An assembly (101) comprising a semiconductor device (110) with solderable bumps (112); a substrate (120) with a layer (130) of a first insulating compound and an underlying metal layer (140) patterned in contact pads (141) and connecting traces (142), the insulating layer having openings (132) to expose the surface (142a) and sidewalls (142b) of underlying traces; the device bumps soldered onto the contact pads, establishing a gap (150) between device and top insulating layer; and a second insulating compound (160) cohesively filling the gap and the second openings, thereby touching the underlying traces, the second insulating compound having a higher glass transition temperature, a higher modulus, and a lower coefficient of thermal expansion than the first insulating compound.