Patent Publication Number: US-2022224019-A1

Title: mmWAVE ANTENNA-FILTER MODULE

Description:
TECHNICAL FIELD 
     This disclosure relates to wireless communications, and in particular to an antenna-filter array module and a method of its manufacture. 
     BACKGROUND 
     The use of integrated low temperature co-fired (LTCC) antenna-filter array modules for millimeter wave (mmWave) Fifth Generation (5G) advanced antenna systems (AAS) has been suggested. Routing circuits for the antenna-filter array can also be integrated together with the antenna-filter array, as shown in  FIG. 1 .  FIG. 1  shows a top view and a sectional side view of an antenna-filter array module  10 . The antenna-filter array module  10  has an antenna-filter array  12  that includes an antenna layer  12   a  and a filter layer  12   b  above a routing layer  12   c . The antenna layer has a plurality of antenna elements in an array of N rows of M elements in each row, where N and M are integers and may be equal. 
     In the example of  FIG. 1 , there are four rows (N=M=4) of cross-polarized antenna elements  14  mounted above a radio printed circuit board (PCB)  16  via solder balls  18 . 
     Although the integrated LTCC antenna-filter array module has advantages in comparison to other antenna-filter integration solutions, such as higher radio frequency (RF) performance, smaller size and lower cost, etc., such design has proven to be unreliable for mmWave 5G AAS. 
     In a study to evaluate the reliability of the LTCC antenna-filter array module mounted on a standard type radio PCB Megatron-6, three sets of dimensions (25×25 mm, 12×12 mm and 6×6 mm) of the LTCC antenna-filter module were tested. Only the smallest dimension 6×6 (mm) module sample corresponding to a 1×1 (i.e., single element) 28 GHz antenna-filter unit dimension showed a reliability result close to the radio requirement. The larger two module samples failed during the test. 
     Technically, module reliability is determined by two main factors: one is the difference of mismatched Coefficients of Thermal Expansion (CTE) between the antenna-filter array  12  and its mounting radio PCB  16 ; another is the dimension of the antenna-filter array  12  that determines a span of solder balls  18  over the radio PCB  16 . 
     Tests show that failure may be due to solder balls cracking. This cracking is directly caused by an alternately changing thermal stress imposed on the solder balls due to the mismatched CTE. The larger the difference between the two CTEs, the stronger the thermal stress on the solder balls. In addition, the larger the span between the solder balls, the stronger the thermal stress imposed on the solder balls. In general, a larger dimension of the module requires a larger span between solder balls. Therefore, to enhance module reliability, the difference of the CTEs should be reduced or the dimension of the LTCC antenna-filter array should theoretically be reduced. 
     However, since the antenna-filter array dimension (also referred to as size herein) is dictated by other engineering concerns, such as avoiding grating lobes and reduction of mutual coupling between antenna elements, reduction of the antenna-filter array dimension is not a desirable option. Altering the difference between CTEs is impracticable, also. Types of standard printed circuit board materials, such as Megatron-6 and FR4 have a similar CTE ˜15 ppm/C. In contrast, the LTCC antenna-filter array usually has a CTE ˜7 ppm/C, which is just a half of the CTE of Megatron-6 or FR4 PCB. Since the Megatron-6 and FR4 are widely used in the radio manufacture industry, it is infeasible to use other materials for the radio PCB that might more closely match the CTE of the LTCC antenna-filter array. 
       FIGS. 2-5  show existing proposals that have attempted to solve these reliability problems. For example,  FIG. 2  shows a proposal that uses the well-known underfill technique, where underfill material  20  lies between the antenna-filter array  12  and the radio PCB  16 . This technique was widely used in the industry for mounting a chip of large dimension on a PCB. This method is not preferred by engineers because it is dirty and, once the chip is mounted, the underfill material  20  cannot easily be removed from the radio PCB  16 . 
