I. Application Background Introduction

PI film (polyimide film) refers to a class of polymers containing imide rings (-CO-N-CO-) in the main chain. It is one of the organic polymer materials with the best comprehensive performance. Its high temperature resistance can reach above 400°C, and the long-term service temperature range is -200 ~ 300°C. Some have no obvious melting point, and it has high insulation performance. The dielectric constant at 103 Hz is 4.0, and the dielectric loss is only 0.004 ~ 0.007, belonging to F-class to H-class insulation. PI heating sheets can provide flexible electric heating plates of various shapes, specifications and power sizes, with a thickness of only 0.25 mm, which can be bent arbitrarily according to the shape of the workpiece to ensure close contact with the workpiece and ensure heat transfer.

Polyimide, as a special engineering material, has been widely used in aerospace, microelectronics, nanotechnology, liquid crystals, separation membranes, lasers and other fields. In the 1960s, countries around the world listed the research, development and utilization of polyimide as one of the most promising engineering plastics in the 21st century. Polyimide, due to its outstanding characteristics in performance and synthesis, whether as a structural material or a functional material, its huge application prospects have been fully recognized, and it is called a "problem solver", and it is believed that "without polyimide, there would be no today's microelectronic technology".

How to test the mechanical properties of PI films such as yield strength, elongation, Poisson's ratio, and Young's modulus in a certain temperature environment, and observe the fracture process of PI films during the test. These parameters are key process parameters for formulating flexible electronic manufacturing processes. Generally speaking, this type of test requires high load precision and small samples. When testing mechanical properties such as yield, strength, and elongation, more importantly, the heating/cooling environment of temperature must be guaranteed, and DIC or visual extensometers also need to be equipped. The heating/cooling in-situ tensile testing system developed by Jingtian Precision Instrument Technology (Suzhou) Co., Ltd. can solve the testing of such small-size variable temperature environments.

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II. Introduction to Heating/Cooling In-situ Tensile Microscopic Testing System

The heating/cooling in-situ tensile testing system is mainly used in scientific research on various materials such as small-size related biology, metals, ceramics, coatings, glass, organic polymers, and fibers. It can achieve a temperature range of -190 ~ 600℃, a temperature control precision of ±0.1℃, and a maximum load of 5kN. The heating/cooling in-situ tensile testing system obtains data such as material deformation and temperature in real time under dynamic load, and combines DIC or video extensometers for material microstructure analysis data, realizing quantitative analysis of material microscopic mechanical properties, phase transition behavior, orientation changes, crack initiation and propagation, material fatigue and fracture mechanisms, material bending, high-temperature creep, delamination, formation of slip planes, and peeling-off phenomena, so as to realize the research on the properties of various materials.

a. It can adapt to the testing of multiple materials and can be used in material science research in multiple disciplines such as metals, semiconductors, ceramics, polymers, and fiber composite coatings.

b. It has a wide variable temperature range, can achieve -190 ~ 600℃, and the temperature control precision is ±0.1℃.

c. It has multiple load loading functions, can replace multiple special fixtures, and can realize tensile, compression, bending tests, etc. of test samples; the maximum tensile load is 5kN, and the load precision is 0.5%.

d. The visual system uses non-contact measurement to avoid the influence of contact on the measurement.

e. It has sub-pixel level precision, using image correlation method, with high calculation precision.

f. It can measure in multiple directions, and calculate longitudinal and transverse strains at the same time.

g. It can measure multiple points simultaneously, and is suitable for multi-sample and multi-gauge length tests.

Figure 1 Video Extensometer (Microscope Version)

The video extensometer adopts a non-contact measurement method, which has relatively few restrictions on the size and material of the sample, and is very suitable for use in small-size sample measurement.

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III. Usage Method Steps

In the testing process of flexible small-size samples, the usage method and steps of the heating/cooling in-situ tensile testing system are as follows:
(1) Make a speckle coating layer through a special small sample speckle spraying device. Of course, marking can also be done by methods such as line drawing, and video extensometers all support it. However, after making the speckle coating layer, it can be extended to other uses, so we recommend processing it as a speckle coating layer. The completed sample is similar to the figure below.

Figure 2 Small-size Sample Speckle Coating Effect

(2) Clamp the sample, place the small sample on the corresponding testing machine normally and clamp it.

Figure 3 Loading Sample into Heating/Cooling In-situ Tensile Testing System

Figure 4 Small-size Sample Using In-situ Fixture

(3)Set up the video extensometer to ensure that the video extensometer is centered with the sample. In the video extensometer software, the centering auxiliary line can be called up through shortcut keys to help adjust the centering conveniently and quickly. At the same time, the algorithm built into the video extensometer software also has the function of eliminating part of the centering deviation.

Figure 5: Centering auxiliary interface of the video extensometer

(4)Select the gauge length. Transmit the deformation value to the testing machine in real time through the video extensometer.

Figure 6 Obtaining Stress - Strain Curve with the Testing Machine

(5) Set the temperature value and tensile speed, and start the test. Generally, enter the target temperature through the temperature control interface, keep it for a period of time, and start the loading test after the temperature stabilizes. The heating/cooling in - situ tensile testing system will obtain the stress - strain curve, and finally calculate parameters such as elastic modulus and elongation at break.

Figure 7 Temperature - Rising Curve

Figure 8 Temperature Fluctuation Curve

Figure 9 Stress - Strain Curve

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IV. Testing Advantages of Heating/Cooling In - situ Tensile Testing System

In the testing of small - size samples, the heating/cooling in - situ tensile testing system has the following advantages:
(1) The heating/cooling temperature control range is wide and precise. It can maintain any temperature value in the range of - 190 - 600°C, with a temperature fluctuation of ±0.1°C.
(2) The heating/cooling block is closely attached to the test sample, and the temperature difference of the sample is small.
(3) Multiple special fixtures can be replaced, and tensile, compression, bending, fatigue tests, etc. of test samples can be realized; the tensile load is 200N - 5000N, and the test range can be replaced.
(4) The system software provides multiple modes of material detection modes, including temperature, load, and displacement threshold settings, curve generation, three - dimensional image generation, automatic data collection, and multiple format outputs.
(5) The test system has a compact structural design and occupies little space.
(6) The highest resolution of the visual extensometer is 0.2μm, and the precision is higher than 1μm.
(7) It can expand strain and displacement cloud map analysis.

Figure 10 Calculated Displacement Field (Upper) and Strain Field (Lower)

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V. Summary

The heating/cooling in - situ tensile testing system can obtain stress - strain data at different temperatures in real time with high precision in flexible small - size samples, which is an excellent solution to the problem of stress - strain measurement of flexible small - size samples in variable temperature environments. Jingtian Precision Instrument Technology (Suzhou) Co., Ltd. can provide users with complete solutions for the measurement difficulties in the mechanical tests of small - size samples, and has relevant solutions in the loading device, fixture design, environment control and other aspects of small samples.