A defective printed circuit board assembly that exhibited excessive current leakage was examined to determine the responsible failure mechanisms. Observation of the failure site (determined electrically) by optical and electron microscopy revealed an area in the circuit board where debonded fiber bundles bridged a from plated-through-hole (PTH) to a copper plane. This phenomenon is highly suggestive of conductive filament formation.
The second level of electronic packaging, printed circuit substrates, involves materials and architecture capable of reliably performing multiple tasks critical to system operation. These tasks include supplying power to single-chip devices, precisely timed signal transmission, structural support, and thermal conduction. In consideration of these performance requirements, printed circuit substrates are laid out in a laminate architecture, consisting of sheets of copper between layers of dielectric material. The circuit interconnection pattern, created from the copper sheets, is used to carry power, signal, and in some cases thermal energy. The dielectric, in most substrates, consists of an organic resin reinforced with highstrength fibers.
The demands of space and performance has led to an increase in the number of components and the necessary interconnection density on printed circuit boards (PCBs). This has driven the evolution of the printed circuit substrate to the multi-layered printed circuit boards in use today. Reduced conductor spacing, small diameter vias, and plated-through-holes (PTHs) on multiple levels can result in PCBs becoming more susceptible to conductive filament formation (CFF).
CFF is an electrochemical process that involves the transportation (usually ionically) of a metal through or across a non-metallic medium under the influence of an applied electric field [1-3]. CFF can result in either leakage currents which reduce performance or catastrophic shorts that cause complete failure. The biased conductors act as electrodes providing a driving potential while ingressed moisture between the organic resin and the fiber reinforcement will serve as an electrolyte (see figure 1). As metallic ions migrate and form a bridge between two biased conductors, the loss of insulation resistance results in a current surge. The current surge will eventually result in a short and a large increase in localized temperature. This increase in localized temperature can manifest itself as a burnt or charred area between the two conductors involved.
The prime factors influencing CFF are board features (resin materials, conformal coatings, and conductor architecture) and operating conditions (voltage, temperature, and relative humidity). Path formation, necessary for the occurrence of CFF, will occur due to interfacial delamination of the interface between the individual fibers and the organic resin matrix. This degradation is often precipitated by poor drilling and thermal cycling (Figures 2a and 2b). Previous studies have gathered data on the quantitative effect of these various elements on the occurrence of electromigration [4-6]. Based upon this empirical evidence, models have been presented that predict the operational lifetime assuming an eventual failure mechanism of CFF [6,7].
More recent experiments have shown that CFF can also occur in the presence of hollow fibers [8,9]. The environmental considerations are the same, however in this case the path formation occurs within the fiber itself, instead of along the fiber/matrix interface. The scenario of electromigration within the fiber and its effect on time-to-failure has also been modeled .
A failure analysis was conducted on the defective PCB that had experienced an electrical short during operation. This 6 layer board is approximately 70 mils thick with 5 mil lines and 5 mil spacing. 1 ounce copper is used for the internal layers and 1/2 ounce copper for the external layers. The PTHs are approximately 12.5 mils in diameter. The distance between the electrically shorted PTH and copper trace of the ground plane is about 14 mils. Conditions suggested possible failure due to CFF. The exact area of the failure was determined by electrical testing. Substrate material was then removed perpendicular to the z-axis of the PCB until the failed copper interconnect plane was located (Figures 3a and 3b). The sample was then sectioned diametrically through one of the plated-through-holes (PTHs) so that the defective area could be observed along with the top view (Figures 4a and 4b). Observations were made using optical and environmental scanning electron microscopes (ESEM).
In Figure 3a, a dark, deformed region can be observed near the PTH. The charred color and deformation of the resin suggests burning of the organic matrix occurred. Images of broken fibers seen in Figure 3b are another indication of large amounts of heat generated in the failure area. The glass fibers used as reinforcements are nominally flexible. However, exposure to high temperatures can cause them to become brittle and thus more susceptible to fracture.
It can also be observed in Figure 4a that this dark, charred region extends from the outer wall of the PTH to a copper trace (top-left corner). The path of damage and the indication of high temperatures suggest that failure is due to conductive filament formation (CFF), which created a conduction path between the PTH and the copper trace of the ground plane.
A failed printed circuit board (PCB) was electrically tested, cross-sectioned at the identified site of electrical failure, and examined using optical and environmental scanning electron microscopy. Based upon the appearance of the epoxy, breakage of several glass fibers, and a damage path from a platedthrough-hole (PTH) and a copper trace, the failure mechanism was identified as conductive filament formation (CFF). The occurrence of conductive filament formation deep within the PCB can easily be misdiagnosed as "Failure Unknown". A complete failure analysis was necessary in order to ensure proper identification of the failure mechanism. Without proper identification, the electronic system will continue to perform below specifications because the user and the vendor are prevented from making the necessary adjustments to the environment or substitutions in material to prevent this sort of failure from reoccurring.
The authors would like to thank R. Uppalapati for his assistance in sample preparation.
E-glass fibers are used as a reinforcement material in the manufacture of laminates used in printed circuit cards and multichip module laminated substrates (MCM-Ls). The principal advantages of using E-glass fibers include high strength, high chemical resistance, and excellent insulating properties at reasonable cost. Although most fibers are solid, hollow fibers can be produced if there is insufficient process control during the manufacture of E-glass fibers.
Increases in failures due to internal board shorting by conductive filament formation have driven glass and laminate manufacturers to consider screens and qualification tests to assess the hollow fiber concentration of circuit cards. This article describes the hollow fiber problem, methods to deter conductive filament formation, a hollow fiber screening technique, and the impact of next generation technology on conductive filament failure mechanism.