Chassis with multi-cantilever spring fingers for EMI shielding and ESD protection of electronic devices

A chassis utilizes spring fingers having contact portions and mid portions where each spring finger forms multiple cantilevers to provide different spring constants. Accordingly, the increase in force resulting from displacement can be better controlled and even minimized for certain ranges of displacement. In particular, each spring finger can be configured to perform in an operating range characterized by a smaller spring constant. As a result, the force increase in this operating range is slower thus accommodating circuit boards with large connector height variations without significantly changing the normal contact force. Such operation enables the chassis to consistently pass EMI and ESD testing, as well as provide more reliable device operation.

BACKGROUND

A metallic spring finger is a device which is capable of biasing an object or providing a holding force on the object in order to maintain the object at a fixed position relative to a main body.FIG. 1shows a side view20of a metallic spring finger22attached to a main body24. As shown, the metallic spring finger22includes (i) a V-shaped end portion26, and (ii) a substantially flat middle portion28which connects the V-shaped end portion26to the main body24. Here, the overall length (L) of the entire spring finger22from the main body24to the end of the spring finger22is based on the length contributions of both the middle portion28and the V-shaped end portion26(seeFIG. 1).

Such a metallic spring finger22applies a reaction spring force (F) in response to a displacement (D), e.g., due to displacement by an object29. The dashed lines show the metallic spring finger22in a new position after being displaced from its original position. In general, the spring force (F) provided by the metallic spring finger22is a function of a spring constant (k) and the displacement (D).
F=k*D(1).
Equation (1) illustrates the function for determining the spring force (F) provided by the metallic spring finger22inFIG. 1.

FIG. 2is a graphical illustration as to how the spring force (F) of the above-described conventional metallic spring finger22increases linearly as the displacement (D) increases. The slope of the curve inFIG. 2is equal to the spring constant (k).

It should be understood that due to the above-described property of the metallic spring finger22, the metallic spring finger22is suitable for positioning or holding an object relative to the main body24. For example, as shown by the solid lines inFIG. 1, the metallic spring finger22may initially extend from the main body24at a 90 degree angle. Then, when an object29is placed at a location30between the metallic spring finger22and the surface32upon which the main body24is mounted resulting in the metallic spring finger22being pushed in a direction away from the surface32to the new position which is slightly greater than 90 degrees (shown in phantom inFIG. 1), the metallic spring finger22responds by providing the force (F) against the object to hold the object at the location30.

SUMMARY

Unfortunately, there are deficiencies to the above-described conventional metallic spring finger22in which the overall spring finger length (L) is based on contributions by both the middle portion28and the V-shaped end portion26as shown inFIG. 1. In particular, the conventional metallic spring finger22is not very well-suited for use in certain chassis designs for electronic devices. For example, in the context of a wireless or non-wireless router, the router manufacturer may contemplate using rows of metallic spring fingers22around the peripheries of port openings in order to provide electromagnetic interference (EMI) shielding and electrostatic discharge (ESD) protection. However, experimental testing has shown that the metallic spring fingers22cannot concurrently provide sufficient EMI shielding and secure contact for proper ESD protection (e.g., due to non-elastic material properties of the metallic spring fingers22).

Additionally, the conventional geometry of the spring fingers22makes it difficult for the manufacturer to properly install circuit boards into metal frames equipped with the spring fingers22. Specifically, the reaction spring force (F) provided by the spring fingers22in response to displacement of the spring fingers22(e.g., by objects29) linearly increases based on the amount of deformation (D) (see Equation (1) andFIG. 2). To avoid installation failures, the spring fingers22have to be manually lifted to gain room for the objects29to enter, and lifting multiple spring fingers22simultaneously is extremely difficult. Moreover, in the case of large tolerance variations in height of objects (e.g., RJ connectors, USB ports, etc.), the spring fingers22must be significantly displaced in order to properly insert a circuit board so that certain circuit board connector ports pass through port openings surrounded by the spring fingers22. If there is too much spring force (F), there is a high probability that the spring fingers22will scratch through nickel plating around the connector ports and thus damage the metallic layers around the connector ports which provide EMI shielding and ESD protection.

