Low
Temperature
Performance
(continued)
Long Term (Days - Weeks)
The limiting factor for long term performance at high temperatures (e.g. 125¡C) is an
oxidation phenomenon which affects those portions of the adhesive having A) exposure
to air (oxygen) and B) as route for the reaction products to escape. The process is
diffusion limited and therefore the oxidation affects a small portion of adhesive around
the perimeter of the bond line. Small parts or poorly bonded parts having air gaps
extending to the edge of the bond area (see cross section below, left) are more
susceptible to oxidation-related failure than well bonded assemblies (below, right).
Choice of adhesive thickness well exceeding the runout of flatness on the part
(especially rigid substrates), coupled with optimal assembly methods can help reduce
air gaps (as discussed in Section I) and increase the high temperature reliability.
The problem of gap filling or air entrapment in pairs of rigid parts assembled with the
adhesive also makes these parts more susceptible to the oxidation phenomenon.
Flexible materials having fully contacted bond areas are less susceptible to reduced
adhesion from the oxidation. the graph below displays data (lbs./in. peel adhesion vs.
time at condition) from peel tests with one-inch wide anodized aluminum foil bonded
with the adhesive to ceramic substrates. These samples showed no net peel adhesion
loss despite a few millimeters of oxidation around the perimeter of the bond line.
6 wk
3 wk1 wk
3 d
1 d
16
14
12
10
8
6
4
2
0
Peel Adhesion, 150°C Aging Condition
lbs. / in.
9890 Tape
9885 Tape
9882 Tape
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Heat and flu g Low Temperatures < 20°c="" temperature="" cycling="">
Technical Bulletin
System Design and Performance for 3MTM Thermally Conductive Tapes 9882, 9885, 9890
II. High/Low
Temperature
Performance
(continued)
Heat and Humidity
The adhesive absorbs very little moisture (< 0.2% by weight), therefore the presence
of humidity does not in itself lead to loss of adhesion. However, metal substrates
prone to formation of loosely bound oxides may suffer accelerated mechanical failure
when in contact with Tapes 9882, 9885, and 9890 Tapes under these conditions.
Examples of such metals include copper, iron, and in some cases, untreated aluminum.
Therefore it is advised that such metals be passivated with appropriate coatings or
treatments before applying the adhesive.
Low Temperatures (² 20¡C)
Temperatures below the glass transition temperature, Tg (-20¡C to 30¡C), cause the
adhesive to become a glassy, rigid material, compared to its rubbery state above Tg.
Low temperatures do not in themselves harm the adhesive or the bond strength.
However, below Tg the bonded parts are more prone to delamination from applied
stresses such as high frequency shock, or from internal stresses such as those due to
mismatched coefficients of thermal expansion of the substrates (e.g. ceramic bonded
to aluminum). Vibration damping, a useful property of the adhesive in the rubber
state, it is also reduced at low temperatures.
Temperature Cycling
Temperature cycles that drop below -20¡C or dwell well above 85¡C may cause
failures as described in the other section above. Delaminations from coefficient of
thermal expansion mismatch may combine with the oxidation effect to cause
additional failures not found when the conditions act separately. For moderate
temperature cycles the adhesive actually can be beneficial to stress relief of rigid
assemblies. Because its modulus is below that of metals or ceramics, some of the
shear stress from temperature cycling may be taken up in strain of the adhesive,
helping to protect the bonded parts.
III. Bond Building/
Rework
Information is presented below on the rates of bond building of the 3Mª Thermally
Conductive Tapes 9882, 9885, and 9890 on various material surfaces and a general
procedure for rework.
In general the adhesion strength of bonds made with these tapes increases over time
(the Òdwell periodÓ) a process called bond building. The bond building effect is not
due to a curing reaction, since the adhesive is a fully cured material. Rather the cause
is due to a combination of mechanical and chemical forces that act over time.
Mechanical interlocking occurs from the flow of the adhesive on a microscopic scale
into pores of the substrate. Surface forces establish in a wetting phenomenon the rate
and extent of which depends on the chemistry of the substrate. The degree of bond
building therefore depends on the details of the substrate surface and chemical
makeup. Examples of quite different bond building characteristics on ceramic and
anodized aluminum substrates are give on the following page.
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Ceramic Substrates DI
Technical Bulletin
System Design and Performance for 3MTM Thermally Conductive Tapes 9882, 9885, 9890
III. Bond Building/
Rework (continued)
At any temperature the bond strength is dependent, among other things, on the
contact area of the bond, which in turn requires flat, clean surfaces and assembly
conditions (temperatures, pressures, dwell times) optimized by the user to the parts
and equipment. The data below were taken from peel test measurements using 1 in.
wide anodized aluminum foil, bonded at room temperature with Tape 9885 to either
ceramic or anodized aluminum substrates. The samples were allow to dwell for
various times at either room temperatures or 70¡C prior to the peel test. Similar trends
were seen in data taken with the 9882 and 9890 Tapes. (Note: The results should be
considered representative or typical and not be used for specification purposes.
