Gap Pad Materials
®
Application Note #116
Compressive Modulus of Elasticity for Gap Pad Materials
Variability in Compressive Modulus Values
Compressive modulus can be measured in various ways yielding
different values for the same material, depending on the test
method and parameters specified. Compressive modulus can be
measured by dynamic and non-dynamic methods. Dynamic or
periodic methods measure the complex modulus, which is made
up of the storage modulus and the loss modulus. Non-dynamic or
transient methods measure the relaxation modulus. Among the
transient methods is the constant rate of strain method.
Since Gap Pad is viscoelastic, its modulus is time dependent.This
means that the complex modulus varies with the frequency of
strain and the relaxation modulus varies with the rate of strain. At
high frequencies and rates of strain, the complex modulus and
relaxation modulus of Gap Pad will be higher and it will appear
harder.At lower frequencies and rates of strain the complex modulus and relaxation modulus of Gap Pad will be lower and it will
appear softer. Viscoelastic materials will stress relieve themselves
over time, so increasing the measurement time interval in a step
strain application test will decrease the modulus measured and the
material will appear to be softer.
Figure 1: Compression vs. Deflection
(Constant Rate of Strain 1.0 in/min)
140
GP3000S30
GP2500S20
GP1500
GP5000S35
GP VO Soft
GP VO Ultra Soft
GP1500S30
120
100
Stress (psi)
The Gap Pad material’s ability to deflect stress over time is
reflected in the compressive modulus (Young’s Modulus) and
compression/deflection data. Modulus is a measure of the hardness or softness of a material and is equal to stress divided by
strain. Stress is equal to pressure. Strain or deflection is equal to
the ratio of the change in thickness to the original thickness of
the material. The lower the modulus the softer the material. In
most cases, soft gap filling material is desired to minimize pressure exerted on printed circuit boards, component leads and
solder joints while the material is being deflected during the
assembly process.
measurement time interval is small and the amount of relaxation
of the test material is low. Therefore, it will give the engineer an
indication of the maximum stress that will be generated in the
application. Here the design engineer needs to remember that by
decreasing the rate at which the Gap Pad is deflected during the
assembly process, the stress induced in the application can be
decreased (Figure 2).
80
60
40
20
0
0
10%
20%
30%
40%
50%
Percent Strain
0.5 in (12.7mm) Circular Test Sample.
All materials tested are 125 mil (0.125 in / 3.175mm) thick.
Figure 2: Gap Pad VO Compression vs. Deflection
(3 Constant Rates of Strain)
100
10.0 in/min
1.0 in/min
0.1 in/min
90
80
Stress (psi)
Introduction
The key to determining the suitability of a material to a given
application is its response to applied stress. In typical applications, screws and clips induce stress on Gap Pad materials.The
moment of assembly is the point at which the material will
experience the highest levels of physical stress. An important
characteristic of any Gap Pad material is its long-term ability to
deflect this stress at low pressures.
70
60
50
40
30
20
10
Constant Rate of Strain Relaxation Modulus
The Constant Rate of Strain Transient Test involves applying an
increasing strain at a constant rate while measuring induced stress.
This test simulates the deflection of a gap filling material during the
course of an assembly process. A design engineer needs to know
the constant rate of strain while the product is being assembled
(Figure 1). During the Constant Rate of Strain Transient Test the
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0
0
10%
20%
30%
40%
Percent Strain
0.5 in (12.7mm) Circular Test Sample.
All materials tested are 125 mil (0.125 in / 3.175mm) thick.
All statements, technical information and recommendations herein are based on tests we believe to be reliable, and
THE FOLLOWING IS MADE IN LIEU OF ALL WARRANTIES, EXPRESSED OR IMPLIED, INCLUDING THE IMPLIED
WARRANTIES OF MARKETABILITY AND FITNESS FOR PURPOSE. Sellers’ and manufacturers’ only obligation shall be to
replace such quantity of the product proved to be defective. Before using, user shall determine the suitability of the product
for its intended use, and the user assumes all risks and liability whatsoever in connection therewith. NEITHER SELLER NOR
MANUFACTURER SHALL BE LIABLE EITHER IN TORT OR IN CONTRACT FOR ANY LOSS OR DAMAGE, DIRECT,
INCIDENTAL, OR CONSEQUENTIAL, INCLUDING LOSS OF PROFITS OR REVENUE ARISING OUT OF THE USE OR
THE INABILITY TO USE A PRODUCT. No statement, purchase order or recommendations by seller or purchaser not
contained herein shall have any force or effect unless in an agreement signed by the officers of the seller and manufacturer.
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