Double-sided cooling package encapsulation process

Abstract

To provide an encapsulation process for double-sided cooled packages.SOLUTION: One or more electronic devices mounted on a substrate, including at least one cooling plate in contact with the one or more electronic devices, are encapsulated. The substrate is clamped between a first mold half and a second mold half which define a molding cavity for molding the one or more electronic devices. A cavity insert movably located in the first mold half is projected into the cavity in order to contact and apply a sealing pressure onto the at least one cooling plate. After introducing a molding compound into the cavity at a first fill pressure, the molding compound in the cavity is packed by applying a second fill pressure, which is higher than the first fill pressure. During this time, the sealing pressure is maintained at values that are higher than the first fill pressure and the second fill pressure.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to encapsulation of electronic devices, and more particularly to encapsulation of electronic packages that facilitates cooling on both sides of the electronic package. [Background technology]

[0002] The hybrid electric vehicle market has been booming in recent years, leading to the exponential growth seen in the automotive industry. As a result, a wider range of vehicles are being electrified, which in turn increases the demand for the production of power semiconductor modules used in inverters. The Double-Sided Cooled package, commonly referred to as "DSC", offers improved performance with higher power density, scalability and space advantages.

[0003] The DSC package may include a direct bonded copper (DBC) insulating ceramic substrate made from aluminum oxide. This is used as a large metal spacer to provide mechanical support and better heat dissipation from electronic components such as insulated gate bipolar transistors ("IGBTs") and diodes. One such DSC package known in the industry has a blocking voltage capability of 750V, and a continuous collector current of 800A. The maximum continuous junction temperature of this DSC package may be about 175°C. This is possible due to the fact that the DSC package uses a hard epoxy molding compound and does not include wire bonds, rather than having a traditional silicone gel filled design. Cooling heat sinks, such as DBC substrates, are placed on both sides of such DSC packages. These double-sided heat sinks are constructed with pin fin structures on the top and bottom surfaces of the DSC package, which greatly improves the thermal performance of the DSC package. The DSC package is a cost-effective solution since better thermal performance can be achieved with less silicon.

[0004] DSC packages must be molded using different encapsulation processes. Traditionally, for DSC packages in the market, overmolding process is a common encapsulation approach. With this approach, the upper cooling plate of the DSC package is covered by molding compound while the lower cooling plate is exposed during molding. However, this approach requires additional grinding and polishing process to expose the upper cooling plate after the overmolding.

[0005] 1 is a cross-sectional view of a conventional molded DSC package 100. The molded DSC package 100 is encapsulated with a molding compound using a molding system that includes an upper die 102 and a lower die 104. The upper die 102 and the lower die 104 cooperate to clamp the substrate 106 such that an electronic device 108 mounted on the substrate 106 is disposed within a cavity formed in the upper die 102 and the lower die 104. On the top and bottom surfaces of the electronic device 108 are an upper cooling plate 110 and a lower cooling plate 112, respectively. Each cooling plate 110, 112 may comprise a DBC insulating ceramic substrate.

[0006] The molding compound 114 is introduced into the cavities of the upper die 102 and the lower die 104 to encapsulate the electronic device 108 and the upper and lower cooling plates 110, 112. Although FIG. 1 shows the lower cooling plate 112 to be relatively flush with the cavity surface of the lower die 104, there is an air gap between the upper cooling plate 110 and the cavity surface of the upper die 102, and thus excess molding compound 114 is trapped between the upper cooling plate 110 and the cavity surface of the upper die 102. To ensure the effectiveness of the upper cooling plate 110 in removing heat from the molded DSC package 100, such excess molding compound 114 molded on the top surface of the upper cooling plate 110 must be removed.

[0007] 2 is a flow chart showing a conventional process flow for manufacturing a DSC package. A molding process 120 is first performed to encapsulate the electronic device 108 on the substrate 106. Then, a post-molding cure 122 is performed to ensure sufficient setting and solidification of the molding compound 114. A top surface grinding 124 must be performed on the molded DSC package 100 to remove excess molding compound 114 from the top surface of the upper cooling plate 110, and then a top surface polishing 126 must be performed to improve the surface quality of the molded DSC package 100.

