JP2019513293A - LED pad configuration to optimize thermal resistance, solder reliability and SMT process yield - Google Patents

Abstract

A cathode and an anode electrode are positioned on the bottom surface along the centerline of the bottom surface of the LED submount. Two thermal pads are positioned on each side of the electrode along both sides of the bottom surface. This configuration reduces the stress of solder bonding of the electrodes to the circuit board. In addition, the heat transfer from the two thermal pads is not choked by the electrodes so that heat is spread evenly by the circuit board. Furthermore, because the outer heat pads are conformal and the metal design is symmetrical, the molten solder has the same shape as the heat pads, and the LED dies and submounts are not inclined with respect to the surface of the circuit board. Other configurations are also disclosed that improve the thermal and electrical properties of the LED die.

Description

  The present invention relates to electronic devices, and more particularly to a semiconductor die or submount soldered to a circuit board pad to improve thermal resistance, solder reliability and surface mount technology (SMT) processing yield. It relates to the configuration of metal pads on the surface. This application claims the priority of US Provisional Application No. 62 / 262,311, filed Dec. 2, 2015. The contents of US Provisional Application No. 62 / 262,311 are incorporated herein by reference.

  FIG. 1 shows a conventional LED die 10. The LED die 10 comprises a semiconductor layer and an optional transparent substrate, as indicated by the LED layer 12. Bottom cathode electrode 14 is electrically connected to the n-type layer in LED die 10, and anode electrode 16 is connected to the p-type layer in LED die 10. The LED die 10 is shown as a flip chip, but may instead have one wire bonded top electrode or two wire bonded top electrodes.

  The LED die 10 can be initially mounted on a thermally conductive submount 18, such as having an AlN body. The use of the submount greatly facilitates the handling of the LED die 10 and its attachment to the circuit board. Submount 18 has top metal pads 20 and 22 bonded to LED die electrodes 14 and 16 by ultrasonic welding or other techniques. The submount metal pads 20 and 22 are formed on the plurality of bottom electrodes 24 (one shown in cross section) by means of vertical vias 26 through metal traces on the top surface of the submount 18 and the submount body. It is electrically connected.

  The submount 18 acts as an interface between the LED die 10 and the printed circuit board 30 to conduct the current and heat generated by the LED die 10. The heat generated by the LED die 10 is conducted to the submount body primarily by the LED die electrodes 14 and 16. The submount 18 conducts heat to the circuit board 30 through an electrically isolated thermal pad 32, typically formed of copper. The electrodes 34 on the circuit board 30 (only one shown in cross section) are soldered to the bottom electrode 24 of the submount 18, the thermal pads 35 on the circuit board 30 on the thermal pads 32 on the submount It is soldered. Electrode 34 is connected to the power supply by trace 36. Circuit board 30 is often highly thermally conductive, as formed from an aluminum or copper core, and has a thin dielectric layer 38 to electrically isolate the electrodes 34 and traces 36 from the metal core.

  The thermal pads 35 on the circuit board 30 may be much larger than the footprint of the submount 18 to diffuse heat laterally.

  FIG. 2 shows the electrodes 24 A and 24 B and the thermal pad 32 on the bottom surface of the submount 18, which correspond to the electrode and thermal pad patterns on the circuit board 30.

  The problem with such a design is that the heat flow is interrupted by the gap between the thermal pad and the electrical pad, so that the thermal diffusion in the Cu layer of the circuit board 30 is partially restricted by the position of the electrode 34 It is to be choked (or cut off). Another problem is that the molten solder 40 (such as from the solder bath) that wets the thermal pad 35 is not flat due to surface tension, and the height of the molten solder 42 on the small electrode 34 is solder 40 on the thermal pad 35. It is different from the height of the As a result, after the solder solidifies due to cooling, the submount 18 and the LED die 10 are slightly inclined when mounted on the circuit board 30. This effect occurs with large submounts, in particular more than 2.5 × 2.5 mm.

  Furthermore, the design of FIGS. 1 and 2 is subject to solder cracking and delamination due to the coefficient of thermal expansion (CTE) mismatch between the submount 18 and the circuit board 30. The reason is that as the distance from the center of the submount 18 increases, the stress increases and the small electrode 24 is the farthest from the center of the submount 18. If the electrodes are not sufficiently electrically connected to one another by solder, an arc is generated at the broken open contacts, high heat is generated at the interface, and the voltage drop is increased.

  The same problem occurs when the bottom surface of the LED die 10 has an electrode and thermal pad configuration similar to that shown in FIG. 2 and the LED die 10 is directly bonded to the corresponding pad on the circuit board.

