Milad Mostofizadeh

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.

Behavior of Humidity Sensors Attached With Anisotropic Electrically Conductive Adhesives in Corrosive Environment

Sensors are used increasingly in different applications. In some of these applications, they need to be able to tolerate harsh environments and also operate for long periods of time. One of the most challenging environments is corrosive media which may be present in many applications. In this paper, the reliability of humidity sensors attached to an FR-4 substrate was studied using a salt spray test to determine their tolerance for corrosion. Because of their structure, the attachment methods for the sensor components were restricted. Consequently, the sensor components were attached using flip chip technology with two different anisotropic conductive adhesives (ACAs). Afterward, the reliability of the sensor components was investigated using the salt spray test. During testing resistance of the ACA interconnection was measured in real time. To extract the failure mechanism, an extensive failure analysis was performed with scanning electron microscopy. The main failure mechanism was found to be corrosion of a polymer film in the sensor component. The results showed that ACA is promising material for sensor attachments, as most of the sensor components were able to withstand a considerable period of time under the salt spray test without failures.

Reliability and shear strength of 42Sn-57Bi-1Ag (wt.%) lead-free solder joints after thermal aging and salt spray testing

Low cost manufacturing in the electronics industry is becoming more demanding, particularly in the production of consumer electronics. Such manufacturing processes require reliable and low-cost lead-free solders. Among the low temperature lead-free solders, Sn-Bi solder has attracted a great deal of interest since it offers good reliability comparable to that of SnPb solders. In this paper the reliability of eutectic 42Sn-57Bi-1Ag (wt.%) lead-free solder was studied using combinations of environmental tests including thermal aging at 100°C, salt spray test, and a sequential combination of thermal aging and salt spray tests. Microstructural studies on samples were performed at different time intervals. To study the effect of salt spray and thermal aging on the mechanical reliability, shear testing was performed on the samples. Failure analysis including fractography was conducted on samples at different time intervals using scanning electron microscope (SEM). Considerable corrosion was observed after the salt spray test. This corrosion also had a marked effect on the shear strength of the solder joints. It was moreover found that the increase in the thickness of intermetallic layers had an adverse effect on the shear strength. To study the effect of adding 1% Ag to the mechanical properties, a comparison between the mechanical reliability of eutectic Sn-58Bi and Sn-Bi-1Ag is presented.

Thermal cycling reliability of Sn-Zn lead-free solders in sensor application

There is a demand for low melting temperature solder in some applications (e.g., sensor attachment) where the components being soldered are temperature-sensitive. However, the same solder needs to meet the life cycle reliability requirements for the entire product. Among low temperature lead-free solders, eutectic Sn-9%Zn (wt.%) lead-free solder offers good mechanical reliability and low melting temperature. However, the presence of Zn makes it susceptible to oxidation especially at elevated temperatures. In this paper, the reliability of sensor attachments using Sn-9%Zn solder and capillary underfills was studied under thermal shock. Three different underfill materials were used with two of them containing fillers. The thermal shock test results showed that the underfills substantially improved the lifetime of the solder joints, and the underfills wth fillers provided the best mechanical support to the solder joints. The reliability of the Sn-9%Zn solder joints with underfills was found to be comparable to that of the Sn-Pb-2%Ag solder joints. Failure analysis revealed that a uniform distribution of the underfill was critical to achieve a reliable sensor attachment.

Effect of Thermal Aging and Salt Spray Testing on Reliability and Mechanical Strength of Sn–58Bi Lead-Free Solder

Low-cost manufacturing in the electronics industry is becoming more demanding, particularly in the production of consumer electronics. Such manufacturing processes require reliable and low-cost lead-free solders. Among the low-temperature lead-free solders, eutectic Sn-Bi solder attracts a great deal of interest as it offers good reliability compared with that of Sn-Pb solders. In this paper, the reliability of eutectic 42%Sn-58%Bi (wt.%) lead-free solder is studied using combinations of environmental tests such as thermal aging (TA) at 100 °C, salt spray test (SST), and a sequential combination of TA and SST. Microstructural studies on the samples are performed at different time intervals. To study the effect of salt spray and TA on the mechanical reliability, drop testing is performed on the samples. Failure analysis is conducted after different tests. Marked corrosion is seen after the SST. This also has a considerable effect on the impact strength of the solder interconnections. TA increases the thickness of the intermetallic layers, which is found to affect the drop performance.

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.