[0008] If there is also excess molding compound 114 on the underside of the lower cooling plate 112, then underside grinding 128 and underside polishing 130 may also be required. To complete the packaging process, each molded DSC package 100 disposed on a substrate 106 containing a plurality of molded DSC packages 100 must be separated by trimming, forming or singulating 132 each of said molded DSC packages 100.

[0009] Several drawbacks are encountered from the above-mentioned overmolding approach. Typically, there is extra mold flash after overmolding, which leads to an increased risk of unreliability due to crack propagation in the molded DSC package. The additional processing as mentioned above results in longer production cycle times and higher costs, such as production, labor, and equipment costs. Furthermore, the package thickness tends to fall outside the acceptable specifications, requiring additional resources to rework it. Thus, the yield rate is reduced as there are more cases of quality rejection.

[0010] It would be advantageous to provide a process for encapsulating a DSC package that avoids at least some of the shortcomings of the prior art discussed above. Summary of the Invention [Problem to be solved by the invention]

[0011] It is therefore an object of the present invention to seek to provide an encapsulation process that ensures robust burr control during the encapsulation process to avoid the risk of defects in the DSC package, and to simplify the process flow by removing the need for subsequent grinding and polishing after the encapsulation process. [Means for solving the problem]

[0012] According to a first aspect of the present invention, there is provided a method of encapsulating one or more electronic devices mounted on a substrate including at least one cooling plate in contact with the one or more electronic devices, the method comprising the steps of: positioning the substrate between first and second mold halves defining a mold cavity for molding the one or more electronic devices; clamping the substrate between the first and second mold halves; projecting a cavity insert movably disposed in the first mold half into the cavity to contact and apply a sealing pressure to the at least one cooling plate; introducing a mold compound into the cavity at a first filling pressure; and thereafter packing the mold compound into the cavity by applying a second filling pressure higher than the first filling pressure, the sealing pressure being maintained at a value higher than the first and second filling pressures during each of the introduction and packing of the mold compound.

[0013] According to a second aspect of the present invention, there is provided a method of manufacturing an electronic package including one or more electronic devices mounted on a substrate including at least one cooling plate in contact with the one or more electronic devices, the method comprising the steps of: positioning the substrate between first and second mold halves defining a mold cavity for molding the one or more electronic devices; clamping the substrate between the first and second mold halves; projecting a cavity insert movably disposed in the first mold half into the cavity to contact and apply a sealing pressure to the at least one cooling plate; encapsulating the one or more electronic devices by introducing a molding compound into the cavity at a first filling pressure; and thereafter packing the molding compound into the cavity by applying a second filling pressure higher than the first filling pressure, the sealing pressure being maintained at a value higher than the first filling pressure and the second filling pressure during each of the introduction and packing of the molding compound.

[0014] It will be convenient hereinafter to describe the invention in more detail by reference to the accompanying drawings, which show preferred embodiments of the invention, the particularities of which and the associated description should not be understood as taking away from the generality of the broad character of the invention defined in the claims.

[0015] An example of an encapsulation process according to the present invention will now be described with reference to the accompanying drawings. [Brief description of the drawings]

