Method of manufacturing semiconductor device, corresponding pre-molded lead frame and semiconductor device

Abstract

The invention relates to a method of manufacturing a semiconductor device, a corresponding pre-molded lead frame and a semiconductor device. An electrically insulating material is molded onto an engraved conductive lead frame structure including a pattern of conductive constructions, e.g., configured to have a die mounting location having at least one semiconductor die disposed thereon, dummy pads disposed on the die mounting location, and tie bars disposed on the dummy pads. And a tie bar extends between the die mounting location and the dummy pad. The pre-molded leadframe structure is created by penetration of an electrically insulating material into spaces between the conductive structures in the conductive structure pattern. At least a portion of the tie bar extending between the die mounting location and the dummy pad is removed to electrically decouple the dummy pad from the die mounting location.

Description

[0001] Priority claim

[0002] This application claims the benefit of priority from Italian Patent Application No. 102023000014049 filed on July 5, 2023, the contents of which are incorporated herein by reference in their entirety to the maximum extent permitted by law. Technical Field

[0003] This description relates to fabricating semiconductor devices.

[0004] The solutions described herein may be applied to power (integrated circuit) semiconductor devices, such as power quad flat no-lead (QFN) packages used in, for example, automotive products. Background Art

[0005] Integrated circuit semiconductor devices are typically mounted to a substrate such as a printed circuit board (PCB) by, for example, soldering.

[0006] To increase the reliability of solder joints, dummy pads may be provided at the bottom surface of the device in addition to the leads providing input / output connections to the device.

[0007] In leadframe based packages, dummy pads may be provided at the tie bars, a bridge-like formation providing mechanical support to the die pad during processing. The tie bars have the same conductive material as the leadframe and thus provide undesirable electrical coupling between the dummy pads and the die pad.

[0008] Furthermore, in some cases, the distance between a dummy pad and a lead (eg, an input / output lead) may be relatively short due to, for example, a relatively large number of leads of a device and / or a reduced size of the device.

[0009] When high voltage differences are involved, such as in power devices operating at high voltages, the short distance between the lead and the dummy pad may increase the risk of discharge.

[0010] To the extent that the dummy pad is electrically coupled to the die pad (via the tie bar), such discharges may damage the device, and discharges between the leads and the dummy pad will also affect the die pad (and possibly the die mounted thereon), leading to failure of the device.

[0011] Reference is made to U.S. Patent Application Publication Nos. 2008 / 00157301, 2022 / 00115303, and 2011 / 0244633, which are incorporated herein by reference, as examples of recent advances in semiconductor devices provided with dummy pads / leads.

[0012] There is a need in the art for solutions directed to addressing the aforementioned problems. Summary of the invention

[0013] One or more embodiments are directed to a method.

[0014] One or more embodiments are directed to corresponding integrated circuit semiconductor devices.

[0015] One or more embodiments are directed to pre-molded lead frames for use in such devices.

[0016] The solution described herein may be advantageously applied to manufacturing processes involving pre-molded lead frames.

[0017] The solution described herein helps to provide a pre-molded lead frame with a dummy pad that is electrically decoupled from a die pad.

[0018] In the solution described herein, the electrical coupling between the dummy pad and the die pad is removed.

[0019] In the solution described herein, removing the electrical coupling between the die pad and the dummy pad leaves a groove / recessed portion in the pre-molded leadframe, which can be filled with a molding compound via a second molding step. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] One or more embodiments will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0021] Figure 1 is a perspective view of a semiconductor device having a dummy pad;

[0022] Figure 2 is a plan view showing steps in a manufacturing process of a substrate for a semiconductor device;

[0023] Figure 3A , Figure 3B and Figure 4A , Figure 4B is indicated by arrow III Figure 2 , showing the bottom surface of the substrate of the semiconductor device being processed ( Figure 3A and Figure 4A ) and top surface ( Figure 3B and Figure 4B );

[0024] Figure 5 shows processing steps according to an embodiment of the present specification; and

[0025] Figure 6 and Figure 7 Along Figure 4B The line VI-VI and Figure 5Cross-sectional view along line VII-VII. DETAILED DESCRIPTION

[0026] Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated.

[0027] The drawings are drawn to clearly illustrate the relevant aspects of the embodiments and are not necessarily drawn to scale.

[0028] The edges of a feature drawn in a drawing do not necessarily indicate the end of the extent of the feature.

