An embodiment of a semiconductor device includes a supporting member, a semiconductor die mounted on a portion of the supporting member, a buffer region, and a plastic encapsulation. The buffer region covers a portion of the die, and includes a resin and filler particles packed within the resin. The filler particles have a mix of filler sizes and are tightly packed within the resin. The buffer region has a first dielectric constant and a first loss tangent. The plastic encapsulation encloses at least part of the supporting member and the die. The plastic encapsulation includes a plastic material of a second dielectric constant and a second loss tangent, where the second dielectric constant is larger than the first dielectric constant and the second loss tangent is larger than the first loss tangent.