Patent Publication Number: US-2022224054-A1

Title: Electrical Connector and Connector Assembly

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of Chinese Patent Application No. 202110045478.6 filed on Jan. 13, 2021 in the China National Intellectual Property Administration, the whole disclosure of which is incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present disclosure relates to an electrical connector, and in particular, to an electrical connector adapted for high-speed signal transmission, and a connector assembly comprising the same. 
     BACKGROUND 
     With development of digital information technology, data transmission rates have greatly increased in recent years. For example, in communications field, a high-speed connector is required to achieve at least 112 Gbps high-speed signal transmission. Since data transmission often requires an electrical connector to connect different interfaces, a signal transmission speed and quality of the electrical connector will greatly affect the speed and stability of data transmission. For example, the electrical connector may be used to realize an electrical connection between two printed circuit boards (PCBs). 
     Generally, electrical connectors suitable for high-speed signal transmission mainly include a base made of insulation material and a plurality of terminal columns mounted on the base. Grounding terminals and differential signal terminal pairs in each of the terminal columns are alternately arranged, wherein the grounding terminals of the adjacent terminal columns correspond to positions at which the differential signal terminal pairs are located to form an independent ground shield for each of the differential signal terminal pairs. In this type of electrical connector, in order to take into account both the high-speed performance and high-density requirements, some of the differential signal terminal pairs are arranged in a staggered manner with the grounding terminals. However, this arrangement does not entirely eliminate crosstalk between the differential signal terminal pairs of one column and the differential signal terminal pairs of the adjacent columns. In order to further reduce this crosstalk, a spacing between the columns may be increased. However, this reduces the density of the transmission channel. 
     SUMMARY 
     According to an embodiment of the present disclosure, an electrical connector includes an insulation housing, a plurality of grounding terminal columns and a plurality of hybrid terminal columns. The plurality of grounding terminal columns are arranged on the insulation housing and comprises a plurality of first grounding terminals. The plurality of hybrid terminal columns are arranged on the insulation housing adjacent respective ones of the plurality of grounding terminal columns and comprises a plurality of second grounding terminals and a plurality of differential signal terminal pairs. Each of the differential signal terminal pairs is located between two adjacent second grounding terminals in one hybrid terminal column and is adjacent to two first grounding terminals of the grounding terminal columns adjacent to the one hybrid terminal column at both sides thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described by way of example with reference to the accompanying Figures, of which: 
         FIG. 1  shows a schematic perspective view of an electrical connector according to an exemplary embodiment of the disclosure; 
         FIG. 2  shows another schematic perspective view of the electrical connector shown in  FIG. 1 ; 
         FIG. 3  shows a schematic enlarged view of a part ‘A’ shown in  FIG. 2 ; 
         FIG. 4  shows a further another schematic perspective view of the electrical connector shown in  FIG. 1 ; 
         FIG. 5  shows a top view of the electrical connector shown in  FIG. 1 ; 
         FIG. 6  shows a transverse cross-sectional view of the electrical connector shown in  FIG. 1 ; 
         FIG. 7  shows a top view of an arrangement of terminals of an electrical connector according to an exemplary embodiment of the disclosure; 
         FIG. 8  shows a schematic perspective view of an arrangement of terminals of an electrical connector according to an exemplary embodiment of the disclosure; 
         FIG. 9  shows a schematic plan view of three types of terminals of an electrical connector according to an exemplary embodiment of the disclosure; 
         FIG. 10  shows a schematic plan view of a first grounding terminal according to another exemplary embodiment of the disclosure; 
         FIG. 11  shows a schematic perspective view of an electrical connector according to another exemplary embodiment of the disclosure; 
         FIG. 12  shows a schematic enlarged view of a part ‘B’ shown in  FIG. 11 ; 
         FIG. 13  shows another schematic perspective view of the electrical connector shown in  FIG. 12 ; 
         FIG. 14  shows a schematic perspective view of an insulation housing according to an exemplary embodiment of the disclosure; 
         FIG. 15  shows a schematic enlarged view of a part ‘C’ shown in  FIG. 14 ; 
         FIG. 16  shows a schematic perspective view of a metallization layer according to an exemplary embodiment of the disclosure, wherein the insulation housing is not shown; and 
         FIG. 17  shows a transverse cross-sectional view of a connector assembly according to an exemplary embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Exemplary embodiments of the present disclosure will be described hereinafter in detail with reference to the attached drawings, wherein the like reference numerals refer to the like elements. The present disclosure may, however, be embodied in many different forms and should not be construed as being limited to the embodiment set forth herein; rather, these embodiments are provided so that the present disclosure will be thorough and complete, and will fully convey the concept of the disclosure to those skilled in the art. 