[0029] In the following description, one or more specific details are shown for the purpose of providing a deeper understanding of the examples of the embodiments of the present description. The embodiments may be obtained without one or more specific details, or with other methods, components, materials, etc. In other cases, well-known structures, materials, or operations are not shown or described in detail so as not to obscure certain aspects of the embodiments.

[0030] References to "one embodiment" or "an embodiment" in the framework of this specification are intended to indicate that a particular configuration, structure, or characteristic described with respect to the embodiment is included in at least one embodiment. Therefore, phrases such as "in an embodiment" or "in an embodiment" that may appear in one or more points of this specification do not necessarily refer to the same embodiment.

[0031] Furthermore, in one or more embodiments, the particular conformations, structures or characteristics may be combined in any suitable manner.

[0032] The headings / reference signs used herein are provided for convenience only and do not limit the scope of protection or the scope of the embodiments.

[0033] For simplicity and ease of explanation, throughout the specification, unless the context otherwise indicates, the same parts or elements are denoted by the same reference numerals in the various drawings, and the corresponding description is not repeated for each drawing.

[0034] Figure 1 There is shown the backside / bottom surface of a power integrated circuit (IC) semiconductor device 10 provided with a package of the type commonly referred to as a quad flat no-lead (QFN) type.

[0035] For example, by soldering the exposed conductive portions on the bottom surface / back side of the plastic (eg, cured epoxy 20), Figure 1 The illustrated device 10 is configured to be mounted on a substrate such as a printed circuit board (PCB).

[0036] As shown, the conductive area may include: a die mounting location (die pad) 12A having an (IC) semiconductor (e.g., silicon) die mounted on a front / top surface (not visible in the figure); leads 12B providing desired input / output (I / O) electrical coupling to the device 10; and a dummy pad 120 providing an additional solder joint to the device 10.

[0037] "Dummy pads" or "dummy leads" are common names in the art for pads / leads that are not used to provide input / output signals to the device. As described above, the dummy pads 120 considered here can have the purpose of enhancing soldering reliability by providing additional solder joints for the device 10 to attach to the PCB.

[0038] Figure 2 The picture is Figure 1 , wherein a plurality of device substrates 12 (commonly referred to as leadframes) are held together in a leadframe strip / reel in order to facilitate simultaneous processing of the plurality of leadframes 12.

[0039] The name "leadframe" (or "leadframe") (see, e.g., the USPC Incorporated Glossary of the U.S. Patent and Trademark Office) refers to a metal frame used to provide support for an integrated circuit chip or die, and the electrical leads that interconnect the integrated circuits in the die or chip to other components or contacts.

[0040] Essentially, the leadframe 12 includes an array of conductive structures (or leads, 12B) extending inwardly from an outline position in the direction of a semiconductor chip or die (not visible in the figure), thereby forming an array of conductive structures from a die pad 12A, which is configured to have at least one (integrated circuit) semiconductor chip or die attached thereto. This can be achieved by conventional means such as a die attach adhesive (e.g., a die attach film (DAF)).

[0041] The leadframe may be of a pre-molded type, ie a leadframe type comprising an embossed metal (eg copper) structure formed by etching a metal sheet and comprising empty spaces filled with an insulating compound (eg resin) "pre-molded" over the embossed metal structure.

[0042] Figure 2 A portion of a lead frame strip / reel is shown, which includes a (sacrificial) tie bar CB (in Figure 3A1 , the connecting bars CB extend at the periphery of each individual lead frame 12. The connecting bars CB are removed in a final singulation step, in which the strips of processed devices 10 are cut (e.g., by sawing with a blade) to obtain a plurality of processed / finished individual devices 10.

[0043] The lead frame 12 as shown may be used as a substrate in a power (integrated circuit) semiconductor device.

[0044] By way of example only, the lead frame 12 may include: a low power portion (in Figure 2 ), i.e., a first die pad 12A configured to accommodate a controller die or chip; and a high power portion (on the left hand side of each individual lead frame 12 of Figure 2 On the right-hand side of each individual lead frame 12), namely, one or more die pads 12A configured to accommodate one or more power integrated circuit dies or chips. As used herein, the terms chip and die are considered synonymous.

[0045] The array of leads 12B provides access to low power sectors and high power sectors at the periphery of the device (at Figure 2 ). On the left-hand and right-hand sides of each individual lead frame 12 shown in FIG.

[0046] Figure 3A and Figure 3B Respectively explain Figure 2 The bottom and top surfaces of the lead frame portion (illustrated in an enlarged scale) are indicated by arrow III in FIG.

