Hollow stem design for high density interconnects

An article includes a plurality of stems projecting from a surface thereof. Each stem includes a hollow stalk projecting orthogonally from the surface and a hollow head defined on a distal end of the hollow stalk. The head includes a flange section extending from the distal end of the hollow stalk and having an outer diameter greater than an outer diameter of the hollow stalk at the distal end. A cap section extends from the flange section, defines a distal end of the stem and has a generally convex contour. The cap section has a first wall thickness at the distal end of the stem, a second wall thickness at a junction of the cap section and the flange section and a third wall thickness at a circumferential section intermediate the distal end and the flange section. The third wall thickness is less than the first and second wall thicknesses.

FIELD OF INVENTION

The present invention relates generally to interconnect systems, and more particularly, to high density electronic interconnect systems.

BACKGROUND

Fasteners, solders and adhesives have been used to attach components to primary hardware and structures. For example, integrated circuit (IC) chips are typically soldered to motherboards. However, solders are subject to problems such as cracking which may cause a circuit board to malfunction or to fail. Furthermore, solders conventionally include lead; which may be undesirable in some applications. Adhesives used to connect a component to a primary hardware often require a bong cure time. Further, adhesives often employ undesirable solvents for attachment. Moreover, once the component is attached to a structure using solder or adhesive, the component may not be repositioned without damaging the component and/or the structure.

Other options include coating the surface of mechanical interconnects with an electrically conductive material. Several of the known mechanical interconnect systems include hock and loop fasteners available under the trademark Velcro® from Velcro USA, Inc., Manchester, N.H. and reclosable fasteners available under the trademark 3M™ Dual Lok™ from 3M, St. Paul, Minneapolis. However, such mechanical fasteners suffer from disadvantages such as unpredictable contact area, which may prove detrimental to consistent high electrical conductivity, and insufficient locking strength. These fasteners are also subject to frictional locking which may deform the fastener upon multiple engagement/disengagement cycles. Such fasteners provide rather limited holding force that rapidly decreases with repeated cycles of engagement and disengagement. Alternatives are, therefore, desirable.

SUMMARY

According to an embodiment of the invention, an article of manufacture includes a plurality of stems projecting from a surface of the article for interconnecting with another body having a corresponding plurality of stems. Each one of the plurality of stems includes a hollow stalk projecting substantially orthogonally, at a proximal end thereof, from the surface and a hollow head defined on a distal end of the hollow stalk. The head is preferably dome shaped, and more preferably of a mushroom shape or a convex structure. The head includes a flange section extending from the distal end of the hollow stalk and having an outer diameter greater than an outer diameter of the hollow stalk at the distal end. The head further includes a cap section extending from the flange section and defining a distal end of the stem. The cap section has an outer surface that is of a generally convex contour and has a first wall thickness at the distal end of the stem, a second wall thickness at a junction of the cap section and the flange section and a third wall thickness at a circumferential section intermediate the distal end and the flange section. The third wall thickness is less than the first and second wall thicknesses.

According to an embodiment of the invention, the wall thickness of the cap section decreases continuously from the first wall thickness at the distal end of the stem to the third wall thickness at the circumferential section. The wall thickness of the cap section increases continuously from the third wall thickness at the circumferential section to the second wall thickness at the junction.

According to another embodiment of the invention, a system includes a first interconnecting article having a first plurality of stems extending from a first surface of the first article and a second interconnecting article configured for connecting with the first interconnecting article and having a second plurality of stems extending from a second surface of the second article. Each one of the first and second pluralities of stems includes a hollow stalk projecting substantially orthogonally, at a proximal end thereof, from one of the first and second surfaces and a hollow head defined on a distal end of the hollow stalk. The head includes a flange section extending from the distal end of the hollow stalk and having an outer diameter greater than an outer diameter of the hollow stalk at the distal end. The head further includes a cap section extending from the flange section and defining a distal end of the stem. The cap section has a generally convex contour and has a first wall thickness at the distal end of the stem, a second wall thickness at a junction of the cap section and the flange section and a third wall thickness at a circumferential section intermediate the distal end and the flange section. The third wall thickness is less than the first and second thicknesses.

