A solar array (SA) mechanical subsystem made of thin and lightweight substrates was developed, built and tested for a small spacecraft. The SA is compactly foldable and deployable to a length of approximately five times the widthof the spacecraft. It has miniature hinges and latches, and deploys freely without dampers and synchronizing mechanisms. The solar cell interconnect harness consists ofthin, laminatedflexible circuits,and the substrates feature a syntactic foam core exposed to large temperature extremes. This developmental technology, currently at TRL 6, when completely proven out, would be viable for small satellites and would enable missions in the Express-class. The Express-class (or Express) refers to satellites in the range of 25kg to 100 kg that are positioned in the gap between 12U CubeSats and small ESPA-class spacecraft. Cornerstones of the SA development were compact packaging, deployment dynamic simulation, and hinge-latch tuning for dynamics and lock-up loads. Dynamic deployment simulations were modeled in Adams to observe the behavior of the unfolding array, to size the hinge springs and to monitor the lockup loads at the substrate to hinge interfaces. Extensive substrate mechanical and thermal tests were conducted to verify the substrate’s structural capability and dimensional stability in its operating environment. Thermal tests were carried out to observe the effect of mismatching coefficients of thermal expansion between the adhered flexible laminated interconnect circuits and the substrate. Gravity-negated wing deployment tests were performed at temperature limits and in vacuum to verify the overall design intent of the deployment. The stowed wing was vibration tested to verify its structural capabilities under launch environments, and then deployment tested again to demonstrate that the array as a mechanism was unaffected by launch loads. Mechanically, the Express SA substrate assembly has been advanced in its development and proven out as a structure and mechanism. Further development of the electrical power system is necessary, and additional testing for mechanical and thermal interactions of the solar cells with the overall SA substrate will need to be done. This SA subsystem would be an essential expansion to the Express hardware developed by The Applied Physics Laboratory (APL) for the advancement and enablement of Express-class missions