Circuit design optimization based on quality cost estimation

The variations in component parameter values embodied in an electronic circuit can result in it failing to conform to its specifications. Such failures can cause the manufacturer to incur significant quality costs, arising from factors including warranty returns and legal liabilities. It can be difficult to predict both the probability that a circuit will fail to meet its specification and the consequent quality costs. It can also be difficult to determine which component parameter variations are most significant in causing faults. In this paper we introduce the concept of functional capability, which is a variation on the familiar process capability, as a measure of the performance variation found in a circuit relative to the specificed range of acceptable performance. Based on this capability measure, along with a failure modes and effects analysis and a cost model, we are able to relate functional capability to quality costs. In addition, we introduce a capability breakdown which allows the effect of different parameter variations to be readily visualized so that the designer may focus effort where it will be most effective in reducing quality costs. The resulting technique is primarily intended to aid the design of printed circuit board level analogue and mixed signal circuits, particularly in safety critical applications. The paper illustrates the technique through analysis of a simple circuit and the analysis of a commercial mixed signal circuit. It also shows how the technique may be integrated within a capable design, manufacture and test analysis process.