  FIG. 3 shows another conventional configuration of electrodes 44 and 46 and thermal pad 48 on the bottom surface of the submount for the LED die. This three-stripe design has the same drawbacks of tilting, throttling of heat diffusion by the electrodes 44/46, and solder cracking / delamination, similar to that described above for the configuration of FIG. Because the electrodes 44 and 46 are off center and partially surround the thermal pad 48.

  What is needed is a configuration of metal electrodes and thermal pads on the bottom surface of the LED die or submount without the drawbacks described above.

  Various metal electrode and thermal pad configurations on the bottom surface of a semiconductor die or submount are described that improve the thermal and electrical properties of electronic or optoelectronic devices.

  In one embodiment, the cathode and anode electrodes of the LED module are disposed along the centerline of the submount bottom surface, the two thermal pads are disposed on opposite sides of the bottom surface, and the electrodes are two thermal pads. In between. Because the stress in the solder is lowest near the center of the submount, less stress is placed on the solder joining the electrodes to the circuit board.

  Furthermore, the heat is more uniformly spread in and out of the circuit board, since the heat (flow) conducted by the two thermal pads is not squeezed by the electrodes.

  Furthermore, because the metal design is symmetrical and the thermal pads are identical, the molten solder has the same shape on both thermal pads, making the die and submount parallel to the surface of the circuit board. Thus there is no die tilt. This is particularly important when the die is an LED die. This is because the slope affects the light emission profile.

  Other configurations are described in which various metal electrodes and thermal pads are disposed on the bottom of the die or submount to achieve the above thermal and electrical improvements.

FIG. 1 is a cross-sectional view of an exemplary LED die mounted on a submount mounted on a circuit board. FIG. 2 shows a conventional metal electrode and thermal pad configuration on the bottom of a conventional LED die or submount. FIG. 3 shows a conventional metal electrode and thermal pad configuration on the bottom of another conventional LED die or submount. FIG. 4 illustrates the configuration of metal electrodes and thermal pads on the bottom of the LED die or submount, in accordance with one embodiment of the present invention. FIG. 5A is a cross-sectional view of an LED die mounted on a submount mounted on a circuit board, the bottom surface of the submount having the metal configuration shown in FIG. It is the figure seen from the side. FIG. 5B is a view from the long side of the thermal pad, showing the structure of FIG. 5A. FIG. 6 shows the configuration of metal electrodes and thermal pads on the bottom of the LED die or submount in accordance with another embodiment of the present invention. FIG. 7 shows the configuration of metal electrodes and thermal pads on the bottom of the LED die or submount in accordance with another embodiment of the present invention. FIG. 8 shows a submount with a plurality of dies attached, the submount having a bottom metal pattern similar to that of FIGS. Elements that are the same or equivalent in the various figures are given the same reference numerals.

  While the invention is applicable to any electronic device having a metal pad to be bonded to a circuit board, an example of an LED module is described.

  FIG. 4 is a view of the bottom surface of an exemplary LED die or submount showing metal cathode electrode 50, anode electrode 52, and thermal pads 54 and 56 in accordance with one embodiment of the present invention. In this example, it is assumed that the LED die is mounted on the submount and the submount is mounted on a circuit board having the same metal pattern. Thus, the configuration shown in FIG. 4 is on the bottom surface of submount 58. Thus, the metal electrode configuration at the bottom of the LED die is not important. Because submount 58 electrically connects the LED die electrode to bottom electrode 50/52 on submount 58 using traces and vertical vias, the heat generated by the LED die is due to the submount body It is because it is conducted to the thermal pad 54/56.

  In another embodiment, the LED die may also include a thermal pad, and the submount may include metal vias leading from the top thermal pad to the bottom thermal pads 54 and 56.

  FIG. 5A shows the LED die 10 and may be the conventional LED die of FIG. 1 mounted on the top surface of the submount 58. This view faces the short sides of the thermal pads 54 and 56. FIG. 5B illustrates the structure of FIG. 5A, which faces the long side of the thermal pad 54.

  The anode electrode 16 of the LED die is shown to be attached to the top metal anode pad 60 on the submount 58 and the cathode electrode 14 is shown to be attached to the top metal cathode pad 61 on the submount 58 . The attachment is done by ultrasonic welding or other techniques. The pad 60 is electrically connected to the bottom anode electrode 52 by vertical metal vias 62 formed in the submount body. The view from the opposite side showing the cathode electrode 14 of the LED die is identical to FIG. 5A.

