Electrical connector assembly with shorting contacts

An electrical connector assembly is provided. The electrical connector assembly includes a housing including a body fabricated at least partially from a dielectric material. The housing body has an opening therein. An electrical contact is held by the housing. The electrical contact includes an intermediate portion and a spring potion each held at least partially within the opening. The spring portion of the electrical contact is engaged by the housing body such that the spring portion is deflected into shorting engagement with the intermediate portion of the electrical contact.

BACKGROUND OF THE INVENTION

The invention relates generally to electrical connectors, and, more particularly, to socket connectors for retaining electronic modules.

Computers and servers may use numerous types of electronic modules, such as processor and memory modules (e.g. Dynamic Random Access Memory (DRAM), Synchronous Dynamic Random Access Memory (SDRAM), or Extended Data Out Random Access Memory (EDO RAM), and the like). The memory modules are produced in a number of formats such as, for example, Single In-line Memory Modules (SIMM's), or the newer Dual In-line Memory Modules (DIMM's), Small Outline DIMM's (SODIMM's) and Fully Buffered DIMM's. Typically, the electronic modules are installed in one or more multi-pin sockets mounted on a system board or motherboard. Each electronic module has a card edge that provides an interface generally between two opposite rows of contacts in the socket.

There is an ongoing trend toward smaller electronic packages. This trend is accelerated by the adoption of certain standards such as the Advanced Telecommunications Computing Architecture (ATCA) standard. In systems that adhere to the ATCA standard, the space provided for electronic modules and socket connectors is limited. Space limitations require that the size of the electronic modules as well as the socket connectors be reduced. At least some known socket connectors include electrical contacts having a vertical contact design. However, such vertical electrical contacts may not provide a socket connector with as low of a vertical profile as desired. A conventional shepherd's hook beam contact may enable a lower vertical profile. However, shepherd's hook beam contacts may have too long of an electrical path length.

There is a need for a lower profile socket connector that may be used in space-limited applications.

BRIEF DESCRIPTION OF THE INVENTION

In one embodiment, an electrical connector assembly is provided. The electrical connector assembly includes a housing including a body fabricated at least partially from a dielectric material. The housing body has an opening therein. An electrical contact is held by the housing. The electrical contact includes an intermediate portion and a spring portion each held at least partially within the opening. The spring portion of the electrical contact is engaged by the housing body such that the spring portion is deflected into shorting engagement with the intermediate portion of the electrical contact.

Optionally, the housing body may include a shoulder. The opening communicates with the shoulder. The spring portion of the electrical contact includes an extension that engages the shoulder such that the spring portion is deflected into shorting engagement with the intermediate portion of the electrical contact.

Optionally, the intermediate portion of the electrical contact extends between mating and mounting contact portions of the electrical contact. The mating contact portion is configured to electrically connect to a first electrical component and the mounting contact portion is configured to electrically connect to a second electrical component.

Optionally, the intermediate portion of the electrical contact comprises an extension that engages the housing body to facilitate preventing and/or reducing movement of the intermediate portion when the spring portion is deflected into shorting engagement with the intermediate portion.

In another embodiment, a socket connector assembly is provided for connecting a module card to a circuit board. The socket connector assembly includes a housing extending along a longitudinal axis between opposite ends. The housing includes a mounting face configured for mounting on the circuit board and a slot configured to receive a mating edge of the module card. The housing body has a plurality of openings therein. A plurality of electrical contacts are held by the housing. Each electrical contact includes an intermediate portion and a spring portion each held at least partially within the corresponding opening. The spring portion of each of the electrical contacts is engaged by the housing body such that the spring portion is deflected into shorting engagement with the intermediate portion of the same electrical contact.

