A mobile wireless communications device may include a housing, a main dielectric substrate carried by the housing, circuitry carried by the main dielectric substrate, and a ground plane conductor on the main dielectric substrate. In one aspect, the test chamber includes a structure defining a work space having air, and a temperature control system. A powder is disposed between the first disk and the second disk. The organic passivation layer may be formed without the need for high-temperature processing. 1% to 10% by weight of a crosslinkable ethylenically unsaturated monomer, and the hydrophilic solvents including a glycol-based solvent and an ethylene glycol ether-based solvent is provided.