Discussion about the Korg PolySix synthesizer

Don't use only solder to bridge the traces across the crack.
Scrape off the solder mask on either side for maybe 1/4" or more
and lay a piece of resistor lead or something similar across the 
crack then solder to the trace on both sides.   In really severe cases,
you could glue something flat like a piece of some old pc board
to this one to span the crack in-between where the traces are to make
it stronger.  In most cases just fixing the traces is enough.