For reflow soldering conditions, please refer to the latest version of the "AN233 Solder Reflow Recommendation" document.
https://www.microchip.com/en-us/application-notes/an233

<Peak temperature>
Peak temperatures vary depending on the package.
Check the Peak package body temperature (Tp) item in Table 1-1. JEDEC Classification Profiles on page 3.
Depending on whether it is leaded or lead-free, it may say "TP shall not exceed TC in Table 1-3" or "TP shall not exceed TC in Table 1-2", so
Table 1-2. Pb-free (SAC Alloys) Process – Classification Temperatures (Tc)
Table 1-3. SnPb Eutectic Process – Classification Temperatures (Tc)
Please refer to the table to check the peak temperature.

Supplementary Note 1
Regarding package thickness:
Documentation > User Guides Tab > PolarFire FPGA Packaging and Pin Descriptions User Guide
You can check the thickness according to the package in the "7. Mechanical Drawings" section.
      https://www.microchip.com/en-us/products/fpgas-and-plds/fpgas/polarfire-fpgas/polarfire-mid-range-fpgas#Documentation

Supplement 2
If there is insufficient information in AN233, please also refer to the document "Solder Reflow Profile for Standard and Lead-Free Packages."
 https://ww1.microchip.com/downloads/aemdocuments/documents/fpga/ProductDocuments/PackagingSpecifications/solder_reflow_leadfree.pdf
      <Maximum number of reflows>
On page 1, it says, "Per package qualification maximum number of reflows can be done on Microsemi SoC Products Group packages is 3." This means the maximum number of reflows is three.