Patent Publication Number: US-2020295491-A1

Title: High performance electronic interposer

Description:
TECHNICAL FIELD 
     The present disclosure relates to interposer assemblies used for forming electrical connections between spaced contact pads on circuit members. 
     BACKGROUND 
     Interposer assemblies form electrical connections between contact pads arranged in very close proximity to each other. The pads are generally arranged on a grid spaced equidistantly from each other. Each assembly may include thousands of contacts. Thus, the contacts must establish a reliable electrical connection with the pads when they are placed between circuit members. In many cases, if a single contact fails to make a reliable connection, it could render the entire assembly useless. 
     Typically, contacts used in assemblies include contact surfaces, which mechanically engage the contact pads and form electrical connections. Conventional formed metal contact interposer assemblies have a surface contacts which engage a mating pad at an upper and lower interface thereby forming an electrical connection between the two pads along a single conduction path. 
     What is needed is an interposer assembly design that can create a higher-signal-speed connection by means of dual conduction paths. As discussed herein, various improvements are provided to enhance the function and reliability of interposer assemblies. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Some embodiments are illustrated by way of example and not limitation in the figures of the accompanying drawings in which: 
         FIG. 1  depicts a top view of contact assembly according to an embodiment. 
         FIG. 2  depicts a side view of a contact according to an embodiment. 
         FIG. 3  depicts a front view of a contact according to an embodiment. 
         FIG. 4  depicts a top view of a contact according to an embodiment. 
         FIG. 5  depicts a sectional view taken along line  1 - 1  of  FIG. 1  illustrating a contact in position to be inserted in a through passage in an interposer plate according to an embodiment. 
         FIG. 6  depicts a sectional view taken along line  1 - 1  of  FIG. 1  illustrating the contact in the plate according to an embodiment. 
         FIG. 7  depicts another sectional view taken along line  1 - 1  of  FIG. 1  illustrating the assembly positioned on a lower substrate according to an embodiment. 
         FIG. 8  depicts another sectional view taken along line  1 - 1  of  FIG. 1  illustrating an upper substrate on the top of the plate supported by a contact with the contact partially compressed according to an embodiment. 
         FIG. 9  depicts another sectional view taken along line  1 - 1  of  FIG. 1  illustrating the upper and lower substrates sandwiched onto the plate and the contact fully compressed to form two circuit paths according to an embodiment. 
         FIG. 10  depicts a front view of a contact according to another embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The present description and claims may make use of the terms “a,” “at least one of,” and “one or more of,” with regard to particular features and elements of the illustrative embodiments. It should be appreciated that these terms and phrases are intended to state that there is at least one of the particular feature or element present in the particular illustrative embodiment, but that more than one can also be present. That is, these terms/phrases are not intended to limit the description or claims to a single feature/element being present or require that a plurality of such features/elements be present. To the contrary, these terms/phrases only require at least a single feature/element with the possibility of a plurality of such features/elements being within the scope of the description and claims. 
     In addition, it should be appreciated that the following description uses a plurality of examples for various elements of the illustrative embodiments to further illustrate example implementations of the illustrative embodiments and to aid in the understanding of the mechanisms of the illustrative embodiments. These examples are intended to be non-limiting and are not exhaustive of the various possibilities for implementing the mechanisms of the illustrative embodiments. It will be apparent to those of ordinary skill in the art in view of the present description that there are many other alternative implementations for these various elements that may be utilized in addition to, or in replacement of, the embodiments provided herein without departing from the spirit and scope of the present disclosure. 
     As used herein, the term “includes” means “comprises.” For example, a device that includes or comprises A and B contains A and B but can optionally contain C or other components other than A and B. A device that includes or comprises A and B may contain A and B, and optionally one or more other components such as C. 
     In accordance with the present invention, various embodiments of a high performance electronic interposer are disclosed herein. In previous solutions, an array of spring metal contacts would be retained in cavities within a molded plastic housing using a molded ramp feature in each housing cavity that engages a gap at the mouth of one or more C-shaped contacts. Various examples of previous designs may be found in U.S. Pat. Nos. 6,290,507; 6,730, 134; and 6,315,576. 
