Abstract:
The invention relates to an integrated circuit assembly and a method of making same. The method according to the invention comprising providing a flex substrate having one or more dielectric tape layers, assembling one or more semiconductor chips to said flex substrate, said semiconductor chips having an active surface and a plurality of contact pads on said active surface, providing one or more conductive layers on said flex substrate, said conductive layers forming the electrical connections required for the assembly, electrically connecting the contact pads to the conductive layers.

Description:
FIELD OF THE INVENTION  
         [0001]    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.  
         BACKGROUND OF THE INVENTION  
         [0002]    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.  
           [0003]    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.  
         BRIEF DESCRIPTION OF THE INVENTION  
         [0004]    It is thus an object of the invention to implement an integrated circuit assembly and a method for making one in such a manner that the above-mentioned problems are solved. This is achieved by a method of making an integrated circuit assembly, the method of the invention comprising providing a flex substrate having one or more dielectric layers, assembling one or more semiconductor chips to said flex substrate, said semiconductor chips having an active surface and a plurality of contact pads on said active surface, providing one or more conductive layers on said flex substrate, said conductive layers forming the electrical connections required for the assembly and electrically connecting the contact pads to the conductive layers.  
           [0005]    The invention also relates to an integrated circuit assembly, the integrated circuit assembly of the invention comprising a flex substrate that comprises one or more dielectric tape layers, one or more semiconductor chips on said flex substrate, said semiconductor chips comprising an active surface having several contact pads, one or more conductive layers on said flex substrate, said conductive layers forming the electric connections required in the assembly, and means for connecting said contact pads directly to the conductive layer of the flex substrate.  
           [0006]    Preferred embodiments of the invention are set forth in the dependent claims.  
           [0007]    The assembly of the invention provides several advantages. One advantage of the invention is that it is possible to have very high component densities on assemblies operating at high frequency ranges. A further advantage is that inexpensive organic materials can be used as the substrates without the material selection impeding the operation of the assembly. The flex substrate used in the solution of the invention receives the stress caused by the different thermal coefficients of expansion of the materials, thus reducing the stress directed to the joint between the circuit and substrate and improving the reliability of the device and saving costs. A yet further advantage of the invention is that the assembly of the invention comprising a flex substrate is suited for use for three-dimensional, non-planar mounting of said components.  
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0008]    The invention will now be described in more detail using as examples the attached drawings showing the preferred embodiments of the invention, in which  
         [0009]    [0009]FIG. 1 shows a top plan view of an assembly of the presented solution comprising flex substrate,  
         [0010]    [0010]FIGS. 2A and 2B show a cross-profile of an embodiment of the presented solution,  
         [0011]    [0011]FIG. 2C shows a top plan view of the embodiment of FIGS. 2A and 2B,  
         [0012]    [0012]FIGS. 3A and 3B show a cross-profile of an embodiment of the presented solution,  
         [0013]    [0013]FIG. 3C shows a top plan view of the embodiment of FIGS. 3A and 3B,  
         [0014]    [0014]FIGS. 4A and 4B show a cross-profile of an embodiment of the presented solution,  
         [0015]    [0015]FIG. 4C shows a top plan view of the embodiment of FIGS. 4A and 4B,  
         [0016]    [0016]FIG. 5A shows a cross-profile of an embodiment of the presented solution,  
         [0017]    [0017]FIG. 5B shows a top plan view of the embodiment of FIG. 5A. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]    [0018]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.  
         [0019]    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.  
         [0020]    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.  
         [0021]    The unpackaged semiconductor chips  90 ,  91 ,  92  can be electrically connected to the flex substrate  102  in several different ways. The semiconductor chips  90 ,  91 ,  92  can be connected in manners known per se, for instance by reflow soldering, microwelding, by using flip chip techniques or large BGA (ball grid array) balls.  
         [0022]    The semiconductor chips  90 ,  91 ,  92  can, according to the presented solution, be microwave chips (MW), for instance. In addition to microwave chips, RF (radio frequency) and DC signals and a ground layer can be integrated to one and the same flex substrate  102 .  