In another embodiment, a socket connector assembly is provided for connecting a module card to a circuit board. The socket connector includes a housing extending along a longitudinal axis between opposite ends. The housing includes a mounting face configured for mounting on the circuit board and a slot configured to receive a mating edge of the module card. The housing body has an opening therein. An electrical contact is held by the housing. The electrical contact includes an intermediate portion and a spring portion each held at least partially within the opening. The spring portion of the electrical contact is in shorting engagement with the intermediate portion of the electrical contact before the mating edge of the module card is inserted within the slot.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1is a perspective view of an exemplary embodiment of a socket connector assembly10. The socket connector assembly10includes a housing12having a dielectric body14that extends along a central longitudinal axis16between opposite end portions18and20. The body14has a mating face22and a mounting face24. The body14includes a slot26that is configured to receive a mating edge portion28(FIG. 2) of an electronic module30(FIG. 2). The housing body14includes a plurality of openings32that each communicate with the slot26and extend through the housing body14from the mating face22to the mounting face24. As will be described in more detail below, each opening32holds a portion of a corresponding inner or outer electrical contact34and36, respectively, therein. Each inner and outer electrical contact34and36, respectively, includes a respective mating contact, portion38and138(FIG. 3) and a respective mounting contact portion40and140. The mating contact portions38and138extend into the slot26to electrically engage contact pads (not shown) on the electronic module30when the electronic module30is installed in the socket connector assembly10. The mounting contact portions40and140extend from the mounting face24of the housing body14and are configured to electrically connect the socket connector assembly10to a circuit board42to enable the connection of the electronic module30to the circuit board42. The inner and outer electrical contacts34and36, respectively, are alternated within adjacent openings32to form inner rows39and41(FIGS. 5 and 6) of the mounting contact portions40and outer rows43and45(FIGS. 5 and 6) of the mounting contact portions140. Specifically, the mounting contact portions40of the inner electrical contacts34are arranged in a pair of opposite inner rows39and41that are offset on opposite sides of the central longitudinal axis16of the housing body14. Similarly, the mounting contact portions140of the outer electrical contacts36are arranged in a pair of opposite outer rows43and45that are offset on opposite sides of the central longitudinal axis16of the housing body14by a greater amount than the inner rows39and41.

Optionally, a key44may be provided at an off-center position in the slot26for reception within a notch (not shown) in the electronic module30to assure that the electronic module30is properly aligned with respect to the connector assembly10. One or more board locks46may optionally be provided to mechanically attach the socket connector assembly10to the circuit board42.

As will be described in more detail below, each of the inner and outer electrical contacts34and36, respectively, is engaged by the housing body14such that each of the electrical contacts34and36electrically shorts against itself. Each of the inner and outer electrical contacts34and36, respectively, are considered to be “pre-shorted” because each of the electrical contacts34and36electrically shorts against itself, via engagement with the housing body14, before insertion of the mating edge portion28of the electronic module30into the slot26. In other words, each of the electrical contacts34and36electrically shorts against itself independent of the electronic module30.

FIG. 2is a perspective view of the socket connector assembly10with an exemplary electronic module30installed thereon. The electronic module30includes a planar substrate48that has the mating edge portion28and a plurality of electrical traces (not shown), each of which terminates at a respective contact pad (not shown) on the mating edge portion28. When the electronic module30is installed on the housing body14as shown inFIG. 2, each of the contact pads on the mating edge portion28of the electronic module30electrically engages the mating contact portion38or138(FIGS. 5 and 6) of a corresponding one of the electrical contacts34or36. The substrate48includes exemplary surface mounted components generally represented at50.

The housing end portions18and20are substantially identical and therefore only the housing end portion18is described in detail. The housing end portion18includes a cavity52between opposed towers54and56that extend outwardly at the mating face22of the housing body14. An extractor58is received in the cavity52. The extractor58is pivotably connected to the housing end portion18for retaining the electronic module30on the housing body14and for extracting the electronic module30from the housing body14. Specifically, the extractor58extends outwardly between the towers54and56and is pivotable between an open position (FIG. 1) for receiving the electronic module30within the slot26and a closed position (FIG. 2) for retaining the electronic module30.

The extractor58includes a pair of opposite side portions60and62that each engages the electronic module30. Specifically, each of the side portions60and62includes a side wall64and66, respectively. The side walls64and66are spaced apart from one another such that an extractor slot68is defined therebetween. The extractor slot68is in communication with the slot26in the housing body14. The extractor slot68receives the mating edge portion28of the substrate48of the electronic module30. Opposite interior surfaces70and72of the side walls64and66, respectively, include ribs74that engage an edge portion76of the substrate48of the electronic module30to stabilize the electronic module30. Optionally, a beveled forward edge (not shown) on the ribs74provides guidance for facilitating entry of the edge portion76of the electronic module30into the extractor slot68. The extractor58may include a latch element (not shown) that engages a notch (not shown) in the edge portion76of the substrate48of the electronic module30to facilitate retaining the electronic module30on the housing body14. Opposite outer surfaces80and82of the side walls64and66, respectively, may include a projection (not shown) that communicates with a retention receptacle (not shown) on inner surfaces84and86of the towers54and56, respectively, to facilitate holding the extractor58in the closed position. A foot (not shown) of the extractor58engages a lower edge87(FIG. 5) of the mating edge portion28of the electronic module30to lift the electronic module30upward when the extractor58is opened to assist in the extraction of the electronic module30from the housing body14. The extractor58may optionally include a thumb pad88for moving the extractor58between the open and closed positions.