  Heat from the LED die 10 is conducted from the bottom surface of the LED die, including the metal electrodes 14/16 of the LED die, to the body of the submount 58 and initially diffuses the heat throughout the submount 58. Next, heat is transferred to metal core circuit board 64 through thermal pads 54 and 56 on submount 58 and corresponding metal pads 66 and 68 on circuit board 64. Anode electrode 52 of submount 58 is electrically connected to anode pad 70 on circuit board 64. Pads 66 and 68 on circuit board 64 may extend beyond the footprint of submount 58 for better heat diffusion, as there are no electrodes to prevent outward expansion of pads 66 and 68. it can.

  The metal configuration shown in FIG. 4 improves the heat sink from the LED die 10, reduces the tilt of the LED die, and reduces the possibility of solder cracking / peeling as described below.

  Since the thermal pads 54 and 56 are symmetrically disposed near both sides of the submount 58 and the electrodes 50 and 52 are disposed along the center, after the submount 58 is mounted on the circuit board 64, The heat can not spread outward. Furthermore, this configuration allows the thermal pads 66 and 68 on the circuit board 64 to extend beyond the footprint of the submount 58 to better spread the heat.

  Since the dimensions of thermal pads 66 and 68 are the same and extend substantially the entire length of submount 58, molten solder 74 and 76 distributed to both pads 66 and 68 have the same height and volume, etc. It has a characteristic. Thus, when the submount 58 is placed over the molten solder, the submount 58 and the LED die 10 do not tilt. When the solder is cooled, the metal pads 70/66/68 on the circuit board 64 are thermally and electrically connected to the respective electrodes 50/52 and the thermal pads 54/56 on the submount 58.

  Due to the inherent CTE mismatch between the submount 58 and the circuit board 64, the solder joints are stressed by thermal cycling. This stress increases with distance from the center of the submount 58. The solder joints connected to the "center" electrodes 50 and 52 have a smaller area than the solder joints connected to the thermal pads 54 and 56 and are thus more susceptible to cracking / delamination and to the LED die Reduce the reliability of electrical connections. Because these electrode solder joints are closer to the center of the submount 58, they receive less stress than the solder joints of the thermal pad. Thus, this configuration improves solder joint reliability.

  The same concept of providing a pad configuration that is more symmetrical and not throttling the heat distribution from the thermal pad can be applied to different metal designs on the surface of the submount or LED die.

  FIG. 6 illustrates another embodiment of metal electrode and thermal pad configurations on the bottom surface of the submount or LED die. The cathode electrode 80, the anode electrode 82, the thermal pad 84 and the thermal pad 86 are rectangular (e.g. square) and located in different quadrants. Circuit boards have corresponding patterns. Since all the electrodes / pads have the same dimensions, the molten solder has the same height on each electrode / pad, so there is no tilt.

  Because the thermal pads 84 and 86 are not blocked by their outer two upper electrodes, heat is more efficiently diffused in the circuit board, and the circuit board extends the thermal pads sufficiently beyond the footprint of the submount It can be used.

  Because the electrodes 80 and 82 are close to the center of the submount, the thermal stress on the solder joints is reduced and the potential for cracking or peeling is reduced.

  FIG. 7 shows another embodiment of metal electrode and thermal pad configurations on the bottom surface of the submount or LED die. The cathode electrode 90, the anode electrode 92, the thermal pad 96 and the thermal pad 98 are triangular and located in different quadrants. The circuit board has a corresponding metal pattern. Since all the electrodes / pads have the same dimensions, the molten solder has the same height on each electrode / pad, so there is no tilt.

  Because the thermal pads 96 and 98 are not blocked by the electrodes outside of them, heat is more efficiently diffused into the circuit board, and the circuit board sufficiently exceeds the footprint of the submount An extended thermal pad can be used.

  Because the electrodes 90 and 92 are close to the center of the submount, the thermal stress on the solder joints is reduced and the potential for cracking or peeling is reduced.

  Other metal pattern configurations are also possible using the guidelines described above in connection with FIGS. 4-7. For example, the thermal pad may be three or more, and symmetrically surround the electrode near the center of the submount. The thermal pad can be of any shape.

  As understood from the above, the thermal resistance between the LED and the circuit board is optimized (i.e. at the system level), the solder reliability is improved, and the SMT processing yield is improved (i.e. bonding to the circuit board) Have made such an LED metal pad configuration possible.

  Either structure can be referred to as an LED module, as the metal pattern is useful for either the LED die or its submount as long as it can be directly attached to the surface of a circuit board or other substrate. The LED die may be a laser diode die or may be a non-laser die.