       FIG. 3  shows a second proposal that uses solder-coated polymer balls  22 . This type of connecting ball is much softer than a conventional solder ball because the solder-coated polymer ball has a polymer core inside. A drawback of this solution is its very high cost. 
       FIG. 4  shows a third proposal that uses an interposer board  24  inserted between the LTCC antenna-filter array and the radio PCB. As the interposer board  24  has a CTE that lies between the CTE of the LTCC antenna-filter array  12  and the CTE of the radio PCB  16 , the imposer board  24  can reduce the thermal stress imposed on the solder balls  18  that are placed between the antenna-filter array  12  and the interposer board  24 . However, there still is a CTE mismatch between the antenna-filter array  12  and the interposer board  24 , and therefore it cannot completely solve the reliability issue of the LTCC antenna-filter array module  10 . 
       FIG. 5  shows a fourth existing proposal, where the LTCC antenna-filter array  12  of  FIG. 1 , is modified by cutting the antenna-filter array  12  in to a plurality of single polarized antenna-filter elements  26  that are then individually mounted on the radio PCB  16  through a standard reflow soldering process. However, during the reflow soldering process, these individual LTCC antenna-filter elements could lose their alignment due to solder melting, as shown in  FIG. 5 . As a result, the entire antenna array could have very bad element alignments that would cause a very poor beamforming performance. 
     SUMMARY 
     Some embodiments advantageously provide an antenna-filter array module and a method of its manufacture. According to one aspect, a method includes identifying a maximum size of an LTCC antenna-filter unit mounted on a radio PCB that does not have a reliability issue. This may be done by experimentation. The method includes soldering a LTCC tile (which may typically be larger than the identified maximum size and has at least two antenna elements) on a selected module PCB that has a CTE that is close to or equal to the CTE of the radio PCB, the closer the two CTEs, the greater the reliability of the antenna-filter array module, in at least some embodiments. When the antenna-filter array module is assembled, the selected module PCB lies between the LTCC tile and the radio PCB. After the LTCC tile is soldered to the module PCB, the tile is diced into antenna-filter units having a dimension that is not greater than the identified maximum size. An antenna-filter unit having a size that is not greater than the identified maximum size is referred to herein as a reliability issue free unit or more simply, a reliability unit. 
     According to one aspect, a method of manufacturing an antenna-filter array module that includes at least two antenna elements on a low temperature co-fired ceramic, LTCC, tile couplable to a radio printed circuit board, PCB, in an antenna array, includes soldering an LTCC tile having the at least two antenna elements to a first side of a module PCB, the soldering including soldering at first soldering sites lying between the LTCC tile and the module PCB, the module PCB having a size at least as great as a size of the LTCC tile. Following the soldering, the method includes cutting the LTCC tile into reliability issue free, RIF, units, each RIF unit having a size not greater than a predetermined largest reliable size. The method further includes forming a plurality of second soldering sites configured to couple with the radio PCB on a second side of the module PCB opposite the first side of the module PCB. 
     According to this aspect, in some embodiments, the method further includes coupling the module PCB to the radio PCB, the coupling including soldering at the plurality of second soldering sites. In some embodiments, a difference between a coefficient of thermal expansion, CTE, of the module PCB and a CTE of the radio PCB is less than a predetermined amount. In some embodiments, the module PCB and the radio PCB are of the same material and have the same CTE. In some embodiments, a size of the module PCB is greater than an area of the LTCC tile. In some embodiments, the size of an RIF unit is a size of one antenna element. In some embodiments, the size of an RIF unit is a size of two rows of two antenna elements per row. In some embodiments, the size of an LTCC tile is N rows of M antenna elements per row, where N and M are integers. In some embodiments, the size of an RIF unit is a size of an antenna element of the at least two antenna elements. In some embodiments, a module PCB has a size of at least two RIF units. In some embodiments, the solder structures are solder balls or bumps. 
