Integrated circuit assembly and method for making same

#The invention relates to electronic circuits, and especially to an assembly of multi-chip circuits operating on microwave, millimeter wave or radio frequency ranges, which assembly is based on a multi-layer flex substrate.

#Monolithic microwave integrated circuits (MMIC) are used in microelectronics at high frequency ranges. During assembly, individual semiconductor chips are typically connected to a base structure, i.e. substrate, which is in turn connected to a circuit panel, such as printed circuit board (PCB). In multi-chip modules, several unpackaged semiconductor chips are placed on one substrate. The substrate is then connected to a common circuit panel and enclosed in a common package. This saves space that would be wasted when using individually packaged semiconductor chips. A multi-chip module (MCM) is usually an assembly made of a rigid material, such as ceramic or other material, which comprises a ceramic substrate and several semiconductor chips on the substrate and in which the connections between the semiconductor chips are implemented by multi-layer circuitries insulated from each other by insulating layers and connected to each other by lead-throughs. In conventional multi-chip assemblies, the adjacent chips are placed on the surface of the substrate by means of a planar technique, and non-planar solutions are impossible.

#One reason for the poor microwave performance in conventional assemblies of monolithic microwave integrated circuits comprising ceramic substrates is the connections between the chip surface and the conductive patterns in the different layers of the multi-layer circuit panel. The insertion loss of a coaxial line or stripline on top of the inter-layer connections increases at high frequencies, which in turn causes a weakening in the signal strength. One of the biggest problems in MMIC assemblies comprising ceramic substrates is also the incompatibility caused by the different thermal coefficients of expansion of the substrate and the semiconductor circuits.

#The invention will now be described in more detail using as examples the attached drawings showing the preferred embodiments of the invention, in which

#FIG. 1 shows a top plan view of an assembly of the presented solution comprising flex substrate,

#FIGS. 2A and 2B show a cross-profile of an embodiment of the presented solution,

#FIG. 1 shows a top view of an assembly 101 according to one embodiment of the presented solution. The assembly 101 comprises a flex substrate 102 that comprises one or more dielectric tape layers. In the embodiment of FIG. 1, said dielectric tape layers are made of a flexible, organic material, such as polyimide, LCP (Liquid Crystal Polymer) or other suitable flex substrates. Several electronic components, such as semiconductor chips 90, 91, 92, are connected to the flex substrate 102. On top of the flex substrate 102, there are conductive layers 104 made of an electrically conductive material, such as copper. Vias 106 are formed through the flex substrate layers 102, and at least some of the vias form an electrical contact between the semiconductor chips 90, 91, 92 and the conductive layers 104. The vias 106 are at least partly filled with a conductive material 105, such as metal. In FIG. 1, the locations of the vias 106 are marked, even though when seen from the top, at least a part of them remain under the conductive layers 104. Some of the conductive layers 104 run between vias 106 and some of them run from the vias 106 at the semiconductor chips 90, 91, 92 to the edge of the flex substrate 102. Thus, some of the conductive layers 104 form an electrical contact between one or more semiconductor chips 90, 91, 92, whereas some of them form an electrical contact from the semiconductor chips 90, 91, 92 to the edges of the flex substrate 102. The conductive layers 104 extending to the outer edges of the flex substrate 102 are used in connecting the assembly 101 electrically to a motherboard, for instance. The conductive layers 104 thus form the necessary electrical connections in the assembly. The conductive layers 104 can for instance form a microstrip, stripline or coplanar wave-guide configuration.

#In FIG. 1 according to the embodiment of the presented solution the semiconductor chips 90, 91, 92 are also connected to a mechanical part 114, such as a mechanical base, a frame or a heatsink.

#In FIG. 1, the visible part of the semiconductor chips 90 91, 92 is shown by a continuous line. The parts of the semiconductor chips 90, 91, 92 that remain under the flex substrate 102 in a top view of the assembly 101 and at which vias 106 are formed in the flex substrate 102, are marked with a dashed line.