SAWTOOTH CARD RETAINER

Abstract: A printed circuit board heat sink retainer is disclosed comprising a mating pair of sawtoothed bars (12,14) with slidably engaging teeth. The bars (12,14) are laterally spread apart and held by a forcing de­ vice (20), which may be a screw or a spring, in order to push against and hold a heat sink (36) in a channel. The distance between successive teeth may be constant in both bars (12,14) or may be constant in one bar and may decrease in a direction toward the forcing device (20) in the second bar.

Description: This invention relates to a printed circuit board heat sink retainer, comprising a first sawtoothed piece fixed to an internal chassis wall between a pair of ribs integral to said wall and butted up against one of the ribs, the teeth of said first sawtoothed piece being constantly spaced from one another; a second sawtoothed piece loosely held between the ribs for mating its sawtooth teeth with the teeth of the first piece; and spread apart and hold means at only one of the ends of the first and second pieces for providing spreading forces to spread said first and second pieces apart such that the second piece pushes against a heat sink inserted between it and the other of said pair of ribs and which forces hold the second piece in place against the heat sink. A printed circuit board heat sink retainer of this type is disclosed in GB-A-2 103 020, in which the first sawtoothed piece is fixedly clamped in position by a lock screw. A clamping screw is provided for applying force to the second sawtoothed piece against the action of a spring to spread the first and second pieces apart. The object of the invention is to provide an improved printed circuit board heat sink retainer of the recited type which is both less expensive to manufacture and provides a more uniform heat transfer path along the length of the printed circuit board heat sink. In accordance with the invention, to achieve this, the printed circuit board heat sink retainer is characterized in that the first sawtoothed piece is resiliently bonded to the internal chassis wall and that the distance between successive teeth of said second sawtoothed piece decreases in the direction of the spread apart and hold means. Due to the use of both a fixed and a varying period toothed bar clamping forces are distributed more uniformly over the entire length of the heat sink to provide a constant heat transfer path. These and other objects and advantages of the printed circuit board heat sink retainer will become more apparent in light of the detailed description of a best mode embodiment thereof, as illustrated in the accompanying drawing, wherein Fig. 1 is an illustration of a first embodiment of the sawtooth card retainer; Fig. 2 is a perspective illustration of a sawtoothed bar shown elevated over the position in which it is to be compliantly bonded to an internal wall of a chassis such that it is compliantly butted against one of two parallel ribs integral to the internal wall; Fig. 3 is an illustration of the embodiment of Fig. 1 shown inserted between the ribs of Fig. 2; Fig. 4 is a section view of the retainer of Fig. 3 along lines 4-4 of Fig. 3; Fig. 5 is an illustration of a second embodiment of the sawtooth card retainer; Fig. 6 is a sectional illustration of the retainer of Fig. 5 along lines 6-6 of Fig. 5; Fig. 7 is a third embodiment of the sawtooth card retainer; Fig. 8 is a sectional illustration of the retainer of Fig. 7 along lines 8-8 of Fig. 7; Fig. 9 is an illustration of a fourth embodiment of the sawtooth card retainer; Fig. 10 is a sectional illustration of the retainer of Fig. 9 taken along lines 10-10 of Fig. 9; Fig. 11 is a sectional illustration of the Allen Head set screw circled in Fig. 9; and Fig. 12 is an illustration of the bonded toothed bar with constant tooth pitch slidably engaged with the moveable toothed bar with decreasing tooth pitch, Fig. 1 is an illustration of the first embodiment 10 of the sawtooth card retainer. The illustrated embodiment is made up of two elongated pieces 12, 14 each having a sawtooth pattern which mates with the sawtooth pattern of the other piece. The two pieces' sawtooth patterns slidably engage one another such that they can be very close together as shown in Fig. 1 or can be spread apart while still maintaining sliding engagement of the teeth. A captivating pin 16 attached to piece 12 is free to move within a slot 18 of piece 14 but is not permitted to leave the slot 18. Piece 14 has a socket head screw 20 captivated in a slot 22 at the end of piece 14 opposite from the end with the captivating pin. The threaded end of the cap screw 2 engages a threaded hole 24 in piece 12. The slot 22 is oriented such that the screw 20 can move in the slot 