     According to another aspect, an antenna-filter array module is provided. The antenna-filter array module includes a module printed circuit board, PCB, having a first side and a second side, the first side having first soldering structures and configured to be soldered to a low-temperature co-fired ceramic, LTCC, tile the second side having second soldering structures, the second side configured to be coupled to a radio PCB. The antenna-filter array module further includes an LTCC tile having at least two antenna elements and corresponding filters, the LTCC tile being soldered to the first side of the module PCB at the first soldering structures and cuttable into reliability issue free, RIF, units, each RIF unit being of a size not greater than a predetermined largest reliable size. 
     According to this aspect, in some embodiments, a difference between a coefficient of thermal expansion, CTE, of the module PCB and a CTE of the radio PCB is chosen to be less than a predetermined amount. In some embodiments, the module PCB and the radio PCB are of the same material and have the same CTE. In some embodiments, a size of the module PCB is greater than an area of an LTCC tile. In some embodiments, the size of an RIF unit is a size of one antenna element. In some embodiments, the size of an RIF unit is a size of two rows of two antenna elements per row. In some embodiments, the size of an LTCC tile is a size of N rows of M antenna elements per row. In some embodiments, the size of an RIF unit is a size of an antenna element of the at least two antenna elements. In some embodiments, a module PCB has a size equal to the LTCC tile before cutting. 
     According to yet another aspect, a method of manufacturing an antenna-filter array module configured to be coupled to a radio printed circuit board, PCB, the antenna-filter array module having a module PCB having a first side on which a first set of solder balls are positioned and having a second side on which a second set of solder balls are positioned, is provided. The method includes bonding a low temperature co-fired ceramic, LTCC, tile having a plurality of antennas and corresponding filters to a first side of the module PCB via a first set of solder balls, a coefficient of thermal expansion, CTE, of the module PCB being within a predetermined amount of a CTE of the radio PCB. The method further includes cutting the LTCC tile into a plurality of reliability units after the bonding, each reliability unit having a size that is less than a predetermined largest reliable size. 
     According to this aspect, in some embodiments, a size of the module PCB is a size of an LTCC tile. In some embodiments, the module PCB and the radio PCB are of the same material and have the same CTE. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the present embodiments, and the attendant advantages and features thereof, will be more readily understood by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein: 
         FIG. 1  shows a top view and a sectional side view of an antenna-filter array module; 
         FIG. 2  shows an application of an underfill technique; 
         FIG. 3  shows an application using solder-coated polymer balls; 
         FIG. 4  shows an interposer board inserted between the LTCC antenna-filter array and the radio PCB; 
         FIG. 5  shows an array of misaligned LTCC antenna-filter elements where the elements are cut first and then bonded to a radio PCB via a set of solder balls; 
         FIG. 6  shows one embodiment of an LTCC antenna-filter made according to principles set forth herein; 
         FIGS. 7A, 7B and 7C  show three steps for forming an antenna-filter array module according to principles set forth herein; 
         FIG. 8  shows an embodiment of an antenna-filter array module where an RIF is a 2×2 array of antenna elements; 
         FIG. 9  shows the embodiment of  FIG. 8  mounted to the radio PCB; 
         FIG. 10  is a flowchart of an exemplary process for manufacturing an antenna-filter array module; and 
         FIG. 11  is a flowchart of an alternative exemplary process for manufacturing an antenna-filter array module. 
     