[0047] As mentioned, the illustrated lead frame 12 may be provided by stamping or etching of a conductive material such as a sheet of copper, and as shown Figure 3A As illustrated in , the leadframe 12 may include portions having different thicknesses, namely: “full” thickness, such as the die pad 12A, leads 12B, and the center portion of the dummy pad 120 ; and “half” thickness, such as the tie bars TB and the connecting bars CB.

[0048] The portion of the leadframe 12 having a "half" thickness may be obtained by "half-etching" a sheet of conductive material (eg, copper).

[0049] The term "half" is a common designation in the art and does not imply that such partial etching or reduced thickness is necessarily exactly half the thickness of the base engraved structure of the lead frame.

[0050] The lead frame 12 may be connected to the connecting bar CB via a so-called tie bar TB, which is a bridge-like structure connecting the die pad 12A (and / or the leads 12B) of the lead frame to the connecting bar.

[0051] Tie bars TB facilitate handling of the lead frame 12 by keeping the die pad 12A and leads 12B flat and in place during a pre-molding step (eg, where relatively strong pressure is applied to the lead frame 12 ).

[0052] As shown, the dummy pad 120 may be disposed at a tie bar TB connecting the die pad 12A to a (sacrificial) tie bar CB, and thus electrically coupled thereto.

[0053] like Figure 3A and Figure 3B The conductive portions of the lead frame 12 (ie, the die pad 12A, the leads 12B, the tie bar TB, and the dummy pad 120) are illustrated in FIG. 1 and are separated by “empty” spaces therebetween.

[0054] To obtain the pre-molded leadframe 12 , an insulating (pre-)molding compound (eg, epoxy) is molded onto the leadframes 12 of the leadframe strip, causing the molding compound to penetrate (and fill) the empty spaces between the conductive portions of the leadframe 12 .

[0055] Figure 4A and Figure 4B 3 shows a pre-molded lead frame 12 obtained through the processing steps described above.

[0056] The pre-molded lead frame 12 has two opposing surfaces: Figure 4B The first (top or front) surface shown in FIG. 1 is configured to have one or more (integrated circuit) semiconductor dies mounted thereon (at die pad 12A); and a second (bottom or back) surface, as shown in FIG. Figure 4A As shown, it is configured to be soldered on a (final) substrate (eg a PCB).

[0057] The die pads 12A, leads 12B and dummy pads 120 are exposed (i.e., not covered by the pre-mold compound) at the second (bottom) surface of the pre-mold lead frame 12 to facilitate soldering thereof to corresponding locations on a final substrate (e.g., a PCB), which may be facilitated by solder paste provided on corresponding locations on the final substrate.

[0058] As discussed above, dummy pads 120 are a desirable feature in IC semiconductor devices, particularly for automotive applications, because they provide an additional solder joint to the device, thereby increasing reliability.

[0059] The dummy pads are exposed at the device package and soldered to the board (eg, PCB) on which the device is mounted.

[0060] If no dummy pads are provided, the encapsulation resin provides external isolation.

[0061] If dummy pads are provided, the large number of leads 12B in the device and the relatively small size of the device may result in a short distance (in the range of 100 to 1000) between the dummy pads 120 and the leads 12B. Figure 4A 12B), thereby increasing the risk of discharge between the pad 120 and the lead 12B when the device operates with a high voltage difference. Figure 4A As can be seen in FIG. 1 , the distance L1 between the dummy pad 120 and the adjacent lead 12B may be shorter than L2 , ie, the distance between the lead 12B and the adjacent die pad 12A.

[0062] Such discharge may damage the device as long as the dummy pad 120 is electrically coupled to the die pad 12A via the bar TB, and the discharge between the dummy pad 120 and the lead 12B will also affect the die pad 12A (and possibly the die mounted thereon), resulting in device failure.

[0063] One possible approach to reducing the risk of unwanted discharge may be to increase the package size, thereby increasing the distance between the dummy pad 120 and the lead 12B. However, this approach may place undue restrictions on the package design, especially when a small size device is required.

[0064] Another approach may be to choose not to include dummy pads 120 in devices that operate at high voltage differentials, thereby forgoing the desirable feature for improving soldering reliability.

[0065] The solution described herein may advantageously be applied to a pre-molded lead frame 12 provided with dummy pads 120 .

[0066] The solution described herein involves removing the electrical coupling between the dummy pad 120 and the die pad 12A, thus conceptually eliminating the problems associated with electrical discharge between the lead 12B and the dummy pad 120 .

[0067] The solution described herein involves removing a portion of the tie bar TB to decouple the dummy pad 120 from the die pad 12A.

