diff --git a/fpga/11_build_bitfile.sh b/fpga/11_build_bitfile.sh
index 2d71673..9c84436 100755
--- a/fpga/11_build_bitfile.sh
+++ b/fpga/11_build_bitfile.sh
@@ -5,7 +5,9 @@ set -e
 . script_env
 setup_vivado
 
-( cd vivado ; vivado -mode batch -source nonproject.tcl )
+( cd vivado
+  mkdir -p output
+  stdbuf -oL vivado -mode batch -source nonproject.tcl | tee output/build_log.txt )
 
 cp -a vivado/output/puzzlefw_top.bit .