Janne Kiilunen

Reliability analysis of an ACA attached flex-on-board assembly for industrial application

Purpose – The purpose of this paper is to examine the long-term reliability of an anisotropic conductive adhesive (ACA) attached polyethylene terephthalate (PET) flex-on-board (FOB) assembly for industrial application used in harsh environments. In addition, the possibility of reducing reliability testing time was studied. Design/methodology/approach – A−40/+125°C thermal cycling test with 5- and 14-minute soak times was used to study the reliability. To study the functionality of the FOB assembly during testing, a real-time resistance measurement was used together with a 90° peel strength test. Failure analysis was performed on samples using scanning electron microscopy and cross sectioning. Findings – No failures or noticeable increase in the measured resistance values were seen during testing. The peel strength, however, decreased significantly with both soak times used. The highest drop in the mechanical strength occurred at the start of the temperature cycling tests. The time spent at the high temper...

Reliability of ICA attachment of SMDs on inkjet-printed substrates

Abstract Printable electronics has been attracting considerable attention in recent years as a technology for flexible production of low-cost electrical devices on flexible substrates. Due to the additive nature of the production process, printable electronics offers to be a simple and effective method to manufacture electronics. Because the complexity and functionality of all-printed electrical devices is highly limited mainly by the low performance of semiconductive inks, external components are necessary for complex functionalities required in today’s electrical devices. Such components must be attached to printed structures with connections having adequate electrical and mechanical performance and good long-term reliability. This study evaluated the reliability of isotropically conductive adhesive connections on inkjet-printed substrates and viewed ICA component connections as viable options for attaching SMD components on inkjet-printed circuits.

Product Level Accelerated Reliability Testing of Motor Drives With Input Power Interruptions

Motor drives utilizing power semiconductors play an important role in modern day electric motor control. Although the reliability of power semiconductors is widely studied, the product level reliability of motor drives has been studied markedly less even though their more complex control and measuring electronics often make them more vulnerable to environmental stresses. In order to advance product level accelerated reliability testing, customized test methods with multiple simultaneous or sequential stresses can be used. However, the knowledge of combined effects of different stresses is still largely unknown. In this research the reliability of a commercial motor drive was studied. Environmental conditions used included an 85 °C constant temperature test and an 85 °C test with 85% relative humidity. Additionally, input power interruptions were included to study the effect of sudden shortages of electricity. The results of the study showed that the mean time to failure for the devices tested with the input power interruptions was notably shorter than that for the test series without them. An especially clear effect of the input power interruptions was seen on the power MOSFETs of the motor drives. Moreover, the humidity was found to play an important role in the reliability of the motor drives.

The Effect of Bonding Temperature and Curing Time on Peel Strength of Anisotropically Conductive Film Flex-On-Board Samples

Today, flex-on-board (FOB) attachments are commonly used in a variety of applications. By using anisotropically conductive adhesives (ACAs), low-cost flex materials that could not withstand typical soldering temperatures can be utilized. This paper examines the effect of manufacturing process parameters on the peel strength of a FOB assembly with an ACA attachment. The effects of bonding time and temperature on the curing properties of the ACA were also studied. The mechanical strength of the assembly was studied using a 90° peel strength test. The results showed that, by elevating curing temperature, better peel test results were obtained due to the higher degree of cure of the ACA. Additionally, longer curing time increased the peel strength. However, it also caused a considerably higher deviation in the results. Failure analysis using scanning electron microscopy showed that, with a higher degree of curing, the failure location in the peel strength test changed from the flex-adhesive interface toward the adhesive-substrate interface.

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.

System Level Reliability Testing for High Reliability Devices

Purpose of this study was to investigate system level reliability for electronic devices intended to be used in industrial use conditions. The reliability of such devices should be extremely high, the intended useful lifetime being in the range of 10 to 30 years. Testing reliability of such devices can be seen difficult with limited amount of time and keeping the failure modes same as in the real use conditions. In this paper, efforts of tackling this problem are described. Compatible modified standard tests and multiple environmental overstress tests (MEOST) are sought for to minimize the test duration and maximizing the achieved result accuracy. In the case of the used devices, MEOST test has given good correspondence between the failure mechanisms comparing the use and test condition failures, and the failures were produced in a relatively short amount of time.

Hygrothermal Aging of an ACA Attached PET Flex-on-Board Assembly

In this paper, accelerated temperature and humidity test methods were used to study the long-term reliability of a flex-on-board assembly attached with an anisotropically conductive adhesive (ACA). Test methods included constant 65°C/90% relative humidity (RH) (65/90) and 85°C/85%RH (85/85) tests, 10°C-65°C/90%RH cycling test with moisture condensation (65/90 cycling), and 85°C/85%RH humidity cycling test (85/85 cycling). The resistance of the ACA joints was measured in real time during testing. On basis of the resistance measurement results, failures occurred in the 65/90 cycling test due to ACA delamination and cracking. In addition, failures were also observed in the constant 85/85 test and 85/85 cycling test. However, these failures were mostly related to the properties of the polyethylene terephthalate flex and its embrittlement or latent defects. In addition, a 90° peel strength test was used to study the changes in the mechanical strength of the assembly. The peel strength results showed that with all the test methods, the largest drop in the mechanical strength occurred at the start of the test. After this, the peel strength in 65/90 and 65/90 cycling tests remained almost unchanged, whereas a more significant drop was observed with the 85/85 cycling test. The 85/85 test caused the flex to become brittle, which prevented peel testing. Based on the failure analysis of the peel tested samples, similar failure modes were observed with all the test methods used. With untested samples, failure mostly occurred at the interface between the adhesive and the flex, although cohesive as well as adhesion failures at the ACA-printed circuit board interface were also observed. After hygrothermal aging, the failure location changed toward the ACA-flex interface as testing progressed and, in most cases, finally resulted in the detachment of conductor traces from the flex.

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.

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.

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.