Strap Spring for Attaching Heat Sinks to Circuit Boards

The present invention relates to securing electronic components together. More particularly, it relates to a strap spring for attaching a heat sink to an electronic device on a printed circuit board.

A heat sink placed in contact with an electronic device transfers heat through conduction from the semi-conductor device contained in the electronic package. U.S. Pat. No. 4,884,331--Hinshaw shows a commonly used pin fin heat sink used to dissipate the conducted heat.

Springs or clips, such as those shown in U.S. Pat. No. 4,745,456--Clemens, or U.S. Pat. No. 4,660,123--Hermann, have been used to secure heat sinks to electronic devices, and a catalog entitled "Thermalloy Semiconductor Accessories," pp. 15-29, Thermalloy, Inc., Dallas, Tex. (USA) also shows clips for attaching heat sinks to electronic devices. Page 17 of the Thermalloy catalog shows two types of wire springs for securing a heat sink to a pin grid array (PGA). U.S. Pat. No. 5,594,624 to Clemens et al., entitled, "Strap For Heat Sink Clip Assembly," shows and describes several different clips for securing a heat sink to an electronic device. Another type of anchor for securing a heat sink to a printed circuit board is shown in U.S. Pat. No. 5,640,305, entitled "Anchor for Securing a Heat Sink to A Printed Circuit Board."

These patents are incorporated herein by reference as if set forth in their entirety.

However, if a heat sink is larger than the microprocessor there is no secure way to attach the heat sink to the microprocessor. The microprocessor is typically, but not necessarily, a socket. Surface mount applications for Ball Grid Array (BGA), Column Grid Array (CGA), Land Grid Array (LGA), Plastic Quad Flat Pack (PQFP), are all examples of devices that are not in sockets.

In the above-mentioned prior art attachment means, which clip the heat sink to the circuit board, the circuit boards often warp due to the spring loading by the attachment device. Therefore, devices that provide attachment to the board normally have to be kept close to the microprocessor, so that the board is not put under large bending stresses. Also, some prior art clips require a soldering operation to affix the hook in the board. This usually would be a wave soldering operation, which is not normally done when surface mount technology is used.

It is therefore an object of the present invention to provide an anchor which can be inserted through a printed circuit board to secure a heat sink and an electronic device to the board.

The foregoing and other objects, features and advantages of the invention will be better understood from the following more detailed description and appended claims. It has now been found that the deficiencies of the prior art can be overcome by a strap spring in which the spring contacts the circuit board directly below the microprocessor. All of the spring loading is in compression through the circuit board is concentrated at the microprocessor. The ends of the clip snap through holes or slots in the heat sink to attach the entire assembly together. Therefore, the board is allowed to "float" and no bending moment is applied. The strap spring of the present invention can be constructed in embodiments that are easily removable by dislocating the inside corners of the attaching portions and popping them back through the holes. Embodiments of the present invention are also disclosed that have barbs or other means to snap through the board on two sides of the clip thereby locking the clip into the heat sink. Since the strap spring of the present invention easily snaps the assembly together, it can be easily automated for high production.

FIG. 1 is an elevation view of the strap spring of the present invention;

FIG. 2 is a top view, broken away, of the strap spring illustrated in FIG. 1;

FIG. 3 is an exploded perspective view of the strap spring of the present invention and related components;

FIG. 4 is a perspective view of an assembled heat sink and electronic device anchored with a strap spring made in accordance with the present invention;

FIG. 5 is a cross-section of the assembly shown in FIG. 4, taken along line 4--4 of an assembly securing a heat sink to a printed circuit board;

FIG. 6 is a top plan view, similar to FIG. 2, of the assembly of FIG. 5; and

FIG. 7 is an elevation view in cross-section, similar to FIG. 5, showing an alternate embodiment of the invention.

Referring now to FIG. 1, a side elevation view of a strap spring 100 made in accordance with the present invention is illustrated. The strap spring 100 is generally comprised of two arms 110, 112 that extend from a central section 120. The arms 110,112 each have an end and are preferably terminated with engagement sections 130,132. As explained in further detail below, various embodiments of the engagement section can be designed to permit the strap spring 100 either to be easily removed or to be permanently or semi-permanently affixed in place. In the preferred embodiment shown in FIG. 1, the central section 120 is connected to the arms 110,112 at a slight bend to create the flattened "V" shape illustrated. This design provides a secure mounting due to the localized force created, as explained below.

