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 Clock Skew
Arrival Variation
Timing variances between clock signals at various flip-flops during a test cycle can lead to data corruption in the scan chain. The presence of scan clock skew is a common cause of hold time violations between adjacent registers in a shift register configuration. This variation occurs due to differences in trace lengths, buffer delays, and the loading on the clock distribution network.
Skew Management
Mitigation strategies involve the use of balanced clock trees and the insertion of hold-fix buffers between scan cells. During the design of the test infrastructure, scan clock skew is monitored to ensure that data remains stable long enough to be captured by the following stage. Tools analyze the worst-case paths between all flip-flops in the scan chain to identify potential timing hazards.
In many cases, a dedicated test clock is used to provide more control over the signal timing during the shift operation. Automatic balancing routines ensure that the clock tree remains symmetrical across the layout.
Structural Validation
Success in the manufacturing test depends on the ability of the clock to reach all registers within a narrow time window. Excessive scan clock skew can lead to unpredictable test results and a reduction in the overall yield of the production line. Verification of the clock tree timing is a mandatory step in the physical design flow for all scan-based integrated circuits.