Chip resistor’s failure phenomenon/mechanism and solutions (1)

Migration mechanism

Migration is generally a phenomenon occurring with a pair of electrodes placed in a high humidity environment. When voltage is applied between these electrodes, the anode metal is ionized and moves to the facing cathode and regenerates as metal again.
The metal generated on the cathode grows into a tree-like shape and reduces insulation between poles, eventually causing a short-circuit.
Migration generally occurs when the three factors of electricity, water and halogen substance co-exist, by generating “Sn migration” on the exterior (solder) of the electronic component and “Ag migration” generated within the component.

Mechanism of electrolytic corrosion

As described in migration, when a halogen substance (F, Cl, Br) residue, contained in flux, adhesive, or cleaning agent, attaches to the surface of a chip resistor, and is exposed to a sulfur-containing atmosphere, the chip resistor surface protection membrane deteriorates, and moisture in the atmosphere containing halogen and sulfur tend to penetrate in the resistive element.
As thin-film chip resistors use a nickel-chrome alloy (Ni-Cr), the resistor element can be corroded by the penetrating moisture containing halogen and sulfur, leading to a resistance value increase and open circuit (wire breakage).

Thick film chip resistors consist of a resistive element made from a sintered mixture of ruthenium oxide and glass, and therefore free from electrolytic corrosion.

Method of solving electrolytic corrosion

The following three methods are applicable in avoiding electrolytic corrosion.

• (1) Remove halogen substance residue with a non-halogen cleaning agent.
• (2) When using flux or adhesive, employ the appropriate heat treatment* to make halogen substances inactivate.

  *For detailed heat treatment conditions, consult with each flux or adhesive manufacturer.

• (3) Remove or isolate from a sulfur-containing atmosphere.

Sulfurization mechanism

When a chip resistor is exposed to a sulfur-containing atmosphere, sulfur content penetrates through the gap between the protective film and plating. When the sulfur content reaches the internal electrodes (silver), silver (Ag) and sulfur (S) cause a chemical reaction and form an insulating substance (Ag2S), thereby corroding internal electrodes and increasing the resistance value, eventually causing an open circuit (wire breakage).

Locations that have a sulfur-rich atmosphere include areas near a volcano or hot spring, automobile exhaust gas, and environments using cutting oil or rubber products. Even in a general environment, some materials sometime contain sulfur content. For example, cooling fan filters, rubber products and sponges used for packing and vibration prevention, even silicon-based coating agent may promote sulfurization.

Sulfurization solution method

The following three methods are thought effective for preventing sulfurization.

• (1) Remove or isolate from sulfur-containing atmosphere.
• (2) Do not use sulfur-containing materials.
• (3) Select components that are strong against sulfur contents.

The most effective method for preventing sulfurization is (3) selecting components highly resistant to sulfur content.
Our company offers a product lineup of “Anti-sulfurization chip resistors” using high palladium-silver electrodes and gold electrodes that are highly resistant to sulfurization.

This article described “Outline of failure phenomena” along with “Migration”, “Electrolytic corrosion” and “Sulfurization”.
The next issue provides explanation of the four items from “Electrode peel-off” to “Resistive element deterioration”.