When the second surface is superposed on the first surface such that the heads of second plurality of stems are in general contact engagement with, but laterally offset from, the heads of the first plurality of stems, a sub-set of stems of the first plurality of stems defines a gap and accommodates a head and a stem of the second plurality of stems therein, upon an application of a first force in excess of a first predetermined threshold on at least one of the first and second articles urging the at least one of the first and second articles toward the other article.

Applying a second force in excess of a second predetermined threshold on at least one of the first and second articles in a direction opposite of the first force pulling the first and second articles away from each other causes the stem of the second plurality of stems to be released from the at least one sub-set of the stems of the first plurality of stems.

An electrical interconnect apparatus includes a substrate and a plurality of stems projecting from the substrate. Each of the plurality of stems includes a hollow stalk projecting generally orthogonally, at a proximal end thereof, from the substrate and a generally convex, hollow head defined on a distal end of the stalk. The hollow head includes a flange section extending from the distal end of the stalk and having an outer diameter greater than an outer diameter of the stalk at the distal end. The hollow head further includes a cap section have a generally convex contour extending from the flange section. A distal end of the cap section is configured to flex about a circumferential section of the cap section along a longitudinal axis of the stalk, responsive to a force acting thereon. The outer diameter of the cap section increases from the circumferential section toward the flange section, defining an engagement section. The plurality of stems is fabricated from at least an electrically conducive material.

According to an embodiment of the invention, the circumferential section has a wall thickness less than a wall thickness of the cap section at any other circumferential section. The cap section has a wall thickness at the distal end greater than the wall thickness of the circumferential section.

DETAILED DESCRIPTION

It is to be understood that the figures and descriptions of the present invention have been simplified to illustrate elements that are relevant for a clear understanding of the present invention, while eliminating, for purposes of clarity, many other elements found in such interconnecting bodies and reclosable fasteners. However, because such elements are well known in the art, and because they do not facilitate a better understanding of the present invention, a discussion of such elements is not provided herein. The disclosure herein is directed to all such variations and modifications known to those skilled in the art.

Referring to the embodiment ofFIG. 1, there is illustrated an interconnecting body100. In the illustrated embodiment, body100includes a substrate or base110and a plurality of hollow stems120. Each one of the plurality of hollow stems120projects generally orthogonally from a first surface115of base110. A second surface117of base110may be adapted to be affixed or otherwise fastened to a component or a structure (not shown). In the illustrated embodiment, base110is a generally flat, planar substrate. In other embodiments, base110may take the form of a substrate having a curved profile. In yet other configurations, base110may take the form of an outer skin of a mechanical structure or an electrical component such as a circuit board or an electrical component. For example, the solder balls in a ball grid array (BGA) package1010(known in the art) may be replaced by hollow stems1020, as illustrated inFIG. 2, to engage hollow stems120of substrate110.

In an exemplary embodiment, body100may be fabricated from engineering plastics using, for example, high tolerance injection molding processes, such as those currently in use for compact disc (CD) and Digital Video Disc (DVD) manufacture. Non-limiting examples of engineering plastics suitable for fabrication of surface100include polycarbonates (PC), acrylonitrile butadiene styrene (ABS), polyamides (PA), polybutylene terephthalate (PBT), polyethylene terephthalate (PET), polyphenylene oxide (PPO), polysulpone (PSU), polyetherketone (PEK), polyetheretherketone (PEEK), polyimides and polyphenylene sulphide (PPS). Other suitable materials include materials having high heat resistance, mechanical strength, rigidity, chemical stability and flame retardance. In an exemplary embodiment, body100including hollow stems120may be fabricated from an electrically conductive material. In an exemplary embodiment, a non-conductive plastic may be surface coated with a nano-composite material such as carbon nanotubes composite or with metallic nano-spheres for imparting and electrical conductivity to the embodiment. Since such coatings are known in the art, they are not described in detail for sake of brevity after the fabrication of hollow stems120. In another embodiment, hollow stems may be fabricated from an electrically conductive composite. Examples of electrically conductive composite include, but are not limited to, plastics that contain additives that impart such conductivity via inclusion of metallic powders, carbon black, carbon fibers, mats, and metalized glass fibers and spheres.