         [0023]    Due to the flexible nature of the flex substrate  102 , the flex substrate  102  according to one embodiment of the presented solution can receive mechanical stress in the semiconductor chip  90 ,  91 ,  92  interconnects. The assembly  101  can also be made three-dimensional depending on the requirements of each assembly, such as thermal solutions and in-out signaling.  
         [0024]    [0024]FIGS. 2A, 2B and  2 C show one embodiment of the invention, in which the conductive layers  104  form a microstrip line configuration. Typically, a microstrip line is made up of a strip line and ground layer having a dielectric substrate between them. FIG. 2A shows an enlarged cross-profile of the embodiment of the presented solution. The active surface  103  of the semiconductor chip  90  has contact pads  108 . Alternatively, the contact pads  108  can also be solder balls or bumps. Vias  106  are formed in the flex substrate  102 , through which the semiconductor chip  90  is electrically connected directly to the conductive layers  104  on top of the vias  106 . The conductive layers  104  are on top of the flex substrate  102  in such a manner that some of the conductive layers  104  come above the vias  106 .  
         [0025]    The flip chip technique used in electrically connecting the semiconductor chips  90 ,  91 ,  92  is a useful alternative in GaAs devices that operate at microwave and RF ranges. In the solder-bump flip chip technique, unpackaged semiconductor chips are directly connected to the flex substrate. A direct connection to the flex substrate is formed through contact bumps made on the active surface of the semiconductor chips. Due to the flexibility of the flex substrate, no underfill is needed. The bumpless universal contact unit (UCU) technique is another flip chip technique. No balls, contact bumps or underfill are needed in connections in the UCU technique. In the UCU technique, contact pads  108  are formed of aluminum or copper, for instance, on the active surface  103  of the semiconductor chips  90 ,  91 ,  92 , and on top of the pads, electrical contacts are formed for instance by means of the conductive material  105  in the vias  106 .  
         [0026]    In the embodiment of FIG. 1, the semiconductor chip  90  is typically reflow soldered to the conductive material  105  in the vias  106  and the conductive layers  104 . Instead of soldering, microwelding or UCU methods known per se can also be used.  
         [0027]    In one embodiment of the invention, a space  110  free of the substrate material, such as an air window, is formed in the flex substrate  102  above the active surface  103  of the semiconductor chip  90 . The purpose of the space  110  free of the substrate material is to minimize the effect of the flex substrate  102  on the performance of the semiconductor chip  90 . The space  110  free of the substrate material is of equal height to one or more flex substrate layers in the presented solution. The height of the space  110  free of the substrate material can be adjusted as required to ensure that the operation of the semiconductor chip  90  is as trouble-free as possible.  
         [0028]    The ground layer  112  is connected to the flex substrate  102  opposite the conductive layers  104  in such a manner that the flex substrate  102  is between the conductive layers  104  and the ground layer  112 .  
         [0029]    In FIGS. 2A and 2B according to one embodiment of the presented solution the semiconductor chip  90  is also connected to a mechanical part  114 , such as a mechanical base, a frame or a heatsink.  
         [0030]    [0030]FIG. 2B shows the embodiment of FIG. 2A from the side. The figure shows that some of the contact pads  108  are connected to the ground layers  112  below the flex substrate  102 . FIG. 2C shows the embodiment of FIGS. 2A and 2B from the top. The part of the semiconductor chip  90  that is visible when seen from the top is marked with a continuous line, and a dashed line shows the part of the semiconductor chip  90  that remains below the flex substrate  102  when seen from the top. The conductive layers  104  run on top of the vias  106  at the location of the semiconductor chip  90  to the edges of the flex substrate  102 .  
         [0031]    As described in FIGS. 2A, 2B and  2 C, the microwave performance of the assembly can be improved considerably by using air windows  110  next to the active surface  103  of the semiconductor chip  90 .  
         [0032]    [0032]FIGS. 3A, 3B and  3 C disclose a solution according to one embodiment of the invention, in which the conductive layers  104  form a stripline configuration. In a stripline configuration, the stripline is typically between two ground layers. In FIGS. 3A, 3B and  3 C, the flex substrate comprises layers  102   a  and  102   b.  FIG. 3A is an enlarged cross-profile of the embodiment of the presented solution. The semiconductor chip  90  is reflow soldered to the conductive material  105  in the conductive vias  106  and to the conductive layers  104 . Instead of reflow soldering, the semiconductor chip  90  can be electrically connected to the conductive layers  104  by brazing or by using flip chip techniques known per se.  
