Kirsi Saarinen

Effects of Cycling Humidity on the Performance of RFID Tags With ACA Joints

Radio frequency identification (RFID) tags with anisotropically conductive joints (ACAs) are used in different applications where the environmental conditions may impair their reliability. Thus the effects of different environmental stresses on reliability need to be investigated. The effects of high temperature and humidity may change the performance of the tags. More- over, the effects of constantly varying temperature and humidity conditions may be even more harmful. In this study, the effects of changing humidity conditions on the performance of a passive ultra high frequency (UHF) RFID tag with ACA joints were studied. The tags were tested in a humidity cycling test where humidity varied from 85%RH to 10% RH, and temperature from 85°C to 25°C. Tags with four different sets of bonding parameters were tested. Significant differences in the reliability between the tags with different bonding parameters were observed. The results were also compared with results from a corresponding constant humidity test where the humidity was 85%RH, and the temperature 85°C. The tags had different failure times, modes, and mechanisms in these two tests. Furthermore, the effects of bonding parameters on the reliability were different in these tests. According to this study, it is important to investigate the effects of changing humidity, when the reliability in different environments is investigated, but the constant humidity test cannot be replaced with the faster humidity cycling test.

Reliability of ACF Interconnections on FR-4 Substrates

The use of anisotropic conductive adhesives (ACA) in flip chip interconnection technology has become very popular because of their numerous advantages. The ACA process can be used in high-density applications and with various substrates as the bonding temperature is lower than that in the soldering process. In this paper, six test lots were assembled using two anisotropic conductive adhesive films (ACF) and four different FR-4 substrates. FR-4 was chosen as it is an interesting alternative for making low-cost high-density interconnections. Some of the chips were thinned to study the effect on reliability. To study the effect of bonding pressure, four different pressures were used in every test lot. The reliability of the assembled test samples was studied in a temperature cycling test carried out between temperatures of -40°C and 125°C for 10 000 cycles. A finite element model (FEM) was used to study the shear stresses in the interconnections during the test. Marked differences between the substrates were seen. The substrate thinning and also the chip thinning increased the reliability of the test samples. From the FEM, it was seen that both decreased the shear stress in the adhesive, which is assumed to be the reason for the increased reliability. A significant difference was seen in the reliability between the ACFs. This was probably caused by differences in the conductive particle materials and the T g values and of the ACFs. In addition, the bump material used with the ACFs varied, which most likely affected the reliability of the test samples.

Changes in Adhesion of Non-Conductive Adhesive Attachments During Humidity Test

Non-conductive adhesives (NCA) are used to attach flip chip components onto substrates. They have many advantages compared to solders. However, in a humid environment the use of NCA may cause some reliability problems. Long-term tests are essential when the reliability of NCA joints is determined. These tests, however, are often very time-consuming. Nevertheless, without the tests it is difficult to predict the failure times and places. One of the long-term failure mechanisms is the deterioration of the adhesion. In order to study the effects of moisture on the adhesion of NCA, strength tests were conducted after a humidity test, and the adhesion was found to decrease as a function of time. Additionally, finite element models of moisture and thermal expansion were formed, and on the basis of the models the most likely places for failures were defined to be the corner joints. Finally, the results of the models were compared to the results of adhesion measurements and the expected failure time was estimated to be 13 500 hours in test conditions. No failures were seen during 4000 hours of testing. Thus, this structure was found to be extremely reliable in a humid environment.

Reliability Analysis of RFID Tags in Changing Humid Environment

Radio frequency identification (RFID) tags with anisotropic conductive adhesive (ACA) joints are used in various applications where the environmental conditions may impair their reliability. Therefore, it is essential to investigate the effects of different environmental stresses on reliability and to analyze failure mechanisms. The purpose of this paper was to study the effects of changing humidity conditions on the performance of a passive ultrahigh frequency RFID tag with ACA joints. The tags were tested in a humidity cycling test where the humidity varied from 85%RH to 10%RH and temperature from 85 °C to 25 °C. Tags with four different sets of bonding parameters were tested. Significant differences in reliability between tags with different bonding parameters were observed. Different failure times, failure modes, and failure mechanisms were observed. Full-wave electromagnetic antenna simulations were used to support the analysis on the failure mechanisms manifesting themselves through changes in the common tag performance parameters. Changes in impedance matching between the chip and the antenna were found to be the most likely failure mechanism. The impedance matching may have changed due to cracks formed in the antenna during testing or due to chemical and mechanical changes in the ACA joints. It was observed that several failure factors might act simultaneously.

