Corrosion in Printed Circuit Boards
Prevent dendrites in accidental PC board batteries
Printed circuit boards are potentially filled with accidental batteries. When there is moisture and voltage, inevitable contaminants will form a battery. Current flowing across these batteries leads to corrosion and failure.
In its simplest form, the migration of metal from the anode to the cathode is a simple plating reaction, where the current of electrons across the potential is balanced by the current of ions from anode to cathode. Electron beam machining uses this effect.
In this simple case, a large voltage is required to move the copper ions, or else no current would flow. The voltage could be reduced by heating the anode. The electrodes need to be kept in a vacuum or inert medium to prevent the copper ions from reacting with something else on their way to the cathode. The temperature and voltage required is described by the work function.
In a moist environment or a plating tank, there are many other chemicals present. The reactions are not as simple.
Metal plating processes use potent chemical solutions to keep the copper ions from reacting on the way to the cathode.
Once circuit boards are built, the reactions are supposed to stop. Any further reactions are called “Electrochemical Migration” or corrosion.
Louis Rossmann’s YouTube channel shows spectacular levels of circuit board corrosion as he repairs liquid-damaged circuit boards.
These examples are from the November 10 and 11, 2016 videos. (Warning: Louis is a colorful New Yorker, and his videos are long.)
The inside of unsealed electronics is dusty. Air flow from a fan or convection draws in dust. If the product creates enough heat, the dust will stay dry. This usually doesn’t cause a problem until it clogs the airflow and the product overheats. When left unpowered in a humid environment, dust absorbs moisture from the air. This can cause corrosion by itself, but more likely the corrosion will happen at power-up. Within seconds, dendrites form and create a short-circuit.
To watch dendrites form and to see the forces that shape them, see this video from my garage:
Design for Humidity
Printed circuit board material choice and layout can minimize humidity-related corrosion.
Electronegativity of Metals
Dissimilar metals in the presence of an electrolyte form a battery. The voltage of this battery depends on the difference in standard electrode potential of the dissimilar metals. Tin, silver, lead, and nickel are all fairly close to copper (0.337V) in electrode potential, while gold is higher. Aluminum is lower than copper in electrode potential. One extreme case to avoid is direct contact between aluminum (-1.662V) and gold (1.52V). This can happen between an aluminum enclosure and a gold-plated circuit board. These electrodes create a 3.182V battery. This doesn’t sound like much, since many power supplies are higher in voltage. However, the accidental battery caused by the dissimilar metals continues to operate when the power supply is turned off. With the power off, there is no extra heat to drive away moisture. Small amounts of moisture captured by dust is enough to cause corrosion.
Electromigration of Silver
Silver is unique in that it can form dendrites on printed circuit boards without liquid water. Humid air is enough to convey silver, and it can creep over wide gaps. See Metallic Electromigration Phenomena for details.
Take special care when using processes that include silver. Silver is more stable when it is dissolved in solder. Silver also reacts with sulfur compounds in air pollution. This same reaction causes silverware to tarnish.
Guard Ring Design
Guard rings protect sensitive circuit nodes by reducing electric fields. An example of a sensitive node is the output of a high-impedance sensor. The sensor creates small amounts of current. Any current due to corrosion will look like sensor current, and create an error. Because of their extra sensitivity, high-impedance circuits tend to fail first in a humid environment.
A photodetector is an example of a high-impedance circuit. An opamp converts the small photocurrent into a voltage.
The photodiode output is protected by a ring at a ground potential. This sensitive high-impedance output node operates at virtual ground, and can easily be protected with a grounded guard ring. This eliminates the electric field around the high-impedance node to prevent corrosion.
The MCP6001 opamp in this example has extra space around the negative input pin, leaving room for the guard ring. The resistor R1 straddles the guard ring. Use at least an 0805 size resistor, because traces underneath smaller resistors cause manufacturing problems.
For more guard ring example circuits and guidelines, see Layout For Precision Op Amps. For a more detailed version of this photodiode circuit, see Hello, Diodes!
Increase Spacing Between High Voltages
As circuit board geometries continue to shrink, watch out for adjacent traces with large voltage differences. Today’s circuit boards can have 3 mil spacing between traces. If these two traces carry +15V and -15V, there is an electric field of 30V/3 mil = 10kV/inch. This could start an arc in a dry air!
Watch the board repair videos to see how much more sensitive high-voltage circuits are to moisture. For example, laptop backlight driver circuits are often the first circuits to fail due to liquid damage.
At lower voltages, narrow spacing is still a challenge. Arcing is less of a problem, but the rate of corrosion increases directly with the electric field. Reduce corrosion failures by increasing spacing between traces at different voltages.
Use Rounded Corners
Oval pads on circuit board ICs help minimize the electric fields that lead to corrosion. Sharp points and corners cause higher electric fields than rounded corners. A sharp point or corner has much higher curvature than a rounded edge. This makes oval pads more reliable than square pads.