22, laterally on the drawing, with lateral and horizontal movement of piece 12 with respect to piece 14. Fig. 2 is a perspective illustration of piece 14 of Fig. 1 elevated above an internal wall 26 of an electronic chassis. Piece 14 is elevated above the position in which it is to be compliantly bonded to the internal wall 26 using silicone or acrylic adhesive. A dotted line outline 28 of piece 14 is illustrated. Piece 14 is bonded so that it is butted up against a first rib 30 integral to the internal wall 26. A second, parallel rib 32 completes the formation of a U-shaped cross-section channel within which the sawtooth retainer of the present invention will reside, along with a printed circuit board heat sink, as is more fully illustrated in Figs. 3 and 4. Fig. 3 is an illustration of the type of retainer embodiment as shown in Fig. 1 mounted between the parallel ribs 30, 32 of Fig. 2. Piece 14 has been bonded to the internal wall 26 and is butted up against rib 30. Piece 14 is free to move slightly to the extent permitted by the resiliency inherent in the bonding material utilized. Piece 12 is not bonded and is free to move with respect to piece 14 by sliding engagement of the teeth. A heat sink 36 is shown sectionally inserted between piece 12 and rib 32. A section view along lines 4-4 is presented in Fig. 4. The section view shows the heat sink 36 butted up against rib 32 by virtue of forces applied by the sawtooth card retainer embodiment 10. A printed circuit board 38 is shown attached to the heat sink 36 and contains a plurality of electronic components which conduct heat to the heat sink for conduction to the chassis wall 26. Referring back to Fig. 3, the screw 20 may be threaded into piece 12 to spread the two pieces 12, 14 of the sawtooth card retainer further apart in order to exert force required to provide a heat conduction path from the heat sink to rib 32 and from the heat sink to rib 30 through pieces 12 and 14 via the slidingly engaged teeth. Since piece 14 is only compliantly, or resiliently, bonded to internal wall 26, there will be movement of piece 14 such that piece 14 can be forced up tighter against rib 30 depending on the degree of force exerted by the screw acting through piece 14 into piece 12. Returning to Fig. 1, the sawtooth card retainer illustrated consists of two machined, rolled or extruded pieces 12, 14 in the form of elongated bars having a four-sided end view with right angled corners. Each bar has a sawtooth pattern on one side that mates with and slidingly engages the teeth of the other bar. The card retainer is mounted as shown in Figs. 2, 3 and 4 to the internal wall of an electronic chassis between two parallel ribs. The card retainer is operated by applying torque to a socket head cap screw or other similar device. As the screw is torqued, the card retainer expands causing the heat sink to be clamped between the opposing ribs. The set screw is torqued in a manner which causes a pulling upwardly, in the direction from bottom to top of the sheet of Fig. 3, on the piece 12. In other words, if the screw 20 of Fig. 3 has a right-handed thread, it will be necessary to turn the screw in a clockwise direction in order to cause expansion of pieces 12 and 14. The tooth angle illustrated in Figs. 1 and 3 is 45°. The tooth angle may be changed to a wide variety of possible different angles but a choice of 30° was made in the embodiments of Figs. 5, 7 and 9 in order to decrease the number of teeth required from the large number shown in Fig. 3. Fig. 5 is an illustration of the second embodiment of the sawtooth card retainer. A first piece 40 is bonded to the internal wall 42 of the chassis, as shown in Fig. 6. Referring back to Fig. 5, the bonded piece 40 butts up against a rib 44 which has a screw 46 inserted and captivated in a slotted end 48. The screw 46 is threaded and screwed into one end of a second saw-tooth piece 50. Piece 40 contains a captivating pin 52 that is fixed at the end opposite the screw. Piece 50 contains a slot to accommodate pin 52 and allows the sawtooth bars to slidably engage the printed circuit board heat sink. A printed circuit board heat sink 36 may be inserted between piece 50 and a second rib 54 for being retained. In that event, screw 46, if it is of the right-handed thread variety, may be turned in the clockwise direction in order to spread apart pieces 40 and 50 thereby compressing the heat sink 36 up against rib 54 and providing a heat transfer path similar to that described in connection with Fig. 3. The main difference between the embodiement of the card retainer of Fig. 5 and that of Fig. 3 is that the screw is held in a slot which is part of rib 44 instead of part of piece 14, as in Fig. 3. This is a matter of design choice. Fig. 