Bent pin analysis is a special kind of failure mode and effect analysis (FMEA) performed on electrical connectors, and by extension it can also be used for FMEA of interface wiring. This analysis is generally applicable to mission-critical and safety-critical systems and is particularly applicable to aircraft, where failures of low-tech items such as wiring can and sometimes do affect safety.
How Connectors Work Electrical connectors carry signals and power between parts of a system that may need to be separated during manufacturing, while in use, or when maintenance is required. Each connector that is part of a mating connector pair may be part of an electrical cable assembly (in which the unmated connector has some freedom of movement), or part of a chassis or other assembly (in which the connector position is fixed). In most pairs of mating connectors, one connector is fitted with an array of socket contacts and the other connector has a corresponding array of pin (or other shaped) contacts, as illustrated in Figure 1. These are sometimes called female and male contacts. Contacts are held in fixed positions within the connector body by a solid rectangular or cylindrical block of insulating material called an insert (shaded red in the illustration). The insert includes holes to accommodate the contacts. In many modern connectors used to pass signals and power in wires, contacts are supplied separately from the connector body. The non-mating ends of the contacts are crimped or soldered to wires, and then the mating ends of the contacts are pushed into the connector inserts with a special tool. A properly inserted contact locks itself in the insert, and another special tool must be used to extract it. In some kinds of connectors the contacts are permanently captured in the insert, so it might be necessary to replace the entire connector if one contact becomes damaged.
Not all connectors are joined to wires as shown in the figure. For example, some connectors may be populated with contacts whose non-mating ends feature printed circuit (PC) tails rather than openings for wires so that the contacts may be joined directly to a printed wiring board.
Most connectors also include an outer metal jacket, called a shell (shaded blue in the illustration), which retains the insert in a fixed position with respect to the shell. A shell provides a means to handle the connector while providing the contacts with some protection against damage. Shells in a mating connector pair are designed to mate in exactly one orientation with respect to each other such that their inserts align the socket and pin contacts for mating without damage as the connectors are pushed together. Shells in most kinds of connectors also provide a mechanism to lock mating connectors together to prevent unintentional de-mating due to stress or vibration. Metal shells are often electrically connected to chassis ground for safety purposes and for control of electromagnetic interference (EMI).
A connector whose shell fits inside the shell of the mating connector is called a plug, and the other connector is called a receptacle. The figure shows a plug with pin contacts and a receptacle with socket contacts, but the opposite arrangement is also common.
How Connectors Fail
Connectors, like any other system parts, are subject to failures. Metal shells can fail mechanically such that connector pairs fail to remain mated. Bent pin analysis examines more common connector failure modes associated with connector contacts. These include loss of electrical conductivity along an intended path due to corrosion on mating surfaces of electrical contacts, wires that have broken away from the contacts, and physically damaged or bent contacts. A bent contact cannot mate with the corresponding contact in the mating connector. These bent contacts are usually called bent pins. While some contacts are not truly pins with circular cross sections, any bendable male contact is typically called a pin.
In most connectors, and as shown in Figure 1, socket contacts are held completely within the insert, with just the mating end of the socket contact accessible at the insert mating surface. With this arrangement, socket contacts have good protection from unexpected damage during handling, and socket contacts with this arrangement are therefore not subject to inadvertent bending. In contrast, the mating ends of pin contacts protrude above the surface of the insert, and mishandling can bend one or more of these pins. For example, bending can occur if a person fails to carefully align the shells of two mating connectors before pushing them together because the shell of the socket connector can sometimes be pushed against exposed pins on the pin connector. Or, a person handling a cable assembly may let an end of the cable with a pin contact connector brush against the corner of a workbench, leaving some bent pins. While one or more pins may suffer only a slight bend due to mishandling, an attempt to mate the two halves can force a slightly bent pin – which no longer aligns with the opening of its socket contact – to slide between the mating surfaces of the two inserts and wind up lying flat between them. Unfortunately, pin contacts are thin and can easily be bent in many kinds of connectors, and the effect of this bending on mating usually isn't noticeable when a human applies the relatively strong force necessary to mate a connector pair. Rather, the damage becomes known only when the system fails to operate as expected.
(Some newer connectors are designed with the exact opposite arrangement – protruding socket contacts and recessed pin contacts. The idea is that the more vulnerable pins are protected and the more rigid sockets are exposed, and if a rigid socket is bent by mishandling the damage becomes immediately obvious because it is virtually impossible to mate the two connectors. Since the damaged connector cannot be mated, and presumably the system would not be operated, there is no reason to apply bent pin analysis for this kind of connector.)
