March 2011

Soft-termination MLCC

Driving MLCC reliability in rugged environments

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The extremely harsh conditions prevailing in automotive electronics place especially high demands on the reliability of the solder joints of the many MLCCs that are used. TDK MLCCs are now available with advanced soft terminations.

Modern cars are typically equipped with a thousand or more MLCCs. While such capacitors are renowned for their long life and high reliability, the operating environment to which automotive electronic devices are exposed is characterized by wide temperature range from −40 to 125 °C (in some applications even up to 150 °C), shocks and vibrations, and many other adverse factors that can affect the solder joint. Furthermore, the increasing use of lead-free solder, which is less elastic than conventional solder, leads to solder joints that are harder and more brittle. As a result, solder cracks can develop at the joints when the PCB bends or flexes due to thermal shock or mechanical influences. Overcoming this problem is a major goal for improving the reliability of the solder joint. The problem of joint reliability can be ameliorated by improving the physical properties of the lead-free solder or by reducing the size of parts, for example, but these measures do not represent a fundamental solution.

 

For this reason, TDK-EPC has developed MLCCs that incorporate soft flexible terminations and are designed to absorb and reduce the deflection stress of the circuit board (Figure 1). They employ a conductive resin electrode layer between the copper base and the nickel plating of the terminal electrode. This layer serves to absorb and soften deflection stress of the circuit board caused by factors such as high temperature and shocks and thus suppresses the formation of solder cracks. The conductive resin itself is made of epoxy or other synthetic resins mixed with a filler of conductive particles (such as silver).

Figure 1: Conductive resin terminal type product
The soft conductive termination is applied between the copper electrode and the nickel barrier. It absorbs mechanical shocks and thermal elongation.

High resistance to thermal shock and board flexing

Japanese Industrial Standards (JIS) specify various test methods for MLCCs soldered to a PCB, to determine characteristics such as resistance to thermal and mechanical influences. In an ECU located in the engine compartment of a car, vibrations, shocks and bending can occur that affect the PCB. In addition, thermal shocks and temperature cycling causing expansion and contraction also increase the risk of cracks. Figure 2 illustrates the results for thermal shock testing (3000 cycles) with a temperature cycle of −55 to +125 °C. While the adherence strength of a conventional MLCC drops by 90 percent, that of an MLCC with conductive resin terminals decreases by only 50 percent. Conventional MLCCs exhibit cracks in the solder. By contrast, an MLCC with soft conductive resin terminations only shows a partial separation of the nickel plating and the conductive resin layers.

Figure 2: Thermal shock (temperature cycling) test

Temperature cycle: −55 to +125 °C

Lead-free solder: 96.5 Sn / 3.0 Ag / 0.5 Cu

 

Comparison of joint strength using applied pressure testing after thermal shock (temperature cycling). The photos on the right show the superior performance of the new conductive resin TDK MLCCs over conventional MLCCs.

A board bending test reveals similar results (Figure 3). The conventional MLCC already exhibits a crack in the ceramic element at a 4 mm deflection, while the MLCC with soft terminations can easily withstand more than twice the deflection. Moreover, when excessive tension is applied, the ceramic element of the conventional MLCC develops a crack, while the MLCC with conductive resin terminals only shows a separation of the nickel plating layer from the conductive resin layer, but no cracking.

Figure 3: PCB bending stress test

Soft termination TDK MLCCs can withstand a deflection of 8 mm, which is twice that of a conventional MLCC.

Preventing cracks to the MLCC

Cracks in the capacitor element itself can represent a more serious problem than solder cracks. When the crack destroys the internal electrode, dielectric breakdown may occur. Cracks in capacitor elements usually follow a certain pattern. When the terminal electrode is firmly joined by solder, deflection stress is concentrated on the joint section of the terminal electrode, and the crack will generally develop starting from the tip of the electrode and advancing through the ceramic element.

 

Cracks in the capacitor element often occur due to improper handling of the PCB after component mounting. In order to achieve higher efficiency during production, the components are all placed on a long continuous board on the mounting line in one operation, and the board is later separated into the individual boards. If the boards are broken off manually rather than by dicing or using special tools, deflection stress can cause cracks in the capacitor element.

 

TDK-EPC has successfully solved the joint reliability problems, which can occur related to the use of lead-free solder, by developing an MLCC with a conductive resin layer within the terminal electrode. This technique also lends itself to the production of large capacitors with high capacitance, thereby increasing the range of options available to design engineers. Therefore, application areas for soft-termination MLCCs include not only automotive electronics equipment but also electronic equipment installed outdoors under demanding environment conditions.

Problems caused by lead-free solder

Conventional solder, which is an alloy of tin and lead, has a low melting point, is inexpensive and easy to work with, but it is also an environmental pollutant that is harmful to humans. Therefore, different types of lead-free solder composed of tin, silver, and copper but no lead are now being used as a replacement. Currently available lead-free solder however has a higher Young’s modulus (measure of stiffness of a material) than conventional lead-based solder, making it harder and more brittle and thus more vulnerable to expansion and contraction. Therefore, when the PCB on which the chip components are mounted is subject to deflection stress due to warping or bending, the solder joint can deteriorate and cracks can appear.

Another defect that can occur with lead-free soldering is the development of microscopic cavities (Kirkendall voids), which in turn lead to reduced cohesion. When two different types of metal that are in close contact are heated, atomic dispersion occurs. This effect is known as the Kirkendall effect. Because the speed of dispersion differs depending on the type of atom, repeated thermal cycling can cause the formation of voids that eventually lead to solder cracks. The engine compartment of a running car regularly reaches temperatures of 100 °C and higher. This causes populated circuit boards to expand and contract, leading to deflection stress that can result in cracks and voids in solder joints, thereby lowering the joint reliability.

Product portfolio: Lineup of TDK MLCCs with soft terminations

Soft terminations are available for:

– All ranges of 2-terminal MLCCs with rated voltages from 6.3 to 630 V DC

– All ranges of array capacitors (dual-element type)

– 150 °C temperature-resistant types (X8R)

Conductive resin terminal electrodes can also be implemented in other mid-voltage MLCCs with C0G temperature characteristics.

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