Furthermore, due to tolerance stack up issues and thus variations in the displacement (D) of each spring finger22, the amount of spring force (F) provided by each spring finger22may vary widely. Such inconsistencies result in some connector ports loosing EMI shielding and ESD protection due to high impedance from the poorly contacting spring fingers22. As a result, the routers may fail EMI and ESD testing as well as perform poorly once the circuit boards are installed and operational.

In contrast to the above-described conventional spring finger22in which the spring finger overall length (L) is based on contributions by both the middle portion28and the V-shaped end portion26(FIG. 1), embodiments of the invention are directed to chassis which utilize spring fingers having contact portions and mid portions where each spring finger forms multiple cantilevers to provide different spring constants. Accordingly, the force increase resulting from displacement can be better controlled and even minimized for certain ranges of displacement. In particular, each spring finger can be configured to perform in an operating range characterized by a smaller spring constant. As a result, the force increase in this operating range is slower thus accommodating circuit boards with large connector height variations and different connector types without significantly changing the normal contact force. Such operation enables the chassis to consistently pass EMI and ESD testing, as well as provide more reliable device operation.

One embodiment is directed to a chassis for a circuit board module having a circuit board and connectors mounted along an edge of the circuit board. The chassis includes a base configured to reside in an installed position relative to the circuit board, and a frame portion coupled to the base. The frame portion defines connector openings corresponding to the connectors of the circuit board. The chassis further includes spring fingers coupled to the frame portion. The spring fingers are configured to provide electrical pathways from the connectors of the circuit board to the frame portion when the base resides in the installed position relative to the circuit board. Each spring finger has a contact portion configured to contact one of the connectors, and a mid portion which interconnects that contact portion with the frame. Each spring finger forms (i) a first cantilever at a first location where the mid portion of that spring finger attaches to the frame portion and (ii) a second cantilever at a second location where the contact portion of that spring finger attaches to the mid portion of that spring finger. A distance between the first and second locations defines an overall length of that spring finger.

DETAILED DESCRIPTION

Embodiments of the invention are directed to chassis which utilize spring fingers having contact portions and mid portions where each spring finger forms multiple cantilevers to provide different spring constants. Accordingly, the force increase resulting from displacement can be better controlled and even minimized for certain ranges of displacement. In particular, each spring finger can be configured to perform in an operating range characterized by a smaller spring constant. As a result, the force increase in this operating range is slower thus accommodating circuit boards with large connector height variations and different connector designs without significantly changing the normal contact force. Such operation enables the chassis to consistently pass EMI and ESD testing, as well as provide more reliable device operation.

FIG. 3shows an electronic system40which utilizes spring fingers that form multiple cantilevers to provide different spring constants. The electronic system40includes a housing42, a metallic chassis44(e.g., tin, copper, nickel plated surfaces, sheet metal, etc.) which installs within the housing42, and a circuit board module46which installs within the metallic chassis44. It should be understood that the electronic system40includes other components (e.g., a top cover, a fan assembly, a power supply, etc.) which are omitted fromFIG. 3to better illustrate spring finger details of the electronic system40.

The circuit board module46includes a circuit board48, I/O connectors50, and a variety of other circuit board components52(e.g., integrated circuit devices, heat sinks, discrete components, etc.) which are shown generally by the arrow52inFIG. 3. The circuit board48is substantially planar in shape and extends in the X-Y plane. The connectors50mount to the circuit board48in a row along a circuit board edge54and pass through portions of both the chassis44and the housing42for external accessibility.

In some arrangements, the electronic system40is a network device (e.g., wireless router which is capable of performing both wireless and non-wireless data communications operations). Along these lines and by way of example, the I/O connectors50include a variety of different connector designs (e.g., RJ11, RJ45, a four-port RJ45 assembly, a USB port, etc.) thus enabling the electronic system40to communicate through a variety of different cables and connectors. Other connector designs are suitable for use as well (DSUB, Firewire, and so on).

