Abstract:
A pressure clip for contacting a heat sink device to a heat sink by means of a pressure bond having thermal resistance of less than about 5 K/kW/cm 2 . The pressure clip includes a mounting block, a clamp block; a spacer disposed between the mounting block and the clamp block forming a channel therebetween, support shoulders in the channel for supporting a heat sink, means for securing the clamp block and the spacer to the mounting block, pressure arm disposed above the mounting block, flexible joint for flexibly attaching the pressure arm to the mounting block, pressure screw disposed between the pressure arm and the mounting block for applying pressure to the pressure arm, and a plunger projecting into the channel between the mounting block and the clamp block for transmitting pressure from the pressure arm.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention pertains to the field of heat sink pressure clip which forms a pressure bond between a heat source and a heat sink. 
     2. Description of Prior Art 
     There is a rapidly increasing demand for efficient IR semiconductor lasers operating at ambient or thermoelectric cooler temperatures. Military needs include countermeasures and communications whereas commercial applications focus on remote chemical sensing and drug monitoring, leak detection, chemical process control, and laser surgery. In both of these markets continuous wave (CW) or quasi-CW laser operation is essential and current thermal management techniques are the primary impediment to these types of operation. 
     Thermal management involves removing heat from a device which, in the case of lasers, critically affects the efficiency and maximum operating temperature. As a semiconductor laser is either electrically or optically excited, excess thermal energy from joule heating, optical heating, hot-carrier relaxation, etc., must be efficiently removed from the laser&#39;s active region to minimize degradation of the laser&#39;s performance at elevated temperatures. Standard techniques to accomplish this involve soldering the laser to a heat sink using one of a variety of soldering alloys. The heat sink is usually a high thermal conductivity material such as diamond or copper. 
     A typical semiconductor laser structure consists of a few microns of epitaxially grown laser material (epitaxial-side) containing the active region disposed on a lattice matched substrate. The substrate can be conveniently thinned to a minimum of about 50 microns. Two configurations for soldering a laser to a heat sink are epitaxial-side-up and epitaxial-side-down. Since most of the heat is generated in the active portion of the epitaxial layer, the heat removal is most efficient when the epitaxial layer directly contacts the heat sink, i.e., epitaxial-side-down. While this configuration is the best thermally, it is technically more complicated than the epitaxial-side-up technique and methods must be employed to insure that the facets of the laser are not obscured or contaminated by the solder or its residue. Even when voids, granularity and/or other imperfections in the solder joint do not significantly impede the heat flow, the intrinsic thermal resistance of a solder layer can be significant. 
     All of the soldering techniques employed for electrically-pumped semiconductor lasers may be used to fabricate optically-pumped lasers as well. A further difficulty occurs when the laser is soldered epitaxial-side-down, in that the only access by the pump laser is through the substrate. This requires that the substrate be transparent to the pump laser, which is often impractical due to other constraints related to fabrication and convenience. 
     Most of the currently-used soldering and mounting techniques require considerable device processing. The semiconductor and heat sink are typically patterned with layers of different metals and the soldering must be done in a highly controlled environment. Some common problems encountered in epitaxial-side-down soldering are degradation of the laser due to stress or high-temperature processing, breaking upon thermal cycling, contamination of the laser facets, and poor yield associated with the critical nature of the alignment between the laser facet and the edge of the heat sink. 
     Although the above discussion focused on the IR semiconductor laser application, it should be understood, however, that similar considerations apply equally to semiconductor lasers emitting in other wavelength ranges and to many other optical and electronic devices for which thermal management issues are important, including nonlinear difference frequency generation and high-power electronic devices. 
     In a specific embodiment, the pressure clip disclosed and claimed herein includes a base and a pressure arm that are connected by a keyhole flexible joint. Force is applied to the pressure arm by the pressure screw which passes through the arm. Mounted at the end of the pressure arm is a plunger which exerts force on the device. The epitaxially grown layer of a laser is pressed against the diamond heat sink which is thermally grounded to the copper mounting block. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     An object of this invention is a means for effecting a pressure bond between a heat source device and heat sink in absence of soldering. 
     Another object of this invention is a pressure clip that delivers adequate force to the interface between a heat sink and a heat source device to ensure good thermal contact 
     Another object of this invention is a pressure clip that applies force to the interface between a heat sink and a heat source device uniformly. 
     Another object of this invention is a pressure clip that applies sufficient force to the interface between a heat sink and a heat source device to form a pressure bond therebetween, the application of force is accomplished in a controlled manner from a stable platform. 