    
    
     DETAILED DESCRIPTION 
     Before describing in detail exemplary embodiments, it is noted that the embodiments reside primarily in combinations of apparatus components and processing steps related to an antenna-filter array module and a method of its manufacture. Accordingly, components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein. 
     As used herein, relational terms, such as “first” and “second,” “top” and “bottom,” and the like, may be used solely to distinguish one entity or element from another entity or element without necessarily requiring or implying any physical or logical relationship or order between such entities or elements. 
     Referring again to the drawing figures, where like reference numerals denote like elements,  FIG. 6  shows one embodiment of an antenna-filter array module  30  that solves the above-mentioned problems of CTE mismatch and large span between solder balls  32  both causing unreliability, without introducing problems such as dirty underfill, expensive solder-coated polymer balls, and antenna element misalignment. Components of the antenna-filter array module  30  include LTCC antenna-filter elements  34 , one module PCB  36  and two layers of solder balls/bumps  32  or other solder structure at suitable soldering sites. As shown in  FIG. 6 , the antenna-filter array module  30  has antenna-filter units (elements)  34 , each antenna-filter unit (element)  34  having an antenna design on a top layer  38  and a filter design on the lower layer  39  of the antenna-filter unit  34 . In some embodiments, routing circuits for the antenna  38  and filter  39  array, such as transmission lines and splitters/combiners, etc., can be designed within the module PCB  36  if preferred. Also, the two layers of solder balls/bumps  32  can have different melting temperatures depending on the radio PCB  40  assembly process. Note that although  FIG. 6  discloses only one antenna element per antenna-filter unit  34 , it is noted that there can be more than one antenna element per antenna-filter unit. Different arrays of antenna elements may make up an antenna-filter unit. For example, see the 2×2 antenna-filter unit of  FIG. 8 , discussed below in detail. Note also that radio PCB  40  can be a known/existing radio PCB such as radio PCB  16 . In other words, the LTCC antenna-filter module disclosed herein is backward compatible, being able to couple to existing radio PCBs, and forward compatible, being able to couple to radio PCBs yet to be developed. 
       FIGS. 7A-7C  show steps of one embodiment of a method for manufacturing the LTCC antenna-filter array module  30  disclosed herein. The method starts with an LTCC tile  42  having an array of at least two antenna elements with their underlying filters (antenna-filter element  34 , for example).
         Step  1  ( FIG. 7A ): Mount the LTCC tile  42  to a first side of a module PCB  36  via soldering structures at soldering sites, where the module PCB  36  is chosen to have a CTE that is the same as or close to the CTE of an expected radio PCB.   Step  2  ( FIG. 7B ): Dice (cut) the LTCC tile  42  into antenna-filter units  34  that have the same dimension as the identified maximum LTCC antenna-filter array size that does not have reliability issues. Such an antenna-filter unit is called a reliability-issue-free (RIF) unit  46 . Note that this step may be performed after Step  1 , to avoid the situation of  FIG. 5 , where the mounting occurs after the cutting. Thus, the dicing lines  44  denote boundaries of RIF units  46 .   Step  3  ( FIG. 7C ): Clean up all dicing debris and create soldering sites on a second side of the module PCB  36  opposite the first side of the module PCB  36 .       