[0016] [Figure 1] FIG. 1 is a cross-sectional view of a conventional molded DSC package. [Diagram 2] 1 is a flowchart illustrating a conventional process flow for manufacturing a DSC package. [Diagram 3] FIG. 2 is a cross-sectional view of a molding apparatus in an open position for enclosing a DSC package according to a preferred embodiment of the present invention. [Figure 4]4 is a cross-sectional view of the molding apparatus of FIG. 3, with the molding apparatus in a closed position. [Diagram 5] 4 is a graph of pressure over time during various stages of the encapsulation process in accordance with a preferred embodiment of the present invention. [Figure 6] FIG. 13 is a side view of the upper cavity insert applying a biasing force to the upper cooling plate during molding. [Figure 7] FIG. 1 is a plan view of the areas subjected to air pressure and packing pressure on the upper cooling plate. [Figure 8] FIG. 2 is an exploded view of a sealing mechanism used in a molding apparatus according to a preferred embodiment of the present invention. [Figure 9] FIG. 13 is a side view of the sealing mechanism when air pressure is applied to the top cavity insert. [Figure 10] FIG. 9 is a side view of an extrusion gap sealed by the sealing mechanism of FIG. 8. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] 3 is a cross-sectional view of a molding apparatus 18 in an open position for encapsulation of a DSC package according to a preferred embodiment of the present invention. The DSC package comprises a substrate 10, one or more electronic devices 12 mounted on the substrate 10, a first or upper cooling plate 14 positioned above and in contact with the electronic devices 12 mounted on the top surface of the substrate 10, and a second or lower cooling plate 16 positioned below and in contact with the electronic devices 12 mounted on the bottom surface of the substrate 10.

[0018] The substrate 10, including the electronic device 12 and the upper and lower cooling plates 14, 16, is placed in a molding apparatus 18 having an upper mold half or die 20 and a lower mold half or die 22. The upper and lower dies 20, 22 define a molding cavity 23 for encapsulating the electronic device 12. A first cavity insert, such as an upper cavity insert 26, is centrally positioned and movably disposed within the upper die 20 to apply a biasing or sealing force to the upper cooling plate 14. Also, a second cavity insert, such as a lower cavity insert 28, is centrally positioned and movably disposed within the lower die 22 to apply a corresponding biasing or sealing force to the lower cooling plate 16. A flexible protective film 24 is also shown separating the upper die 20 and the upper cooling plate 14. The lower die 22 and the lower cooling plate 16 may be separated by a similar flexible protective film.

[0019] 4 is a cross-sectional view of the molding apparatus 18 of FIG. 3 with the molding apparatus 18 in a closed position. In this position, the substrate 10 is clamped between the upper die 20 and the lower die 22. The upper cavity insert 26 is extended into the molding cavity to contact and apply an upper biasing force 30 to the upper cooling plate 14, thereby closing the gap between the upper cavity insert 26 and the upper cooling plate 14. If a flexible protective film 24 is used to separate the upper die 20 and the upper cooling plate 14, the protective film 24 acts to contact the upper cooling plate 14 and form a sealing arrangement around the top surface of the upper cooling plate 14 when the upper biasing force 30 is applied by the upper cavity insert 26.

[0020] Correspondingly, the lower cavity insert 28 is protruded into the mold cavity 23 to contact and apply a lower biasing pressure or force 32 to the lower cooling plate 16 to close the gap between the lower cooling plate 16 and the lower die 22. The lower cavity insert 28 likewise applies a sealing pressure to the lower cooling plate 16. Thus, the mold compound cannot also flow into the interface between the lower cooling plate 16 and the lower die 22.

[0021] While the upper and lower cavity inserts 26, 28 apply upper and lower biasing forces 30, 32, respectively, to the upper and lower cooling plates 14, 16, molding compound is introduced into the space or void around the electronic device 12, as defined by the mold cavity 23 created by the upper and lower dies 20, 22. The upper biasing force 30 is generated from air pressure acting on the upper cavity insert 26 (see FIG. 6), and the lower biasing force 32 is generated from air pressure that may similarly act on the lower cavity insert 28. Thus, while the space around the electronic device 12 is filled with molding compound, the upper surface of the upper cooling plate 14 and the lower surface of the lower cooling plate 16 are not covered with molding compound, thus ensuring that the outer surfaces of the upper and lower cooling plates 14, 16 are not covered with molding compound, which may impede the heat dissipation of the molded DSC package. Furthermore, the additional steps of grinding and polishing the upper and lower surfaces of the molded DSC package are not required, resulting in cost savings.

[0022] 5 is a graph of pressure over time during various stages of the encapsulation process according to a preferred embodiment of the present invention. The pressure-time graph 40 is divided into three stages: a first stage when the mold compound is initially introduced into the molding apparatus 18, a second stage when a packing pressure is applied on the mold compound to ensure that it completely fills the space in the mold cavity, and a third stage when the mold compound is cured under heat and pressure to ultimately produce a molded DSC package. Two lines are plotted on the pressure-time graph 40 showing the encapsulation pressure, which correlate to the air pressure 42 and the packing pressure 44 that are simultaneously exerted in the molding apparatus 18.

