Abstract:
A laser diode bar, solder preform and heat sink are assembled prior to reflowing the solder in a manner to prevent molten solder from being drawn by capillary action over the light emitting end of the diode bar. A recessed pin  30  locates the ends of the laser bar and solder preform with respect to an edge of the heat sink so that the laser bar and solder preform overhang the heat sink edge by respective amounts. When molten, the solder will not be drawn by capillary action to obscure the light emitting end of the laser diode bar.

Description:
FIELD OF THE INVENTION 
     This invention relates to laser diode bars mounted on heat sinks and, more particularly, to avoiding the introduction of defects when soldering laser diode bars to heat sinks. 
     BACKGROUND OF THE PRIOR ART 
     It is current practice to solder laser diode bars to copper heat sinks, sometimes using an intermediate layer of CuW between the diode bar and the heat sink. Because of the wide angle of emission, it is also current practice to mount the emitting edge of the laser bar at the very edge of the heat sink so that none of the emerging rays are blocked. In the soldering operation, the laser diode bar is placed on a solder preform of AuSn atop the heat sink The assembly is aligned at the emitting edge of the laser bar using a planar datum, clamped together and placed in an oven to reflow the solder Because of the small dimensions involved, the molten solder is liable to flow by capillary action up and over the emitting face of the laser bar, spoiling the light emission from the diode. It would be of great advantage to improve the soldering of laser bars to heat sinks by preventing such unwanted solder overrun. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, a laser diode bar, solder preform and heat sink are assembled prior to reflowing the solder in a manner to prevent molten solder from being drawn by capillary action over the light emitting end of the diode bar. A pair of recessed alignment pins  37  establish a datum that locates the emitting end of the laser bar, solder preform and an edge of the heat sink without contacting the emitting surface of the laser bar. Advantageously, the recesses in the alignment pins allow the laser bar and solder preform to overhang the heat sink edge by respective amounts. When molten, the solder will not be drawn by capillary action to obscure the light emitting end of the laser diode bar. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The foregoing and other objects and features of the present invention may become more apparent from a reading of the ensuing description together with the drawing in which: 
     FIG. 1 is an isometric view of a prior art laser diode bar mounted on a heat sink; 
     FIG. 2 is an idealized side view of the configuration of FIG. 1; 
     FIG. 3 shows the preferred method of aligning a laser diode bar and heat sink in accordance with the invention; and 
     FIG.  4 . is a top view corresponding to FIG.  3 . 
    
    
     DESCRIPTION 
     Referring now to prior art FIG. 1, a laser diode bar  14  and submount  12  are soldered to a massive heat sink  10 . Laser diode bar  14  typically has a thickness of only 0.005″ and submount  12 , typically made of copper tungsten (CuW) is used to minimize mechanical stress on bar  14  which arises because the thin laser diode bar has a thermal coefficient of expansion (CTE) of about 6 ppm while copper block  10  has a CTE of 16 ppm. A CuW submount  12  which has a CTE similar to that of the laser diode bar. An insulating standoff  15  separates electrode  16  from block  10 . Wires conventionally used to connect electrode  16  to laser diode bar  14  are omitted for clarity. 
     An idealized side view of the assembly is shown In FIG. 2. A solder layer  13  secures laser diode bar  14  to submount  12  and a solder layer  11  secures submount  12  to heat sink block  10 . The emitting facet of the laser bar is aligned to the solder preform and the end of the heat sink using a planar datum (not shown). It is customary to use solder in the form of sheets called preforms which are melted when the assembly is placed in an oven to reflow the solder. It should be noted that laser diode bar  14  is soldered so that the diode surface closest to its quantum well layer QW adjoins heat sink  10 . Typical solder preforms may be an 80/20 AuSn or an In alloy. 
     When the assembly is connected to a source of electrical power, the emitting facet at the (left-hand) emitting end of quantum well layer QW radiates a beam of laser light that diverges along a “fast axis” and a “slow axis”, the angle “theta fast” of the fast axis being depicted. Ideally, the laser diode facet should be aligned flush with the end of submount  12  and solder preform  13  so that no part of the emitted rays are intercepted or obscured by the submount or solder. To achieve such an alignment, laser diode bar  14 , solder preform  13 , submount  12 , solder preform  11  and heat sink  10  may be placed in a jig so that their left-hand ends are butted against one or more cylindrical alignment pins (not shown). Unfortunately even when properly aligned for the solder reflow operation the molten solder may be drawn by capillary action between the surfaces of the alignment pins and the facet at the emitting end of the laser bar&#39;s QW layer, blocking some or all of the laser bar&#39;s light output The height of undesired solder rise by capillary flow up the emitting facet is determined by several factors, and may be expressed as:          h   =       2        γ   lv        cos                 θ       ρ                 gd         ,                          
     where γ lv  is the liquid-vapor surface energy; 
     θ is the contact angle; 
     ρ is the density of the molten solder; 
     g is the acceleration of gravity; and 
     d is the separation between the surfaces of the alignment pin and the laser bar facet. 
