Method for forming hermetic package for a power semiconductor

Abstract

A method for fabricating a hermetic electronic package includes providing a package body; hermetically coupling a package base plate to the package body; thermally coupling a substrate to the base plate; thermally mounting a semiconductor device to the substrate; bonding at least one high-current input/output (I/O) terminal to the first metalized region of the substrate by a strap terminal that is an integral high current heatsink terminal. A ceramic seal surrounding the at least one high-current I/O terminal is hermetically bonded to an outer surface of the package body. A metal hermetic seal washer surrounding the at least one high-current I/O terminal is hermetically bonded to the ceramic seal and to a portion of the at least one high-current I/O terminal. A lid is seam welded onto the package body.

Claims

1. A method of fabricating a hermetic electronic package comprising: providing a package body; hermetically coupling a package base plate to a first end of the package body; thermally coupling a substrate to the base plate, the substrate having a plurality of metalized regions; mounting a semiconductor device to the substrate, the semiconductor device having at least one high-current output electrically bonded to a first metalized region of the substrate; bonding at least one high-current input/output (I/O) terminal to the first metalized region of the substrate by a strap terminal that is an integral high current heatsink terminal, the at least one high-current I/O terminal passing through a respective hole formed in the package body; hermetically bonding to an outer surface of the package body a ceramic seal surrounding the at least one high-current I/O terminal by brazing a first surface of the ceramic seal to the outer surface of the package body; hermetically bonding a metal hermetic seal washer surrounding the at least one high-current I/O terminal to the ceramic seal and to a portion of the at least one high-current I/O terminal that passes through the metal hermetic seal washer by brazing the metal hermetic seal washer to a second surface of the ceramic seal and to the portion of the at least one high-current I/O terminal that passes through the metal hermetic seal washer; and seam welding a lid onto the package body at a second end opposite the first end to form a hermetic seal with the package body.

2. The method of claim 1 wherein the ceramic seal is formed from one of alumina and silicon nitride and the brazing the first surface of the ceramic seal to the outer surface of the package body uses a CuAg braze.

3. The method of claim 1 wherein the brazing the metal hermetic seal washer to the second surface of the ceramic seal and to the portion of the at least one high-current I/O terminal that passes through the metal hermetic seal washer uses a CuAg braze.

Description

BRIEF DESCRIPTION OF THE DRAWING FIGURES

(1) The invention will be explained in more detail in the following with reference to embodiments and to the drawing in which are shown:

(2) FIG. 1 is a top cross-sectional view of a representative prior-art semiconductor package;

(3) FIG. 2 is a magnified side cross-sectional view of a portion of the prior-art semiconductor package of FIG. 1 taken through lines 2-2 of FIG. 1;

(4) FIG. 3 is a magnified side cross-sectional view of the prior-art semiconductor package of FIG. 2 showing a typical hermetic seal failure mode;

(5) FIG. 4 is a top cross-sectional view of a representative semiconductor package in accordance with the present invention;

(6) FIG. 5 is a magnified side cross-sectional view of a portion of the semiconductor package of FIG. 4 taken through lines 5-5 of FIG. 4;

(7) FIG. 6 is a flow diagram illustrating a method for fabricating the semiconductor package of the present invention in accordance with an aspect of the invention; and

(8) FIG. 7 is a flow diagram shows an illustrative method for brazing hermetic seals to the outer wall of the package body and to the I/O terminals.

DETAILED DESCRIPTION

(9) Persons of ordinary skill in the art will realize that the following description is illustrative only and not in any way limiting. Other embodiments will readily suggest themselves to such skilled persons.

(10) Referring to FIGS. 4 and 5, a top cross-sectional view shows a representative semiconductor package 50 in accordance with the present invention and a magnified side cross-sectional view shows a portion of the semiconductor package of FIG. 4 taken through lines 5-5 of FIG. 4. The semiconductor package 50 of FIGS. 4 and 5 include some of the same elements of the prior-art semiconductor package depicted in FIGS. 1 and 2. Those elements will be identified in FIGS. 4 and 5 using the same reference numerals that were used to identify the corresponding elements in FIGS. 1 and 2.

(11) The semiconductor package 50 of FIGS. 4 and 5 includes a package body 10 shown in cross section in FIG. 4 that is formed from a material such as Kovar, Alloy 42, Alloy 46, or Alloy 52. A metallized substrate 12 is thermally and mechanically bonded by preform soldering it to an inner surface of a base plate 14 which is formed from a material such as a copper tungsten alloy, molybdenum, a Cu/Mo/Cu composite, or AlSiC and the base plate 14 is brazed to the package body 10 to hermetically bond and seal it to the base plate. A semiconductor die 16, in the case of a power MOSFET illustrated in FIGS. 4 and 5, has its drain connection on the bottom side of the die electrically connected to the metallized substrate 12 by preform soldering it to the metallized substrate 12. In one illustrative non-limiting embodiment of the invention, a 95Pb/5In solder may be employed.

(12) Where a MOSFET device formed on the semiconductor die 16 is to be placed in the package 10, the metallization on the metallized substrate 12 is formed into three conductive regions 52, 54, and 56, that serve, respectively, as connection locations for source, drain, and gate terminals of the MOSFET device that is formed on the semiconductor die 16. The drain of the MOSFET device is electrically connected to the bottom side of the semiconductor die 16 which is bonded to the conductive region 54 of the metallized substrate 12 to provide both electrical and thermal connection between the semiconductor die 16 and the metallized substrate 12. The source of the MOSFET device is electrically connected to the top side of the semiconductor die 16 and is electrically connected to the conductive region 52 of the metallized substrate 12 by a plurality of bonding wires (indicated by the dashed oval lines 58) stitched between the conductive region 52 of the metallized substrate 12 and source connection points on the top side of the semiconductor die 16 in order to provide the current carrying capacity needed for the high power MOSFET device formed on the semiconductor die 16.