As further shown inFIG. 3, the metallic chassis44includes a base60, a frame portion62, multi-cantilevered spring fingers64, and tab members66(shown generally by the arrow66inFIG. 3) which are integrated together to form a metallic unitary body. The base60attaches to the housing42and forms a portion of an EMI barrier around the circuit board48. The frame portion62couples to the base60, and defines connector openings68which correspond to the connectors50of the circuit board module46. The multi-cantilevered spring fingers64and the tab members66fasten to the frame portion62around the connector openings68. As a result, the multi-cantilevered spring fingers64and the tab members66form EMI seals around the connectors50, and further provide electrical pathways from the connectors50to the frame portion62and the base60.

At this point, it should be understood that the use of the multi-cantilevered spring fingers64facilitates installation of the circuit board module46during assembly of the electronic system40. Such use further enables the chassis44to accommodate large tolerance variations in the connectors50(e.g., connector height) but still provide constant and reliable contact force to the connectors50for reliable and consistent EMI shielding and ESD protection. Such a design guarantees competent long term electrical conductivity. Further details will now be provided with reference toFIGS. 4 through 6.

FIG. 4is a perspective view of a portion of the housing42and the chassis44of the electronic system40prior to installation of the circuit board module46.FIG. 5is a side view of a multi-cantilevered spring finger64when undergoing a small displacement (D1).FIG. 5is a side view of a multi-cantilevered spring finger64when undergoing a further displacement (D2) resulting in an overall displacement (DT).

As shown inFIG. 4, the frame portion62of the chassis44defines a single-port connector opening68(S) and a multi-port connector opening68(M). Both openings68(S),68(M) are rectangular in shape to mirror the shapes of their corresponding connectors50(FIG. 3). The-single port connector opening68(S) is skirted with multi-cantilevered spring fingers64on three sides, i.e., with two spring fingers64on each lateral side and three spring fingers64on a top side. Similarly, the multi-port connector opening68(M) is skirted with spring fingers64on three sides, i.e., with a series of three spring fingers64on each lateral side and a series of nine spring fingers64on a top side. The spring fingers64and tab members68are capable of being used around other openings68as well.

As shown inFIG. 5, each multi-cantilevered spring finger64includes a contact portion80, and a mid portion82which interconnects that contact portion80with the frame portion62. The spring finger64forms a first cantilever84at a first location86where the mid portion82attaches to the frame portion62. The cantilever84is substantially at 90 degrees to properly frame a connector50(also seeFIG. 3). Furthermore, the spring finger64forms a second cantilever88at a second location90where the contact portion80attaches to the mid portion82. The cantilever88is substantially less than 90 degrees to enable the contact portion80of the spring finger64to extend back over the connector50and to provide a point of contact between the spring finger64and the connector50which is relatively central to the body of the connector50(FIG. 3). A measured distance between the first and second locations defines an overall length (OL) of the spring finger64.

At this point, it should be understood that the multi-cantilevered spring finger64is capable of actuating at two points, i.e., the location86and the location90. It should be further understood that the spring constant (k1) at the location86and the spring constant (k2) at the location90are different thus enabling the cantilevers84,88to compress at different times in response to a large displacement. In particular, the spring constant (k1) is larger than the spring constant (k2). Accordingly, the cantilever84returns more force upon compression than the cantilever88. This feature enables the cantilever88to operate first in response to a small displacement (D1).FIG. 5shows the spring finger64with no displacement using solid lines, and the spring finger64with the small displacement (D1) in dashed lines.

Once the cantilever88of the spring finger64has been displaced to its maximum extent, the cantilever84with the greater spring constant (k1) operates to accommodate further displacement (D2). That is, if the total displacement (DT) is beyond the small displacement (D1), the spring finger64is still capable of complying due to subsequent operation of the cantilever84.FIG. 6shows the spring finger64with the small displacement (D1) using solid lines (also shown inFIG. 5in dashed lines), and the spring finger64with total large displacement (DT) in dashed lines. The total displacement (DT) equals the small displacement (D1) and the further displacement (D2).

FIG. 7is a graphical illustration as to how the spring force (F) of the multi-cantilevered spring finger64increases as the displacement (D) increases. There are two slopes to the curve inFIG. 7which correspond to the two deformation regions ofFIGS. 5and6. In particular, due to displacement of the spring finger64in the range of no displacement to D1, the spring force (F) increases proportionately to the displacement by the spring constant (k2). This response is due to compression of the spring finger64predominantly at the cantilever88(also seeFIG. 5) with the spring force (F) of the cantilever88being characterized by the spring constant (k2) which is purposefully configured to be smaller than the spring constant (k1) and the spring constant (k) for the earlier-described conventional spring finger22(FIGS. 1 and 2).

