February_EDFA_Digital

edfas.org 23 ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 23 NO. 1 components and short-circuit protection in SMPS. As with using injections instead of ingesting pills to treat disease, the proposed direct current injection method directly injects current on the power rails of a PCBA that show signs of being shorted. Injections using power supplies with precision controlled voltage and current energize shorted components. While the power is being injected, an IR imaging camera is used to locate abnormally hot components on PCBAs. BASIC PRINCIPLE When current is injected into shorted power rails, the component power dissipation can be calculated as follows: (1) where: P = Component power dissipation (W) U = Input voltage (V) R = Component DC resistance (Ω) Among components connectedonpower rails inparal- lel, the component with the smallest resistance will have the biggest power dissipation and high case temperature, which can be estimated as follows: T C = ( P ⋅ θ C −A ) + T A (2) where: P = Component power dissipation (W) θ C-A = thermal resistance, case-to-ambient (°C/W) T C = Component case temperature (°C) T A = Ambient temperature (°C) When θ C-A and T A are not changed, the steady-state case temperature will increase when input power P is increased, so the case temperature of the components can be controlled by adjusting the input power. For most electrical components, the case temperature is recom- mended to not exceed 70°C to prevent thermal damage during analysis. The equipment required for performing this method are: • Infrared imaging camera • DC regulated power supply • Digital multimeter • Hand soldering machine This equipment is typically found in failure analysis or development labs, so additional investment in equipment may not be required to use this failure analysis method. ADVANTAGES OF IR-BASED DIRECT CURRENT INJECTION METHOD Although a digital multimeter can be used to locate shortedpower rails, it cannot determine theactual shorted component. Removal of components one by one to confirmwhich components are shorted is time consuming and can be extremely difficult if the PCBA is conformally coated. The advantages of the IR-based direct current injection method (Fig. 1) are: • Intuitive and efficient: Shorted components are more quickly identified over traditional static mea- surements. There is no need to guess and remove suspected shorted components one by one to validate. • Nondestructive: Controlled current and voltage injection are performed while component tempera- tures are monitored to prevent thermal damage. This method can even prevent damage for faulty components. Fig. 1 IR-based direct current injection method.