     As discussed herein, various improvements are possible over the above-listed designs. For example, in one embodiment, the contact is retained in the housing cavity by means of a molded ramp that engages a detent in the spine of the contact. Thus, in some embodiments, the front arms of the C-shaped contact are allowed to deflect freely in the cavity (i.e., without having to deflect along the molded ramp) during contact deflection. 
     In a further embodiment, the freely acting arms of the contact may have various improvements, such as, for example a tab-and-slot design, or matching offset protrusions. Thus, when pressure is applied to the contact (e.g., via dielectric plates), the mouth of the contact is closed, and the two ends securely interlock with each other. It should be understood that, although various specific examples may be used herein, the contact may sit within the interposer and between a variety of planar surfaces. Some non-limiting examples of planar surfaces may be circuit boards, flexible circuits, integrated circuits, and/or any other electronic surface that includes wiring and/or pads suitable for connection to another surface. 
     As would be understood by one skilled in the art, the interlocked ends of the contact arms ensure that the arms do not slip off of each other and/or that one arm does not flex or move more than the other. In some embodiments, when the contact arms meet, the force applied may cause the arms of the contact to deflect inward toward the center, thereby significantly adding to the contact&#39;s resultant normal force and meeting minimum standard normal force requirements for a low-voltage electronic connector. Thus, ensuring a secure connection between the two arms is critical not only for improved conductivity, but also to ensure that each contact maintains the proper required resultant force. The interlocked ends of the contact arms also ensure a consistent electrical path length, as compared to the variable electrical path length of a similar contact without interlocking ends and rather a ‘rolling’ contact end interface. 
     In a further embodiment, one or more domes may be placed at the upper and lower contact points of the contact. As discussed herein, such domes may allow for an increased point-contact force at the contact interface and for easier alignment with the one or more mating contact pads. 
     Referring now to  FIG. 1 , a top view of an interposer assembly  100  is shown. As illustrated in  FIG. 1 , each passage  101  may have a maximum width between side wall portions  102  at a first end wall  103  and a minimum width between the sidewall portions at a second end wall  104 . It should be understood that various features, such as a protrusion or tip, may be discussed herein by reference to the first end wall  103  and/or the second end wall  104 , but that these examples are for illustrative purposes only. Thus, the orientation and/or placement of a contact  200  may be discussed relative to the first end wall  103  and/or the second end wall  104 ; however, one of ordinary skill in the art will realize that alternative embodiments are possible in which the first end wall  103  represents the narrow end of the passage  101  and the second end wall  104  represents the wide end of the passage, or vice versa. 
     It should also be noted that although  FIG. 1  shows one passage  101  as being empty, this is for illustrative purpose only. Generally, each passage  101  of the assembly  100  will contain a contact  200 . Additionally or alternatively, some embodiments may exist wherein one or more passages  101  are left empty by design. 
     Referring generally to  FIGS. 2, 3, and 4 , in some embodiments, the contacts  200  may be stamp-formed from gold and nickel plated beryllium copper strip stock of uniform thickness. However, it should be understood that various other metals or metal alloys capable of meeting the conductive requirements of electronic circuitry may be used (e.g., silver, copper, gold, aluminum, zinc, nickel, brass, bronze, iron, platinum, steel, lead, stainless steel, or any combination thereof). In some embodiments, the stock may have a thickness between about 0.001 mm and about 0.1 mm. In another embodiment, the stock may have a thickness between about 0.04 mm and about 0.06 mm. Each contact  200  has a flexible vertical spine  201  with a rounded upper contact support  202  and a rounded lower contact support  203  at the ends of the spine. 
     In a further embodiment, one or more contacts may have a first upper flat spring arm  204  that angles upwardly and inwardly from an upper contact support  205  to the rounded upper contact support  202  at the top of the contact  200 . In another embodiment, a second upper flat spring arm  206  may angle downwardly and outwardly from the rounded upper contact support  202  to an upper support bend  207  which faces away from the spine  201 . A contact  200  may also have a upper end strip  208  that extends downwardly and inwardly from upper support bend  207  to an upper tip  260 . 
     In a further embodiment, the contact  200  may be vertically symmetrical to either side of the center point of the spine  201 , such that the lower half of the contact has a lower contact support  209 , first lower flat spring arm  210 , rounded lower contact support  203 , a second lower flat spring arm  211 , lower support bend  212 , lower end strip  213  and a lower tip  280 . In a further embodiment, the contact  200  may have a pair of domes (i.e., upper dome  214  and lower dome  215 ) provided on the opposed edges of rounded upper contact support  202  and rounded lower contact support  203 . 
