Investigating the Malfunctioning of Whisker Growth in AF114 and Similar Transistors
In the 1960s, the AF11x family of germanium transistors, including the AF114, were commonly used for high-frequency PNP applications such as radios and RF or audio circuits. However, these transistors have developed a reputation for failure, a problem that is now understood to be due to the growth of zinc whiskers on their metal cans.
Zinc whiskers are fine, conductive crystals that can grow on the zinc can that shields the transistor inside. This can shielding, intended to improve reliability, ironically promotes the growth of these whiskers due to the metal's properties. These whiskers can short internal connections, causing the transistor to fail.
When the cans were opened for inspection, the inside was found bristling with these whiskers, which grow over time and bridge connections that shouldn't be electrically linked. This failure is inherent because the whiskers keep growing, so the problem is persistent and difficult to avoid for vintage devices using these components.
Attempts at repair involve melting the whiskers by applying a high current pulse from a capacitor. This can temporarily restore the transistor's functionality, though often with altered characteristics. For example, a component tester might recognize the AF114 transistor as a transistor again after the whiskers are melted, but it might function as a confused NPN transistor instead of the intended PNP. However, this fix is not permanent since the whiskers will regrow, and the transistor will ultimately fail again.
The phenomenon of zinc whisker growth is well-documented in other electronic components as well but is a notable failure mode specifically for these AF11x germanium transistors. A video by Anthony Francis-Jones demonstrates that new-old-stock AF-series transistors all test faulty and show whisker growth on the can's exterior.
In summary, the cause of failure for the AF114 germanium transistor and related AF115 through AF117 transistors is the growth of zinc whiskers on their metal cans. The whiskers keep growing, making the problem persistent and difficult to avoid for vintage devices using these components. Repair attempts involve melting the whiskers with high current pulses, but this is only a temporary solution. The long-term reliability of these transistors is problematic due to their natural, irreversible failure mode.
Electronics and science intertwine in the growth of zinc whiskers on the metal cans of AF11x germanium transistors, a known failure mode particularly prevalent in the AF114 model. These whiskers, caused by the properties of the zinc can intended to shield the transistor, can short internal connections, causing the transistor to malfunction, and they keep growing over time, making the problem persistent and difficult to avoid, especially for vintage devices using these components.
