Hi Guillaume1978,
According to your description, you want to create a sub report which contains detail of each occurrence of one synthesis board. Right?
In your scenario, how you specified your sub report? Since you want to show detail of each occurrence of synthesis board, I would suggest you put the sub report within detail row level on your synthesis board. Also you can use data field as parameter directly.
Please refer following sample:

The result show like this:

If you still have any questions, please share us more information or screenshots if possible.
Thanks,
Xi Jin