Ceramic Substrates
As shown in the data below, the initial room temperature bond to ceramic substrates
is typically only about 40% of the final levels, 85% of which is reached after about
three days dwell. The final values of the two conditions differ Ð the final bond
strength approached at 70¡C appears to be 40% higher than that approached at room
temperature. Perhaps due to the higher final levels, the bond building process is
accelerated by heat: after 30 minutes to an hour at 70¡C the bond strength is
equivalent to that obtained after a one day dwell at room temperature. One day at
70¡C corresponds to three days at room temperature.
10
8
6
4
2
0Init 0.5 h 1 h 24 h 72 h 1 wk
RT Dwell
70°C Dwell
Peel Strength (lbs./in.) of 9885 Tape on Ceramic
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Anodized Aluminum Substraies DI Effen on Thermal Resistance Re“ ork
Technical Bulletin
System Design and Performance for 3MTM Thermally Conductive Tapes 9882, 9885, 9890
III. Bond Building/
Rework (continued)
Anodized Aluminum Substrates
As one can see from the data below, anodized aluminum substrates show very rapid
building of the peel strength. The initial bond is nearly 80% of the final levels and the
effect of elevated temperatures is diminished. Both the final peel strengths obtained,
as well as the rate of bond building appear to be the same for either room temperature
dwells or 70¡C dwell.
10
8
6
4
2
0Init 0.5 h 1 h 24 h 72 h 1 wk
RT Dwell
70°C Dwell
Peel Strength (lbs./in.) of 9885 Tape on Ceramic
Effect on Thermal Resistance
In a spot check over a 24 hour period, the thermal properties of the adhesive did not
appear to change beyond the 20% - 25% margin of error of our experiments.
However it is believed that wetting of the surfaces by the adhesive and strong
adhesion levels contribute to yield optimum thermal properties.
Rework
The procedure for reworking bonds made with the adhesive.
A) Mechanically separate the parts, using torque for rigid parts and peel for flexible
ones. The amount of force required will depend on the bond area and the extent
to which the bond has built, as described above. Heating the substrates and
adhesive (with a hot air gun or other means) to 70¡C - 100¡C will soften the
adhesive bond and allow the amount of force to be reduced.
B) Remove the adhesive by rubbing if off with a tool. Use solvent only as a last
(messy) resort.
C) Clean the surfaces using isopropyl alcohol. If gross contamination exists, use
another solvent such as acetone or methyl ethyl ketone, followed by isopropyl
alcohol (to remove the solvent residue).
D) Apply fresh adhesive and repeat the bonding procedure.
Note: Carefully read and follow the manufacturerÕs precautions and directions for use
when using solvents.
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Technical Bulletin
System Design and Performance for 3MTM Thermally Conductive Tapes 9882, 9885, 9890
To request additional product information or to arrange for sales assistance, call toll free 1-800-362-3550.
Address correspondence to: 3M Bonding Systems Division, 3M Center, Building 220-7E-01, St. Paul, MN
55144-1000. Our fax number is 612-733-9175. In Canada, phone: 1-800-364-3577. In Puerto Rico, phone:
1-809-750-3000. In Mexico, phone: 5-728-2180.
For Additional
Information
3M MAKES NO WARRANTIES, EXPRESS OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, ANY
IMPLIED WARRANTY OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. User is
responsible for determining whether the 3M product is fit for a particular purpose and suitable for users
method of application. Please remember that many factors can affect the use and performance of a 3M
product in a particular application. The materials to be bonded with the product, the surface preparation
of those materials, the product selected for use, the conditions in which the product is used, and the time
and environmental conditions in which the product is expected to perform are among the many factors
that can affect the use and performance of a 3M product. Given the variety of factors that can affect the
use and performance of a 3M product, some of which are uniquely within the user’s knowledge and
control, it is essential that the user evaluate the 3M product to determine whether it is fit for a particular
purpose and suitable for the users method of application.
Important Notice
If the 3M product is proved to be defective, THE EXCLUSIVE REMEDY, AT 3M’S OPTION, SHALL BE
TO REFUND THE PURCHASE PRICE OF OR TO REPAIR OR REPLACE THE DEFECTIVE 3M
PRODUCT. 3M shall not otherwise be liable for loss or damages, whether direct, indirect, special,
incidental, or consequential, regardless of the legal theory asserted, including negligence, warranty, or
strict liability.
Limitation of Remedies
and Liability
This Bonding Systems Division product was manufactured under a 3M quality system registered to ISO
9002 standards.
ISO 9002
3
Bonding Systems Division
3M Center, Building 220-7E-01
St. Paul, MN 55144-1000
Printed in U.S.A.
©3M 1999 70-0704-8798-1
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