     In the following detailed description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the disclosed embodiments. It will be apparent, however, that one or more embodiments may be practiced without these specific details. In other instances, well-known structures and devices are schematically shown in order to simplify the drawing. 
     According to an embodiment of the present disclosure an electrical connector includes an insulation housing, a plurality of grounding terminal columns arranged on the insulation housing and including a plurality of first grounding terminals, and a plurality of hybrid terminal columns arranged on the insulation housing and including a plurality of second grounding terminals and a plurality of differential signal terminal pairs. The plurality of grounding terminal columns and the plurality of hybrid terminal columns are arranged adjacent to each other, respectively. Each of the differential signal terminal pairs is located between two adjacent second grounding terminals in one hybrid terminal column and is adjacent to two first grounding terminals of the grounding terminal columns adjacent to the one hybrid terminal column at both sides thereof. 
     According to another embodiment of the disclosure, there is provided a connector assembly including two electrical connectors as described above. The grounding terminals and the differential signal terminal pairs of the two electrical connectors are electrically connected with each other. 
       FIG. 1  shows a schematic perspective view of an electrical connector according to an exemplary embodiment of the disclosure,  FIG. 2  shows another schematic perspective view of the electrical connector shown in  FIG. 1 ,  FIG. 3  shows a schematic enlarged view of a part ‘A’ shown in  FIG. 2 ,  FIG. 4  shows a further another schematic perspective view shown in  FIG. 1 , and  FIG. 5  shows a top view of the electrical connector shown in  FIG. 1 . 
     According to an exemplary embodiment of the disclosure, as illustrated in  FIGS. 1-5 , an electrical connector  100  is utilized in a communication system to transmit signals at a high speed, for example, at a speed of no less than 112 Gbps. The electrical connector  100  includes an insulation housing  1 , a plurality of grounding terminal columns  21  and a plurality of hybrid terminal columns  22 . The plurality of grounding terminal columns are arranged on the insulation housing  1  and include a plurality of first grounding terminals  211  adapted to transmit ground signals. The plurality of hybrid terminal columns  22  are arranged on the insulation housing  1  and include a plurality of second grounding terminals  221  and a plurality of differential signal terminal pairs  222  adapted to transmit differential signals. The plurality of grounding terminal columns  21  and the plurality of hybrid terminal columns  22  are arranged adjacent to each other, respectively. Each of the differential signal terminal pairs includes two adjacent differential signal terminals. Each of the differential signal terminal pairs  222  is located between two adjacent second grounding terminals  221  in one hybrid terminal column  22  and is adjacent to two first grounding terminals  211  of the two grounding terminal columns adjacent to the one hybrid terminal column  22  at both sides thereof. In this way, each of the differential signal terminal pairs is adjacent to the grounding terminals in both a column direction and a row direction; that is, each of the differential signal terminal pairs is surrounded by the grounding terminals. In this way, a signal crosstalk between different differential signal terminal pairs can be suppressed. Further, it is also possible to allow the grounding terminals and the differential signal terminals to be arranged at a higher density while ensuring a high-speed signal transmission performance of the electrical connector. 
       FIG. 7  shows a top view of an arrangement of terminals of an electrical connector according to an exemplary embodiment of the disclosure, and  FIG. 8  shows a schematic perspective view of an arrangement of terminals of an electrical connector according to an exemplary embodiment of the disclosure. 
     In an exemplary embodiment of the disclosure, as illustrated in  FIGS. 1-5, 7 and 8 , the grounding terminal columns  21  do not include the differential signal terminals. The plurality of hybrid terminal columns  22  include the plurality of second grounding terminals  221  and the plurality of differential signal terminal pairs  222 , and each of the differential signal terminal pairs  222  is located between two second grounding terminals  221 . With this arrangement, there are no two hybrid terminal columns directly adjacent to each other. Each of the differential signal terminal pairs  222  includes two differential signal terminals. 
       FIG. 6  shows a transverse cross-sectional view of the electrical connector shown in  FIG. 1 . 
