Patent EP-3890008-A1 (EP)
Inventor: BRANDELERO JULIO
IPC: H01L23/488
Filed: 2020-03-31
Abstract: A power module assembly comprises a power die (5) attached to a substrate (1) through a joint layer (41) comprising solid pads (4, 4') of metal alloy material connecting electrically and thermally said power die (5) to said substrate (1), wherein said solid pads (4, 4') are located within cells (200, 201, 202) of a grid (2, 2') made of an elastic material and having cell walls (20) extending between the surface of the substrate (1) and the surface of the die (5) facing each other. A process for manufacturing the power module assembly comprises: providing an elastic grid (2, 2') having a length and a width corresponding to the length and width of the semiconductor die (5) to attach; placing and attaching said elastic grid (2, 2') on the substrate (1); placing a stencil (3) forming a frame with a shape adapted to a contour of the semiconductor die (5) on the substrate (1) around the grid (2, 2') and applying paste (9) with a screen printing technique in the area defined by the stencil (3); removing the stencil (3); placing the power semiconductor die (5) on top of the paste (9); applying pressure on the die (5) to compress the assembly made of the die (5), the grid (2, 2') and paste (9) and the substrate (1); heating (10) the assembly either to melt the metal particles of the paste (9) or to sinter the particles; and cooling the assembly to provide solid pads (4, 4') embedded in said grid (2, 2'). The grid (2, 2') acts as an elastic barrier, preventing crack propagation within the joint layer (41) while providing little thermal resistance. The grid (2, 2') may be a honeycomb shaped (2) or square shaped (2') lattice. A grid cell (200, 201, 202) size may be at least ten times bigger than a grid wall (20) width. The width of walls (20) of the grid cells (200, 201, 202) may be larger than half of the thickness of the joint layer (41). The grid material may be such that the walls (20) of the grid (2, 2') contract with an increase of temperature, helping applying a high pressure on the paste (9) during the heating process phase and allowing the solid paste (4, 4') to expand in the hot areas without extra constraint during the operation of the power semiconductor die (5), leaving room to the rest of the joint layer (41) to expand under high temperature, relaxing the stress. The grid (2, 2') may be a polyimide grid, in particular, made out of a single- or double-faced adhesive polyimide tape. The grid (2') may comprise peripheral cells (201) of a reduced width with respect to central cells (202) of the grid (2'), limiting propagation of cracks so that the cracks cannot grow from the sides or corners more than the width of the reduced width cells (201) while inner cells (202) may be larger to reduce the degradation of the overall thermal resistance. The cells (200, 201, 202) of the grid (2, 2') may be shaped through laser cutting. The elastic grid (2, 2') may be formed as a tape with adhesive on both sides and be glued on both the substrate (1) and the die (5). The elastic grid (2, 2') may be placed and attached on the substrate (1) with a screen printing technique using an ink charged with high thermal conductive particles.