Various test and measurement methods of RFID tags

In the past few years, RFID technology has been continuously developing. RFID has evolved from a specific application in the past to a technology commonly used by logistics companies, for example, for reading encrypted information in parcel tags or airport luggage tags. A market survey by IDTechEx shows that the RFID market, including tags, card readers, and software/services, is expected to reach US$5.56 billion in 2009 (US$5.25 billion in 2008). The market size of RFID tags will increase from 1.97 billion in 2008 to 2.35 billion in 2009.
    New substrates such as PVC or PV are also being gradually adopted like increasingly efficient and miniaturized chips. Although there are many types of RFID tags, their basic structure is the same-that is, the microchip is connected to the coil or antenna of the electromagnetic coupling unit.
    Ensuring reliable fixation and contact between the chip and the antenna is a necessary condition for the normal operation of RFID. The commonly used method is to glue the RFID chip to the tag antenna; this way, on the one hand, it realizes the conductive function, on the other hand, this bonding method can realize the yield in mass production. Less than 1 milligram of glue per unit component can establish a tight material connection between the chip and the antenna substrate within a few seconds after hot pressing. In view of this, it is particularly important to select appropriate bonding process parameters, otherwise, RFID tags may not meet the requirements that should be met. For example, the new PVC and PC materials are more sensitive to temperature rise. As a result, the curing process at high temperatures is more complicated.
    Preparation: glue selection and process parameters
    Since many parameters must be considered, such as chip type, substrate, assembly equipment, adhesive, and subsequent requirements for products, it is extremely difficult to develop a feasible bonding solution in the RFID field, and the bonding process is therefore far away. It is impossible to do simple "plug and play". Substrate manufacturers, adhesive suppliers and plant builders must work closely together in order to jointly develop chemical solutions.
    When selecting the adhesive, you should not only consider the characteristics of bonding strength and good temperature and humidity resistance, but also ensure that the adhesive meets the process requirements of the fully automated production process (see Figure 1).

    Curing within a few seconds. First, apply glue on the chip bonding reserved position on the surface of the antenna (see Figure 2). And repeatable dispensing control has become the first problem to be solved. According to customer requirements and actual equipment conditions, a variety of dispensing methods can be used, such as time pressure control, screen printing or jet dispensing. The amount of glue is usually controlled at 0.1 mg per part. In order to ensure that the bonding is in place, the amount of glue should not be too small, but for cost considerations, too much glue needs to be avoided.

    After the glue is dispensed, the chip to be bonded is placed on the liquid glue with a pick-and-place tool (see Figure 3). To ensure chip positioning, a die bonder with a positioning accuracy of <15 μm is usually used. Today's flip-chip equipment can achieve this accuracy. After the chip is placed, use, for example, Mühlbauer’s hot press equipment (see Figure 4) to cure the adhesive. In the laboratory, a small machine is usually used for initial testing to debug various parameters. However, this hot pressing method does not comply.

    Cure in a few seconds
    First, apply glue on the chip bonding reserved position on the surface of the antenna (see Figure 2). And repeatable dispensing control has become the first problem to be solved. According to customer requirements and actual equipment conditions, a variety of dispensing methods can be used, such as time pressure control, screen printing or jet dispensing. The amount of glue is usually controlled at 0.1 mg per part. In order to ensure that the bonding is in place, the amount of glue should not be too small, but for cost considerations, too much glue needs to be avoided.
    After the glue is dispensed, the chip to be bonded is placed on the liquid glue with a pick-and-place tool (see Figure 3). To ensure chip positioning, a die bonder with a positioning accuracy of <15 μm is usually used. Today's flip-chip equipment can achieve this accuracy.
    After the chip is placed, use, for example, Mühlbauer’s hot press equipment (see Figure 4) to cure the adhesive. In the laboratory, a small machine is usually used for initial testing to debug various parameters. However, this hot pressing method does not conform to the actual situation on the production line. The curing step in the laboratory must also be debugged according to the actual operating conditions using hot pressing; therefore, various parameters such as temperature, pressure and time measured in the laboratory can be directly used on the actual operating equipment.
    Various measurement and test methods
    Since the bonding parts are subjected to a variety of stress tests in actual use, we will conduct different tests in the laboratory to ensure the bonding quality. A common practice is to test RFID tags made with current production equipment. The location of the chip can be detected by the vision system, and the performance of the tag can be tested by the card reading system. Figures 5a and 5b show some of the bonding errors that can be avoided by using cameras or computer monitoring equipment during the production process.

    In addition to the quick test methods provided with these production equipment, there are more detailed follow-up test methods to test the bonding quality.
    Chip shear force: Use the cutter of the shear force tester to push the chip away from the substrate. In the shear test, the ideal value of the adhesive force between the adhesive, chip and substrate should not be less than 25 N/mm2.
    The degree of curing of the adhesive: DSC analysis (differential scanning calorimetry) can be used to detect whether the adhesive is completely cured within the selected parameter range (see Figure 6). This test method can reflect the abnormal situation that occurs due to the curing time is too short or the temperature is too low.

    Micrograph: The micrograph of the chip and substrate can show the degree to which the chip and its bumps are pressed into the antenna (see Figure 7). Insufficient pressure will lead to poor chip contact, and too much pressure will cause chip or substrate damage.

    Measure the reading distance: In this test, keep the power of the card reader unchanged, and keep the label to be tested away from the card reader until a card reading error is prompted. Or, continue to increase the transmit power of the reader until the tag starts to send data; in this case, the distance between the tag and the reader has been set in advance.
    Measuring contact impedance: In addition to determining the reading distance, analyzing the contact impedance between the chip bumps and the substrate can also be used to test the connection quality (see Figure 8). To measure the value, we use the 4-point test method here.
    Aging test: More continuous aging tests can be used to test the stability of RFID (see Figure 9).

    Different humidity/high temperature storage, such as 85oC/85% R.H., 60oC/90% R.H.. The temperature and humidity are applied together on the antenna substrate and the bonding part to simulate the accelerated aging process.
   Temperature shock test, such as -40... +85oC. Different materials (antenna substrate, chip, adhesive) used in RFID tags stretch differently when the temperature changes. This test is used to test the adhesive's ability to balance various stresses. If a bad situation occurs, these stresses will weaken the bonded part and even debond.

    Bending test: The pre-tensioned antenna is rotated by different reels to simulate various bending forces that may occur in actual use (see Figure 10). This test is particularly suitable for testing the bending properties of adhesives. In the test, the substrate on which the chip is attached is subjected to stress in different directions to simulate the peeling stress. Tag composite: Each test method is different, which mainly depends on whether the test is composite or non-composite RFID tags. For composite tags and non-composite tags, the following test curves can also be used to evaluate the impact of the composite process (see Figure 11 and Figure 12).

    Multiple tests to ensure high-quality bonding

    The above-mentioned various detection and testing methods can be combined to make an effective assessment of the bonding of the adhesive in a specific application. In addition, these tests and analyses are for process monitoring of large-volume, stable production, which means many advantages for customers, because DELO not only provides customers with adhesives, but also provides curing and reliability testing. Extensive information.

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