(13) The gate of the MOSFET device is a geometrically distributed gate as is known in the art and is shown connected to the conductive region 56 of the metallized substrate 12 by bonding wires (indicated by the dashed oval lines 60).

(14) The source and drain connections from the conductive regions 52 and 54 of the metallized substrate 12 are connected to the high-current I/O terminals 22 and 28, respectively, by strap terminal conductors 62 and 64. The strap terminal conductors 62 and 64 also serve as thermal conductors providing integral high current heatsink terminals to help in transferring heat from the high power MOSFET device to the outer of the package 50 and may be formed from a material such as copper, or copper zirconium and Beryllium copper (BeCu).

(15) The conductive region 56 of the metallized substrate 12 is connected to a gate I/O terminal 66 by a bonding wire 68. A Kelvin sense wire 70 is connected to a Kelvin current sense I/O terminal 72.

(16) Each of the high-current I/O terminals 22 and 28 and the gate and Kelvin current sense I/O terminals 66 and 72 communicates with the outside of the package body 10 through hermetic seals 74 that are bonded to an outer wall of the package body 10. The hole in which the high-current I/O terminal 22 is disposed is identified in FIG. 4 and FIG. 5 by reference numeral 38a. The hole in which the high-current I/O terminal 28 is disposed is identified in FIG. 4 by reference numeral 38b. The holes in which the gate and Kelvin current sense I/O terminals 66 and 72 are disposed are identified in FIG. 4 by reference numerals 38c and 38d, respectively. A lid 40 shown in FIG. 5 is seam welded onto the package body 10 to bond and hermetically seal it to the package body 10.

(17) The hermetic seals 74 through which the high-current I/O terminals 22 and 28 and the gate and Kelvin current sense I/O terminals 66 and 72 are designed to match the high coefficient of thermal expansion (CTE) of the I/O terminals and internal terminals to the CTE of the package body 10 and are multi-part structures. Ceramic seals 76, which form a portion of hermetic seals 74 through which the terminals 22, 28, 66, and 72 pass, are formed from a material such as alumina or silicon nitride and a first surface of ceramic seals 76 are each attached to the outer wall of the package body 10 by a hermetic seal joint in the form of a high-temperature braze 78 of a material such as CuAg. A metal hermetic seal washer 80 formed from a material such as Kovar, Alloy 60, Alloy 46, or Alloy 52 is attached to a second surface of the ceramic seal 76 and to the I/O terminal which passes through it by a hermetic seal joint 82 in the form of a high-temperature braze of a material such as CuAg. Because of the smaller diameters of the I/O terminals 66 and 72, the maximum thermal stress occurs in the high temperature braze 78 between the ceramic seal 76 and the outer wall of the package body 10 in the smaller I/O terminals 66 and 72.

(18) In one instance of the present invention where the high-current I/O terminals 22 and 28 are formed to a diameter of 0.125″, the ceramic seal 76 may have a diameter of about 0.380″ and a thickness of about 0.050″, the thickness of the high temperature braze joint 78 may be about 0.010″, and the metal hermetic seal washer 80 may have a diameter of about 0.250″ and a thickness of about 0.020″. In one instance of the present invention where the signal I/O terminals 22 and 28 are formed to a diameter of 0.040″, the ceramic seal 76 may have a diameter of about 0.200″ and a thickness of about 0.050″, the thickness of the high temperature braze joint 78 may be about 0.010″, and the metal hermetic seal washer 80 may have a diameter of about 0.150″ and a thickness of about 021″.

(19) Referring now to FIG. 6, a flow diagram shows an illustrative method 90 for fabricating the semiconductor package of the present invention in accordance with an aspect of the invention. The method begins at reference numeral 92.

(20) At reference numeral 94 the base plate is brazed to the package body. At reference numeral 96 the various I/O terminals are positioned in their respective holes in the wall of the package body. At reference numeral 98 the hermetic seals are brazed to the outer wall of the package body and to the I/O terminals. All of the aforementioned processes can be performed at a package fabrication facility.

(21) At reference numeral 100, the metallized substrate is bonded to the package base plate by a process such as preform soldering. At reference numeral 102, semiconductor die is bonded to the metallized substrate by a process such as preform soldering.

(22) At reference numeral 104 the top bonding wires are bonded to connection pads on the top surface of the semiconductor die and to a respective area of the metalized substrate. At reference numeral 106, the I/O terminals are bonded to the metallized substrate. At reference numeral 108, the lid is seam welded to the package body to hermetically seal the package. The method ends at reference numeral 110.

(23) Referring now to FIG. 7, a flow diagram shows an illustrative method 120 for performing the processes shown at reference numeral 98 of FIG. 6. The method begins at reference numeral 122. At reference numeral 124 the ceramic seals are positioned around the I/O terminals. At reference numeral 126 the ceramic seals are brazed to the outer wall of the package body. At reference numeral 128 the metal hermetic seal washers are positioned over the ceramic seals and around the I/O terminals. At reference numeral 130 the metal hermetic seal washers are brazed to the I/O terminals and the ceramic seals. The method ends at reference numeral 132.

(24) While embodiments and applications of this invention have been shown and described, it would be apparent to those skilled in the art that many more modifications than mentioned above are possible without departing from the inventive concepts herein. The invention, therefore, is not to be restricted except in the spirit of the appended claims.