     As shown in  FIG. 5 , in some embodiments, an interposer assembly  100  may have a flat dielectric plate  105  with parallel top and bottom surfaces  106  and  107  having a uniform thickness  108  and closely spaced contact passages  101  arranged in intersecting land grid array rows and columns (see  FIG. 1  for detail). The plate  100  may have a thickness  108  between about 1.0 mm and about 2.0 mm. In a further embodiment, one or more formed metal contacts  200  may be positioned in the passages  101 . 
     Referring generally to  FIGS. 1 and 5 , in some embodiments, each passage  101  in plate  105  may have a wide end wall  103  and an opposed narrow end wall  104 . The wide end wall  103  may be flat and extend perpendicularly between the parallel top  106  and bottom  107  surfaces. The narrow end wall  104  may include a contact retention protrusion  109  which extends into passage  101 . The protrusion  109  may have two flat and inwardly angled cam surfaces that meet to form a tip  110 . In some embodiments, the tip  110  may be located equidistant between the top surface  106  and the bottom surface  107 . 
     The flat cam surfaces on wall  104  extend from the tip  110  toward the top and bottom surfaces  106  and  107  at a shallow outward angle. As shown in  FIG. 5 , wall  103  is perpendicular to surfaces  106  and  107 , and the width of passage  101  increases to either side of tip  110 . 
     In a further embodiment, passages  101  have opposed sidewalls extending between end walls  103  and  104 . In a further embodiment, each sidewall may include a flat portion extending perpendicularly between the top surface  106  and the bottom surface  107  and perpendicularly from one edge of wide end wall  103 . A yet further embodiment may have flat, inwardly tapered sidewall portions extend from the edge of second end wall  104  away from first end wall  103 . As illustrated in  FIG. 1 , each passage  101  has a maximum width proximate to one of the first  103  and second  104  end walls (e.g., the first end wall  103  as shown), and a minimum or reduced width proximate to the other one of the first  103  and second  104  end walls (e.g., the second end wall  104  as shown). 
     In some embodiments, the one or more contacts  200  may be inserted into one or more passages  101  by positioning a contact above a passage, as shown in  FIG. 5 , and then lowering the contact down into the passage to the position shown in  FIG. 6 . In one embodiment, during and after insertion, the contact  200  may be positioned such that the detent of the spine  201  encompasses and/or contacts the tip  110  of the protrusion  109 . During insertion of the one or more contacts  200 , the upper contact support  205  may move down along wall  104  and engage the upper cam side of protrusion  109 , while the lower contact support  209  engages the lower cam side of the protrusion. In a further embodiment, the upper support bend  207  and the lower support bend  212  may slide down the first end wall  103 , as shown in  FIG. 6 . 
     In some embodiments, the contact  200  may have a loose fit in passage  101  with limited free vertical movement. Accordingly, at least a portion of the upper contact support  205  and the lower contact support  209  may rest on a portion of wall  104 , specifically the contact retention protrusion  109 . In a further embodiment, at least a portion of the upper support bend  207  and the lower support bend  212  rest on a portion of wall  103 . As shown in  FIG. 6 , the contact  200  is in a gravity down position with the upper contact support  205  resting on the upper cam side of protrusion  109  and lower contact support  209  resting on the lower cam side of protrusion  109 . 
     In one or more embodiments, the interposer assembly  100  may have one or more contacts  200  inserted and in the gravity down position as shown in  FIG. 6 . The interposer assembly  100  may then be placed on a lower substrate  111  as illustrated in  FIG. 7 . The lower substrate  111  may have a contact pad (not illustrated) located below each contact  200  for engagement with the lower dome  215 , as illustrated. When the assembly is placed on substrate  111 , the plate  105  may be lowered until the lower dome  215  engages the contact pad on the substrate, which creates an upward biasing force. As discussed herein, the upper  214  and lower  215  domes allow for a higher point-contact force at the contact pad of the substrate. Moreover, the domes also allow for easier alignment of the one or more mating contact pads with the one or more contacts, specifically the upper  214  or lower  215  domes on the contact  200 . 
