Structural innovations reduce capacitor cracking
30 November 2010
In harsh automotive environments, reliability of electronic systems depends on the properties of the individual components, down to the smallest ceramic capacitor.
In this article, Naoyuki Kobayashi details two innovations from Murata that contribute to reliability of MLCCs.
Despite many advances in plastic film and electrolytic capacitors, the monolithic multilayer ceramic capacitor is by far the most common type found in automotive applications. Ceramic capacitors present an interesting trade-off between size, capacitance, stability and break-down voltage, which are all aspects of the dielectric.
The ceramic material used defines the dielectric constant, which in turn has an impact on the other parameters. The higher the dielectric constant of the ceramic insulating material used between a capacitor’s plates, the higher the capacitance for a given plate area and distance between the plates.
The ceramic ‘mix’ determines the dielectric constant. Materials with a higher dielectric constant exhibit less stable temperature characteristics than those with a low dielectric constant, but higher values of capacitance can be achieved within a given volume, so the choice of dielectric is a compromise.
Also, the more stable the dielectric, the more expensive the capacitor, not least because, for a given value, more material has to be used.
Thinner dielectric layers increase the capacitance for a given plate area and dielectric type, but thinner layers exhibit lower breakdown voltages.
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 with fewer layers – and therefore reduced thickness – to be manufactured.
Apart from capacitance, size and breakdown voltage, another key property that automotive manufacturers value highly is mechanical strength; demands for enhanced mechanical strength in automotive MLCCs have never been greater. In today’s vehicles, capacitors are subject to severe flexing, vibration and mechanical stress in normal use and this causes cracking. A cracked capacitor can cause a potentially fatal short circuit, so the ability to withstand mechanical force is paramount in these applications. 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.
Murata has developed two new capacitor technologies that help to meet automotive manufacturers’ demands for mechanical strength. Both are changes to the structure of the MLCC rather than dielectric materials development.
Figure 1a shows a typical Murata GCM series MLCC with the type of cracking damage visible when the component is subject to severe bending stress. The usual mode of failure is a crack across the ceramic material from the point of largest stress (marked on the figure), diagonally upwards, effectively snapping the corner off the device. To try to combat this mode of failure, Murata has developed 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 (see figure 2a). 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 (shown in figure 1b).
This results in excellent mechanical performance, particularly with respect to bending stress. Soft termination technology is incorporated into Murata's newly introduced GCJ series of MLCCs, which is AEC-Q200 qualified.
Figure 3 is a comparison of bending strength data between GCM series capacitors (MLCC without soft termination) and GCJ series (MLCC with soft termination technology), as measured by Murata. The x-axis shows millimeters of deflection of the PCB and the samples were tested up to 8mm deflection, which was the limit of the test equipment. All the GCM series capacitors survived 2 mm of deflection, and 0% survived up to 8 mm.
Soft termination technology enables almost all the GCJ series capacitors to survive beyond 8 mm deflection.
The demands from automotive manufacturers also included a specific request: they wanted an automotive grade MLCC for direct connection to the battery or generator in a vehicle, whose design would 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. Murata developed a device with simple structure that contains two capacitors in series, called MLSCs (multi-layer series capacitors), shown in figure 2b. When the board is exposed to bending stress or extreme temperatures during the soldering process, for example, the device may become damaged. For an ordinary MLCC the capacitor, when cracked, may fail to a short circuit. In Murata's MLSC, the crack damages only one of the two capacitors.
Despite one capacitor being shorted, the other remains in operation, avoiding 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 only. If this crack happens on one side, the mechanical stress on the device is mitigated and it's unlikely that a similar crack will affect the other capacitor.
In the excitement of great advances in semiconductor technology for automotive electronics, it’s easy to overlook the substantial progress that’s also been made in ceramic components in recent years.
Ceramic capacitors are now used in vehicle power trains, comfort systems, safety circuits and infotainment systems. The reliability of each of these systems depends on the individual properties of all of its components, down to the smallest capacitors. It is simple innovations like soft termination technology and multi-layer series capacitor layout through which Murata helps automotive electronics manufacturers achieve reliability for their products.
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