Chip resistor's failure phenomenon/mechanism and solutions (2)

The mechanism by which electrode separation occurs

A reflow process involves solder shrinking excessively, the printed board's bending, or the molding resin setting/shrinking. These phenomena create strong stress applied to the intermediate electrode, causing it to separate from the inner electrode. This increases the resistance value and ultimately leads to an open circuit failure (wire-breaking). This failure occurs when too much solder is used, or the chip resistor is mounted under reflow conditions not conforming to recommended reflow conditions. In addition, using a special solder type that suppresses crack formation, such as high-temperature solder, may also cause electrode separation. Incidentally, the molding resin setting/shrinking leads not only to cracking of the terminal but also to cracking of the chip itself.

Solutions to prevent electrode separation

Preventing electrode separation requires using a proper amount of solder and chip mounting under conditions conforming to recommended reflow conditions.

The following are criteria for determining the proper amount of solder.

T ≧ t ≧ T/3

T: chip resistor height
t: solder filet height

Recommended reflow conditions for our products are included in the product specifications available to the public. Please check the product specifications.

The mechanism by which solder cracking occurs

Cycles of temperature changes (temperature cycles) repeated in a service environment may cause the chip resistor and the printed board to thermally expand and shrink at respective expansion/shrinkage rates different from each other. This creates stress that may cause the solder to crack. This is a problem that often occurs in applications involving large temperature differences, such as using the chip resistor as an automotive component, thus it requires special caution to implement. Solder cracking increases the resistance value, leading to an open-circuit failure (wire-breaking) in the end.

A solution to prevent solder cracking

To alleviate the effects of temperature changes (temperature cycles) repeated in a service environment, it is important to use the proper amount of solder and select appropriate materials (PCB, solder). In addition, adopting components highly resistant to thermal shocks (a downsized component, a resistance with elongated electrodes, etc.) are also effective for reducing temperature cycle effects.

Our company adopts a soft electrode material to improve the product's resistance to stress, thereby suppressing crack development. We have a "highly heat-resistant chip resistor" on our product lineup. This product has achieved guaranteed performance for 1000 temperature change cycles in a -55°C/+175°C thermal shock test*.

*The product achieves its guaranteed performance for 1000 temperature change cycles in a -55°C/+175°C thermal shock test* when mounted under the recommended usage conditions for the solder, board, land, etc. For detailed information about this, please check the product specifications.

The mechanism by which resistive element burnout occurs

An excessive load, ESD, surge pulse, etc., causes a large current to flow instantaneously through the resistive element, generating heat as a consequence. This damages the resistive element, increasing its resistance or, worst case, leading to breaking the circuit (open circuit failure).

A solution to prevent resistive element burnout

The rated power is specified for the chip resistor, and using the chip resistor with power equal to or lower than the rated power is a basic requirement. So design the circuit where the chip resistor is supplied with power equal to or lower than the rated power. It is difficult, however, to keep an instantaneous power load, such as electrostatic discharge (ESD) and surge (on-off surge), equal to or lower than the rated power, so the following two measures to deal with a chip resistor exposed to ESD or surge should be taken.

1. (1) Provide the circuit with anti-ESD/surge components to minimize the overall effects of ESD/surge on the circuit (use an ESD suppressor, varistor, etc.)
2. (2) Select a component highly resistant to ESD/surge.

Our product lineup has a "highly resistant/highly reliable thin-film chip resistor" and a "small-sized high-power chip resistor." These chip resistors show high resistance to ESD/surge.