Moisture absorption, which is the percent of moisture absorbed by material is also a factor in board flatness. Moisture can cause the board to expand during thermal cycling. The end result could be catastrophic as de-laminating, barrel cracking may surface during the process but may go unnoticed until the boards start failing in the field.
A wide variety of materials are available to help combat or limit this problem.
Manufacturing can also take additional steps to help minimize boards that deviate from the flatness specification by pre-baking the boards under pressure for several hours, typically 6-8 hrs.