     These and other objects of this invention are achieved by a pressure clip which includes a mounting block, a clamp block, a spacer disposed between the mounting block and the clamp block forming a channel therebetween, shoulders in the channel for supporting a heat sink, screws for securing said clamp block and the spacer to the mounting block, pressure arm disposed above the mounting block, flexible joint for flexibly attaching the pressure arm to the mounting block, pressure screw disposed between the pressure arm and the mounting block for applying pressure to the pressure arm, and a plunger projecting into the channel between the mounting block and the clamp block for transmitting pressure from the pressure arm. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is the front view of the pressure clip that can be used to apply sufficient pressure to a substrate and heat sink to form the pressure bond at the interface thereof to allow for electrical or optical actuation of the device. 
     FIG. 2 is the top view of the pressure clip shown in FIG.  1 . 
     FIG. 3 is an enlarged view of the interaction between a heat sink and a heat source device which can be actuated electrically or optically. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     This invention pertains to a pressure clip which can be used to provide a pressure bond between a heat source device and a heat sink. The pressure clip is characterized by the fact that it delivers adequate force to the semiconductor/heat sink interface to ensure good thermal contact, that the force is applied uniformly along the interface to prohibit damage to the semiconductor substrate, and that the force is applied in a controlled manner from a stable platform to ensure that the semiconductor substrate does not shift in the process. 
     The mounting apparatus is a pressure clip which is designed to fulfill three critical requirements for the pressure bond: it provides a method to deliver adequate force to the semiconductor/heat sink interface to ensure good thermal contact; it applies force uniformly along the interface to prohibit damage to the semiconductor substrate; it applies force in a controlled manner from a stable platform to ensure that the substrate does not shift during the process. 
     The pressure clip  800  shown in FIGS. 1,  2  and  3  includes a mounting rectangular block  802 , a separate clamp block  804  which forms channel  806  within which is disposed spacer  902 . Screws  904 , 906  pass through clamp block  804 , through spacer  902  and into mounting block  802  to secure spacer  902  in channel  806 . Spacer  902  is flush with rear edge  803  of mounting block  802  and extends longitudinally in channel  806  from its rear edge  803  toward the front of pressure clip  800  to within about 3 mm of front edge  807  of clamp block  804 . Front edge  807  is flush with front edge of mounting block  802 . 
     Openings  905 , 907  in mounting block  802  are intended for screws for attaching pressure clip  800  to an underlying cooling structure. 
     The purpose of spacer  902  is to space clamp bar  804  from mounting block  802  wide enough for heat sink  808  to fit loosely therein, as shown in FIGS. 1 and 3. Width  908  of spacer  902  should be uniform throughout its length in order to position therein heat sink  808  with its parallel sides. If heat sink  808  is 2.5 mm wide, then spacer  902  and channel  806  should be about 2.5 mm wide to accommodate the heat sink. Spacer  902  can be thicker or thinner or be of the same thickness as the mounting block  802 . Heat sink  808  rests in channel  806  on shoulders  1002 ,  1004 , as shown in FIGS. 1 and 3. Shoulder  1002  forms a ledge in clamp block  804  and shoulder  1004  forms a ledge in mounting block  802 . Length or depth of shoulders  1002 ,  1004  depends on the depth dimension of heat sink  808 . If depth dimension of heat sink is 2.5 mm, then the length or depth of shoulders  1002 ,  1004  should be at least 2.5 mm, such as about 3 mm, in order to fully accommodate the depth dimension of the heat sink. With provision of the shoulders in the front portions of mounting block  802  and clamp block  804 , width of the channel is narrower in the section where the shoulders are, as shown in the enlargement in FIG.  3 . 
     Secured to mounting block  802  is pressure arm assembly  810  which includes pressure arm  812 , attachment arm  914 , screws  916 , 918  for securing pressure arm assembly  810  to mounting block  802  and flexible joint  814  connected the pressure arm  812  of the pressure arm assembly  810 , as shown in FIG.  1 . Pressure arm assembly  810 , typically a unitary structure, is a mirror image of letter “L”, as shown in FIG. 2 with pressure arm  812  extending leftward at right angle to the attachment arm  914 . The flexible joint  814  includes a circular bore  816  in the pressure arm  812  in the far or right extremity of the pressure arm  812 . Diameter of the bore  816  is less than the width  818  of the attachment arm  914 . The top surface of attachment arm  914  is below the top surface of the pressure arm  812 . Bore  816 , therefore, extends just above the top surface of attachment arm  914 , which is indicated by the dotted line  824  in FIG.  1 . Slot  820  in pressure arm  812 , which extends leftward from bore  816  to edge  822  of attachment arm  914 , provides flexibility to pressure arm  812  together with bore  816 . 
     There is a pair a spaced openings in the attachment arm  914  and a corresponding pair of spaced openings in the mounting block  802  through which pass screws  916 ,  918  to secure the. pressure arm assembly  810  to the mounting block  802 . Elements  917  and  919  are washers for screws  916 ,  918 . 