     It is noted that Step  2  and Step  3  in  FIGS. 7B and 7C  depict only one embodiment, where the reliability-issue-free unit is a single antenna element (a 1×1 array), the smallest array size. In general, the reliability-issue-free unit can be an N×M array, where N and M are integers that may be equal. The size of the RIF unit may depend on what LTCC material and what PCB material are used. 
       FIG. 8  illustrates a top view and sectional side view of an LTCC antenna-filter array module  30 , where the RIF unit  46  is a 2×2 array. Thus, in the example of  FIG. 8 , the LTCC tile that has 16 dual-polarized antenna elements, and which is cut into four quadrants each quadrant occupied by a different RIF unit  46 . Note that in some embodiments, the 16 dual-polarized antenna elements may each consist of two perpendicular antennas. Other differently polarized antenna elements may be employed. 
     Thus, once bound to the module PCB  36 , the LTCC tile  42  is diced to create small mechanically independent units, defined in  FIG. 7  by dicing lines  44 , where each such unit is reliability-issue-free (RIF). For these units, there will be no reliability issue due to thermal expansion mismatch or too large of a span between edge soldering sites (such as soldering balls or bumps) on the first side of the module PCB  36 , since no unit is greater than the identified maximum LTCC antenna-filter array size that does not have reliability issues. 
     When the LTCC antenna-filter module is mounted on the radio PCB through the second side solder sites as shown in  FIG. 6  and  FIG. 9 , for example, and when the module PCB  36  has a CTE that is equal to or close to the CTE of the radio PCB  40 , there is no thermal mismatch or small thermal mismatch between the module PCB  36  and the radio PCB  40 . Thus, the second set of solder structures on the second side of the module PCB  36  should not have a reliability issue. 
     As result of the above, the entire LTCC antenna-filter array module  30  as manufactured according to the above recited steps should not have reliability issues when mounted on the radio PCB  40 . Thus, some embodiments provide a comprehensive approach to solving the reliability issues of the LTCC antenna-filter array module  30  mounted on a radio PCB  40 . The LTCC antenna-filter array module  30  described herein may be mounted simply on the radio PCB  40  at low cost. Also, in at least some embodiments, the LTCC antenna-filter array module  30  has higher beamforming performance than the existing solution proposals described above, because all antenna-filter units are aligned and because the gaps between adjacent antenna-filter units impede surface-traveling electromagnetic waves that degrade beamforming performance. Further, because the LTCC antenna-filter array module is a physical module, the assembly yield of the radio manufacture will not be affected by the presence of the module. 
       FIG. 10  is a flowchart of an exemplary process for manufacturing an antenna-filter array module. The process includes soldering an LTCC tile  42  having the at least two antenna elements  34  to a first side of a module PCB, the soldering including soldering at first soldering sites lying between the LTCC tile  42  and the module PCB  36 , the module PCB  36  having a size at least as great as a size of the LTCC tile  42  (Block S 100 ). The process also includes cutting the LTCC tile  42  into reliability issue free, RIF, units  46 , each RIF unit  46  having a size not greater than a predetermined largest reliable size (Block S 102 ). The process further includes forming a plurality of second soldering sites (e.g., solder balls/bumps  32 ) configured to couple with the radio PCB  40  on a second side of the module PCB  36  opposite the first side of the module PCB (Block S 104 ). 
       FIG. 11  is a flowchart of an alternative exemplary process for manufacturing an antenna-filter array module  30 . The process (Block S 106 ) includes bonding a low temperature co-fired ceramic, LTCC, tile  42  having a plurality of antennas and corresponding filters (to form an antenna-filter element  34 ) to a first side of the module PCB  36  via a first set of solder balls  32 , a coefficient of thermal expansion, CTE, of the module PCB  36  being within a predetermined amount of a CTE of the radio PCB  40 . The process further includes cutting the LTCC tile  42  into a plurality of reliability units  46  after the bonding, each reliability unit  46  having a size that is less than a predetermined largest reliable size. 
     Therefore, some embodiments described herein include LTCC antenna-filter modules designed at low cost, small size and with high-performance in the mmWave 5G spectrum with NR AAS, thereby removing a last reliability problem of the LTCC module over the radio PCB. 
     According to one aspect, a method of manufacturing an antenna-filter array module  30  that includes at least two antenna elements on a low temperature co-fired ceramic, LTCC, tile  42  couplable to a radio printed circuit board, PCB  40 , in an antenna array, includes soldering an LTCC tile  42  having the at least two antenna elements to a first side of a module PCB  36 , the soldering including soldering at first soldering sites lying between the LTCC tile  42  and the module PCB  36 , the module PCB  36  having a size at least as great as a size of the LTCC tile  42 . Following the soldering, the method includes cutting the LTCC tile  42  into reliability issue free, RIF, units  46 , each RIF unit  46  having a size not greater than a predetermined largest reliable size. The method further includes forming a plurality of second soldering sites configured to couple with the radio PCB  40  on a second side of the module PCB  36  opposite the first side of the module PCB  36 . 