Reliability Analysis of UHF RFID Tags Under a Combination of Environmental Stresses

Accelerated environmental tests can be used to study the effects of environmental stresses on reliability. The environmental tests are commonly run in parallel. One test is performed for one set of test samples, and new sets of test samples are used for other possible tests. However, the subsequent use of different tests for the same set of test samples may describe the operational environment of the samples more accurately and give more reliable results in a short testing time. In addition, the different stresses may accelerate the effects of other stresses, and such behavior can only be perceived if combinations of environmental tests are used. Passive ultrahigh-frequency radio-frequency identification (RFID) tags were tested with different combinations of environmental tests. Some of the tags were subjected first to a bending test, to a constant humidity test, or to a temperature cycling test and then subjected to a humidity cycling test. As a reference, some of the tags were tested only in the humidity cycling test. Changes in the performance parameters of the RFID tags were examined during testing. Clear changes in the reliability of RFID tags with different test combinations were observed. The exposure to constant humidity test before the humidity cycling test significantly impaired the reliability of tags in the humidity cycling test. On the other hand, bending testing before the humidity cycling test had no noticeable effect. The temperature cycling test only slightly impaired the reliability of the tags in the humidity cycling test. In the failure analysis, no clear failure mechanisms were found. It is possible that several failure mechanisms affected the tags simultaneously during the humidity testing. This study demonstrated the importance of studying combinations of different environmental tests one after another on the same test samples. Although exposure to certain environmental stresses may not immediately cause a device to fail, it may impair the reliability when the device is later exposed to another environmental stress. Combinatory stress may be critical for the actual product, and therefore, their discovery during environmental testing is crucial.

Reliability Testing and Modeling of Anisotropic Conductive Adhesive Joints Under Temperature Cycling Test

The use of anisotropic conductive adhesives (ACAs) in flip-chip interconnection technology has several advantages compared to solders with underfills. However, when ACAs are used with organic substrates, their reliability may cause problems. Temperature cycling testing can be used to study the effect of fluctuating temperatures on the reliability of ACA joints. However, this is fairly time consuming. If modeling can be utilized in the evaluation of reliability, significant time savings can be achieved. In this paper, the prospects of using shear stress modeling in predicting the reliability of two different anisotropic conductive adhesive films (ACFs) with an organic FR-4 substrate in temperature cycling testing are examined. Finite element models were made for the test structures. Stresses and strains during the cycling tests ae calculated. To estimate the validity of the models, experimental testing is also done. Test samples with two different ACFs are tested in a temperature cycling test where the temperatures vary from -40°C to 125 °C. Testing is carried out for 10000 cycles. In addition, failure analysis is conducted on the test samples. To model the test samples accurately, the material properties of the ACFs are tested using a dynamic mechanical analyzer and a thermomechanical analyzer. The results of the modeling are compared with those of the experimental tests by comparing typical failure locations, failure modes, and the failure order of the test samples with different ACFs. According to the modeling, the highest shear stress seems to indicate the failure location. In addition, it seems that shear stress models can be used to rank ACF flip-chip joints in the order of the anticipated failure times. On the other hand, normal stress and strain models seem to describe failure mechanisms different from that observed in the experimental tests, and they are observed to be impractical for our purposes. Delamination, which may be one manifestation of failure mechanisms related to shear stress, is observed in all samples with ACF1. However, as no delamination is found in the samples with ACF2, it cannot be positively stated whether the failure mechanism of ACF2 joints is related to shear stress or not. If the samples with different ACFs have different failure factors, simple stress or strain models seem to be impractical for reliability studies, as the results of different models cannot be easily compared.