         [0033]    Conductive vias  106  are formed in the lower flex substrate layer  102   a  above the active surface  103  of the semiconductor chip  90 . The conductive layers  104  are above the conductive vias  106 , and thus between the flex substrate layers  102   a  and  102   b.  The space  110  free of the flex substrate material, such as an air window, at the location of the active surface  103  of the semiconductor chip  90  is formed by making an opening through both flex substrate layers  102   a  and  102   b  or just through the flex substrate layer  102   a.  The upper ground-layer  112   b  is on top of the upper flex substrate layer  102   b  located above the conductive layers  104  and the lower ground-layer  112   a  is below the lower flex substrate layer  102   a.    
         [0034]    [0034]FIG. 3B shows the embodiment of FIG. 3A from one side. As can be seen in the figure, at least some of the contact pads  108  of the semiconductor chip  90  are electrically connected to the conductive layer  104  and some of the contact pads  108  are connected to the lower ground-layer  112   a.  The upper ground-layer  112   b  is electrically connected to the lower ground-layer  112   a  through the conductive vias  106  formed through the flex substrate layers  102   a,    102   b.  FIG. 3C shows a top view of the embodiment of FIGS. 3A and 3B. The upper ground-layer  112   b  covers most of the figure. In a top view, a part of the active surface of the semiconductor chip  90  and a part of the upper flex substrate layer  102   b  are visible. The figure also shows the locations of the vias  106  formed through the upper flex substrate layer  102   b,  which remain under the upper ground-layer  112   b.    
         [0035]    [0035]FIGS. 4A, 4B,  4 C show a solution according to one embodiment of the invention, in which the conductive layers  104  form a coplanar transmission line, such as a coplanar waveguide line, configuration. In a coplanar line, there are typically ground layer halves on both sides of a stripline. FIG. 4A shows an enlarged cross-profile of the flex substrate  102 . There are contact pads  108  on top of the active surface  103  of the semiconductor chip  90 . The flex substrate  102  comprises conductive vias  106 , through which the semiconductor chip  90  is electrically connected directly to the conductive layers  104  on top of the conductive vias  106 . The semiconductor chip  90  is reflow soldered to the conductive material  105  in the conductive vias  106  and to the conductive layers  104 . Instead of reflow soldering, the semiconductor chip  90  can be electrically connected to the conductive layers  104  by brazing or by using flip chip techniques known per se. The conductive layers  104  are on top of the flex substrate in such a manner that some of the conductive layers  104  are above the conductive vias  106 . In a preferred embodiment of the invention, a space  110  free of the flex substrate material, such as an air window, is formed at the location of the active surface  103  of the semiconductor chip  90  in the flex substrate  102 .  
         [0036]    [0036]FIG. 4B shows the embodiment of FIG. 4A from one side. As can be seen in the figure, some of the contact pads  108  are connected to the ground layers  112   a  and  112   b  located on top of the flex substrate  102  through the conductive vias  106  formed through the flex substrate  102  in such a manner that the ground layers  112   a  and  112   b  are on both sides of the conductive layer  104 . FIG. 4C shows a top view of the embodiment of FIGS. 4A and 4B. The section of the semiconductor chip  90  that is visible as seen from above is marked with a continuous line and the sections marked with a dashed line show the sections of the semiconductor chip  90  that remain under the ground layer  112  when seen from above. The ground layers  112   a  and  112   b  are on both sides of the conductive layers  104 . The vias  106  formed through the flex substrate layer  102  remain under the ground layers  112   a,    112   b  and the conductive layers  104  when seen from above. A part of the flex substrate layer  102  is visible when seen from above.  