     In a further embodiment, the upward biasing force may cause the lower support bend  212  to further engage the first end wall  103 , thereby forcing the tip  280  to move in an upward and inward direction as shown in  FIG. 8 . In some embodiments, the contacts  200  may be raised up in passages  101  to an elevated position. The dialectic plate  105  may have a distance  112  above the lower substrate  111  as illustrated in  FIG. 7 . The contact  200  may be raised up in passage  101  with the rounded upper contact support  202  or the dome  214  at a distance  113  above the top of the plate which is greater than the distance  112  between the substrate  111  and the dielectric plate  105 . 
     In some embodiments, when the contact  200  is in the position shown in  FIG. 7 , an upper substrate  114  is placed on the top of plate  105 . As shown in  FIG. 8 , the upper substrate  114  may comprise one or more contact pads (not illustrated) that engage the upper dome  214 . The upper substrate  114  may then be moved toward the lower substrate  111  to compress the domes  214  and  215 , thereby compressing the rounded upper contact support  202  and the rounded lower contact support  203  into passage  101 . The contact  200  may then be in the position shown in  FIG. 8  when first engaged by upper substrate  114  and lower substrate  111 . In another embodiment, the upper contact support  205  and the lower contact support  209  may slide along wall  105  thereby interacting with the protrusion  109 . In one embodiment, the upper support bend  207  and the lower support bend  212  may be bent down into passage  101  with further lowering of the upper substrate  114 . 
     Accordingly, in some embodiments and as shown in  FIG. 8 , as the contact  200  compresses due to the force applied at the upper substrate  114  and/or the lower substrate  111 , the upper support bend  207  and the lower support bend  212  are forced toward each other while still being contained by wall  103 . Thus, in at least one embodiment, the upper tip  260  may contact the lower tip  280 . In a further embodiment, tips  260  and  280  may be formed with various interlocking or connection geometries as disclosed in further detail herein. In a further embodiment, as the upper support bend  207  and the lower support bend  212  move towards each other, the upper and lower domes  214  and  215  move or slide along the contact pads on both the upper substrate  114  and lower substrate  111 , and thereby wipe or scratch the contact pads to ensure proper electrical connections between the two points of contact at the upper and lower domes  214  and  215 . 
     In additional embodiments, as the contact  200  is compressed, the spine  201  may remain in approximately the same location during compression. As shown in  FIG. 8 , the upper contact support  205  and the lower contact support  209  contact the contact retention protrusion  109  of the second end wall  104 , thereby creating tension in the contact  200 . This further creates a biasing force in a direction distal to the contact retention protrusion  109 . This biasing force ensures that the contact  200  is not moved or reoriented as forces are applied via the upper substrate  114  and the lower substrate  111 . 
     Referring now to  FIG. 9 , in some embodiments, further downward movement of the upper substrate  114  and/or upward movement of the lower substrate  111  may result in the substrates being located against the top  106  and bottom  107  surfaces of the dialectic plate  105 , respectively. During this movement, the contact  200  may be further compressed causing the point of physical engagement between tips  260  and  280  to move inwardly toward the center of the contact (e.g., compare  FIGS. 8 and 9 ). Thus, in some embodiments, the electrical connection between the tips  260  and  280  creates a redundant or second electrical circuit path (i.e., in additional to the path created by the spine  201  of the contact  200 . 
     Referring briefly to  FIG. 2 , in some embodiments, a first continuous conductive circuit path may exist between the upper dome  214  and the lower dome  215  via the pathway created from the upper substrate  114  through the upper dome, the first upper flat spring arm  204 , the upper contact support  205 , the spine  201 , the lower contact support  209 , the first lower flat spring arm  210 , and the lower dome to the contact point on the lower substrate  111 . A second or redundant continuous conductive circuit path may exist between the upper dome  214  and the lower dome  215  via the pathway created from the upper substrate  114  through the upper dome, the rounded upper contact support  202 , the second upper flat spring arm  206 , the upper support bend  207 , the upper end strip  208 , the compression connection of the upper tip  260  and the lower tip  280 , the lower end strip  213 , the lower support bend  212 , the second lower flat spring arm  211 , the rounded upper contact support  203 , and the lower dome to the contact point on the lower substrate  111 . 