     Threaded pressure screw  826  passes through pressure arm  812 , which extends parallel and spaced above mounting block  802 , and into mounting block  802 . Screw head  828  on the top end of pressure screw  826  can control pressure on the pressure arm by screwing the pressure screw in or out. Element  830  is a washer around the pressure screw  826  between the screw and the pressure arm  812 . Pressure screw  826  is disposed vertically at about the midpoint of the pressure arm  812 , as shown in FIG.  1 . 
     At the end of pressure arm  812  is vertically disposed threaded plunger  832  which extends through pressure arm  812  and into channel  806 . Plunger  832  is provided with screw head  834  at its upper extremity, washer  836  around the plunger  832  and on the pressure arm  812 , and lock nut  838  disposed around the plunger and on the washer. The function of the lock nut  836  is to lock plunger  832  in place and prevent its up and/or down movement. 
     Function of plunger  832  is to transmit pressure imparted by the pressure screw  826  to the heat source device disposed on the heat sink in channel  806 , and thus form a pressure bond between the heat source device and the heat sink. 
     The structure forming the pressure bond is shown in the encircled section in FIG. 1, which is shown enlarged in FIG.  3 . FIG. 3 shows heat sink  808  resting on shoulders  1002 ,  1004  in channel  806  with the heat sink device  22  resting on top of the heat sink and well below the top surfaces of mounting block  802  and clamp block  804 . If the heat sinking device  22  is a semiconductor laser, its dimensions are typically 2 mm×0.5 mm×150 μm. Plunger  832  converges through a conical section  840  to a point  842  which presses on the heat source device  22  to form the pressure bond. Typically, the heat generating region of the device is in contact with the heat sink 
     A key feature of the pressure clip  800  is the flexibility of the keyhole joint  814  in combination with rigidity of the of the pressure arm  812  and the stability of the mounting block  802 . Flexibility of the joint is provided by its thin walls of 0.032″, i.e., dimensions  844 ,  846  in FIG. 1, in combination with the thickness of 0.090″ of its pressure arm  812 . This combination insures that as pressure is applied to the pressure arm and it is consequently displaced, most of the flexing occurs in the joint. This minimizes the angular displacement of the tip of the plunger as the pressure arm applies pressure. The angular displacement of the plunger tip is approximately 1.5 milliradians per 0.001″ vertical displacement of the plunger. 
     Minimizing the angular displacement of the tip is important to insure that pressure is applied uniformly to the semiconductor substrate. To compensate for the slight angular displacement, which will inevitably occur, the tip of the plunger is coated with approximately 0.002″ of Indium or some other soft Indium alloy or a suitable soft metal to form gasket  848 . This gasket  848  deforms to both fill the space and transmit the force between plunger tip  842  and heat source device  22 . The length of the hole in the keyhole joint provides lateral and torsional stability for the pressure arm. This provides a stable platform for the process wherein pressure is applied to the arm and is transmitted through the plunger to the semiconductor and insures that the heat source device is not displaced during the process. 
     Procedure for using the pressure clip in order to form a pressure bond between the heat sink and the heat source device include the following steps: 
     (a) position and mount heat sink  808  in channel  806  on shoulders  1002 ,  1004  while applying pressure from the top, and tightening screws  904 ,  906 ; 
     (b) place the heat source device  22  on the heat sink by means of a vacuum pick-up tool or otherwise; 
     (c) install pressure arm assembly  810  without the pressure screw  826  and with plunger  832  in a raised position; 
     (d) lower plunger  832  by screwing it in by ½ turns as close to the heat source device  22  as visually possible without touching the devices; 
     (e) tighten the lock nut  838  on the plunger to render the plunger stationary; 
     (f) remove the pressure arm assembly  810  from the pressure clip and apply an indium film  848  to the tip  842  of the plunger  832 ; 
     (g) re-install the pressure arm assembly  810  without the pressure screw  826 ; 
     (h) install pressure screw  826  and tighten it in order to contact the heat source device  22 , as verified by visual observation, which results in pressure arm  812  rotating around the keyhole joint  814  and bringing down with it plunger  832  against top of the heat source device; 
     (i) tighten pressure screw  826  until the interface between the heat sink  802  and the heat source device becomes dark, indicating formation of the pressure bond; 
     (j) operate the heat source device; 
     (k) make necessary adjustments from time to time due to thermal cycling and other reasons by re-tightening the pressure screw  826  to re-establish the optimum pressure for the pressure bond. 
     The main advantage of the pressure clip is that it allows the controlled application of pressure in a manner such that the pressure can be varied without appreciably changing the angle of the plunger tip. The soft metal gasket distributes the pressure uniformly. 
     While presently preferred embodiments have been shown of the novel invention, and of the several modifications discussed, persons skilled in this art will readily appreciate that various additional changes and modifications may be made without departing from the spirit of the invention as defined and differentiated by the following claims.