[0023] During the first stage of the encapsulation process when the mold compound is introduced into the mold cavity 23, a first fill pressure 44a is applied to the mold compound as it is introduced into the mold cavity, and the first fill pressure 44a may be maintained at a substantially constant value. During this stage, the first air pressure 42a is set to be maintained at a value slightly higher than the first fill pressure 44a. In FIG. 5, the first air pressure 42a is set at a fixed increment relative to the first fill pressure 44a so that it is also maintained at a first constant value that is substantially constant. However, the first air pressure 42a may vary relative to the first fill pressure 44a, so long as it is consistently maintained at a value higher than the first fill pressure 44a.

[0024] At the start of the second stage, the mold cavity is filled and a higher packing pressure must be applied to the mold compound. Therefore, the second packing pressure 44b is steadily increased to apply more pressure to the mold compound to fill all of the spaces or voids in the mold cavity 23. While the second packing pressure 44b is steadily increased, the second air pressure 42b should also be correspondingly steadily increased so that it is always higher than the second packing pressure 44b. It should be understood that the difference between the second air pressure 42b and the second packing pressure 44b need not remain constant and may vary. Thus, the difference between the second air pressure 42b and the second packing pressure 44b may begin to narrow as shown in FIG. 5. As shown in the graph, the first sealing pressure or air pressure 42a falls below the second sealing pressure or air pressure 42b.

[0025] After the fill pressure 44 reaches a predetermined maximum, a maximum third fill pressure 44c is maintained in the third stage of the process as the molding compound is cured. At the same time, heat is applied so that the molding compound is cured and solidified. During the third stage, the system continues to ensure that the third air pressure 42c exerted on the cooling plates 14, 16 exceeds the third fill pressure 44c. This difference may vary or remain constant during the course of the third stage. As shown in FIG. 5, the difference between the third air pressure 42c and the third fill pressure 44c may be maintained at a second constant value throughout the third stage, the second constant value being higher than the first constant value. Once the molding compound is cured, the encapsulation of the DSC package is complete and the molding apparatus 18 may be opened for removal of the molded DSC package.

[0026] 6 is a side view of the upper cavity insert 26 exerting a biasing force on the upper cooling plate 14, such as a DBC substrate, during molding. A diaphragm 52 can be seen separating the upper cavity insert 26 from an air pressure chamber 50, which contains a gas for exerting a downward air pressure 51 on the diaphragm 52, and thus the upper cavity insert 26, so as to exert an upper biasing force 30 on the upper cooling plate 14. Although not shown in the drawings, it will be understood that the lower cavity insert 28 must also include a diaphragm disposed between the lower cavity insert 28 and the lower air pressure chamber, which similarly contains a gas for exerting an upward air pressure on the diaphragm and the lower cavity insert 28.

[0027] The upper cavity insert 26 is in direct contact with the upper cooling plate 14, which in turn contacts electronic devices such as diode dies 56 and transistor dies 58 that rest on the substrate 10. Additionally, there may be standoffs 54 that act as spacers between the upper cooling plate 14 and the substrate 10.

[0028] 7 is a plan view of the areas where air and packing pressures are exerted on the DBC device. It will be appreciated that the sealing or air pressure 42 is exerted on the areas where the biasing force is applied by the upper cooling plate 14, such as the posts 54, the diode die 56 and the transistor die 58. Meanwhile, there are spaces 62 where the mold compound is filled, and these are the areas where the packing pressure in the form of an increased filling pressure 44 is exerted on the mold compound to fill the entire space 62.