     To prevent such capillary action by the reflowed molten solder, the arrangement of FIGS. 3 and 4 is used. FIG. 3 is a side view and FIG. 4 is a top view of a heat sink  30 , a first solder preform  31 , a submount  32 , a second solder preform  33  and a laser diode bar  34  aligned for soldering in accordance with the invention. Laser diode bar  34  is placed on solder preform  33  so that its quantum well layer QW is closest to preform  33 . 
     Proper alignment for avoiding capillary action is achieved with the aid of a pair of polished alumina (Al2O3) alignment pins  37 - 1  and  37 - 2 . Alignment pins  37  exhibit three different diameters,  37   a ,  37   b  and  37   c . It should be noted that this invention teaches that rather than attempting to flush align the end of laser bar  21  with the end of heat sink block  20 , as in the idealized but unachievable alignment depicted in FIG. 2, the alignment pins  37  create a condition where laser bar  34  is deliberately allowed to overhang the ends of heat sink  30  and submount  32  by a minute amount, identified in FIGS. 3 and 4 as “OH  34 ”. Also, the solder sheet or preform  33  is allowed to overhang the end of heat sink block  30  by a small amount identified as “OH  33 ” in order to ensure that the solder, when melted, will flow to the end of bar  34  closest to the emitting end. The solder at the end of the laser bar assists in conducting heat away from the emitting facet into the heat sink when the laser bar is in operation. Accordingly, the solder securing the laser bar to the heat sink must extend to the end of the laser bar but must not be permitted to rise up on the emitting facet as solder flow up the emitting facet poses the danger of obscuring the light emitted from the active regions of the facet and thereby degrading laser operation. The amount of solder sheet  33  overhang OH  33  may thus be greater than the amount OH  34  by which laser bar  34  overhangs the ends of submount  32  and heat sink  30 . 
     The degree of overhang OH  34  is determined by the difference in diameters of portions  37   a  and  37   c  of alignment pin  37 . Major diameter  37   a  is dimensioned to butt against the left-hand edge of heat sink  30 , solder preform  31  and submount  32  while the diameter of section  37   c  is designed to butt against the left-hand end of laser diode bar  34 . It is important that section  37   c  not contact the emitting end of the quantum well portion QW of laser bar  34  as established by dimension d 2 . 
     To prevent capillary action obscuring the end of the QW layer, portion  37   b  of pin  37  is recessed. The height of the recess portion is identified by dimension d 1 . The depth of the recessed portion is dimensioned to accommodate the solder preform overhand OH  33 . Advantageously, the depth of the recess portion is such that solder preform may bottom in the recess when the end of laser bar  34  makes contact with portion  37   c  of pin  37 . When the parts are offset as shown in FIGS. 3 and 4, they may advantageously be held in position by a weight W and/or clamp (not shown) to prevent inadvertent movement until the solder is reflowed and allowed to cool to secure the parts together. When the solder sheets are reflowed in an oven (not shown), the portion of solder sheet  33  overhanging the end of heat sink block  30  will be pulled downward by gravity, toward the end of submount  32 , rather than flowing upward by capillary action toward the light emitting facet end of laser diode bar  34 . This safeguards the light emitting end of bar  34  from being overrun by molten solder. Thus, in accordance with the invention, the light emitting end of the laser bar is protected from being obscured by molten solder without resort to any shielding or protective coating. 
     In one illustrative embodiment, the following dimensions were employed: 
     OH  34 =0 to 8 μ maximum. (Greater than an 8 μ overhand is not desired since it reduces the conduction of heat transfer away from the emitting facet into the heat sink) 
     d 1 =0.010″. 
     OH  33 =≅0.001′. 
     What has been described is deemed illustrative of the principles of the invention. It should be apparent to those skilled in the art that instead of using recessed pin  30  to locate the ends of the laser bar and solder preforms with respect to an edge of the heat sink, as shown in FIGS. 3 and 4, the overhangs may be accomplished with the use of a “pick and place” machine. It should also be apparent that instead of a solder preform, a predeposited solder layer may be used on the submount and/or heatsink with equally advantageous results. Further, pin  30  may be mounted on a carriage that slides away after the laser bar and submount have been positioned and clamped together before the assembly is placed in an oven to reflow the solder. 
     Further and other modifications will be apparent to those skilled in the art and may be made without, however, departing from the scope of the invention.