     As shown in  FIG. 9 , in a further embodiment, during compression of the contact  200 , the upper support bend  207  and the lower support bend  212  may rotate out of engagement with wall  103 . In some embodiments, the contacts  200  are free to move vertically in the passage. This freedom of vertical movement desirably equalizes the pressures at the top (e.g., where the upper dome  214  contacts the upper substrate  114 ) and bottom (e.g., where the lower dome  215  contacts the lower substrate  111 ) of the contact to ensure that a reliable electrical connection is formed between the contact pads on both substrates. 
     Thus, according to some embodiments, the upper dome  214  and/or the lower dome  215  may wipe or scrape along the contact pad of the upper substrate  114  and/or the lower substrate  111 , which can improve the quality of the electrical connection. Over time, impurities, oxides, or contaminants may build up on either the contact surface or the pad and can impair the electrical connection. As should be understood by one of ordinary skill in the art, the redundant circuit paths described herein reduce electrical resistance between the upper substrate  114  and the lower substrate  111  to help reduce high speed inductance between contacts in plate  105 . 
     In some embodiments, when the contact  200  is being compressed and the second redundant electrical connection between tip  260  and tip  280  is formed, variables inherent in the manufacture of interposer assemblies, their components, and the location of the components may affect the connection between the substrates. Generally, mating with contact pads (not shown) on opposing substrates (e.g.,  111  and  114 ) is unlikely to be perfectly symmetrical. This is because, in some embodiments, the manufactured parts may have dimensional tolerances or design characteristics that may affect mating. In a further embodiment, the result of having these manufacturing variables is that during compression of the contact  200  and formation of the connections between tips  260  and  280 , the upper  107  and lower  212  support bends may engage the end wall  103  at different times, such that tips may not align and perfectly deflect together. 
     As illustrated in  FIG. 9 , in some embodiments, the compressed contact  200  may include a first and second circuit path, described in detail herein, extending across the thickness of plate  105  from a first contact point (i.e., where the upper dome  214  contacts the upper substrate  114 ) to a second contact point (i.e., wherein the lower dome  215  contacts the lower substrate  111 ). The two connectivity paths substantially extend through the full length of the contact  200 , which may eliminate any substantial antenna or stub portion of the contact being outside of the electrical path. Elimination of the stubs prevents radiation of current passing through the contact and, as a result, increases signal integrity at the contact. 
     As discussed herein, the tips  260  and  280  may not perfectly align during compression between the upper substrate  114  and the lower substrate  111  based on manufacturing tolerances. As would be understood by one of ordinary skill in the art, the upper and lower tips  260  and  280  could potentially slide off of each other or lose connection in another way if the tips are blunt and/or not interlocking. Accordingly, in some embodiments, the tips  260  and  280  may have corresponding interfaces that improve the connection strength of the tips and decrease the likelihood of a failed connection (i.e., where upper tip  260  and lower tip  280  do not touch and thus fail to create a conductive path). 
     As shown in  FIG. 3 , the upper tip  260  may have one or more protrusions  261 , which mate with a corresponding recession  281  in the lower tip  280  (or vice versa). Accordingly, in some embodiments, even if the upper tip  260  and the lower tip  280  do not perfectly align while moving in an inward direction, due to the compression of the contact  200 , the interlocking nature of the upper and lower tips ensures that a consistent and long-term connection is maintained. It should be understood that the figures and description of the interlocking pattern and/or method of upper tip  260  and lower tip  280  may be of any form or corresponding shape. Accordingly,  FIG. 10  illustrates an additional or alternative non-limiting embodiment where the upper tip  260  may have a lowered portion  262  and a raised portion  263 , and the lower tip  280  may have a correspondingly shaped raised portion  282  and lowered portion  283 . It should be understood that the number of raised/lowered areas and/or the number of protrusions and/or recesses may vary based on the specific situation and need. 
     In the above detailed description, reference is made to the accompanying drawings, which form a part hereof. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. The illustrative embodiments described in the detailed description, drawings, and claims are not meant to be limiting. Other embodiments may be used, and other changes may be made, without departing from the spirit or scope of the subject matter presented herein. It will be readily understood that the aspects of the present disclosure, as generally described herein, and illustrated in the Figures, can be arranged, substituted, combined, separated, and designed in a wide variety of different configurations, all of which are explicitly contemplated herein. 