In another configuration, body100including hollow stems120may be fabricated from a thermally conductive material, for example, from carbon nanotube based composites. In another embodiment, the surface of body100or at least stem elements120may be coated with a thermally conductive material such as a carbon nanotube composite or with metallic spheres.

Referring now toFIG. 1A, hollow stem120includes a hollow stalk210and a hollow bulbous head230defining inner space220. A proximal end212of stalk210is coalesced to and extends from surface115(ofFIG. 1) of base110(ofFIG. 1). Head230is coalesced to and extends from an end214of stalk210. In the illustrated embodiment, hollow stem120is a unitary, monolithic structure coalesced to and extending from base110(ofFIG. 1). The height of stalk210is the distance between ends212and214.

FIG. 3Ashows a partial perspective cross-sectional view of a hollow stem120.FIG. 3Bshows a cross-sectional view of hollow stem120along lines3B-3B, according to an embodiment of the invention. With reference toFIGS. 3A and 3B, stalk210has a generally uniform wall thickness216, a generally uniform inner diameter350, and a generally uniform outer diameter352between ends212,214. In other embodiments, stalk210may have a non-uniform wall thickness216and/or a non-uniform inner diameter350and/or a non-uniform outer diameter352, depending on the requirements of a given application. In one configuration, head230includes a flange section314and a cap section310. Flange section314has an outer diameter375greater than an outer diameter352at distal end214of hollow stalk210. In an exemplary configuration, the outer surface of cap section310has a generally convex contour318. Cap section310has a circumferential section315intermediate of flange section314and a distal end or apex316. Distal end316defines a distal end of stem120. An engagement section320is defined on cap section310between intermediate circumferential section315and flange section314. A disengagement section or locking section340is defined between flange section314and distal end214of stalk210. Engagement section320has a first end322and a second end324. Engagement section320extends between cap section310, at second end324, and disengagement section340, at first end322. Disengagement section has a first end342and a second end344. Disengagement section340, in turn, extends between engagement section320, at second end344, and stalk210, at first342.

Cap section310acts as an alignment mechanism for guiding a counterpart bulbous head (not shown) of a hollow stem (not shown) projecting from a second corresponding interconnecting body (not shown) of like dimensions and superposed on body100toward a gap (not shown) defined by adjacent hollow stems120. Cap section310may also provide structural rigidity to hollow stem120. Engagement section320has a generally increasing outer diameter from circumferential section315to flange, section314. Engagement section320is configured to engage a corresponding engagement section (not shown) of a hollow stem (not shown) projecting from the second body superposed on body100and, responsive to the engagement therebetween, provides a bending force to stalk210in a radial direction. Engagement section320, thus, serves to bend stalk210in a radial direction, to facilitate the entry of a bulbous head (not shown) into a gap (not shown) defined by adjacent hollow stems120by widening the gap (not shown). Engagement section320, by providing the bending forces, also reduces the kinematic friction between the engaging hollow stems. Disengagement section340has a generally decreasing outer diameter from flange section314to distal end214of hollow stalk210. Disengagement section or locking section340serves to engage a corresponding disengagement section340of a hollow stem (not shown) extending from the second body superposed on body100and provide a holding force resisting the movement of the bulbous head (not shown) of a hollow stem of a second interconnecting body out of the gap defined by adjacent hollow stems120, when the interconnecting bodies are subject to a force pulling them away from each other.

Cap section310is coalesced to and extends from a second end324of engagement section320to apex316. It will be understood that apex316may have a convex contour or may be flat relative to cap section310. Cap section310has a tapered cross-section with a maximum first thickness365about apex316and a second minimum thickness370about intermediate circumferential section315. Second thickness370of cap section310is thinner than first thickness365of cap section310as well as a first thickness360of engagement section320or flange section314. Such a configuration enables circumferential section315to function as a hinge. For instance, upon application of a force on hollow head230toward stalk210in the form of a corresponding hollow bulbous head (not shown) of a hollow stem (not shown) projecting from the superposed second body (not shown), cap section310may reversibly bend or flex about circumferential section315relative to engagement section320and move along central or longitudinal axis380of stalk210.