     In an exemplary embodiment of the disclosure, referring to  FIGS. 1-6 , the insulation housing  1  includes a bottom wall  11  and a plurality of protruding bars  12 . The grounding terminals and the differential signal terminal pairs  222  extend from a first side to a second side of the bottom wall  11  in a first direction (a height direction). The plurality of protruding bars  12  protrude from the second side of the bottom wall  11  and extend in a second direction (a length direction) perpendicular to the first direction. The grounding terminals and/or the differential signal terminals protruding from the second side of the bottom wall are held on side walls of the respective protruding bars  12 . 
     The plurality of protruding bars  21  include a first outer protruding bar  121 , a second outer protruding bar  122  and at least one middle protruding bar  123  located between the first outer protruding bar and the second outer protruding bar. One of the two adjacent terminal columns is the grounding terminal column  21 , and the other is the hybrid terminal column  22 . The first outer protruding bar  121  is provided with the grounding terminal column  21  on an inner side thereof. The second outer protruding bar  122  is provided with the grounding terminal column  21  and the hybrid terminal column  22  on an inner side and an outer side thereof, respectively. The at least one middle protruding bar  123  each is provided with the grounding terminal column  21  and the hybrid terminal columns  22  on both sides thereof, respectively. In this way, the grounding terminal column  21  is arranged on one of the two side walls, extending in the second direction, of each protruding bar expect for the first outer protruding bar, and the hybrid terminal column  22  is arranged on the other side of the two side walls. Further, there is no protruding bar provided with the grounding terminal column or the hybrid terminal columns on the both opposite sides thereof. In this way, the grounding terminals are located on the outermost side, and no signal terminal is located on the outermost side, thereby avoiding the crosstalk between the signal terminals and other external terminals. 
     In an exemplary embodiment of the disclosure, referring to  FIGS. 1-6 , an insertion slot  13  is formed between two adjacent protruding bars  12 . The grounding terminal column  21  and the hybrid terminal column  22  are arranged on both sides of the insertion slot  13 , respectively. In this way, the grounding terminal column  21  is arranged on one of two side walls of the insertion slot  13 , and the hybrid terminal column  22  is arranged on the other of the two side walls. There is no such an insertion slot that is provided with the grounding terminal column or the hybrid terminal columns on the both opposite sides thereof. 
       FIG. 17  shows a transverse cross-sectional view of a connector assembly according to an exemplary embodiment of the disclosure. 
     Referring to  FIG. 17 , according to an exemplary embodiment of another aspect of the disclosure, there is provided a connector assembly including two electrical connectors  100  and  100 ′ according to any one of the embodiments as described above, wherein the grounding terminals and the differential signal terminal pairs of the two electrical connectors are electrically connected to each other to realize an electrical connection of the two electrical connectors with each other. That is, the first grounding terminals  211  of one electrical connector  100  are electrically connected with the first grounding terminals  211 ′ of the other electrical connector  100 ′, the second grounding terminals  221  of the one electrical connector  100  are electrically connected with the second grounding terminals  221 ′ of the other connector  100 ′, and the differential signal terminal pairs  222  of the one electrical connector  100  are electrically connected with the differential signal terminal pairs  222 ′ of the other electrical connector  100 ′. 
     Further, the bottom wall  11 ,  11 ′ of each of the electrical connectors is provided with a circuit board  3 ,  3  electrically connected to the grounding terminals and the differential signal terminals on the first side of the bottom wall, respectively, so that the electrical connection between the two circuit boards is realized. In this way, a signal transmission between the two circuit boards can be realized through the electrical connectors according to the embodiments of the disclosure. 
     In an exemplary embodiment of the disclosure, referring to  FIGS. 6 and 17 , the insertion slot  13  has a width approximately equal to or slightly greater than that of each of the protruding bars  12 , so that the protruding bars  12  of the one electrical connector  100  are insertable into the respective insertion slots of the other electrical connector  100 ′ to assembly the one electrical connector and the other electrical connector together. In this way, when the two circuit boards  3 ,  3 ′ are electrically connected with each other, only one type of electrical connector is needed. The protruding bars and insertion slots of the two electrical connectors  100 ,  100 ′ are engaged with each other, which reduces manufacturing cost of the electrical connector. 
     In an exemplary embodiment of the disclosure, referring to  FIGS. 1 and 2 , a projection width of each of the differential signal terminal pairs  222  in a third direction (a width direction) perpendicular to the first direction and the second direction is less than that of the first grounding terminal  211  in the third direction. In other words, the projection width of each of the differential signal terminal pairs  222  in the third direction is totally projected within a projection range of the first grounding terminal  211  in the third direction. 
