To confirm it is actual stress within the shield, send three or four through the reflow oven and see if they warp.
If they warp try the following
Put some samples in an oven and set the oven temp to 76F.
Put the parts in the oven for one hour per inch thickness (you want to make sure the entire part has reached 76F)
Let the oven and the parts cool overnight. This process should release 90% of the stress from the part.
To confirm, place the parts in the reflow oven and compare them to the non-heat treated shields.
Finally, call the supplier and ask if they can heat treat them prior to packaging. If they cannot, get a bigger baking oven. I would also test the solderability of the shield afterwards. Depending on the material or plating it may not solder well.
From: TechNet [mailto:[log in to unmask]] On Behalf Of Brian Chandler
Sent: Thursday, July 07, 2016 7:19 AM
To: [log in to unmask]
Subject: Re: Shielding cans deformation during reflow
Sounds like residual stress as other have mentioned. You could try annealing some samples to see if that helps. You will need to look up the recommended annealing schedule for the material used. If this works, add this requirement to the part drawing for your supplier to perform.