Naoyuki Kobayashi, Senior Product Manager at Murata Europe explores how, by minimising ceramic capacitor cracks due to board bending in harsh environments such as automotive, can improve device reliability
The reliability of electronic systems in harsh environments, such as in automotive applications, will depend on the properties of individual components, right down to the smallest ceramic capacitor.
Ceramic capacitors have made a strong case for selection over alternative technologies for both power electronics and standard circuitry used in harsh environments. Across all applications, monolithic multilayer ceramic capacitors (MLCC) are the most used capacitor technology at 1, 10 and 100uF values.
A similar trend is also happening in automotive – ceramic has a market share up to 10uF values of more than 90 percent, and is growing fast for 22uF and 47uF values, in particular. Consequently, ceramic is now the most common type of capacitor found in automotive applications.
Given the necessary trade-offs between size, capacitance, stability and breakdown voltage, ceramic capacitors offer excellent overall performance over the alternatives. All of these factors are aspects of the capacitor dielectric.
Recent developments in ceramic materials have led to dramatic increases in the dielectric constant and capacitance that can be achieved for a given capacitor size and this has allowed capacitors to be manufactured with fewer layers, and therefore with reduced thickness.
However, in addition to size, capacitance, stability and breakdown voltage, a key property that automotive manufacturers value highly is mechanical strength. A critical issue is the prevention of short circuits, particularly in harsh environments such as in automotive; and a cracked capacitor can cause a short circuit, so demands have never been greater for enhanced mechanical strength offered by automotive MLCCs.
In vehicles, typically, when subjected to high mechanical stress, induced by board flexing, or vibration, or a wide variation of temperature, the ceramic capacitor body can crack, causing a short circuit. The failure mode of the ceramic capacitor can be short, and where a capacitor is directly connected to the battery, for example, in the worst-case scenario, board burnout could occur.
However, unlike capacitance, size and breakdown voltage, the mechanical strength of a capacitor is not simply a property of the dielectric material. The structure and construction of the capacitor play an important part in preventing cracks.
Harsh environment capacitors
Two capacitor technologies developed by Murata, that help to meet automotive manufacturers’ demands for mechanical strength are changes to the structure of the MLCC rather than dielectric materials development.
The company’s GCJ, GCD and GCE series of MLCCs have been specifically designed to accommodate the bending, vibration and thermal conditions experienced in under-the-bonnet automotive applications to provide a fail-safe against short circuits.
The usual mode of failure is a crack across the ceramic material from the point of largest stress diagonally upwards, effectively snapping the corner off the device.
To combat this, a technology called ‘Soft Termination’, which is effectively a layer of conductive resin between the copper electrode and the nickel/tin plating of the termination is used.
In the event of bending stress, this layer will start to peel off the ceramic device, mitigating the bending stress on the ceramic and avoiding cracking so the device can continue to function. This results in excellent mechanical performance, particularly with respect to bending stress.
In a comparison test of bending strength between the GCM series capacitors (without soft termination) and the GCJ series (including soft termination technology) tested up to 8mm deflection, (which was the limit of the test equipment), showed that all the GCM series capacitors survived 2mm deflection, but none survived at 8mm. Soft-termination technology enables almost all the GCJ series capacitors to survive beyond the 8mm deflection level.
Demands from automotive manufacturers have also included a request for an automotive-grade MLCC for direct connection to the battery or generator in a vehicle. The design of the MLCC has to reduce the effects of bending fracture and solder-shock cracks by preventing the device from failing in a way that would produce a short circuit.
The company therefore developed a device with a simple structure that contains two capacitors in series, known as the MLSC (multi-layer capacitor range).
This approach uses an internal ‘floating’ plate, which creates an equivalent circuit of two capacitors in series. In this way, a short-circuit condition is highly unlikely to happen due to component failure.
Damage to the device can occur when the board is exposed to bending stress or extreme temperatures during the soldering process, for example.
An ordinary MLCC capacitor, when cracked, may fail to a short circuit. However, in the MLSC, the crack damages only one of the two capacitors.
Despite one capacitor being shorted, the other remains in operation and avoids a short circuit failure of the whole device. In many cases, a typical mode of failure would be a crack starting from the edge of the termination and penetrating one side of the capacitor.
If this crack happens on one side, the mechanical stress on the device is mitigated and it is unlikely that a similar crack will affect the other capacitor.
In addition to the GCJ and GCD series, the company has also announced a series, which implements both of these important developments in one capacitor range, including both soft termination technology and the MLSC approach.
Ceramic capacitors are now widely used in the vehicle powertrain, safety applications and infotainment systems. It is innovations such as soft-termination technology and multi-layer series capacitor layout that helps automotive electronics manufacturers achieve the required levels of high reliability.