       FIG. 9  shows a schematic plan view of three types of terminals of an electrical connector according to an exemplary embodiment of the disclosure. 
     In an exemplary embodiment of the disclosure, referring to  FIGS. 9 and 14 , the first grounding terminal  211  includes a first body portion  2111 , and a first elastic portion  2112  extending from the first body portion  2111  and having a free end formed as an arc-shaped first contact portion  2113 . The second grounding terminal  221  includes a second body portion  2211  and a second elastic portion  2212  extending from the second body portion  2211 , and having a free end formed as an arc-shaped second contact portion  2213 . The differential signal terminal includes a third main body portion  2221 , and a third elastic portion  2222  extending from the third main body  2221  and having a free end formed as an arc-shaped third contact portion  2223 . In addition, each of the first body portion  2111  of the first grounding terminal  211 , the second body portion  2211  of the second grounding terminal  221  and the third body portion  2221  of the differential signal terminal is provided with a soldering portion  2115 . After each terminal is mounted onto the insulation housing, solder balls  4  may be pre-arranged on the soldering portions  2115  to be soldered with electrical contacts of the circuit board. 
     According to the electrical connector as described in the above embodiments, when the one electrical connector  100  is connected with the other electrical connector  100 ′, the contact portion of the terminal of the one connector  100  is brought into contact with the elastic portion of the terminal of the other electrical connector. At the same time, the contact portion of the other electrical connector  100 ′ is brought in contact with the elastic portion of the one electrical connector  100 . For example, when the one electrical connector  100  is connected with the other electrical connector  100 ′, the first contact portion  2113  of the first grounding terminal  211  of the one connector  100  is brought into contact with the first elastic portion  2112 ′ of first grounding terminal  211  of the other electrical connector  100 ′. At the same time, the first contact portion  2112 ′ of the other electrical connector  100 ′ is brought into contact with the first elastic portion  2112  of the one electrical connector  100 . Therefore, two first grounding terminals  211  mated with each other of the two electrical connectors are brought into electrical contact with each other at four elastic first contact portions thereof, that is, two sets of the first contact portions mated with each other of the two first grounding terminals  211  form four electrical contact points totally. The differential signal terminals in contact with each other are brought into contact with each other at the third contact portion and form two contact points. In this way, the reliable electrical connection may be realized by the terminals of the two electrical connectors corresponding to each other. 
     In an exemplary embodiment of the disclosure, referring to  FIG. 9 , the first elastic portion  2113  includes two sub-elastic portions  2114  separated from each other to reduce an elastic force of the first elastic portion so as to facilitate an insertion of the two electrical connectors. 
     In an exemplary embodiment of the disclosure, referring to  FIG. 9 , the first body portion  2111  of the first grounding terminal  211  has a maximum width W 1  greater than a total width W 2  of the two third body portions  2221  of the differential signal terminal pair. The second body portion  2211  of the second grounding terminal  221  has a width W 3  greater than a width W 4  of one of the third body portions. The width W 3  of the second body portion  2211  of the second grounding terminal  221  is less than the total width W 2  of the two third body portions  2221  of the differential signal terminal pair. Each of the sub-elastic portions  2114  of the first grounding terminal  211  has a width greater than a width W 6  of one of the third elastic portions. 
       FIG. 10  shows a schematic plan view of a first grounding terminal according to another exemplary embodiment of the disclosure. A first body portion  2111  of the first grounding terminal  211  includes two sub-body portions  2111 ′ separated from each other. In an exemplary embodiment of the disclosure, referring to  FIGS. 1, 2, 4 and 5 , the insulation housing  1  is provided with a guide groove  14  and a guide post  15 , and the guide post  15  of the one electrical connector  100  may be inserted into the guide groove  14  of the another electrical connector  100 ′. When two electrical connectors are connected with each other, the two electrical connectors can only be plugged together when the guide post and guide slot of the two electrical connectors are aligned with each other. Otherwise, the two electrical connectors cannot be plugged into each other. Therefore, the guide post and the guide groove not only have a guiding function, but also can avoid incorrect connection of the two electrical connectors. In an exemplary embodiment of the disclosure, the guide groove  14  and/or the guide post  15  has a height equal to or greater than that of the protruding bar  12 . 