FIG. 3is a perspective view of an exemplary embodiment of an inner electrical contact34and an exemplary embodiment of an outer electrical contact36of the socket connector assembly10(FIGS. 1 and 2). Each inner electrical contact34includes the mating contact portion38, the mounting contact portion40, a spring portion90, and an intermediate portion92. Similarly, each outer electrical contact36includes the mating contact portion138, the mounting contact portion140, a spring portion190, and an intermediate portion192. In the exemplary embodiment, the spring portions90and190each include a respective end portion91and191of the respective inner and outer electrical contacts34and36, and the mounting contact portions40and140each include a respective opposite end portion93and193of the inner and outer electrical contacts34and36, respectively. Moreover, the spring portions90and190each include the respective mating contact portions38and138. The intermediate portion92of the inner electrical contact34extends between the spring and mounting contact portions90and40, respectively. Similarly, the intermediate portion192of the outer electrical contact36extends between the spring and mounting contact portions190and140, respectively. The mounting contact portion40of the inner electrical contact34is offset from the intermediate portion92in a direction generally towards the spring portion90. In contrast, the mounting contact portion140of the outer electrical contact36is offset from the intermediate portion192in a direction generally away from the spring portion190. The exemplary geometry of the inner and outer electrical contacts34and36, respectively, results in approximately equal electrical path lengths between the mounting contact portions40and140.

The intermediate portion92of the inner electrical contact34includes a stem94extending a length L1between a pair of opposite end portions96and98. In the exemplary embodiment, a pair of extensions100extend outwardly from opposite side surfaces102and104of the stem94adjacent the end portion96. The extensions100are “paired” in that the extensions100extend from the opposite side surfaces102and104at approximately the same location along the length L1of the stem94. Similarly, the intermediate portion192of the outer electrical contact36includes a stem194extending a length L2between a pair of opposite end portions196and198. A pair of extensions200extends outwardly from opposite side surfaces202and204of the stem194adjacent the end portion196. As will be described in more detail below, the extensions100and200are configured to engage the housing body14to facilitate preventing and/or reducing movement of the intermediate portions92and192within the corresponding opening32(FIGS. 1 and 2) of the housing body14.

The intermediate portion92also includes a pair of extensions106that extend outwardly from the opposite side surfaces102and104of the stem94adjacent the end portion98. Similarly, the intermediate portion192includes a pair of extensions206that extend outwardly from the opposite side surfaces202and204of the stem194adjacent the end portion198. As will be described in more detail below, the extensions106and206are configured to engage the housing body14to facilitate retaining the inner electrical contact34in position within the corresponding opening32of the housing body14using an interference-fit arrangement. Optionally, each of the extensions106and206includes a plurality of respective projections108and208that engage the housing body14.

The spring portion90of the inner electrical contact34includes a stem107that extends from the end portion96of the stem94of the intermediate portion92. The stem107extends from the end portion96to the end portion91of the inner electrical contact34. The stem107includes a plurality of joints110,112,114, and116that define a plurality of arms118,120,122, and124, respectively. The arm118extends from the end portion96of the intermediate portion92, while the arm124includes the end portion91of the inner electrical contact34. The joint114and adjacent portions of the arms120and122define the mating contact portion38. The contact pads (not shown) of the electronic module30(FIG. 2) engage a surface115of the stem107at the joint114when the electronic module30is installed on the housing body14(FIGS. 1 and 2). Similarly, the spring portion190of the outer electrical contact36includes a stem207that extends from the end portion196of the stem194of the intermediate portion192. The stem207extends from the end portion196to the end portion191of the outer electrical contact36. The stem207includes a plurality of joints210,212,214, and216that define a plurality of arms218,220,222, and224, respectively. The arm218extends from the end portion196of the intermediate portion192, while the arm224includes the end portion191of the outer electrical contact36. The joint214and adjacent portions of the arms220and222define the mating contact portion138. The contact pads of the electronic module30engage a surface215of the stem207at the joint214when the electronic module30is installed on the housing body14.

