Added 81:20 (pad center ID) to the list of ignored layer:purpose
pairs in the magic tech file.

sky130/magic/sky130.tech

diff --git a/sky130/magic/sky130.tech b/sky130/magic/sky130.tech
index 121c5ca..8ec8675 100644
--- a/sky130/magic/sky130.tech
+++ b/sky130/magic/sky130.tech
@@ -2394,6 +2394,7 @@
  ignore HVNWELLID
  ignore PADDIFFID
  ignore PADMETALID
+ ignore PADCENTERID
 
  layer pnp NWELL,WELLTXT,WELLPIN
  and PNPID
@@ -4031,6 +4032,7 @@
  calma MET5BLOCK 72 10
  calma PADDIFFID 81 6
  calma PADMETALID 81 8
+ calma PADCENTERID 81 20
  
  # LICON
  calma CONT 66 44