Still referring toFIGS. 3A and 3B, at a second end324, engagement section320is coalesced to and extends from intermediate circumferential section315of cap section310and, at first end322, coalesces into second end344of disengagement section340. Engagement section320has an inclined outer surface for engaging a corresponding engagement section (not shown) of a hollow stem (not shown) projecting from the second body (not shown) superposed on body100(ofFIG. 1). Engagement section320has a tapered cross-section having a first maximum thickness360at first end322and a second minimum thickness370at second end324. The thickness of engagement section320, thus, varies in a non-linear, continuously increasing fashion from second thickness370at circumferential section315to first thickness360at flange section314. Thus, engagement section320has a first gradient defined from second end324to first end322. The first gradient of engagement section320may be derived as the difference, between an outer diameter of circumferential section315and outer diameter375of flange section314, divided by a height326of engagement section320.

At a second end344, disengagement section340is coalesced to and extends from first end322of engagement section320and, at a first end342, coalesces into end214of stalk210. Disengagement section340has an inclined outer surface for engaging a corresponding disengagement section (not shown) of a bulbous head (not shown) of a hollow stem (not shown) projecting from the second body (not shown) superposed on body100(ofFIG. 1), when the bulbous head (not shown) is lodged in the gap (not shown) defined by adjacent hollow stems120. Disengagement section340has a tapered cross-section having a first maximum thickness360at second end344and a second minimum thickness216at first end342. The thickness of disengagement section340, thus, varies in a non-linear, continuously decreasing fashion from first thickness360to second thickness216at first end342. Thus, disengagement section340has a second gradient defined from first end342to second end344. In an exemplary embodiment, the second gradient of disengagement section340is greater than the first gradient of engagement section320. The second gradient of disengagement section340may be derived as the difference, between outer diameter375of flange section314and outer diameter352of stalk210at distal end214, divided by a height346of disengagement section340.

In one configuration, flange section314may have a ring-like bulging profile. Flange section314has thickness360which is greater than minimum thickness370of engagement section as well as minimum thickness216of disengagement section. Flange section314serves to limit the local deformation of head230when engagement section320engages a corresponding engagement section (not shown) of a bulbous head (not shown) of a hollow stem (not shown) projecting from the second body (not shown) superposed on body100(ofFIG. 1) as well as when disengagement section340engages a corresponding disengagement section (not shown) and a force is pulling the second body away from body100(ofFIG. 1).

In an exemplary embodiment, hollow head230may have diameter375of about 490 microns (μm) and stalk210may have height355of about 260 μm, wall thickness216of about 25 μm, and inner diameter350of about 400 μm, by way of non-limiting examples only. In one configuration, stalk210may have a ratio of height355to wall thickness216of about 10 and may range from about 7 to 13. Cap section310may have first thickness365of about 60 μm and second thickness370of about 16 μm. In an exemplary configuration, the ratio of first thickness365to second thickness370may range from about 3 to 5. In one configuration, engagement section320may have second thickness370of about 16 μm, first thickness360of about 45 μm and a height326of about 80 μm. Disengagement section may have first thickness216of about 25 μm, maximum thickness360of about 45 μm and a height346of about 40 μm in an exemplary embodiment.

In an exemplary embodiment, engagement section320may have a first gradient of about 0.15 and disengagement section340may have a second gradient of about 0.8. In one configuration, the ratio of second gradient of disengagement section340to first gradient of engagement section320may range between about 4 to 6. It will be understood that different dimensions and ratios may be selected for hollow stem120, including hollow stalk210and hollow head230, depending on the requirements of a given application. One skilled in the art will further appreciate that the given dimensions may be scaled down to nanometer levels by a factor of about 1000 as well as scaled up to millimeter levels by a factor of about 1000.