Reliability study of isotropic electrically conductive adhesives under thermal cycling testing

Electrically conductive adhesives (ECA) have potential for low cost, high reliability, and simple processing. Additionally, an important advantage with ECA materials is the possibility for low bonding temperature. Therefore, they are especially well suited for low cost applications. ECA materials are prepared by mixing polymer matrix with electrically conductive particles. In isotropic conductive adhesives (ICA) concentration of the conductive particles is high and they conduct in all directions. Several materials can be used to manufacture ICAs. The most widely used ICAs in the electronics industry are silver-filled epoxies. However, other polymers can also be used. Currently, ECAs are increasingly used under demanding environments, inh which fluctuation of temperature is a common environmental stress. Such fluctuation causes stresses to form in the interconnections and are a common reason for failures in electronics devices. The interconnections formed with ICA are often not as mechanically robust as those with solders making it especially important to study how thermal fluctuations affect the ICA materials. In this work eight different commercial ICA materials were studied using two different thermal cycling tests. Additionally, low temperature tin-bismuth (Sn-Bi) solder was studied as a reference material. To study the behavior of the ICAs and the solder they were used to attach zero ohm resistors onto FR-4 test boards. After assembly testing of the samples was conducted in thermal shock and thermal cycling tests between -40°C and +125 °C. Marked changes were seen in the resistance values of the test samples during the test. Additionally, clear variation was seen between the ICAs. Apart from one ICA slow thermal cycling test was found to be more detrimental than the faster shock testing.

Moisture effects on adhesion of non-conductive adhesive attachments

Purpose – Personal and portable electronic devices are becoming an important part of the everyday lives. Electronics miniaturization has been and continues to be the most important driver in this development. The current level of miniaturization has made the use of solder joints challenging and new methods, such as adhesive attachments, have been developed. The applicability of these methods depends crucially on their long‐term reliability. Typical failures mechanisms in adhesive connection include cracking, open joints and delamination. Moisture is the principal cause of failures in adhesive attachments. The purpose of this paper is to examine the long‐term effects of moisture and extended temperature on non‐conductive adhesive (NCA) attachments.Design/methodology/approach – Moisture and extended temperature on NCA attachments are examined by strength tests and by finite element models.Findings – The increase in temperature and moisture induces stresses on the interface of the adhesive and the chip. In t...

Reliability of UHF RFID tags in humid environments

Radio frequency identification (RFID) is an emerging technology in the field of identification and security. With RFID tags it is possible to identify objects individually and reliably using radio waves. However, due to numerous applications, RFID tags are exposed to various environmental conditions which may impair their reliability. Long-term reliability is generally studied using accelerated environmental tests. In earlier studies a combination of high humidity and high temperature has been found to be very harmful to RFID tags. Consequently, in this study failure times and mechanisms between three accelerated humidity tests were compared to find a test which is most suitable for RFID tags. A passive ultra high frequency (UHF) RFID tag with a polyethylene terephthalate (PET) substrate was tested using two constant humidity tests and a humidity cycling test. According to this study, the selection of test conditions for reliability studies in humid environments should be carefully considered. PET substrate is susceptible to hydrolysis when a combination of high humidity and high temperature is present. Consequently, a standard 85°C/85% test is very harsh for RFID tags with PET substrate. On the other hand if a cycling test is used instead of a constant test to acclerate the test, the failure mechanism may alter. Therefore it is important to investigate the effects of changing humidity, but a constant humidity test cannot be directly replaced by the faster humidity cycling test due to the different stresses caused by the tests.

Reliability of isotropic electrically conductive adhesives under condensing humidity testing

Electrically conductive adhesives (ECA) are considered to be one of the future technologies due to their potential for low cost, high reliability, and simple processing. Additionally, an important advantage with ECA materials is the possibility for low bonding temperature. Therefore, they are especially well suited for low cost applications. ECA materials are prepared by mixing polymer matrix with electrically conductive particles. In isotropic conductive adhesives (ICA) concentration of the conductive particles is high and they conduct in all directions. Several materials can be used to manufacture ICAs. The most widely used ICAs in the electronics industry are silver-filled epoxies, which also provide a high level of thermal conductivity. However, other polymers can also be used. All polymer materials used in ICAs absorb moisture, which affects their mechanical behavior. Additionally, the electrical properties of the ICA may change. Therefore it is important to study how different ICA materials behave under humid conditions. Especially, if the humidity levels are high, these changes may occur very rapidly. In this work 14 different commercial ICA materials were studied under condensing humidity conditions. To study the behavior of the ICAs they were used to attach zero ohm resistors onto FR-4 test boards. To study the effect of glob top on the behavior of the ICAs, two additional test series were assembled with two epoxy ICAs using a glop top material to protect the components and the interconnections. Marked changes were seen in the resistance values of the test samples during the test. Additionally, considerable variation was seen between the ICAs. Some ICAs showed increased resistance values very quickly after the testing was started. The two ICAs not shown did not show failures during testing.