The nickel layer is very brittle and cannot be subjected to stress or strains in the Z axis without cracking. Flexible PCBs are especially susceptible to this with all areas subject to potential bending supported with rigidizing materials. Caution: Improperly controlled ENIG processing can result in weak solder connections which may not be visible and/or result in failure. A typical sign of failure is a flat black copper pad after the attached component has been forcibly removed.
In today’s complex circuits, this surface finish is very limited because it requires that all surfaces to be plated have to be electrically connected (i.e. an electrical charge must be present for plating). These interconnections must then be broken to make the circuit functional. The plated nickel is very solderable and not subject to the solderability issues of ENIG. The plated gold has no limits on thickness and can support wire-bonding processes like Thermo Compression Bonding (i.e. ball bonding).