As further shown inFIG. 7, once the displacement of the spring finger64goes beyond (D1) and enters the range between (D1) and (D2), the spring force (F) continues to increase proportionately by the spring constant (k1). This change in response is due to compression of the spring finger64at the cantilever84(also seeFIG. 6) with the spring force (F) of the cantilever84being characterized by the spring constant (k1). Here, the spring force (F) increases faster due to the cantilever84having a higher spring constant than that of the cantilever88, i.e., the spring constant (k1) was purposefully configured to be higher than the spring constant (k2).

At this point, it should be understood that installation of the circuit board module46within the chassis44may require a relatively large displacement in the range between (D1) and (D2). As illustrated inFIG. 7, the overall spring force (FT) is nevertheless lower than the spring force of the earlier-described conventional spring finger22. Accordingly, there is less likelihood of causing damage to the spring fingers64(FIGS. 3 and 4) of the chassis44or to the connectors50during installation.

It should be further understood that following installation of the circuit board46within the chassis44, the connectors50continue to displace the spring fingers64but less than (D1). At this point, a connector50(e.g., a single port connector, a multi-port connector, etc.) resides at a location92either between a spring finger64and the base60of the chassis44, or between two spring fingers64which apply force toward each other. In either situation, the amount of spring force (F) provided by each spring finger64of the chassis44is more accurately controlled. In particular, there is not such a wide range of spring forces (F) provided by the spring fingers64. Rather, the exact amount of spring force (F) provided by each spring finger64falls within a narrow band thus enabling the manufacturer to enjoy robust EMI shielding and ESD protection due to consistent spring forces (F) applied to the connectors50.

Further details of the tab members66of the chassis44will now be provided with reference toFIGS. 4 and 8.FIG. 4shows a perspective view of the tab members66which extend from the frame portion62of the chassis44in a direction (the negative X-direction away from the circuit board module46inFIG. 4) which is opposite the direction of the spring fingers64(the positive X-direction toward the circuit board module46inFIG. 4). Furthermore,FIG. 8is a side view of a tab member66in contact with a partially shielded circuit board connector50. The tab members66are configured to provide additional electrical pathways from the connectors50(also seeFIG. 3) to the frame portion62when the circuit board module46resides in the chassis base60.

As shown inFIG. 8, each tab member66defines a domed portion100which provides a precise point of contact between that tab member66and metallic shielding102of the corresponding connector50for a low impedance interface. Accordingly, the metallic shielding102is essentially extended to the chassis44thus providing enhanced EMI shielding and ESD protection to the corresponding connectors50(e.g., a partially shielded RJ11 connector with only limited area102being shielded by metal). In some arrangements, there are tab members66along each side of an opening68. In other arrangements, there are tab members on less than each side, e.g., only on three sides to accommodate a circuit board, etc.

It should be understood that the above-described spring fingers64and tab members66are capable of being easily integrated with the other parts of the chassis44in order to form the chassis44s a cohesive unitary body. In some arrangements, the spring fingers64are provided onto the frame portion62using an extrusion riveting process (e.g., for providing zero Ohm impedance), and the tab members66are simply stamped, pressed and angled from a metal sheet into appropriate orientations relative to the frame portion62. Such a manufacturing technique provides suitable mechanical strength and electrical properties for robust EMI and ESD characteristics.

As described above, embodiments of the invention are directed to a chassis44which utilizes a spring finger64having a contact portion80and a mid portion82where the spring finger64forms multiple cantilevers84,88to provide different spring constants (k1), (k2). Accordingly, the rate of increase in force resulting from displacement can be better controlled and even minimized for certain ranges of displacement. In particular, the spring finger64can be configured to perform in an operating range characterized by a smaller spring constant (k1). As a result, the force increase in this operating range is slower thus accommodating a circuit board module46with potentially large connector height variations and different types of connectors without significantly changing the normal contact force. Such operation enables the chassis44to consistently pass EMI and ESD testing, as well as provide more reliable device operation.