  FIG. 8 shows a submount 102 having a bottom metal pattern that matches any of the metal patterns described above for bonding to a circuit board. A plurality of dies 104 and 106 representing any number of dies can be interconnected by a trace pattern on top of submount 102. The heat generated by the dies 104 and 106 is combined and transferred to the circuit board through thermal pads on the bottom of the submount. In one embodiment, any number of electrodes for dies 104 and 106 may be disposed between the thermal pads on the bottom of submount 102 consistent with the concept of FIG. In another embodiment, it may be distributed with the thermal pad in a manner as shown in FIGS. 6 and 7.

  Placing all the electrodes near the center of the submount and placing thermal pads on both sides of the electrodes or around all the electrodes reduces the thermal (CTE) stress on the electrodes (because the electrodes are near the center) , Increase the reliability of solder bonds and reduce the possibility of tilting. With larger submounts, such as supporting multiple LED dies (or other heat generating dies), the thermal stress problem is increased, and the invention is particularly beneficial for larger submounts.

  The metal pattern design is useful for dies and / or submounts that generate heat and are not limited to LEDs. For example, power transistors and other high heat producing dies can benefit from the present invention.

  While particular embodiments of the present invention have been shown and described, it would be obvious to those skilled in the art that changes and modifications can be made without departing from the scope of the invention and, therefore, the appended claims are Cover all changes and modifications.

Claims (15)

A semiconductor diode module having a semiconductor light emitting layer (12) having at least a p-type layer and an n-type layer, light is emitted through the first surface of the semiconductor light emitting layer, and the second surface of the module is on the substrate (64). Have a metal pattern to connect to the metal pads (66, 68, 70) of the
The metal pattern is:
A cathode electrode (50) electrically connected to the n-type layer;
An anode electrode (52) electrically connected to the p-type layer; and two thermal pads (54, 56) insulated from the cathode electrode and the anode electrode, the second surface of the module A thermal pad positioned along both sides of the and having substantially the same shape;
Have
module. A module according to claim 1, further comprising an LED die (10) mounted to the submount (58).
The second surface of the module is the bottom surface of the submount
module. A module according to claim 1, further comprising an LED die (10).
The second surface of the module is the bottom surface of the LED die,
module. The module according to claim 1, wherein the substrate (64) is a circuit substrate.
module. The module according to claim 1,
The cathode electrode (50) and the anode electrode (52) are located along the centerline of the second surface, and the thermal pads (54, 56) are located on both sides of the centerline, A cathode electrode and the anode electrode are between the thermal pad and the centerline,
module. The module according to claim 1,
The thermal pad (84, 86) is rectangular and located at opposing corners of the second surface, and the cathode electrode (80) and the anode electrode (82) are rectangular; Located at opposing corners,
module. The module according to claim 1,
The thermal pad (96, 98) is triangular and is located on the opposite side of the second surface, and the cathode electrode (90) and the anode electrode (92) are triangular; Located on the opposing side,
module. An electronic device comprising an electrical device having at least a first terminal (14) and a second terminal (16), the surface of the electronic device being connected to a metal pad (66, 68, 70) on a substrate (64) Have a metal pattern for
The metal pattern is:
A first electrode (50) electrically connected to the first terminal;
A second electrode (52) electrically connected to the second terminal; and at least two thermal pads (54, 56) insulated from the first electrode and the second electrode, the metal pattern A thermal pad positioned along both sides of the surface of the electrical device having substantially the same shape;
Have
Electronic device. An electronic device according to claim 8;
Said electrical device comprises a semiconductor die (10) mounted on a submount (58),
The surface of the electrical device is the surface of the submount,
Electronic device. An electronic device according to claim 8;
The electrical device comprises a semiconductor die (10)
The surface of the electrical device is a bottom surface of the semiconductor die.
Electronic device. An electronic device according to claim 8;
The electrical device comprises a plurality of dies (10) mounted to a submount (58);
The surface of the electrical device is the bottom surface of the submount,
Electronic device. An electronic device according to claim 8;
The first electrode (50) and the second electrode (52) are located along the centerline of the surface, and the thermal pads (54, 56) are located on both sides of the centerline, A first electrode and the second electrode are between a thermal pad and the centerline,
Electronic device. An electronic device according to claim 8;
The thermal pad (54, 56) is larger than the first electrode (50) and the second electrode (52),
Electronic device. An electronic device according to claim 8;
The thermal pad (84, 86) is rectangular and located at opposing corners of the surface, and the first electrode (80) and the second electrode (82) are rectangular, opposing the surface Located in the corner,
Electronic device. An electronic device according to claim 8;
The thermal pad (96, 98) is triangular and is located on opposite sides of the surface, and the first electrode (90) and the second electrode (92) are triangular and opposite of the surface Located on the side,
Electronic device.

Images