Printed circuit boards can become unusable after warping too much by FPC supplier. However, every single FPC will show some degree of warping due to the nature of their manufacturing process. Generally speaking, any warpage under 0.75% is considered in range for industry specifications, although this may differ depending on the end use of a product. This warping is generally invisible to the untrained eye.
FPC warpage occurs due to the presence of imbalanced copper percentages in different layers of the board. Laminate is made out of several different types of materials: resin, glass fibers, and copper. Although these layers are flat, they also have “locked-in” stress – when heated to extremely high temperatures, thermal expansion causes movement in the direction of that stress. FPC board layers thus need to account for this movement in a balancing act.
In multilayer printed circuit boards, it is thus essential that the layers of copper are properly balanced around the central layer, and distributing equal amounts of weight around the center helps minimize the warpage.
The amount of warp and weft associated with different prepregs can be calculated in advance to prevent major warpage. The closer a prepreg’s warp fill is to another, the less chance for warpage, whereas prepregs with significantly different warp fills in the x and y directions create the potential for warpage.

As soon as a board begins warping, it will continue to warp further throughout the soldering process thanks to the introduction of heat activating the locked-in stress of the board.
Other causes of warping include:
Heat Sinks and FPC Warping – Warping primarily occurs during preheating, and can also occur during the soldering process which applies localized heat to specific areas of the board. But warping isn’t only specific to these stages. When a FPC has heavy heat sinks, it can result in localized expansion on the printed circuit board and can sag during the transition phase.
Operating Temperatures – When printed circuit boards are fabricated, they will undergo several periods of intense heating and cooling. Fabricators must be careful to minimize this processing time as much as possible, while evenly heating both sides of a board. A good fabricator will communicate with the manufacturer to ensure they understand warp and weft directions before lamination.