  One of the most promising routes towards scalable quantum computing is a
modular approach. We show that distinct surface code patches can be connected
in a fault-tolerant manner even in the presence of substantial noise along
their connecting interface. We quantify analytically and numerically the
combined effect of errors across the interface and bulk. We show that the
system can tolerate 14 times higher noise at the interface compared to the
bulk, with only a small effect on the code's threshold and sub-threshold
behavior, reaching threshold with $\sim 1 \%$ bulk errors and $\sim 10 \%$
interface errors. This implies that fault-tolerant scaling of error-corrected
modular devices is within reach using existing technology.