When the inner electrical contact34is not held by the housing body14, the spring portion90is in a non-shorting position (FIG. 3) wherein a gap125is defined between a surface126of the stem107at the joint116and a surface128of the stem94of the intermediate portion92. Similarly, when the outer electrical contact36is not held by the housing body14, the spring portion190is in a non-shorting position (FIG. 3) wherein a gap225is defined between a surface226of the stem207at the joint216and a surface228of the stem194of the intermediate portion192. The spring portions90and190are each configured to bend along at least a portion of the respective stem107and208such that the spring portions90and190move from the non-shorting position to a shorting position (FIGS. 5 and 6). With respect to the inner electrical contact34, in the shorting position the surface126of the stem107at the joint116engages the surface128of the stem94of the intermediate portion92. Similarly, with respect to the outer electrical contact36, in the shorting position the surface226of the stem207at the joint216engages the surface228of the stem194of the intermediate portion192.

In the exemplary embodiment, the inner electrical contact34includes a pair of extensions130that extend outwardly from opposite side surfaces127and132of the stem107at the arm124. Similarly, the outer electrical contact36includes a pair of extensions230that extend outwardly from opposite side surfaces227and232of the stem207at the arm224. As will be described in more detail below, the extensions130and230are configured to engage the housing body14to move the respective spring portions90and190from the non-shorting position to the shorting position and to retain the respective spring portions90and190in the shorting position.

In the exemplary embodiment, the spring portions90and190generally bend at the respective joints110and112and210and212to move from the non-shorting position to the shorting position. However, the spring portions90and190may bend along any portion(s) of the length of the respective stem107and208that enables the spring portions90and190to move from the non-shorting position to the shorting position. Moreover, although the surface126of the stem107engages the surface128of the stem94of the intermediate portion92at the joint116, the spring portion90may engage any portion of the inner electrical contact34at any location along the stem107of the spring portion90. Similarly, although the surface226of the stem207engages the surface228of the stem194of the intermediate portion192at the joint216, the spring portion190may engage any portion of the outer electrical contact36at any location along the stem207of the spring portion190.

Although one pair of two of the extensions100, the extensions106, the extensions200, the extensions206, the extensions130, and the extensions230are shown, the respective portions92,192,90, and190may each include any number of pairs of the respective extensions100,106,200,206,130, and230, and any number of the respective extensions100,106,200,206,130, and230overall, that enable the extension(s)100,106,200,206,130, and230to function as described and illustrated herein. In some embodiments, some or all of the extensions100,106,200,206,130, and/or230are not paired with another respective extension100,106,200,206,130, and230on the opposite side surface102or104,202or204,127or132, and227or232, respectively. Moreover, each extension100,106,200, and206may extend from either of the side surfaces102or104and202or204, respectively, and may be located at any portion of the respective length L1and L2of the respective stem94and194(whether or not the extension100,106,200, and/or206is paired with another extension100,106,200, and206, respectively, on the respective opposite side surface102or104and202or204). Similarly, each extension130and230may extend from either of the side surfaces127or132and227or232, respectively, and may be located at any portion of the length of the respective stem107and208at any of the arms118,120,122,124,218,220,222, and/or224, respectively (whether or not the extension130and/or230is paired with another extension130and230, respectively, on the respective opposite side surface127or132and227or232).

The size, shape, and location on the electrical contacts34and36of each of the respective portions38,40,90,92,138,140,190, and192, as well as the overall size and geometry of the electrical contacts34and36overall, is not limited to the embodiments described and illustrated herein, but rather may be any suitable size, shape, location, and/or geometry that enables the electrical contacts34and36to be engaged by the housing body14such that each of the electrical contacts34and36electrically shorts against itself.

