February_EDFA_Digital

edfas.org ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 23 NO. 1 22 FINDING SHORTED COMPONENTS ON PRINTED CIRCUIT BOARDS BY INFRARED-BASED DIRECT CURRENT INJECTION METHOD Zhifeng Zhu 1 and Garron K. Morris 2 1 Rockwell Automation, Shanghai, China 2 Rockwell Automation, Mequon, Wisconsin jzzhu@rockwellautomation.com EDFAAO (2021) 4:22-26 1537-0755/$19.00 ©ASM International ® INTRODUCTION Switch mode power supplies (SMPS) and step-down voltage regulators are commonly used on printed circuit board assemblies (PCBAs) to supply power to other cir- cuits at multiple voltages via power rails. These power supplies often have integrated short circuit protection. Many components on the power rails tend to fail with electrical shorts initially, such as integrated circuits (ICs), transistors, diodes, and multilayer ceramic capacitors (MLCCs). [1-2] These short-circuit protection features can disable voltage output in the event of a short on the power rails to prevent further thermal damage to shorted com- ponents and keep them in short state, so usually there is no visual sign of component damage, which increases the difficulty of locating faulty components. In the literature,magneticmicroscopyhasbeenused to detect the location of short-circuits in PCBAs. [3] However, magnetic microscopy requires specialized equipment, which is quite expensive. Cost-effective commercially available equipment for short circuit identification, such as Esamber M&S 45 and Toneohm 950, use a magnetic field probe to track the path of current in the PCBA. [4] Disadvantages of themagnetic fieldmethod are that it can only indicate the current path and it will take significant time to scan the full current path on the circuit board. Additionally, experience is still needed toconvertmagnetic distributions to potentially shorted components and it may be affected by complicated trace layouts or multi- layer PCBAs. Finally, magnetic field methods are highly dependent on removing components for confirmation. Another common method for short circuit identifi- cation is to use a milliohm meter. This is better than a multimeter because it can locate shortedcomponents that are soldered on the same trace by leveraging the equip- ment’s ability to sense very small differences in resistance. However, milliohmmeters may not be able to apply high enough measurement voltage to forward bias the diode to find an active short. Specially designed equipment that uses similar principles tomeasure a voltage drop to detect short circuit locations that overcome milliohm meter shortcomings are available. [5] The voltage drop method needs to measure all possible shorted components on a shorted power rail. The resistance measurements among the components are compared and the smallest one is assumed to be the shorted component. The risk with this method is that if the usermissesmeasuring evenone com- ponent, the resultsmay be incorrect. Thismethod requires a schematic and layout drawing, as well as removal of the suspected components to confirmthe presence of a short. Most PCBAs used in industrial or transportation prod- ucts are conformally coated, so it is extremely difficult and labor intensive to remove the coatings to provide access to locations for componentmeasurements. The preferred approach is to identify shorted components with as few measurements as possible. The proposed infrared-based (IR), direct current injection method requires very few accessible measurement points, overcomes the issues with magnetic field and voltage drop methods, and ulti- mately is an efficient, low-cost, nondestructive method that can be used to identify failures in modern PCBAs. IR-BASED DIRECT CURRENT INJECTION METHOD Applying power at a power input may not be able to energize faulty components because of shorted