     The present disclosure is not to be limited in terms of the particular embodiments described in this application, which are intended as illustrations of various aspects. Many modifications and variations can be made without departing from its spirit and scope, as will be apparent to those skilled in the art. Functionally equivalent methods and apparatuses within the scope of the disclosure, in addition to those enumerated herein, will be apparent to those skilled in the art from the foregoing descriptions. Such modifications and variations are intended to fall within the scope of the appended claims. The present disclosure is to be limited only by the terms of the appended claims, along with the full scope of equivalents to which such claims are entitled. It is to be understood that this disclosure is not limited to particular methods, reagents, compounds, compositions or biological systems, which can, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting. 
     With respect to the use of substantially any plural and/or singular terms herein, those having skill in the art can translate from the plural to the singular and/or from the singular to the plural as is appropriate to the context and/or application. The various singular/plural permutations may be expressly set forth herein for sake of clarity. 
     It will be understood by those within the art that, in general, terms used herein, and especially in the appended claims (for example, bodies of the appended claims) are generally intended as “open” terms (for example, the term “including” should be interpreted as “including but not limited to,” the term “having” should be interpreted as “having at least,” the term “includes” should be interpreted as “includes but is not limited to,” et cetera). While various compositions, methods, and devices are described in terms of “comprising” various components or steps (interpreted as meaning “including, but not limited to”), the compositions, methods, and devices can also “consist essentially of” or “consist of” the various components and steps, and such terminology should be interpreted as defining essentially closed-member groups. It will be further understood by those within the art that if a specific number of an introduced claim recitation is intended, such an intent will be explicitly recited in the claim, and in the absence of such recitation no such intent is present. 
     For example, as an aid to understanding, the following appended claims may contain usage of the introductory phrases “at least one” and “one or more” to introduce claim recitations. However, the use of such phrases should not be construed to imply that the introduction of a claim recitation by the indefinite articles “a” or “an” limits any particular claim containing such introduced claim recitation to embodiments containing only one such recitation, even when the same claim includes the introductory phrases “one or more” or “at least one” and indefinite articles such as “a” or “an” (for example, “a” and/or “an” should be interpreted to mean “at least one” or “one or more”); the same holds true for the use of definite articles used to introduce claim recitations. 
     In addition, even if a specific number of an introduced claim recitation is explicitly recited, those skilled in the art will recognize that such recitation should be interpreted to mean at least the recited number (for example, the bare recitation of “two recitations,” without other modifiers, means at least two recitations, or two or more recitations). Furthermore, in those instances where a convention analogous to “at least one of A, B, and C, et cetera” is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (for example, “a system having at least one of A, B, and C” would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, et cetera). In those instances where a convention analogous to “at least one of A, B, or C, et cetera” is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (for example, “a system having at least one of A, B, or C” would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, et cetera). It will be further understood by those within the art that virtually any disjunctive word and/or phrase presenting two or more alternative terms, whether in the description, claims, or drawings, should be understood to contemplate the possibilities of including one of the terms, either of the terms, or both terms. For example, the phrase “A or B” will be understood to include the possibilities of “A” or “B” or “A and B.” 
     In addition, where features or aspects of the disclosure are described in terms of Markush groups, those skilled in the art will recognize that the disclosure is also thereby described in terms of any individual member or subgroup of members of the Markush group. 
     As will be understood by one skilled in the art, for any and all purposes, such as in terms of providing a written description, all ranges disclosed herein also encompass any and all possible subranges and combinations of subranges thereof. Any listed range can be easily recognized as sufficiently describing and enabling the same range being broken down into at least equal halves, thirds, quarters, fifths, tenths, et cetera. As a non-limiting example, each range discussed herein can be readily broken down into a lower third, middle third and upper third, et cetera. As will also be understood by one skilled in the art all language such as “up to,” “at least,” and the like include the number recited and refer to ranges that can be subsequently broken down into subranges as discussed above. Finally, as will be understood by one skilled in the art, a range includes each individual member. Thus, for example, a group having 1-3 cells refers to groups having 1, 2, or 3 cells. Similarly, a group having 1-5 cells refers to groups having 1, 2, 3, 4, or 5 cells, and so forth. 
     Various of the above-disclosed and other features and functions, or alternatives thereof, may be combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art, each of which is also intended to be encompassed by the disclosed embodiments.