FIG. 4is a perspective view illustrating a cross section of the housing body14of the socket connector assembly10taken along line4-4ofFIG. 1. The housing body14is illustrated inFIG. 4with the inner and outer electrical contacts34and36(FIG. 3), respectively, removed therefrom. The extractor58(FIGS. 1 and 2) has also been removed from the housing body14inFIG. 4. Each of the openings32within the housing body14includes a pair of opposite side walls300and302, which may engage any of the side surfaces of the corresponding electrical contact34or36held therein. Each of the opposite side walls300and302includes a shoulder304formed therein. As will be described in more detail below, the shoulders304are configured to engage a corresponding one of the pair of extensions100or200(FIG. 3) of the corresponding electrical contact34or36to facilitate preventing and/or reducing movement of the corresponding intermediate portion92or192(FIG. 3).

Each of the opposite side walls300and302also includes a shoulder306formed therein. As will be described in more detail below, the shoulders306are configured to engage a corresponding one of the pair of extensions130or230(FIG. 3) of the corresponding electrical contact34or36to move the corresponding spring portion90or190(FIG. 3) from the non-shorting position to the shorting position and to retain the corresponding spring portion90or190in the shorting position. The side walls300and302also include a recess308formed therein. The recesses308are each configured to receive a corresponding one of the end portions98or198(FIG. 3) of the intermediate portions92or192(FIG. 3) of the corresponding electrical contact34or36(FIGS. 1-3). A corresponding one of the pair of extensions106or206(FIG. 3) engages the side walls300and302to facilitate retaining the corresponding electrical contact34or36in position within the opening32using an interference-fit arrangement. Optionally, the projections108or208(FIG. 3) of the corresponding extension106or206may engage the side walls300and302to facilitate retaining the corresponding electrical contact34or36using an interference-fit arrangement.

Although the exemplary embodiment includes two, each side wall300and302of each opening32may include any number of recesses308for cooperating with any number of extensions106or206on the corresponding intermediate portion92or192. Each recess308may include any suitable shape, and may have any suitable location on the corresponding side wall300or302, that enables the recess308to cooperate with the corresponding extension106or206to facilitate retaining the corresponding electrical contact34or36as described and illustrated herein. Moreover, although each opening32includes two shoulders304and two shoulders306in the exemplary embodiment, each side wall300and302of each opening32may include any number of shoulders304and any number of shoulders306for cooperating with any number of respective extensions100or200and130or230. Although the shoulders304are each shown as having a generally planar surface, each shoulder304may include any suitable shape, and may have any suitable location on the corresponding side wall300or302, that enables the shoulder304to cooperate with the corresponding extension100or200to facilitate preventing and/or reducing movement of the respective intermediate portion92or192as described and illustrated herein. Similarly, each shoulder306may include any suitable shape, and may have any suitable location on the corresponding side wall300or302, that enables the shoulder306to cooperate with the corresponding extension130or230to move the corresponding spring portion90or190as described and illustrated herein.

FIG. 5is a cross-sectional view of the socket connector assembly10taken along line5-5ofFIG. 2.FIG. 6is a perspective view of the cross section ofFIG. 5with the electronic module30removed from the slot26. The housing body14is illustrated inFIGS. 5 and 6with the inner and outer electrical contacts34and36, respectively, positioned within the corresponding openings32. As each electrical contact34and36is inserted within the corresponding opening32(from the mounting face24, and before the electronic module30is inserted within the slot26), the spring portion90or190is deflected into shorting engagement with the intermediate portion92or192of the same electrical contact34or36. Specifically, each of the extensions130or230of the electrical contact34or36is engaged by the corresponding shoulder306such that the spring portion90or190moves from the non-shorting position (FIG. 3) to the shorting position wherein the surface126or226(FIG. 3) of the joint116or216engages the surface128or228(FIG. 3) of the intermediate portion92or192. As the spring portion90or190is deflected, the extensions100or200of the electrical contact34or36are engaged by the corresponding shoulder304to facilitate preventing and/or reducing movement of the corresponding intermediate portion92or192. Specifically, the shoulder304facilitates preventing and/or reducing rotational movement in the direction D1or D2of the intermediate portion92or192, respectively, caused by the deflection of the spring portion90or190toward the intermediate portion92or192. As can be seen fromFIG. 5, once the electronic module30is inserted within the slot26, engagement between the contact pads of the electronic module30and the mating portions38and138cause the intermediate portions92and192to deflect in the directions D2and D1, respectively. Because each of the electrical contacts34and36electrically shorts against itself via engagement with the housing body14instead of engagement with the electronic module30, each of the electrical contacts34and36is “pre-shorted” before insertion of the mating edge portion28of the electronic module30into the slot26.