Referring now toFIGS. 4,5A and5B, first and second interconnecting bodies410,510are illustrated, according to an embodiment of the invention. Each of first and second interconnecting bodies410,510has a corresponding first and second pluralities of hollow stems420,520, of like dimensions, projecting generally orthogonally from a first surface412and a second surface512respectively and facing each other. Second interconnecting body510is superposed on first interconnecting body410such that heads230of first and second pluralities of stems420,430are in general contact engagement with each other. In an exemplary configuration, heads230of first and second pluralities of stems420,430may be laterally offset from one another. Stalk210(ofFIG. 2) of hollow stem420has a radius r and bulbous head230(ofFIG. 2) of hollow stem420has a radius R. Each of the first and second pluralities of hollow stems420,520is spaced apart from one another by a predetermined distance D. The adjacent bulbous heads of respective adjacent stems420a,420bare separated by a distance430whereas the adjacent stalks of respective adjacent stems420a,420bare separated by a distance440. Distance430between two adjacent bulbous heads of respective adjacent stems420a,420bis smaller than diameter375(ofFIG. 3B) of head230, whereas distance440between two adjacent stalks of respective adjacent stems420a,420bis greater than diameter375(ofFIG. 3B). Likewise, a distance435between two diagonal stems420b,420chas a value less than diameter375of head230and more than an outer diameter of stem210, i.e., inner diameter350plus two times wall thickness216of stem210.

In the illustrated embodiment, a sub-set420a,420b,420c,420dof plurality of hollow stems420of body410define a central gap425therebetween to receive and accommodate one hollow stem520aof surface510. Likewise, a sub-set (not shown) of plurality of hollow stems520of body510define a central gap (not shown) therebetween to receive and accommodate at least one hollow stem420aof body410. First and second bodies410,510are positioned so that heads430of first plurality of stems420are in general contact engagement with heads530of second plurality of stems520. Upon application of an engagement force F on at least one of first and second bodies410,510, pushing first and second bodies410,510toward each other, at least one of second plurality of hollow stems520ais received and accommodated by central gap425defined by a sub-set420a,420b,420c,420dof first plurality of hollow stems420and at least one of first plurality of hollow stems420is received and accommodated by the central gap (not shown) defined by a sub-set (not shown) of second plurality of hollow stems520. Thus, bodies410,510are interconnected with each other via first and second pluralities of hollow stems420,520.

The application of engagement force F on at least one of surfaces410,510causes a bulbous head530aof hollow stem520ato splay each of hollow stems420a,420b,420c,420daway from their respective central axes380(ofFIG. 3B). Bulbous head530ais then lodged in central gap425defined by the sub-set of hollow stems420a,420b,420c,420d. Once bulbous head530ais accommodated in central gap425, hollow stems420a,420b,420c,420drevert to their respective initial positions, thereby locking bulbous530ain central gap425. It is understood that because of the hollow nature of stem420, stem420is relatively stiffer in an axial direction along stem420as compared to the stiffness in a radial direction. Thus, engagement force F on surfaces410,510transmitted via hollow stem520acauses hollow stems420a,420b,420c,420dto bend away from their respective central axes380(ofFIG. 3B) rather than buckling inward along central axes380or providing resistance to bulbous head530a. As will be described later, solid stems may be subject to friction locking, thereby preventing effective engagement between two interconnecting bodies under certain circumstances. Furthermore, first thickness365(ofFIG. 3B) of head230resists inward buckling of head230as well as of stalk210of hollow stem420, when bodies410,510are subjected to engagement force F.

Tapered engagement sections320(ofFIG. 3A) of bulbous heads230of hollow stems420a,420b,420c,420dand520afacilitate the entry of head230of hollow stem520ainto gap425by providing bending forces on hollow stems420a,420b,420c,420d. Tapered disengagements sections340(ofFIG. 3A) and flange sections314of heads230of hollow stems420a,420b,420c,420dand520aprovide resistance to the movement of bulbous head230of hollow stem520aout of central gap425. Because the gradient of disengagement section340is relatively greater than the gradient of engagement section320, a disengagement force required to pull bulbous head230of hollow stem520aout of central gap425is greater than the engagement force required to insert bulbous head230of hollow stem520ainto central gap425. Referring toFIG. 6, simulation results for an exemplary embodiment indicate that the required disengagement force, illustrated as peak620, in a direction opposite of the engagement force is almost double the engagement force, illustrated as trough610.

