9890 (1"X36YDS) - 3M - Datasheet.Live

April, 1999

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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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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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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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

Peel Adhesion, 150°C Aging Condition

lbs. / in.

9890 Tape

9885 Tape

9882 Tape

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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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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.

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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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.

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 user’ s

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 user’ s 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 T O 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

Bonding Systems Division

3M Center, Building 220-7E-01

St. Paul, MN 55144-1000

Printed in U.S.A.

Recycled Paper

40% pre-consumer

10% post-consumer

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