Effect of Epoxy Flux Underfill on Thermal Cycling Reliability of Sn-8Zn-3Bi Lead-Free Solder in a Sensor Application

The use of sensors has significantly increased in both domestic and industrial applications. In some applications, the sensor component is used along with a heat-sensitive component, therefore, the attachment process using common lead-free solders that have high melting temperature (e.g., Sn-Ag-Cu, Tm = 217 °C) may be challenging. Among lead-free solders with low melting temperature, Sn-8%Zn-3%Bi (wt.%), lead-free solder has a rather similar melting temperature to that of typical Sn-Pb solders. In addition, it offers good mechanical properties. However, the presence of Zn makes it prone to oxidation especially at high temperatures. In this paper, the reliability of sensor attachments using Sn-8%Zn-3%Bi solder and epoxy flux underfill was studied under thermal cycling. Thermal cycling results showed that the lifetime of the lead-free solder joint was lower than that of the Sn-Pb-2Ag solder joints. Failure analysis revealed that the dominant failure mode in lead-free samples was delamination of the sensor pad. In contrast, the failure mode of Sn-36%Pb-2%Ag samples was fatigue crack inside the solder. Additionally, it was found that Sn-Zn-Bi lead-free solder was compatible with epoxy flux underfill.

Use of polymer and plastic materials in electronics applications from the reliability point of view

Polymer and plastic materials are used increasingly in electtonics applications. Not only are they used as insulating materials, but they are also widely used as structural components, protective materials, and even as electrically conductive materials. Polymers have many considerable advantages compared to metals and ceramics. These include for example lightness, low cost, ease of manufacturing, and high versatility. New polymer materials have been developed for many applications and this has further increased their use in electtonics for example as structural parts. Furthermore, polymers are commonly used as a part of composite materials which enables their use in new applications such as electrically conductive materials. From the reliability point of view polymers may be problematic as they are typically less stable than the ceramic and metal materials they are used to replace. Most polymers have poor thermal properties, which needs to be taken into account when they are used. Additionally, humidity and other environmental conditions may markedly decrease the mechanical and electrical properties of these materials. Accelerated life testing (ALT) is commonly used to assess the reliability of various structures and materials used in electtonics. This is important especially when new structures and materials are designed. When metals and ceramics are replaced with polymers, environmental test conditions may need to be considered again, as the standard test conditions or previously used test conditions may not be meaningful anymore. Consequently, careful consideration of accelerated test conditions is needed with polymer materials to ensure that they accelerate correct failure mechanisms.

RELIABILITY ENHANCEMENT OF ACF JOINED FLIP CHIPS WITH PARYLENE C AS A PROTECTIVE COATING

Electronic devices are entering every field of life nowadays. Also medicine uses electrical instruments and devices every day, and the safety and reliability are the key factors in that sector. Also the operational aspects of devices are of course vital and make the device useful and worth wile. However, the hardware of the device plays also a key role, since without it the programs cannot function. Furthermore, the use of different packaging technologies determines the size and weight of the device. As the devices get more complicated, the packaging of electronics becomes more important. In this study the reliability of anisotropical ly conductive adhesive joined flip chip components is considered with a parylene C protective coating on top. With this coating the compatibility with human body and the reliability in medical devices are ensured. Reliability testing was made in 8585 –constant humidity test with two different test substrate layouts and test chips. These contained thin chip and substrate structure and thick chip and substrate layouts. The results show that with thin, more space saving and lighter structure, the reliability can be dramatically improved. Furthermore, parylene C proves to be an excellent protection against humid environment enhancing the long term reliability even more.

Static Bending of Flip-Chip Structures Under Humid Conditions

The miniaturization of electronics poses challenges to reliability demands on devices. In applications where both high reliability and small size are demanded, reliability testing plays a critical role. Medical applications are one such example, and in this case, not only must the durability of electronics be considered but the safety of the device to the human body is also of vital importance. This paper considers the use of thin FR-4 substrates and thin silicon chips in medical applications. The joining is made using anisotropically conductive adhesives. The assemblies are protected using a parylene C conformal coating, which has been proved to be a biocompatible material and, thus, safe for medical applications. The assemblies are bent to a fixed radius during humidity testing. Nonbent and noncoated test lots are also studied for comparison. Two different humidity testing methods are used: the 85/85 constant humidity test and humidity cycling test. Furthermore, the test structures are simulated using finite element analysis. The results show the protective nature of parylene C coating and the differences between bent and nonbent structures. Parylene C coating seems to support the structure by lowering the shear and von Mises stresses formed, resulting in better reliability. Furthermore, the results show a clear difference in the two humidity testing methods, giving value to the standard 85/85 testing.