When the electrical contacts34and36are received within the corresponding openings32, the end portions98and198are received within the corresponding recess308and the extensions106or206engage the side walls300and302to facilitate retaining the electrical contact34or36in position within the opening32using an interference-fit arrangement. Optionally, the projections108and208(FIG. 3) of the extensions106and206may engage the side walls302and304to facilitate retaining the corresponding electrical contact34or36using an interference-fit arrangement.

In the exemplary embodiment ofFIGS. 1-6, the mounting contact portions40and140of the electrical contacts34and36, respectively, are configured as solder tails that electrically connect to the circuit board42using solder. However, the mounting contact portions40and140are not limited to being solder tails, but rather may have any suitable arrangement, configuration, structure, geometry, and/or the like that enables the mounting contact portions40and140to electrically connected to the circuit board42and/or another electrical component, such as, but not limited to, using a press-fit arrangement and/or a surface mount arrangement. For example,FIG. 7is a perspective view of a cross-section of an exemplary alternative embodiment of a socket connector assembly400. The socket connector assembly400includes a plurality of electrical contacts536each held within a corresponding opening432within a housing body414. Each of the electrical contacts536includes a mating contact portion538, a mounting contact portion540, a spring portion590, and an intermediate portion592. Each of the mounting contact portions540of the electrical contacts536is configured to electrically connect to a circuit board or other electrical component using a surface mount arrangement.

FIG. 8is a perspective view of a cross-section of another exemplary alternative embodiment of a socket connector assembly600. The socket connector assembly600includes a plurality of inner and outer electrical contacts634and636, respectively, each held within a corresponding opening632within a housing body614. The electrical contacts634and636each include a respective mating contact portion638and738, mounting contact portion640and740, spring portion690and790, and intermediate portion692and792. Each of the mounting contact portions640and740is configured as to electrically connect to a circuit board or other electrical component using a press-fit arrangement.

The embodiments described and illustrated herein provide a lower profile socket connector that may be suitable for use in limited space applications. The connector meets ATCA height restrictions. The connector includes a contact designed to be engaged by the housing body to electrically short against itself to accommodate a lower profile housing for ATCA as well as other low profile applications.

The connector embodiments described and/or illustrated are not limited to use with any exemplary type of electronic module described and/or illustrated herein, but rather may be used with any suitable type of electronic module, such as, but not limited to, processor modules and/or memory modules, such as, but not limited to, Dynamic Random Access Memory (DRAM), Synchronous Dynamic Random Access Memory (SDRAM), Extended Data Out Random Access Memory (EDO RAM), Single In-line Memory Modules (SIMM's), Dual In-line Memory Modules (DIMM's), Small Outline DIMM's (SODIMM's), Fully Buffered DIMM's, and/or the like. Although described and illustrated herein as a socket connector, the embodiments described and/or illustrated herein are not limited to socket connectors, but rather may be any suitable type of connector, such as, but not limited to, a plug connector and/or a surface connector.

Exemplary embodiments are described and/or illustrated herein in detail. The embodiments are not limited to the specific embodiments described herein, but rather, components and/or steps of each embodiment may be utilized independently and separately from other components and/or steps described herein. Each component, and/or each step of one embodiment, can also be used in combination with other components and/or steps of other embodiments. For example, although specific sensor elements are described and/or illustrated with specific attachment devices, each described and/or illustrated sensor element may be used with any of the described and/or illustrated attachment devices as is appropriate. When introducing elements/components/etc. described and/or illustrated herein, the articles “a”, “an”, “the”, “said”, and “at least one” are intended to mean that there are one or more of the element(s)/component(s)/etc. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional element(s)/component(s)/etc. other than the listed element(s)/component(s)/etc. Moreover, the terms “first,” “second,” and “third,” etc. in the claims are used merely as labels, and are not intended to impose numerical requirements on their objects. Further, the limitations of the following claims are not written in means—plus-function format and are not intended to be interpreted based on 35 U.S.C. § 112, sixth paragraph, unless and until such claim limitations expressly use the phrase “means for” followed by a statement of function void of further structure.