Juha Pippola

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.

Power MOSFET failure and degradation mechanisms in flyback topology under high temperature and high humidity conditions

This article describes observations about power MOSFET failures and degradation experienced during accelerated testing involving high temperature and high humidity stresses. The examined power MOSFETs were operated in commercial variable speed drives in flyback transformer topology as power switches. Known power MOSFET failure mechanisms are summarized and electrical stresses typical for the flyback topology are reviewed. In addition, effects of electrical stresses due to power interruptions were studied. The power MOSFET failure analysis results are presented. The visual appearance of the samples with catastrophic damage was examined with such analysis methods as X-ray, acoustic microscopy (SAM) and optical microscopy. The samples with no obvious failures were also analyzed in research for electrically measurable failure precursor parameters to characterize the physical degradation of the devices. Under these test circumstances, the power MOSFET channel off-resistance RDS-off was discovered to have degraded. This resistive leakage phenomenon was also visualized with backside OBIRCH technique.

Fretting corrosion: Analysis of the failure mechanism for low voltage drives applications

Abstract In many power electronics products, such as low voltage LV motor drives, the mechanical and electrical connection between printed circuit board assemblies (PCBAs) is obtained though board-to-board connectors. A common failure mechanism for these components is fretting corrosion. This failure mechanism results from mechanical micro-motion of two parts with respect to each other and causes a wear damage at the asperities of the contact surface resulting in an increase of the ohmic resistance which finally triggers various device failures. Failures resulting from interrupted communication path may create different effects at the product level depending on the power electronic circuit diagram. In this paper investigation on fretting corrosion on board-to-board connectors was performed. The chemical composition and microstructural analysis at the junction part of the connectors were performed for as new samples and after aging testing which includes stressors like humidity, temperature, temperature cycling and vibration. Scanning Electron Microscopy (SEM) and energy dispersive X-ray analysis (EDX) showed aging effect on the intermetallic compound (IMC) material correlated to intermetallic growth and oxidation.

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.

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.

Development of dust test method for motor drives

Motor drives are used in various industrial processes. Some of the operational environments of the drives may contain contaminations like dust, salts and gases. Dust combined with humidity may cause electrical shorts, decreased insulation resistance, and corrosion on the electronics of the device. Therefore, it is essential to study the behavior of the devices under such conditions. In this study, a product level dust test method for low voltage motor drives was developed. Method was successfully used to study the accumulation of the dust inside devices and the behavior of the devices under dusty and humid conditions.

Temperature cycling of low voltage motor drives

Motor drives are used in a wide range of industrial applications, many of which impose high reliability demands on the electronics used in them. Moreover, the environmental conditions in industrial applications may vary a lot and therefore reliability studies for the motor drives are essential. In this study the reliability of a low voltage motor drive was studied with two different temperature cycling profiles. The results of the study showed that temperature cycling triggered some reversible device faults during testing, but catastrophic IGBT failures, common for power electronics, were less seen. More detailed analysis suggests that the reason for the device faults may have been cracks in the solder joints of the pinheader connector between two PCBs of the device, or fretting corrosion on the connector contact materials.

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.

Sequential stress combinations in product level reliability testing of industrial electronics

The reliability requirements for industrial electronics are high and in many cases electronics devices are used in harsh and varying conditions. To study the reliability of electronics in laboratory conditions accelerated life test methods are typically used. However, usually only one or two stresses are used in these studies even though electronics may encounter several sequential and simultaneous stresses during their lifetime. To achieve more accurate results or greater acceleration from the accelerated tests combinational testing methods are a good alternative to tests using only one stress. However, the knowledge about how different stresses affect each other is still highly unknown and this needs to be studied more. In this study the sequential combination effects of high temperature, high temperature high humidity, and temperature shock were studied using five different test sets. The studies showed that exposure to humid high temperature conditions had a greater effect on failure times in a temperature shock test than did dry elevated temperature. Moreover, the use of a temperature shock test prior to the high temperature high humidity test was found to accelerate certain failure modes. Consequently, such combinatory testing may be used as a highly accelerated test method for high reliability devices.