Failure Modes
There are numerous reasons why components fail. Some failures are slow and graceful, where there is time to identify the component and replace it before it fails, and the equipment is down. Other failures are rapid, violent, and unexpected, all of which are tested for during product certification testing.
Component Package Failures
The package of a component provides two core functions: it protects the component from the environment and provides a way for the component to connect to the circuit. If the barrier protecting the component from the environment breaks, outside factors such as humidity and oxygen accelerate the aging of the component and cause it to fail faster. Mechanical failure of the package results from several factors, including thermal stress, chemical cleaners, and ultraviolet light. These causes can be prevented by anticipating these common factors and adjusting the design accordingly. Mechanical failures are only one cause of package failures. Inside the package, defects in manufacturing can lead to shorts, the presence of chemicals that cause rapid aging of the semiconductor or package, or cracks in seals that propagate as the part goes through thermal cycles.
Solder Joint and Contact Failures
Solder joints provide the primary means of contact between a component and a circuit and have their fair share of failures. Using the wrong type of solder with a component or PCB can lead to electromigration of the elements in the weld. The result is brittle layers called intermetallic layers. These layers lead to broken solder joints and often elude early detection. Thermal cycles are also a prime cause of solder joint failure, especially if the thermal expansion rates of the materials—component pin, solder, PCB trace coating, and PCB trace—are different. As these materials heat up and cool down, massive mechanical stress forms between them, which can break the solder connection, damage the component, or delaminate the PCB trace. Tin whiskers on lead-free solders can also be a problem. Tin whiskers grow out of lead-free solder joints that can bridge contacts or break off and cause shorts.
PCB Failures
Printed circuit boards suffer several common sources of failure, some stemming from the manufacturing process and some from the operating environment. During manufacturing, the layers in a PCB board may be misaligned, leading to short circuits, open circuits, and crossed signal lines. Also, the chemicals used in PCB board etching may not be entirely removed and create shorts as traces are eaten away. Using the wrong copper weight or plating issues can lead to increased thermal stresses that shorten the life of the PCB. Despite the failure modes in the manufacturing of a PCB, most failures do not occur during the manufacture of a PCB but rather in later use. The soldering and operational environment of a PCB often leads to a variety of PCB failures over time. The solder flux used in attaching the components to a PCB may remain on the surface of a PCB, which will eat away and corrode any metal contact. Solder flux is not the only corrosive material that often finds its way on to PCBs as some components may leak fluids that can become corrosive over time. Several cleaning agents can have the same effect or leave a conductive residue, which causes shorts on the board. Thermal cycling is another cause of PCB failures, which can lead to delamination of the PCB and play a role in letting metal fibers grow in between the layers of a PCB.