         [0037]    [0037]FIGS. 5A and 5B show one embodiment, in which the semiconductor chip is replaced by surface mount device (SMD) packages  196 ,  197 ,  198  which comprise a semiconductor chip or other components. In FIG. 5A, an active SMD package  198  is directly connected to the conductive layers  104  on top of the flex substrate  102  by means of large contact material components  109 , such as pads, leads, BGA (Ball Grid Array) balls or similar. Alternatively a QFP (Quad Flat Package) package technique can be used. In FIG. 5A, two passive SMD packages  196 ,  197 , such as chip capacitors, are also on top of the flex substrate  102 . The two passive SMD packages  196 ,  197  in FIGS. 5A and 5B are identical, but in respect of each other they are positioned in different directions. The SMD packages  196 ,  197  are typically reflow soldered to the conductive layers  104 . FIGS. 5A and 5B also show the solder joints  194  of the passive SMD packages  196 ,  197 . For simplicity, not all the conductive layers and ground layers on top of the flex substrate  102  are shown in FIGS. 5A and 5B.  
         [0038]    In FIGS. 5A and 5B a passive component  195  is integrated in the flex substrate  102 . In FIGS. 5A and 5B the passive component  195  is a coil, made up from some of the conductive layers  104  on the flex substrate  102 . It is also possible to integrate directly to the flex substrate  102  other passive components, such as capacitances, resistors, filters, and couplers, using metal tracks, dielectrics, vias, air, and other materials.  
         [0039]    [0039]FIGS. 5A and 5B also show a patch matrix antenna  199  integrated to the flex substrate  102 . In FIGS. 5A and 5B the patch matrix antenna  199  is on the other side of the flex substrate  102  than the SMD packages  196 ,  197 ,  108  and the passive component  195 . The patch matrix antenna  199  is made up of some of the conductive layers  104  on the flex substrate  102 .  
         [0040]    [0040]FIG. 5B shows a top view of the embodiment of FIG. 5A. The locations of the BGA balls  109  of the active SMD package  198  are marked in FIG. 5B even though in reality they remain under the SMD package  198  when seen from above. In a top view, the location of the patch matrix antenna  199 , which remains under the flex substrate  102 , is also marked. The FIG. 5B also shows the passive SMD packages  196 , 197  and the passive component  195 , such as a coil.  
         [0041]    In FIG. 5B some of the conductive layers  104 , forming for example metal tracks, on top of the flex substrate layer  102  run from the SMD packages  196 ,  197 ,  198  to the edges of the flex substrate  102  forming the required connections in the assembly. Some of the conductive layers  104  run from the active SMD package  198  to the solder joint  194  of the passive SMD package  196  and some to the patch matrix antenna  199  through the flex substrate  102 . One of the conductive layers  104  also runs from one passive SMD package  196  to the other passive SMD package  197  and from there to the edge of the flex substrate  102 . In FIG. 5B the spiral shaped coil  195  can be seen. The conductive layers  104  connect the coil  195  to the other passive SMD package  197  and to the edges of the flex substrate  102 . The inner part  180  of the spiral shaped coil  195  is connected to the assembly for example by using a connective via  106 .  
         [0042]    In the embodiments according to FIGS. 5A and 5B both active and passive components are integrated to one flex substrate  102 , whereby it is possible to have very high component densities on assemblies operating at high frequency ranges. The passive components, such as inductors or capacitors, to be integrated to the flex substrate  102  can be made up of conductive layers  104  and/or dielectric tape layers of the flex substrate  102 . In addition, resistive layers or patches can also be added to form resistors, which passive components can comprise RF elements made without active components.  
         [0043]    In the solutions according to the embodiments described above, patch-type and/or area-matrix-built-type antennas, for instance, can be integrated to one and the same flex substrate  102 , where it is also possible to have for example spaces  110  free of the substrate material, such as air windows, to minimize the effect of the flex substrate  102  on the performance of the assembly.  
         [0044]    In the solutions according to the embodiments described above, the flex substrate  102  forms a flexible protection for electric connections and receives the stress caused by the different thermal coefficients of expansion of the used materials, thus improving reliability and saving costs. Due to the flexibility of the flex substrate material  102 , it can also be bent three-dimensionally around a bending point, and the components can also be located in arbitrary (3D) positions with respect to each other. Non-planar configurations are thus possible. By means of the presented solutions, it is possible to have very high component densities for microwave circuits.  
         [0045]    Even though the invention has been explained in the above with reference to examples in accordance with the accompanying drawings, it is obvious that the invention is not restricted to them but can be modified in many ways within the scope of the inventive idea disclosed in the attached claims.