       FIG. 11  shows a schematic perspective view of an electrical connector according to another exemplary embodiment of the disclosure,  FIG. 12  shows a schematic enlarged view of part B shown in  FIG. 11 ,  FIG. 13  shows another schematic perspective view of the electrical connector shown in  FIG. 11 ,  FIG. 14  shows a schematic perspective view of an insulation housing according to an exemplary embodiment of the disclosure,  FIG. 15  shows a schematic enlarged view of part C shown in  FIG. 14  and  FIG. 16  shows a schematic perspective view of a metallization layer according to an exemplary embodiment of the disclosure in which the insulation housing is not shown for clarity. 
     In an exemplary embodiment of the disclosure, as shown in  FIGS. 6, 11-16  and with reference to  FIG. 3 , the bottom wall  11  of the insulation housing  1  is formed with a plurality of first through holes  125  and a plurality of second through holes  126 . The side walls of each of the protruding bars is formed with a plurality of first grooves  124  and a plurality of second grooves  127  in communication with the first through hole  125  and the second through hole  126 , respectively. The first grounding terminals  211  and the second grounding terminals  221  are mounted in the first through holes  125  and the first grooves  124 , respectively. The differential signal terminals of the differential signal terminal pairs  222  are mounted in the second through holes  126  and the second grooves  127 , respectively. The body portions of the first grounding terminal  211 , the second grounding terminal  221  and each terminal of the differential signal terminal pairs  222  are mounted in the first through holes  125  and the second through holes  126 , respectively, and the elastic portion and the contact portion are at least partially received in the first grooves  124  and the second grooves  127 . When the two electrical connectors  100 , 100 ′ are electrically connected with each other, the elastic portion and the contact portion of each terminal may be further at least partially biased into the first groove  124  and the second groove  127 . Thus, it is possible to facilitate a plugging operation of the two electrical connectors. An electrical connection layer  16  extends into the first through holes  125  to achieve a reliable electrical connection between the grounding terminals and the electrical connection layer. 
     In an exemplary embodiment of the disclosure, the insulation housing  1  is provided with an electrical connection layer through which at least two of the plurality of first grounding terminals  211  and the plurality of second grounding terminals  221  are electrically connected to each other. The electrical connection layer  16  is electrically insulated from the differential signal terminal pair  222 . The electrical connection layer includes a metallization layer  161  applied on the insulation housing  1  and a conductive layer  162  covering the metallization layer. 
     The metallization layer includes a plastic layer having the conductive particles. For example, the conductive particles are palladium particles. The conductive layer comprises a nickel layer or a copper layer. The at least two grounding terminals including the first grounding terminal  211  and the second grounding terminal  221 , and even all the grounding terminals, are connected with each other through the electrical connection layer. Thus, it is possible to reduce sensitivity of dimensional manufacturing tolerances of components, such as the grounding terminals or the through holes for holding the grounding terminals, to transmission high-frequency performance, while improving a resonance generated when transmitting high-frequency signals to make the signal transmission more stable. It should be understood that the insulation housing  1  is also provided with through holes or grooves adapted to hold the differential signal terminals, and there is no electrical connection layer provided on a surface of these through holes or grooves. 
     Further, the electrical connection layer  16  extends to a region of the bottom wall  11  expect for a region where the differential signal terminal pairs  22  are located. Since there is no plastic layer and conductive layer on the region where the differential signal terminals are located, i.e., the electrical connection layer  16  is provided with windows  1611  at positions where the differential signal terminal pairs  222  are located, so that the different differential signal terminals are electrically insulated from each other, and the differential signal terminals are also electrically insulated from the grounding terminals. In this way, each of the terminals can be electromagnetically shielded at the bottom of the electrical connector to further suppress the signal crosstalk. 
     In a process of manufacturing the electrical connector  100 , referring to  FIGS. 14 and 15 , firstly, forming a insulation housing  1  of a Liquid Crystal Polymer (LCP), for example, through an injection molding process (first injection molding process); applying a plastic layer including conductive particles on the insulation housing  1  through a further injection molding process (second injection molding process); applying a conductive layer on the plastic layer to form an electrical connection layer  16 ; mounting a plurality of grounding terminals (including a first grounding terminal  211  and a second grounding terminal  221 ) on the insulation housing  1 , respectively, so that at least two of the plurality of grounding terminals are electrically connected with each other through the electrical connection layer  16 . Since the insulation housing  1  is made of plastic material, it is difficult to directly plate a surface of the insulation housing  1  with metal material. The plastic layer including the conductive particles is applied on the surface of the insulation housing  1  so that the plastic layer has certain properties of a metal layer. Therefore, a conductive layer  162  may be plated on the insulation housing  1  with the plastic layer to realize an electrical connection of the plurality of grounding terminals. The plastic layer including the conductive particles is applied on the insulation housing  1  through the further injection molding process (second injection molding process). 