6 is a section view taken along lines 6-6 of Fig. 5 and shows the various pieces of Fig. 5 from a bottom view and also shows a printed circuit board 38 mounted on the heat sink 36. Fig. 7 shows the third embodiment of the sawtooth card retainer. In this case, a first piece 56 is bonded against a first rib 58 and the first piece 56 has an oversized end portion 60 for providing a slot for captivating an oversized screw 62. The rib 58 is not as long as a corresponding rib 64 so that the oversized end 60 can extend outward from the confines of the U-shaped channel. A second piece 66 has a threaded end 68 for receiving the threaded end of the screw 62. The cap screw may be rotated in a clockwise direction, assuming a right-handed thread, and the two pieces 56, 66 will spread apart and press a heat sink 36 against rib 64 in a manner similar to that described in connection with Figs. 3 and 5. A captivating pin 70 holds the two pieces 56, 66 together in case the screw is loosened and the heat sink is removed. Fig. 8 is a section view taken along lines 8-8 of Fig. 7 showing a top view of the ribs 58, 64 and many of the other numbered pieces of Fig. 7. Fig. 9 shows the fourth embodiment of the sawtooth card retainer. This particular embodiment utilizes an Allen Head set screw 72 for mechanizing the card retainer. Referring to Fig. 11, the set screw 72 has a captivating pin 74 mounted through its centerline that mates to a slotted steel bearing plate 76. Referring back to Fig. 9, the retainer is held captive by a second captivating pin 78 at the opposite end of the retainer. This holds the two pieces 80, 82 together in the event that the Allen screw is loosened and the heat sink removed. It will be observed that the second piece 82, closest to the heat sink 36, has an oversized end 83 which contains the Allen Head set screw. Assuming a right-hand thread, if the screw is rotated in a clockwise direction as viewed from the top of Fig. 9, piece 82 will be pulled upwardly and heat sink 36 will be pushed up against rib 86 and piece 80 up against rib 84 to the extent that its resilient bonding permits. Fig. 10 is a section view along lines 10-10 of Fig. 9 showing a bottom view of the heat sink 36, the attached printed circuit board 38, ribs 84, 86, pieces 80, 82 and the internal wall 90 upon which piece 80 is resiliently bonded as is similarly illustrated in Fig. 2 for the case of the embodiment of Fig. 1. In the embodiments illustrated in Figs. 3, 5, 7 and 9, the period between teeth on the bonded sawtooth pattern pieces 14, 40, 56 and 80, respectively, is the same. On the mating extrusion, 12, 50, 66 and 82, the period is decreased slightly from tooth to tooth as one progresses from the bottom of the sheet toward the top where the screw is located. This is shown in Fig. 12 in more detail where a decreasing period is shown in the top piece (the moveable piece) and a constant period is shown in the bonded piece at the bottom. The force directional arrow (F) points toward the set screw, where the pulling force originates. The result is that the teeth mate farthest from the screw first and progressively mate as the screw is tightened. This leads to a more constant load distribution and helps to address thermal gradients along the length of the card retainer.

Claims: A printed circuit board heat sink retainer, comprising: a first sawtoothed piece (14; 40; 56; 80) fixed to an internal chassis wall (26) between a pair of ribs (30, 32; 44, 54; 58, 64; 84, 86) integral to said wall (26) and butted up against one of the ribs, the teeth of said first sawtoothed piece being constantly spaced from one another; a second sawtoothed piece (12; 50; 66; 82) loosely held between the ribs for mating its sawtooth teeth with the teeth of the first piece; and spread apart and hold means (20; 46; 62; 72), at only one of the ends of the first and second pieces for providing spreading forces to spread said first and second pieces, apart such that the second piece pushes against a heat sink (36) inserted between it and the other of said pair of ribs and which forces hold the second piece in place against the heat sink, characterized in that the first sawtoothed piece (14; 40; 56, 80) is resiliently bonded to the internal chassis wall (20) and that the distance between successive teeth of said second sawtoothed piece (12; 50; 66; 82) decreases in the direction of the spread apart and hold means (20; 46; 62; 72). Retainer according to claim 1, characterized in that the spread apart and hold means (20; 46; 62; 72) is formed to apply pulling force to the second sawtoothed piece (12; 50; 66; 82).

Inventors: MCNULTY, CHRISTOPHER THOMAS
Applicants: UNITED TECHNOLOGIES CORPORATION
Application year: 1987
Main classification: H05K7
IPC codes: H05K7