How does the hybrid Custom Router and Zroute flow split the work, and why?

Short Answer

In the hybrid flow the Custom Router routes the trunks of special nets and deliberately leaves some or all pin connections unfinished, and Zroute completes them. The two routers connect to pins differently, so letting Zroute make the pin connections avoids inconsistencies and DRCs. The split is set with custom.route.skip_connect_pin_type.

Technical Explanation

• custom.route.skip_connect_pin_type defaults to none, which disables the hybrid flow. Setting it tells the Custom Router which pin connections to leave for Zroute.
• The values are all (the Custom Router completes no pin connections), stdcell (all except standard cell pins), io (all except I/O cell pins), macro (all except macro pins) and auto (it judges pin routability and completes what it can).
• custom.route.distance_to_net_pin limits how far the trunk may stop from an unconnected pin, per net.
• After route_custom, run route_eco on the same nets. That is where Zroute makes the pin connections.
• Then remove_redundant_shapes on those nets cleans up leftover pieces, and route_detail -incremental true fixes remaining DRCs.
• Layer rules are often set with -min_layer_mode allow_pin_connection so trunks stay high but pin connections can drop to the pin layers.
• The benefit is that each router does what it is best at: the Custom Router handles shielding, matching and special geometry on long trunks, and Zroute handles dense, rule-heavy pin access.

What To Check

• Open pins after route_custom, which should match the pins you chose to leave.
• Opens after route_eco, which should be zero.
• DRCs at pin connections after incremental detail routing.
• That the trunk kept its constraints after Zroute's changes.

Healthy, Suspicious & Hard-stop Results

• Healthy (illustrative): Trunks meet their constraints, route_eco closes every pin connection, and the final check is clean.
• Suspicious (illustrative): A few DRCs at pins where the trunk stopped too close, fixed by tuning the distance to pin.
• Hard stop: Hybrid flow left at none while the two routers both try to connect pins, producing mismatched pin connections.

What The Interviewer Is Testing

• Whether you know why the pin connection split exists.
• Do you know the option values and when to pick each?
• A strong answer shows you remember the clean-up sequence after the handoff.

Follow-up Question & Model Response

"When would you choose stdcell rather than all?"

Candidate Model Response: When the net connects to macros or I/O cells where the Custom Router's connection style is fine or even preferred, but also fans out to many standard cell pins, where Zroute's pin access handling is better. stdcell lets the Custom Router finish macro and I/O connections along with the trunk and hands only the standard cell pins to Zroute. all is the simplest choice when you want every pin handled the same way.