A microvalve assembly (30) is integrated into a printed circuit board (PCB) substrate (31). An aperture (32) in the PCB substrate includes a closure member (33) extending into the aperture. A flexible fluid pipe (38) is disposed between the closure member and a closure edge (37) of the aperture. A displacement member (34) is coupled to the PCB and is configured for thermal actuation to displace the closure member so as to vary the cross-sectional profile of the flexible fluid pipe. The closure member may be a cantilever section of the PCB substrate. Displacement of the closure member to vary the cross-sectional profile of the flexible fluid pipe can occur entirely within the plane of the PCB substrate and movement of the displacement member to displace the closure member can also occur entirely within the plane of the PCB substrate, providing a very low profile of microvalve particularly suited for integration into a fuel cell stack (1).