Sanna Lahokallio

Effects of different test profiles of temperature cycling tests on the reliability of RFID tags

Abstract Passive UHF radio-frequency identification (RFID) tags are used for object identification in various environmental conditions, which may affect the reliability of these tags. The effects of different environmental stresses can be studied with accelerated life tests (ALT). Choosing the most suitable test may be challenging: The results are needed as fast as possible, but the failure mechanisms must replicate those occurring in the real operating environment. Here the effects of different temperature cycling profiles were studied by altering temperature ranges, extreme temperatures, soak times to extreme temperatures and transition times between extreme temperatures. Failure times clearly differed between the tests. The test with the fastest transition time and the shortest soak time seemed to have the most acceleration. It was also observed that the different temperature cycling profiles affected the failure mechanisms detected. Cracking of the antenna was observed with lower temperature extremes or shorter soak and transition times. However, with longer soak and transition times, cracks were seen in the RFID interconnections. Both cases led to changes in the impedance matching and consequently to failures. The totally different failure mechanisms clearly demonstrate the importance of carefully determining the test parameters in order to achieve the correct failure mechanism.

High-Temperature Storage Testing of ACF Attached Sensor Structures

Several electronic applications must withstand elevated temperatures during their lifetime. Materials and packages for use in high temperatures have been designed, but they are often very expensive, have limited compatibility with materials, structures, and processing techniques, and are less readily available than traditional materials. Thus, there is an increasing interest in using low-cost polymer materials in high temperature applications. This paper studies the performance and reliability of sensor structures attached with anisotropically conductive adhesive film (ACF) on two different organic printed circuit board (PCB) materials: FR-4 and Rogers. The test samples were aged at 200 °C and 240 °C and monitored electrically during the test. Material characterization techniques were also used to analyze the behavior of the materials. Rogers PCB was observed to be more stable at high temperatures in spite of degradation observed, especially during the first 120 h of aging. The electrical reliability was very good with Rogers. At 200 °C, the failures occurred after 2000 h of testing, and even at 240 °C the interconnections were functional for 400 h. The study indicates that, even though these ACFs were not designed for use in high temperatures, with stable PCB material they are promising interconnection materials at elevated temperatures, especially at 200 °C. However, the fragility of the structure due to material degradation may cause reliability problems in long-term high temperature exposure.

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.

Usability of ECA materials in high temperature sensor applications

In electronics applications sensors are often used in challenging environments such as high temperature and humidity. For a sensor to perform reliably under such conditions stable attachment methods are needed. However, sensor structures may differ from typical silicon chip configuration and materials, which may make their attachment challenging. In this work the attachment of temperature sensors with electrically conductive adhesives (ECAs) was studied. The sensors were attached to a flexible polyimide substrate with different ECAs and exposed to three different step stress tests. In the step stress tests the temperature was ramped from 180°C to 260°C with different dwell times. Typically ECAs are not applied at this high temperature region. However, it was found that the adhesives could withstand exposure to high temperatures for surprisingly long periods of time. The weakest link in the structure was not necessarily the ECA but the copper pad, which oxidized at the high temperature and reacted with the adhesive layer of the substrate. This reaction formed a mix of copper oxide and the adhesive which migrated within the structure. Severe delamination between the sensor chip and the copper pads was also observed.

Performance of passive RFID tags in a high temperature cycling test

Radio frequency identification (RFID) tags are typically used for object identification in environments in which they are not exposed to very harsh conditions. However, there is an increasing demand for inexpensive RFID tags for use in harsh industrial environments, but the adequate performance of the materials used in them needs to be verified in such conditions. This paper reports the reliability of passive RFID tags studied in a high temperature cycling test combined with water immersion. According to the threshold power measurements taken in between the test periods, the RFID tags were able to withstand high temperature cycling. However, cycling testing combined with frequent water immersion impaired their reliability, leading mostly to intermittent failures.

Performance of electrically conductive adhesive attached sensors in high temperature cycling

This paper studies the high temperature performance and reliability of electrically conductive adhesive (ECA) attached temperature sensors. Testing of the sensors was conducted using a high temperature cycling test with long transition time between the extreme temperatures of -55°C and +180°C. Several different ECA materials were used as attachment materials. Additionally, the effects of the thickness and material of a substrate were studied. Real-time measurements were conducted during testing and these showed that ECA interconnections could withstand harsh high temperature cycling for long periods of time when proper materials and structures were chosen. Significant differences between the ECA materials were seen, thus the selection of the ECA material was found to be very important for the interconnection reliability. Additionally, the thickness and material of the substrate had a marked effect on the reliability. In most cases a flexible substrate yielded a better performance, but the effect differed between the ECA materials. The ECAs studied were mostly anisotropic conductive adhesives (ACAs). However, two isotropically conductive adhesives (ICAs) were also studied, one of them also with an underfill. Although underfill material is commonly used with ICAs, in this study it was observed to impair the reliability. The results showed that analysis of the reliability of all materials and structures is essential when adhesives are used in high temperature applications.

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.

Reliability of ICA attached sensor components under prolonged humid conditions

Sensor components are used increasingly in various applications. The structure of a sensor component often differs from that of a typical silicon chip used in electronics, which may make their attachment and packaging challenging. Isotropic conductive adhesives (ICA) may be used in many applications which have such restrictions. ICAs consist of polymer binder, into which conductive particles are added. One of the main advantages of ICAs is their low process temperatures. Furthermore, they can be used with many different component structures and materials. In this study sensor components were attached onto flexible PI substrates using ICA flip chip technique. Five ICA materials were tested. Additionally, three different protective materials were studied. One material was underfill and the other two were glob tops. The effect of harsh humidity conditions on the sensor interconnections was studied using a constant humidity test. The test temperature was 85°C and relative humidity 85%. The overall test duration was 10,000h. The resistance values of the ICA materials were found to be very stable during testing. Three of the five ICAs had no failures during testing and two had only one failure. The protective materials were found to be very important for the reliability, as all samples without them failed during testing. The results showed that ICA materials can offer excellent reliability in humid conditions and are interesting interconnection materials for sensor components.

High density R2R screen printed silver interconnections for hybrid system integration

In this case study, we investigated the minimum line width and spacing for roll-to-roll (R2R) screen printed silver interconnections on polyethylene terephthalate (PET). This initial pilot investigation involved rotary screen printing 16 high density I/0 patterns while alternating pitch (150, 175, 200, and 250 μm) and nominal screen linewidth (50, 75, 100, 150 μm).

Thermal cycling reliability analysis of an anisotropic conductive adhesive attached large-area chip with area array configuration

The reliability of adhesive flip chip attachments was studied. A large-area chip with a great number of contacts was attached onto a flexible polyimide substrate using anisotropic conductive adhesive film (ACF). The test samples were manufactured using various bonding forces and the reliability of the assemblies was examined using a thermal cycling test. Two temperature change rates were used in the cycling test to study the effect of the change rate on the observed failure times and modes. The results show that the ACF flip chip attachment of large-area chips with matrix array interconnections is an applicable technique. Furthermore, a significant increase in the reliability of the assemblies was obtained by increasing the bonding force. However, early failures were observed in all the samples, especially in the outermost adhesive interconnections. Failure analysis performed on the samples exhibiting early failures showed signs of adhesive delamination and silicon chip cracking. No clear differences in the results between the two temperature cycling tests used were observed. However, the faster temperature change rate seemed to cause a higher number of early failures.