[0029] 8 is an exploded view of a sealing mechanism used in a molding apparatus according to a preferred embodiment of the present invention. The upper cavity insert 26 is placed on the cavity block 70 and can be inserted into a through hole formed in the cavity back plate 72. A resilient seal, which may be in the form of an O-ring 78 or a silicone seal, is positioned to surround the upper cavity insert 26 and rest on an inner wedge ring 74 that surrounds the O-ring 78 and has an inclined surface. The inner wedge ring 74 can slide against the inclined surface of an outer wedge ring 76, which surrounds the inner wedge ring 74. The inner wedge ring 74 and the outer wedge ring 76 cooperate to enhance the sealing effect of the O-ring 78. The O-ring 78, the inner wedge ring 74 and the outer wedge ring 76 are received in grooves in an airbag block 80.

[0030] 9 is a side view of the sealing mechanism when air pressure is applied to the top cavity insert 26. When air pressure is exerted on the top cavity insert 26, the high air pressure emanating from the direction of the top cavity insert 26 presses the O-ring 78 against the inner wedge ring 74. When the high air pressure presses the O-ring 78 against the inner wedge ring 74, the inclined surface of the inner wedge ring 74 slides against the opposing inclined surface of the outer wedge ring 76.

[0031] Figure 10 is a side view of the extrusion gap being sealed by the sealing mechanism of Figure 8. As the inner wedge ring 74 slides upward against the outer wedge ring 76, the top of the inner wedge ring 74 closes the extrusion gap 82 that exists between the inner wedge ring 76 and the cavity backplate 72 (and thus between the O-ring 78 and the outside environment), completely sealing the extrusion gap 82. This sealing effect prevents gas from leaking through the extrusion gap 82 to the outside environment and ensures that the air pressure 42 is not reduced by unwanted gas leakage.

[0032] It should be appreciated that the DSC package encapsulation process flow as described above applies multiple stages of predetermined air pressure 42 and mold compound filling pressure 44 at predetermined points in the mold cavity filling process, thereby ensuring a balance between mold flow and the movement of the movable cavity insert mechanism. The process helps maintain robust flash control and eliminates the risk of crack propagation in the DSC package. Additionally, the process flow is simplified as there is no need for additional grinding and polishing processes after encapsulation of the DSC package.

[0033] It should be understood that the invention described herein is susceptible to variations, modifications and / or additions other than those specifically described, and the invention includes all such variations, modifications and / or additions, which fall within the spirit and scope of the invention as described above. [Explanation of symbols]

[0034] 10 Substrate 12 Electronic Devices 14 Upper cooling plate 16 Lower cooling plate 18 Molding equipment 20 Upper Die 22 Lower Die 23 Molding Cavity 24 Protective film 26 Upper cavity insert 28 Lower Cavity Insert 30 Upper biasing force 32 Lower Forces 40 Pressure-Time Graph 42 Air pressure 42a First Air Pressure 42b Secondary air pressure 42c 3rd Air Pressure 44 Filling Pressure 44a First filling pressure 44b Secondary filling pressure 44c Third filling pressure 50 Air Pressure Chamber 51 Downward Air Pressure 52 Bulkhead 54 Post 56 Diode Die 58 Transistor Die 62 Space 70 Cavity Block 72 Cavity Backplate 74 Inner wedge ring 76 Outer wedge ring 78 O-ring 80 Airbag Block 82 Extrusion Gap

Claims

1. 1. A method of encapsulating one or more electronic devices mounted on a substrate, comprising at least one cooling plate in contact with the one or more electronic devices, the method comprising: placing the substrate between first and second mold halves defining a mold cavity for molding the one or more electronic devices; clamping the substrate between the first mold half and the second mold half; protruding a cavity insert movably disposed within the first mold half into the mold cavity to contact and apply a sealing pressure to the at least one cooling plate, the sealing pressure including a first sealing pressure and a second sealing pressure higher than the first sealing pressure; introducing a mold compound into the mold cavity at a first packing pressure; After that, packing the mold compound into the mold cavity by applying a second packing pressure greater than the first packing pressure; Including, the first sealing pressure is applied and maintained at a value higher than the first filling pressure during introduction of the mold compound, and the second sealing pressure is applied and maintained at a value higher than the second filling pressure during packing of the mold compound. method.

