Quality Metric
The density of nonconforming printed circuit board assemblies that bypass all inspection stages and are shipped to the customer is expressed in a normalized ratio. This baseline measure, termed escape rate dppm, is calculated by dividing the number of defective units found after shipment by the total number of units shipped, multiplied by one million. Low values indicate highly effective testing and inspection processes, while high values point to inadequate screen methods.
Understanding this metric helps manufacturing organizations evaluate the performance of their automated optical inspection and functional test stations.
Statistical Derivation
Inspection efficiency and test coverage are the major factors that determine the volume of faulty boards leaving a factory floor. In a high-volume assembly line, escape rate dppm depends heavily on the capability of in-circuit testing and automated x-ray inspection. If an automated optical inspection machine suffers from poor programming or high false-call rates, operator fatigue can lead to genuine faults being erroneously cleared as acceptable.
Additionally, defect types that cannot be detected by optical means, such as internal delamination or marginal solder joints, will contribute directly to the undetected failures that reach the customer.
Operational Influence
Inadequate defect screening results in elevated warranty expenses and damaged client trust. Reducing escape rate dppm requires continuous improvement of testing protocols and the integration of closed-loop feedback systems between final inspection and the screen printers.