     In an exemplary embodiment of the disclosure, the step of forming the insulation housing  1  through the injection molding process includes: forming second through holes  126  adapted to mount differential signal terminals on a bottom wall  11  of the insulation housing  1 ; forming first grooves  124  adapted to accommodate the grounding terminals (the first grounding terminal and the second grounding terminal) and second grooves  127  in communication with the second through holes  126  and adapted to accommodate the differential signal terminals on the protruding bars of the insulation housing  1 . 
     In an exemplary embodiment of the disclosure, as shown in  FIGS. 14 to 16 , the step of applying the plastic layer  161  including conductive particles on the insulation housing  1  through the further injection molding process includes forming first through holes  125  in communication with the first grooves  124  and adapted to accommodates the grounding terminals so that the metallization layer is formed in the first through holes. That is to say, during forming the insulation housing  1  through the first injection molding process, only the second through holes  126  adapted to accommodate the differential signal terminals are formed in the bottom wall  11 , and the first through holes  125  are not formed. The first through holes  125  adapted to mount the grounding terminals are formed during forming the plastic layer  161  through the second injection molding process. The first through holes  125  pass through the bottom wall  11  of the insulation housing  11  and are brought into communication with the first grooves  124 . 
     In an exemplary embodiment of the disclosure, the conductive layer is plated on the plastic layer through a molded interconnect device (MID) molding process, or the conductive layer is deposited on the plastic layer through a physical vapor deposition (PVD) process. 
     In an exemplary embodiment of the disclosure, the bottom wall is provided with an isolation pad  3  for covering the electrical connection layer. After the grounding terminals and the differential signal terminals are installed on the insulation housing  1 , the isolation pad  3  is mounted on a first side (an upper side of  FIG. 4 ) of the bottom wall  11  of the electrical connector  100 , and the soldering portion  2115  of each terminal passes through the isolation pad  3 . Thereafter, the solder ball  4  made of solder material is formed on the soldering portion  2115  to prepare for electrical connection with an electrical contact of a circuit board. 
     According to another exemplary embodiment of the disclosure, referring to  FIGS. 1-17 , and particularly referring to  FIGS. 1-3 and 9 , there is provided an electrical connector  100  comprising: an insulation housing  1 ; a plurality of grounding terminal columns  21  arranged on the insulation housing and including a plurality of first grounding terminals  211 ; and a plurality of hybrid terminal columns  22  arranged on the insulation housing and including a plurality of second grounding terminals  221  and a plurality of differential signal terminals pairs  222 . The plurality of grounding terminal columns and the plurality of hybrid terminal columns are arranged adjacent to each other, respectively. Each differential signal terminal pair  222  is located between two second grounding terminals  221  in one hybrid terminal column. Each of the first grounding terminals  211  has a width W 1  greater than a width W 3  of each of the second grounding terminals  211 . 
     In addition, those areas in which it is believed that those of ordinary skill in the art are familiar, have not been described herein in order not to unnecessarily obscure the invention described. Accordingly, it has to be understood that the invention is not to be limited by the specific illustrative embodiments, but only by the scope of the appended claims. 
     It should be appreciated for those skilled in this art that the above embodiments are intended to be illustrated, and not restrictive. For example, many modifications may be made to the above embodiments by those skilled in this art, and various features described in different embodiments may be freely combined with each other without conflicting in configuration or principle. 
     Although several exemplary embodiments have been shown and described, it would be appreciated by those skilled in the art that various changes or modifications may be made in these embodiments without departing from the principles and spirit of the disclosure, the scope of which is defined in the claims and their equivalents. 
     As used herein, an element recited in the singular and proceeded with the word “a” or “an” should be understood as not excluding plural of the elements or steps, unless such exclusion is explicitly stated. Furthermore, references to “one embodiment” of the present disclosure are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. Moreover, unless explicitly stated to the contrary, embodiments “comprising” or “having” an element or a plurality of elements having a particular property may include additional such elements not having that property.