FIG. 2 is a top plan view showing the detail of one of the ends 130 of the embodiment of the strap spring 100 that is illustrated in FIG. 1. Referring to both FIG. 1 and FIG. 2, it can be seen that in this embodiment, the flat strap that forms the arm 110 is bent into an arcuate shape. As will be recognized by those of skill in the art, such a shape adds rigidity and strength to the engagement portion. Although an arcuate shape is generally preferred, it will be further realized that any number of shapes, e.g., square, "V" shapes, curled edges, etc. can be used in place of the arcuate "U" shape shown in FIG. 2. As also seen in FIGS. 1 and 2, the engagement portion has a generally wedge-shaped profile with an undercut surface 140 that engages the heat sink, as explained in further detail below.

The strap spring described above is used in an assembly along with other components. Thus, referring now to FIG. 3, an exploded view of a typical assembly is illustrated. In FIG. 3, a heat sink 10, circuit board 20 and strap spring 100 are shown. The circuit board 20 is cut away to better illustrate the assembly. A socket 22 is attached to the circuit board 20, and a device 50, in the illustrated example a BCA chip, is inserted into the socket 22. Referring now to FIG. 4, an illustration of the preferred embodiment shown in FIG. 3 is provided. As shown, the strap spring 100 is preferably disposed on the side of the circuit board 20 that is opposite the heat sink 10. The engagement portions 120,122 extend through apertures in the circuit board 20, and through apertures 11 in the heat sink 10. Engagement of the undercut surface 140, combined with the force created by the flexure of the arms (not visible) combine to lock the assembly together. However, because the undercut surface 140 presents basically a sliding surface to the surface of the heat sink 10, the engagement is not permanent, and the heat sink 10 can be removed with relatively little effort by displacing the engagement sections 130,132.

Further details of the assembly illustrated in FIG. 4 are seen in the cross-sectional view of FIG. 5, which is taken along line 5--5 in FIG. 4. As seen therein, in preferred embodiments of the present invention, the microprocessor 50 is smaller than the heat sink 10, although it is possible for it to be larger in other embodiments. However, one problem solved by the present invention is its ability to provide an attachment design that can effectively retain an assembly that retains a microprocessor that is smaller than the heat sink without creating undue bending stresses and creating a stable thermal interface by mechanically forcing the microprocessor 50 into the heat sink 10. As seen in FIG. 5, the engagement portions 130,132 are "outboard" of the microprocessor 50, while the upwardly bent central section 120 ensures that any forces generated are concentrated near the microprocessor 50.

As mentioned above, the undercut surfaces 140 engage the circuit board 20 and resist the forces described immediately above. This engagement is further illustrated in the top plan view shown in FIG. 6. As noted above, the engagement may be one of degree, and can be easily disengaged, semi-permanent, or even permanent if desired.

An alternate embodiment of the strap spring 100 of the present invention is shown in FIG. 7. In those instances where it is desired to more or less permanently secure the heat sink 10 to the microprocessor, a barb 140' can be provided at the end 130 of the strap spring 100. In the illustrated embodiment, two barb points 141, also known as locking tips are provided. It will be understood that the number, size and geometry of the barbs may be varied to provide proper securement based upon the design of the heat sink and the degree of security required.

The present invention thus also discloses improved methods of assembling electronic components in an electronic device is placed in a socket mounted to a circuit board and a heat sink is placed on the electronic device. In accordance with the invention, a strap spring is pushed though apertures in the circuit board, and then through apertures in the heat sink that are in registration with holes in the heat sink. Finally, the strap spring is engaged with the heat sink. As explained above, the engagement can be semi-permanent or permanent, and in the latter case, permanent affixation can be obtained by mechanical engagement, such as barbs.

Although the invention has been described in terms of an exemplary embodiment, the spirit and scope of the appended claims are not to be limited by any details not expressly stated in the claims. Upon review of the foregoing, numerous alternative embodiments will present themselves to those of skill in the art. It will be realized that the principles set forth above are applicable to a wide variety of situations, and are not limited to the electronic devices illustrated, the types of heat sinks illustrated. Accordingly, reference should be made to the appended claims in order to determine the full scope of the present invention.