Still referring toFIGS. 5A-5B, hollow stems420are arranged in a square pattern as described below, in an embodiment of the invention. Any four adjacent stems of the plurality of stems420, for instance, stems420a,420b,420c,420dwhich define a central gap425for receiving and accommodating bulbous head230of hollow stem530a, define the four vertices of a square. A first distance D between stems420aand420cis equal to a second distance D between stems420cand420d, to a third distance D between stems420aand420band to a fourth distance D between stems420band420d. In an exemplary embodiment, distance D may have a value lying between about one time diameter375of bulbous head230and about two times diameter375of bulbous head230.

Now referring toFIGS. 7A-7B, hollow stems720a,720b,720c, of interconnecting body710are arranged in a triangle pattern as described below, according to another embodiment of the invention. Any three adjacent stems of the plurality of stems720, for instance, stems720a,720b,720c, which define a central gap725for receiving and accommodating a head230of a hollow stem720a, define the three vertices of an equilateral triangle. Gap625receives and accommodates a hollow stem920aof a superposed interconnecting body (not shown). A first distance720between stems720aand720bis equal to a second distance720between stems720band720cand to a third distance720between stems720cand720a. In other embodiments, hollow stems720may be arranged in different patterns, such as pentagon, hexagon and other geometrical patterns, depending on the requirements of a given application. It will be appreciated that square and triangle patterns of hollow stems result in tighter patterns as compared to those resulting in other polygonal patterns.

Referring now toFIG. 8, two conventional solid stems810,830in prior art reclosable fastener surfaces are schematically illustrated. Stem810has a mushroom head820and stem830has a mushroom head840. For a given radius R of mushroom heads820,840and a given distance850between stems810,830, an overlap860is given by:
Overlap=2R−Distance between two stems.
FIGS. 9A,9B illustrate the correlation between the engagement force, the extent of overlap of two mushroom heads and the coefficient of friction between the two mushroom heads.FIG. 9Aillustrates the increase in engagement forces as the extent of overlap increases for a constant coefficient of friction 0.1.FIG. 9Afurther illustrates that for a coefficient of frication of 0.1, engagement is not possible when overlap is 0.06 times radius R of mushroom head820,840due to friction locking between mushroom heads820,840. For instance, curve870illustrates the engagement force for an overlap of 0.05 times radius R and curve880illustrates the engagement force for an overlap of 0.06 times radius R.

Likewise,FIG. 9Billustrates the increase in the engagement forces as the coefficient of friction increases for a constant overlap of 0.05 times radius R of mushroom heads820,840.FIG. 9Bfurther illustrates that for a constant overlap of 0.05 times radius R, engagement is not possible when coefficient of friction exceeds 0.2 due to frictional locking between mushroom heads820,840. For example, curve875represents the engagement force for coefficient of friction of 0.1 whereas curve885represents the engagement force for coefficient of friction of 0.2. Thus, for conventional solid stems, friction locking may occur any of the extent of overlap and the coefficient of friction of the stem material increases beyond a threshold.

An advantage of hollow stems is that the engagement and disengagement forces are independent of surface coefficient of friction between the bulbous heads. Therefore, interconnecting bodies with hollow stems are not subject to friction locking encountered in the known prior art interconnecting systems with solid stems under certain circumstances. Another advantage of the hollow stems is that during engagement, entire stem including the stalk and the bulbous head is subjected to deformation. Such deformation of the entire hollow stem keeps maximum strains in the stem within the material elastic limits. Yet another advantage of the hollow stems is that the engagement and disengagement forces may be controlled by changing the tapers of the engagement and disengagement sections of the bulbous head. As is known in the art, BGA technique requires heating of the BGA assembly to solder a BGA package to the circuit board having complementary copper pads. An advantage of the interconnecting hollow stems is that the heating step is eliminated, making the assembly step simpler and cheaper.