2. a first cooling plate disposed on a first side of the substrate and a second cooling plate disposed on a second side of the substrate opposite the first side; a first cavity insert is movably positioned within the first mold half and a second cavity insert is movably positioned within the second mold half; protruding the first and second cavity inserts into the mold cavity to apply a sealing pressure to the first and second cooling plates, respectively; The method of claim 1.

3. 2. The method of claim 1, further comprising: a flexible protective film separating the first mold half and the at least one cooling plate, the flexible protective film acting to contact the cooling plate to form a sealing configuration on the surface of the at least one cooling plate when the cavity insert applies a sealing pressure to the surface of the at least one cooling plate.

4. 2. The method of claim 1, comprising exerting a first sealing pressure when introducing the mold compound into the mold cavity and exerting a second sealing pressure when packing the mold compound into the mold cavity, wherein the first sealing pressure is less than the second sealing pressure.

5. 5. The method of claim 4, further comprising the step of steadily increasing the second fill pressure while steadily increasing the sealing pressure so as to always maintain the sealing pressure at a value higher than the second fill pressure when packing the mold compound into the mold cavity.

6. 5. The method of claim 4, further comprising: maintaining a third sealing pressure, higher than a third filling pressure, on the at least one cooling plate while curing the mold compound while the mold compound is heated at the third filling pressure after packing the mold compound.

7. 7. The method of claim 6, wherein the first sealing pressure is maintained at a first constant value during introduction of the mold compound into the mold cavity, and the third sealing pressure is maintained at a second constant value during curing of the mold compound, the second constant value being higher than the first constant value.

8. The method of claim 1 , wherein applying the sealing pressure further comprises generating air pressure on the cavity insert such that the cavity insert contacts and applies the sealing pressure to the at least one cooling plate.

9. 9. The method of claim 8, wherein the air pressure is maintained at a value greater than the first and second fill pressures during each of the introduction and packing of the mold compound.

10. The method of claim 8 , wherein the air pressure is generated by the air pressure chamber exerting air pressure on a bulkhead separating the cavity insert and the air pressure chamber.

11. The method of claim 8 , wherein the cavity insert is disposed on a cavity block, the cavity insert being insertable into a through hole formed in a cavity backplate.

12. 12. The method of claim 11, further comprising a resilient seal surrounding the cavity insert, the resilient seal being positioned to rest against an inner wedge ring having an inclined surface surrounding the resilient seal, the inner wedge ring being slidable against an inclined surface of an outer wedge ring surrounding the inner wedge ring.

13. The method of claim 12 , wherein the elastomeric seal, the inner wedge ring, and the outer wedge ring are received within grooves in an airbag block.

14. 13. The method of claim 12, wherein when the air pressure is applied to the cavity insert, the resilient seal is forced against the inner wedge ring by the air pressure, causing the angled surface of the inner wedge ring to slide against the angled surface of the outer wedge ring.

15. 15. The method of claim 14, wherein the sliding of the inner wedge ring relative to the outer wedge ring closes an extrusion gap that exists between the elastomeric seal and an external environment, thereby preventing gas from leaking through the extrusion gap to the external environment.

16. 1. A method of manufacturing an electronic package including one or more electronic devices mounted on a substrate including at least one cooling plate in contact with the one or more electronic devices, comprising: placing the substrate between first and second mold halves defining a mold cavity for molding the one or more electronic devices; clamping the substrate between the first mold half and the second mold half; protruding a cavity insert movably disposed within the first mold half into the mold cavity to contact and apply a sealing pressure to the at least one cooling plate, the sealing pressure including a first sealing pressure and a second sealing pressure higher than the first sealing pressure; encapsulating the one or more electronic devices by introducing a molding compound into the mold cavity at a first filling pressure; and thereafter packing the mold compound into the mold cavity by applying a second packing pressure greater than the first packing pressure; Including, the first sealing pressure is applied and maintained at a value higher than the first filling pressure during introduction of the mold compound, and the second sealing pressure is applied and maintained at a value higher than the second filling pressure during packing of the mold compound. method.

Images