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Electromigration Affects SMT PCB Boards

When current passes through a conductor, electromigration causes metal atoms to move. This can cause gaps in the metal and interrupt current flow, leading to performance issues or failure of the circuit board. To prevent this, smt pcb board manufacturers must use thorough testing and qualification methods, including accelerated aging tests to ensure the reliability of their products.

During the manufacturing process, the PCB is first coated with solder paste, which is applied using stencils to ensure that the paste only reaches designated pads. The components are then placed onto the paste, and the entire assembly is sent through a reflow oven to melt the solder and connect the component to the circuit board. After the reflow process, the assembly is tested to verify that it meets its specified electrical and mechanical specifications. These tests include accelerated aging and electromigration, which can be used to detect and mitigate potential problems with the circuit board.

The most common type of accelerated aging test is the EMAP, which measures the effect of moisture and temperature on the conductive properties of the copper. This is an important test because it can identify moisture and temperature-related degradation, such as electromigration, which can lead to short circuits or poor connections in the assembly. It can also help determine the effectiveness of anti-corrosive treatments and other environmental protection processes.

How Electromigration Affects SMT PCB Boards

In a typical EMAP test, the PCB is exposed to a high-temperature environment and a constant current is passed through it for a defined period of time. The resistance of the circuit board is measured after the test has been completed, and any significant increase in resistance can indicate the presence of a problem such as electromigration. The most common type of electromigration damage is the formation of stubs or whiskers that bridge two conductors, creating a short circuit. Another common issue is the depletion of metal at the anode side of a via, which can result in a long-term open circuit.

Electromigration can occur in narrow conductive paths, such as thin traces and small vias, because of the high current density that flows through them. The higher the current density, the more likely it is that the conductor will fail due to electromigration. It is also possible for the metal atoms to diffuse into adjacent conductors, which can lead to intermetallic compounds (IMC) that can cause problems in electronic devices.

The best way to avoid this is to design your PCB with adequate widths for the current density you expect to see, and to check that the wires actually have enough width after chemical-mechanical polishing. There is a limit to the length of a conductor that can tolerate electromigration, which is known as the Blech length. If a metallization line is shorter than the Blech length, it will not experience electromigration damage. This is because the stress gradient within the conductor will not exceed the critical value for damage. However, if the metallization line is longer than this, it may experience electromigration damage and fail over time.

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