Reducing Solder Cracks in MLCCs Due to Thermal Expansion

Multilayer ceramic chip capacitors (MLCCs) are the most widely used surface-mount capacitor technology in the electronics industry, and over time they have experienced additional growth by replacing other capacitor dielectrics due to their evolving capacitance/voltage (CV) capabilities. Some of the applications they are used in — for example, automotive (especially under the hood), drilling and mining, and aerospace — experience rapidly changing thermal environments. In these markets, resistance to heating and cooling cycles is a very important requirement, because the difference between the thermal expansion of the PCB and the termination and mounting methodology of the MLCC can result in a solder failure, especially after numerous cycles.

Vishay has developed a polymer termination system with extended bend capabilities that allow it to absorb both board flexure stress and the stress from thermal expansion and contraction, making this termination method more suitable for environments with temperature variations.

To demonstrate the stable performance of this termination technology during thermal fluctuation, a thermocycle test was chosen that followed the AEC-Q200 and JESD22 Method JA-104 standards with temperature cycles from -55 °C to +125 °C. However, the number of cycles was increased to 3000.

Two termination electrodes were measured and compared: the standard metallic and the polymeric version for extended bending capability. Four different case sizes — 0603, 0805, 1206, and 1812 — terminated with both types of pastes and soldered to a PCB using lead (Pb)-free solder — were used and placed in the thermocycle chamber.

Following assembly the initial shear force was measured during a push test. This measurement was repeated after 1000, 2000, and 3000 thermal cycles. Cross-sections of several capacitors were prepared at each measurement stage to study the degradation mechanism.

Graph: average results of shear test, normalized to initial values. (Image source: Vishay)

The data showed that the shear force degradation is linear up to 3000 cycles, as presented in the above graphs. There is a difference in the length of expansion and contraction of a PCB compared to the MLCC device and the solder, which is more pronounced in the larger body sizes. Therefore, the stability of thermal fluctuation is lower for such case sizes. Over the 3000 cycles, the bonding strength of the standard metallic termination decreased by approximately 80%, while the polymer termination system degraded by less than 50%. This is because the MLCCs with flexible polymer termination can partially absorb the stress developed by thermocycling.

Cross-section view after temperature cycles, 0805 body size. (Image source: Vishay)

For this test, the most popular and environmentally friendly solder paste, tin-silver-copper (SAC) was used. Evaluation of the cross-sectioned parts showed that the failure mode was a cracking of the lead (Pb)-free solder fillet.

In Conclusion:

Cracks in the lead (Pb)-free solder used in the surface-mount production assembly of MLCCs can occur during the high number of thermal cycles that are often seen in automotive and other high-temperature applications. Polymer terminations used to enhance the bending capability of the MLCCs demonstrate an improvement in device flexibility by absorbing stress and reducing the mismatch of shrinkage/elongation between the solder, the capacitor, and the PCB caused by changing temperatures.

Using MLCCs with these polymer terminations is therefore an excellent potential solution for these applications, and those where high vibration or board flexure stress (ex: during PCB assembly and soldering) are involved, and for other environments with severe and continuous thermal fluctuations.