## 3 Identification of the Failure Mechanism  
To understand the failure mechanism, a detailed failure analysis was performed. To begin, the structure of the PCB near the burn-out area was studied, as shown in Figure 2. A V-groove penalization was used here; it can protect the PCBs from shocks and vibration experienced during paste printing to component assembly, soldering, and even testing [5].  
After the PCB fabrication processes are completed, de-penalization is performed to split them into individual PCBs. Currently, there are various types of de-penalization; namely, hand break, pizza cutter/V-cut, punch, router, saw, and laser [5]. In this work, a pizza cutter/V-cut was employed for the de-penalization.  
Upon careful examination of the PCBs that were not short-circuited or burned out, the presence of many crack lines could be seen (Figure 3). These crack lines were found after the de-penalization. With these crack lines, moisture from the environment could penetrate and diffuse into the PCB easily. In the presence of the applied voltage, the electric field could cause the copper ions from the PCB copper planes to drift across either as a dendrite formation [1; 5; 6; 7; 8] or as a CAF formation [9; 10]. This was believed to be the failure mechanism.  
Dendritic growth usually occurs when metal ions move into the electrolyte layer near the anode and then deposit near the cathode and grow in a tree-like or needle-like formation. IPC-5704 defines dendritic growth as the growth of conductive metal filaments on a PCB through an electrolytic solution under the influence of a DC voltage bias and it is  
Figure 3: Cross-section of the PCB: (**a**) potential crack location; (**b**) after pizza cutter (V-cut) de-panel process.  
Figure 2: Cross-section of the board near the V-cut scoring area.  
usually formed on the surface of a PCB [11]. Heat can be generated from the electrical ionic conduction current during the migration process or from the electrical current conducted through the dendrites when they bridge the conductors. Eventually, the dendrites can develop into substantially low resistance with significant heat dissipation and can result in a propagating fault in a PCB [1].  
The CAFs are formed as anode metal gradually becomes ions and begins to migrate under the effects of the bias voltage. It is formed within the PCB insulation materials. Unlike dendrite formation, the conductive filament growth forms from the anode to the cathode, hence the name conductive anodic filament [1]. Failure symptoms of a CAF range from intermittent resistive faults under current limit conditions to catastrophic propagating circuit board faults if the fault current is not sufficiently limited [1]. Basically, CAF formation follows a two-step process; namely, the formation of a microcrack that bridges two conductors to establish the migration path and an electrochemical reaction with the presence of a DC bias and moisture to drive the migration along the cracks between the two conductors [12]. This was observed in our experiments, as will be shown later.  
The formation of dendrites and CAFs can seriously influence the reliability of electronic systems. From our failure analysis observations, the failure mechanism was likely to be a CAF as the short-circuit paths were not on the surface although this PCB was CAF-resistant.