The effects of a bent pin on system operation may or may not be immediately obvious, but they are potentially catastrophic.Airlinesafety.com. “1999, January 31. An American Airlines MD-11 made an emergency landing... after discovering smoke in the cabin… Examination of the VSCU by FAA investigators, revealed that part of a circuit board was charred. Examination of the entire video system revealed ... a short circuit between two pins of a cannon plug that linked the damaged components.” Article There are several possible failure modes. If a pin that normally carries a signal or power has bent, the electrical path is now broken. If the bent pin does not touch a neighboring pin or a grounded shell, then there are no shorts to other paths. Figure 2 shows how pin spacing and diameter in one common military-type connector are such that a bent pin can fall between two others without making contact. If the bent pin touches the grounded shell, then the pin's signal is now shorted to chassis ground. If the bent pin is touching another contact (or two other contacts), then there is an electrical short between two (or three) paths (Figure 3). In some very commonly used miniature D connectors, it is possible that a bent pin can touch two neighboring contacts plus a grounded connector shell, thus shorting chassis ground to three electrical paths. The connector in Figure 3 is an example: its mating plug connector (not shown) fits inside the Figure 3 receptacle shell, and the plug's shell is therefore closer to the pins than the receptacle shell. This means that the bent pin in the figure can touch a shell.
Special Considerations in Bent Pin Analysis
As with any kind of FMEA, bent pin analysis considers only one failure mode at a time. A simple (and traditional) bent pin analysis looks at consequences of each pin bending to each of its neighbors and to the shell. However, as noted above, a bent pin can sometimes touch more than one electrical path at once, so a more complete analysis also considers multiple simultaneous failures caused by the singular failure mode of one bent pin.
Bent pin analysis also determines effects of unused pins that can bend. An unwired but bent “spare” pin may cause no noticeable effect at all, but it may also short two other paths together, or it may short a neighboring path to a grounded shell.
Non-Bending Failure Modes
Bent pin analysis also considers open paths between mating contacts. While an open path may be caused by a bent pin that doesn't touch any neighboring contact (depending on pin density, this is possible in some connectors and impossible in others), but an open path may also be caused by failure modes other than bending. As noted above, one common failure mode is corrosion of the mating surfaces of contacts, but corrosion may also affect the interface where the wire is joined to the contact. Another failure mode is an improperly seated contact (one that has not been properly locked into place in its insert during manufacturing, or one whereby the contact's locking mechanism fails), so that the contact is pushed out of the insert during the mating process, or it can "walk out" as result of pull from its attached wire. At some point in time, the improperly seated contact moves away from its mating contact and breaks the electrical path.
Performing Bent Pin Analysis As with any other FMEA, bent pin analysis consists of two parts: determining failure modes, and determining the consequences (failure effects) on system behavior.
Determining Failure Modes
The failure modes of a particular pin always include (a) open circuit due to corrosion or other non-bending failure, and at least one of the following if the pin is bendable: (b) bending to nothing, (c) bending to one neighboring pin, (d) bending to one neighboring pin and the shell, (e) bending two neighboring pins, (f) bending to two neighboring pins and the shell, and (g) bending to the shell.
In bent pin analysis, as it is usually performed, failure modes of each pin are determined using a scaled drawing of the connector and its pins. The analyst considers each bendable pin, one at a time, and determines which neighboring pins (if any) the selected pin can reach if bent, and whether the selected bent pin can reach the shell. The analysis usually does not include failure modes in which a bent pin simultaneously touches more than one other pin or a pin and the shell. If the analysis requires failure rates, an approximation is usually made by assigning an average failure rate to each failure mode based on the overall connector failure rate and the number of pins.
Since this approach relies on human judgment there can be errors in the conclusions. Even with a conservative approach to cover "worst case" outcomes of bending, concluding that a bent pin can reach another pin (or the shell) when that failure mode is physically impossible is just as much an error as concluding that a bent pin cannot reach another pin (or the shell) when that failure mode is in fact possible.
A more mathematical approach can be applied to determine bending failure modes and the failure rate of each. The approach is to compute the maximum reach of a bent pin as a radius from the pin's center in the insert, then to compute the distance from the bent pin's center to the closest part of each neighboring pin (and the shell). If the bent pin's radius can reach a neighboring pin (or the shell), then the probability of contact with that item can be computed given that the pin is bent. Probability is computed from items 1, 2, and 3 in the following list. Failure rate is computed from the probability and items 4 and 5.“What’s Wrong with Bent Pin Analysis and What to Do About It”. Journal of System Safety, Sept-Oct 2008.
1. Shell and pin dimensional data from military or manufacturer's drawings.