We have a general question about the relationship between PCB stencil thickness and BLR test results. For example, I mount QFN6x6 parts on boards with thicknesses of 1.2mm, 1.6mm, and 2.4mm. all boards have 6 layers of copper. Studies have shown that thicker boards can reduce first failure cycles (worse BLR). What is the main reason for this? I would also like to know if the thickness is the same, say 1.6mm, but one board has 4 layers of Cu and another 6 layers (all 1 oz). Which one is more likely to fail?
It could be a CTE mismatch between the QFN and the PCB stencil material, the thicker the pcb stencil, the higher the failure rate.
Typically, the reliability of the pcb stencil is significantly affected by the increased aspect ratio. Choosing a high thickness board is not preferable. However, I would like to point out that your assumption seems to be wrong because the board layer does not directly affect the BLR test results.