A new approach to investigate conductive anodic filament (CAF) formation

Purpose – This paper aims to describe the development of an approach that uses a flexible substrate to investigate the mechanism of conductive anodic filament (CAF) growth and effect of different material and manufacturing variables. Design/methodology/approach – A new approach using a simulated test vehicle (STV) has been developed to study the CAF phenomena. The STV can be easily built under controlled conditions in the laboratory using different glass fibres and resin powder to investigate the effect of different variables separately on CAF. The advantage of the STV is that CAF can be formed in relatively short period in a controlled way, and CAF growth can be easily identified using a back-lighting under a microscope due to the thin flex material used as the test sample. Findings – STV has been used to investigate a number of effects on CAF formation: different glass fibres, reflow process, acid contamination in drilled holes, desmear process and glass bundle size. The results demonstrate that for finished fibres acid contamination (plating solution) at the electrode was necessary for CAF formation. However, for unfinished glass fibres (loom state and heat cleaned) CAF can be formed without acid contamination. The reflow process significantly increases CAF formation. Running an aggressive desmear process and using large glass fibre bundle also increased CAF formation. Originality/value – This new approach will be of benefit for printed circuit board (PCB) supplier to evaluate CAF performance on different resin systems and glass fibres to provide high CAF resistance quality PCBs. The test period (168 hours) would be much shorter than the traditional CAF testing (1,000 hours).

Failure Localization of Intermittent Short Failures Caused by Vertical Conductive Anodic Filament Formation

Conductive anodic filament (CAF) formation is a mechanism caused by an electrochemical migration of metals from a metal trace in ICs or in PCBs. This is commonly caused by the moisture build-up in the affected metal terminals in an IC package or PC board caused by critical temperature, high humidity and high voltage gradients conditions. This phenomenon is known to have caused catastrophic field failures on various OEMs electronic components in the past [1,7]. Most published articles on CAF described the formation of the filament in a lateral formation through the glass fiber interfaces between two adjacent metal planes [1-6, 8-12]. One common example is the CAF formation seen between PTH (Plated through Hole) in the laminated substrate with two different potentials causing shorts [1-6, 8-12]. In this paper, the Cu filament grows in a vertical fashion (z-axis formation) creating a vertical plane shorts between the upper and lower metal terminals in a laminated IC package substrate. The copper growth migration does not follow the fiber strands laterally or vertically through them. Instead, it grows through the stress created gaps between the impregnated carbon epoxy fillers from the upper metal trace to the lower metal trace with two different potentials, between the glass fibers. This vertical CAF mechanism creates a low resistive short that was sometimes found to be intermittent in nature. This paper presents some successful failure analysis approaches used to isolate and detect the failure locations for this type of failing devices. This paper also exposes the unique physical appearance of the vertical CAF formation.

PCB Related Failures in Electronic Assemblies

The complexity of Printed Circuit Boards (PCBs) has increased dramatically over the last three decades with the development of surface mount technology (SMT). The typical manufacture of rigid multilayer PCB contains many process procedures, which makes manufacture and application much more challenges. This paper focuses on some typical PCB related failures. Recommendations are provided on optimizing PCB manufacture process and material application. Microvia crack, black pad, galvanic attack, pad design, conductive anodic filament and pad crater are presented in detail.