Ceramic capacitors for automotive electronics: Lifting quality to ppb level

EPCOS ceramic capacitors satisfy the quality requirements of automotive electronics (Figure 1). All its product series satisfy the AEC-Q 200 standard. However, with the Advanced series and—at its core—the ppb (parts per billion)-level assurance system, the company offers a series of products that even exceed the AEC-Q 200 requirements. It offers customers the multilayer serial ceramic capacitor (MLSC) for special and highly critical points on the board, such as those with a direct connection to a battery or generator. This solution is designed for particularly critical applications, but is also the right product for wide use in many safety-relevant applications.

CRITERIA FOR HIGH RELIABILITY
Reliable process control is the basis of high reliability for both OEMs and component manufacturers. If we assume that the component manufacturer supplies a perfect product, then further correct processing by customers is the precondition for smooth operation of the components—and hence the entire application. Thus apart from the high quality of an individual component such as a ceramic capacitor, the process control along the entire production chain up to the end product is decisive. This requires a high mutual understanding of the process between the individual partners such as the component manufacturers and ESMs—as well as joint definition of the quality guidelines.
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THE PPB LEVEL
AEC-Q 200 is a standard that stipulates quality requirements on components in various technologies. Because of its superordinate character, this standard does not do equal justice to every component. Especially when it comes to approaching zero defects, process control moves to center stage and becomes the critical element for assuring and continuously improving maximum reliability. Thus, EPCOS has laid the basis for its zero-defect strategy with its concept of the ppb-level assurance system.

To reach the ppb-level, the entire production process is systematically divided up into sub-processes that are continuously checked by statistical methods on the basis of quantifiable parameters. Within the scope of its Advanced series, EPCOS applies these analysis methods to all production steps: from the component design up to the 100% electrical check of the manufactured products. Among the key preconditions for high component reliability is the company’s own manufacture of ceramic powder. Only the knowledge of all inter-relationships and factors from the raw material to the finished component allow the self-defined quality criteria to actually be satisfied in all production steps.

Illustrative examples of these statistical analyses are the distribution of the loss factor tan δ and the evaluation of a solder shock test followed by a highaccelerated life test (HALT).

Figure 2 shows the statistical distribution of the loss factor tan δ in a production lot of the kind carried out in the Advanced series. Simply checking the specified limits no longer suffices for the zero defects requirement. The purely qualitative evaluation of the parameter tan δ was consequently replaced by a dynamic measurement. This goes beyond the previous stipulation that the parameter simply lies within or outside the limits. The mean and variance of the distribution become the new guiding values for the quality evaluation of the loss factor tan δ. The statistical distribution that is considered to be good when it lies within the specification is now additionally assessed with respect to the deviation of the measured values from the statistical mean. Any deviating parts are eliminated. This is done with the aid of a dynamic test limit (green marking) as a supplement to the fixed specification limits.

Whereas some measurements, such as that of the loss factor tan δ, can be integrated into the production process, there are values that can only be determined by random sampling: a defined percentage of components in a lot are tested up to possible failure by increasing the stress factors. The determined distribution is then used to draw conclusions about the properties of many components or the quality of an entire lot. Among the destructive tests used for ceramic capacitors are the solder shock test followed by a HALT or a bending test. Both tests may be used to draw conclusions about the quality of an individual lot and its development over time. Such destructive tests often stress the components far beyond their specified limits. This already allows existing or impending quality defects to be identified at a very early stage and countermeasures to be initiated even before the specifications are violated. The statistical evaluation is performed with the aid of a Weibull analysis.

SOLDER BATH
In the solder shock test followed by a HALT, the ceramic capacitors on the board traverse a solder bath (360 °C/5 s) with no prior heating. This already stresses the components strongly due to the high temperature gradient. No failures may occur here.



In the subsequent HALT, the capacitors are stressed to failure under relevant temperature and voltage conditions. The result is a statistical curve of the pattern of failures over time, which in turn allows conclusions to be drawn about the quality of the components. This introduces a further measurable and quantifiable criterion that goes beyond a good or bad decision, namely the failure time. With the aid of the Arrhenius equation and a knowledge of relevant material parameters, conclusions can thus be drawn about the expected service life under relevant operating conditions.

The ppb level assurance system is the core element of the Advanced series, for which it is specified. This series allows an error rate of 10 ppb or lower to be realized as regards 0km or field failures. The advanced series is predestined for use in safety-relevant applications such as ABS, ESP or airbag systems, as well as in industrial and consumer applications and in telecommunications electronics with their particularly high qualitative requirements. The Advanced series allows the zero-error strategies of the OEMs to be implemented with greater efficiency across all applications.