Modeling Dynamic Parasitic Loading Effects on Scan Network Timing Margins
Dynamic parasitic extraction during ATPG shift prevents false timing violations by accounting for state-dependent capacitance and power rail collapses.
Scan Enable Skew
Control Delay
Propagation delays affecting the control signal that toggles flip-flops between shift and capture modes can result in improper logic states during structural testing. If scan enable skew is not managed, some parts of the chip might remain in shift mode while others have already transitioned to functional mode. This mismatch prevents the accurate capture of test results and can lead to false failures in the manufacturing screen.
Signal Distribution
Designing the scan enable network requires attention to ensure uniform signal arrival. When a test pattern is executed, scan enable skew must be minimized so that the transition occurs simultaneously across the entire device. High fan-out buffers and dedicated routing layers are often used to maintain the integrity of this signal.
Failure to synchronize this transition can cause the chip to enter an indeterminate state during the capture window. Timing tools calculate the arrival times at every scan cell to confirm that the signal falls within the required window.
Test Integrity
Reliability of the structural test process is confirmed by verifying the timing of the scan enable signal across all operating conditions. If the scan enable skew exceeds the allowable limit, the test patterns will not execute correctly on the silicon. Designers use specialized timing analysis tools to check for these violations before finalizing the layout.