A top-gate molding system for encapsulating semiconductor devices includes a plurality of mold cavities formed between a middle plate and a bottom plate, and a runner system formed between an upper plate and the middle plate. The runner system includes a runner with a plurality of reservoirs along its length, with a gate extending from each of the reservoirs to one of the cavities. A particle trap is positioned on the bottom of the runner between a sprue and a first one of the reservoirs, to capture contaminating particles in a flow of molding compound before the particles enter any of the reservoirs. The particle trap can be, for example, a notch or a channel extending transversely across the bottom of the runner, or a dummy reservoir upstream of the first of the plurality of reservoirs.