     According to this aspect, in some embodiments, the method further includes coupling the module PCB  36  to the radio PCB, the coupling including soldering at the plurality of second soldering sites. In some embodiments, a difference between a coefficient of thermal expansion, CTE, of the module PCB  36  and a CTE of the radio PCB is less than a predetermined amount. In some embodiments, the module PCB  36  and the radio PCB  40  are of the same material and have the same CTE. In some embodiments, a size of the module PCB  36  is greater than an area of the LTCC tile  42 . In some embodiments, the size of an RIF unit  46  is a size of one antenna element. In some embodiments, the size of an RIF unit  46  is a size of two rows of two antenna elements per row. In some embodiments, the size of an LTCC tile  42  is N rows of M antenna elements per row, where N and M are integers. In some embodiments, the size of an RIF unit  46  is a size of an antenna element of the at least two antenna elements. In some embodiments, a module PCB  36  has a size of at least two RIF units. In some embodiments, the solder structures are solder balls or bumps. 
     According to another aspect, an antenna-filter array module  30  is provided. The antenna-filter array module includes a module printed circuit board, PCB  36 , having a first side and a second side, the first side having first soldering structures and configured to be soldered to a low-temperature co-fired ceramic, LTCC, tile  42 , the second side having second soldering structures, the second side configured to be coupled to a radio PCB. The antenna-filter array module further includes an LTCC tile  42  having at least two antenna elements and corresponding filters, the LTCC tile  42  being soldered to the first side of the module PCB  36  at the first soldering structures and cuttable into reliability issue free, RIF, units  46 , each RIF unit being of a size not greater than a predetermined largest reliable size. 
     According to this aspect, in some embodiments, a difference between a coefficient of thermal expansion, CTE, of the module PCB  36  and a CTE of the radio PCB is chosen to be less than a predetermined amount. In some embodiments, the module PCB  36  and the radio PCB  40  are of the same material and have the same CTE. In some embodiments, a size of the module PCB  36  is greater than an area of an LTCC tile  42 . In some embodiments, the size of an RIF unit is a size of one antenna element. In some embodiments, the size of an RIF unit is a size of two rows of two antenna elements per row. In some embodiments, the size of an LTCC tile  42  is a size of N rows of M antenna elements per row. In some embodiments, the size of an RIF unit is a size of an antenna element of the at least two antenna elements. In some embodiments, a module PCB  36  has a size equal to the LTCC tile  42  before cutting. 
     According to yet another aspect, a method of manufacturing an antenna-filter array module configured to be coupled to a radio printed circuit board, PCB, the antenna-filter array module having a module PCB  36  having a first side on which a first set of solder balls are positioned and having a second side on which a second set of solder balls are positioned, is provided. The method includes bonding a low temperature co-fired ceramic, LTCC, tile having a plurality of antennas and corresponding filters to a first side of the module PCB  36  via a first set of solder balls, a coefficient of thermal expansion, CTE, of the module PCB  36  being within a predetermined amount of a CTE of the radio PCB  40 . The method further includes cutting the LTCC tile  42  into a plurality of reliability units after the bonding, each reliability unit having a size that is less than or equal to a predetermined largest reliable size. 
     According to this aspect, in some embodiments, a size of the module PCB  36  is a size of an LTCC tile  42 . In some embodiments, the module PCB  36  and the radio PCB  40  are of the same material and have the same CTE. 
     Many different embodiments have been disclosed herein, in connection with the above description and the drawings. It will be understood that it would be unduly repetitious and obfuscating to literally describe and illustrate every combination and subcombination of these embodiments. Accordingly, all embodiments can be combined in any way and/or combination, and the present specification, including the drawings, shall be construed to constitute a complete written description of all combinations and subcombinations of the embodiments described herein, and of the manner and process of making and using them, and shall support claims to any such combination or subcombination. 
     
       
         
           
               
               
               
             
               
                   
                   
               
               
                   
                 Abbreviation 
                 Explanation 
               
               
                   
                   
               
             
            
               
                   
                 AAS 
                 Advanced Antenna System 
               
               
                   
                 CTE 
                 Coefficient of Thermal Expansion 
               
               
                   
                 EM 
                 Electromagnetic 
               
               
                   
                 LTCC 
                 Low Temperature Co-fired Ceramics 
               
               
                   
                   
               
            
           
         
       
     
     It will be appreciated by persons skilled in the art that the embodiments described herein are not limited to what has been particularly shown and described herein above. In addition, unless mention was made above to the contrary, it should be noted that all the accompanying drawings are not to scale. A variety of modifications and variations are possible in light of the above teachings without departing from the scope of the following claims.