automated equipment

Within automated equipment, due to discharges occurring, some devices may be more voluntarily damaged, while others may be more prone to damage from handling by personnel. We will cover the models and test procedures used to characterize, determine, and classify items' sensitivity to ESD in Part Five. Nowadays, these test procedures are based on the two primary models of ESD events: the Human Body Model (HBM) and the Charged-Device Model (CDM). The models used to execute component testing cannot copy the full spectrum of all possible ESD events, and there is no direct correlation among discharges in the field and a test system. Nevertheless, these models have been proven to reproduce over 99% of all ESD field failure signatures. Typically, the ESD withstand voltages obtained by models in test systems are worst-case compared to real-world events with the same discharge voltage. With the use of regular test procedures, the industry can: ● Increase and measure suitable on-chip protection. ● Enable comparisons to be made among devices. ● Give a system of ESD sensitivity classification to assist in the ESD design and monitoring requirements of the manufacturing and assembly environments. ● Have documented test procedures to make sure reliable and repeatable results. HUMAN BODY MODEL (HBM) TESTING One of the most common causes of ESD damage is the direct transfer of electrostatic charge from the human body or a charged material to the ESDS item. When one walks across a floor, an electrostatic charge accumulates on the body. Easy contact (or even proximity) of a finger to an ESDS item's leads allows the body to discharge, perhaps causing device damage. The model used to replicate this event is the Human Body Model (HBM). The Human Body Model is the oldest and most frequently used model for classifying device sensitivity to ESD. The HBM testing model represents the discharge from the fingertip of a standing individual delivered to the device. It is modeled by a 100 pF capacitor, charged by a high-voltage supply through a high-ohmic resistor (typically in the megohm regime), and then discharged through a switching component and a 1.5 kW (1,500 ohms) series resistor through the component to the ground or a lower potential. This model, which dates from the nineteenth century, was developed to investigate gas mixtures' explosions in mines. It was adopted by the armed forces in MIL-STD-883 Method 3015 and is referenced in ANSI/ESDA/JEDEC JS-001: Electrostatic Discharge Sensitivity Testing – Human Body Model (HBM) Component Level. This document replaces the previous ESDA and JEDEC methods, STM5.1-2007 and JESD22-A114F, respectively. The primary Human Body Model circuit without any parasitics from the test system is presented in Figure 1. Figure 1: Typical (simplified) Human Body Model Circuit A classic HBM waveform has a rise time of 2–10 ns, a peak current of 0.67 amps/kilovolts, and a double-exponential decay with a width of 200 ns. Typically, the decisive parameter which causes the failure is the energy of the HBM pulse. Testing for HBM ESD vulnerability is typically performed using automated test systems. The machine is placed in the test system and contacted through a relay matrix. One pin is connected to the HBM network ("zap pin"), and one or several other pins are connected to tester ground ("ground pins"). With today's high-pin calculating devices, a full test of all possible stress combinations is no longer possible; thus, pin combinations have to be chosen, which guarantees sufficient coverage to detect weak stress combinations. These pin combinations, which must be stressed, are defined in the current HBM standard. Electrostatic discharges (ESD) are functional with a waveform generated by a Human Body Model network. A tool is determined to have failed if it does not meet the datasheet parameters using parametric and functional testing. One has to state undoubtedly that the Human Body Model, according to JS-001, addresses handling issues. Sometimes, the well-known IEC 61000-4-2 is also called the "Human Body Model." Still, that model addresses ESD events in a system under diverse operating conditions and should be applied to systems only. The waveform and the severity of the IEC 61000-4-2 and the JS-001 cannot be compared. For handling issues, only JS-001 is significant.CHARGED DEVICE MODEL (CDM) TESTING The transmission of charge from an ESDS item to a conductive surface at a lower potential is also an ESD event. A tool may become charged, for example, from sliding down the part feeder in automated handling tools. If it then contacts the insertion head or another conductive surface at a lower potential, a rapid discharge may occur from the device to the conductive surface. This discharge event is known as the Charged-Device Model (CDM) event and can be more damaging than the HBM for some devices. While the discharge duration is minimal – often less than one nanosecond – the peak current can reach several tens of amperes, causing significant voltage drops in the device and eventually resulting in the breakdown of dielectrics (e.g., gate oxides) due to the excessive current. The machine testing standard for CDM is ANSI/ESDA/JEDEC JS-002: Electrostatic Discharge Sensitivity Testing – Charged-Device Model (CDM) Device Level. This document replaces the previous ESDA and JEDEC methods, STM5.3.1 and JESD22-C101, correspondingly. The test process involves placing the device on a field plate with its leads pointing up, then charging it, and discharging the device. All pins are treated evenly and are discharged after positive and negative charging. Figure 2 illustrates a simplified CDM test circuit with the direct charging of the device. Figure 2: Typical Charged-Device Model Test OTHER TEST METHODS Machine Model (MM) Testing The industry shifted away from MM as a qualification test. It moved towards using the qualification documents of ANSI/ESDA/JEDEC JS-001 (HBM) and ANSI/ESDA/JEDEC JS-002 (CDM) over the years to characterize a component's ESD susceptibility fully. Discharge can happen from an isolated conductor or a charged conductive object, such as a metallic tool, or a piece of automatic equipment or fixture. Therefore the CDM method simulates metal to metal discharges better. Originating in Japan, trying to create a worst-case HBM event, the model is recognized as the Machine Model. This ESD model consists of a 200 pF capacitor discharged directly into a component with no series DC resistor in the output circuitry. The discharge waveform can be oscillating with the rise time and pulse width, as like as HBM. The Machine Model typically addresses the same material failure mode as the Human Body Model; therefore, it is useful when determining failure mode and producing HBM-like ESD effects at significantly lower levels. Testing of devices for MM sensitivity using ESD SP5.2: Electrostatic Discharge Sensitivity Testing – Machine Model (MM) Component Level is similar in procedure to HBM testing. The essential test equipment and the stress combinations are the same, but the test head is very different. The MM version does not have a 1,500-ohm resistor, but otherwise, the test board and the socket are often the same as for HBM testing. As shown in Figure 3, the series inductance is the dominating parasitic element that shapes the oscillating machine model waveform. The sequence inductance is indirectly defined through the specification of various waveform parameters like peak currents, rise times, and waveform period. However, the inductance is not well defined. Hence, for diverse testers, the MM withstand voltage might differ by at least a factor of 2–5, although both test systems comply with the current standard. The need for reproducibility of test results and the fact that the well reproducible HBM addresses the same failure mode as HBM are the most important reasons that the industry shifted away from MM qualification. JEDEC and ESDA do not suggest to qualify products with Machine Model, but qualifying with HBM and CDM instead. Additionally, the device Model risk is now referred to as Isolated Conductor risk mitigation in the ANSI/ESD S20.20. Machine Model testing of integrated circuits (ICs) should be restricted to failure analysis without correlation of withstand voltages and charging in the field. (Megohms) Figure 3: Typical Machine Model Circuit DEVICE SENSITIVITY CLASSIFICATION The HBM and CDM methods contain a classification system for defining the component sensitivity to the specified model (See Tables 1 and 2). These classification systems have numerous advantages. They let easy grouping and comparing components according to their ESD sensitivity. The classification gives you an indication of the level of ESD defense that is required for the component. Table 1: ANSI/ESDA/JEDEC JS-001 Table 3: HBM ESD Component Classification Levels Classification Voltage Range (V) 0Z