[0068] In the following description, reference is made to a power IC semiconductor device having a dummy pad 120 provided at a corner of a lead frame 12; as will be appreciated by those skilled in the art, this is merely exemplary because: the tie bar TB (and therefore the dummy pad 120) may be located elsewhere than at the corner of the lead frame 12; and the solution described herein may be advantageously applied to devices having a structure different from that of the power device exemplified herein.

[0069] Figure 5 is a plan view of a first top / front surface of a pre-molded lead frame 12 processed in accordance with an embodiment of the present invention.

[0070] By removing the (conductive) portion 100 of the tie bar TB connecting the dummy pad 120 and the die pad 12A, the dummy pad 120 is separated from the die pad 12A.

[0071] Removal of the tie bar TB may be limited to a portion 100 thereof, thus leaving a remaining portion of the tie bar (indicated in the figures with reference TB′) in the pre-molded lead frame 12 .

[0072] The portion 100 of the tie bar TB may be removed via any method known to a person skilled in the art. In some embodiments, a (second) etching step may also be advantageously used to remove the portion 100 of the tie bar TB, which is usually already envisaged for other purposes in the current assembly flow.

[0073] In other embodiments, portion 100 of tie bar TB may be removed via laser ablation.

[0074] It should be noted that part 100 of the tie bar TB is removed after the (pre)molding step; indeed, as mentioned, the tie bar TB facilitates the handling of the pre-molded lead frame 12 by keeping parts of the lead frame flat and in place.

[0075] However, performing the removal of the conductive material (eg, metal such as copper) portion 100 of the tie bar TB forms a recessed portion (referenced with the same reference numeral 100 for simplicity) in the (otherwise substantially flat) pre-molded leadframe 12 .

[0076] In response to the removal of the (half-etched) portion of the tie bar TB, the pre-mold insulating compound 22 is exposed at the bottom surface of the recessed portion 100 .

[0077] This is Figure 6 and Figure 7 It is shown in Figure 6 and Figure 7 Along Figure 4B and Figure 5 Cross-sectional view along lines VI-VI and VII-VII.

[0078] like Figure 6 As shown in , the pre-molded lead frame 12 has a "full" thickness T1 between its two opposing surfaces, and the tie bar TB has a thickness T2 that is less than T1 (not necessarily half of T1).

[0079] like Figure 7 As shown in , a portion 100 of the tie bar TB is removed so that the pre-mold insulating compound is exposed at the bottom surface 1000 of the recessed portion 100 .

[0080] In summary, the pre-molded leadframe 12 as described herein may be obtained via molding an electrically insulating material 22 (eg, epoxy) onto an embossed conductive leadframe structure including a pattern of conductive features (die pad 12A, leads 12B, tie bars TB, and dummy pads 120 ).

[0081] The conductive structure includes a die mounting location 12A (eg, a die pad) configured to arrange at least one semiconductor die thereon, a dummy pad 120 , and a tie bar TB extending between the die mounting location 12A and the dummy pad 120 .

[0082] The pre-molded leadframe structure results from electrically insulating material 20 penetrating into the spaces between the conductive structures.

[0083] At least a portion of the tie bar 100 extending between the die mounting site 12A and the dummy pad 120 is removed to electrically decouple the dummy pad 120 from the die mounting site 12A.

[0084] The pre-molded lead frame 12 is further processed as described above to obtain a finished device 10 (e.g., Figure 1 ) may include: mounting / attaching one or more dies to corresponding mounting locations (i.e., die pads 12A); providing required electrical coupling via wire bonding (e.g., coupling a microcontroller die to low power leads and coupling a microcontroller die to a power die) and / or via clip / ribbon bonding (e.g., coupling a power die to a power lead); providing a protective electrically insulating package by molding a molding compound 20 (e.g., epoxy, not necessarily the same type of epoxy as the pre-mold compound 22) onto the assembly; and singulating (e.g., by sawing with a blade) the lead frame reel / strip processed as previously described to obtain a plurality of individual (finished) devices 10.

[0085] It should be noted that the final molding step fills the recessed portion 100 of the pre-molded lead frame 12 with molding compound.

[0086] The claims are an integral part of the technical teaching provided with respect to the embodiments.

[0087] Without prejudice to the essential principles, the details and embodiments may vary, even significantly, with respect to what has been described merely by way of example, without departing from the scope of protection, which is determined by the appended claims.

Claims

1. A method comprising: molding an electrically insulating material onto the embossed conductive leadframe structure including the pattern of conductive features to form a pre-molded leadframe structure; wherein the conductive configuration of the pattern includes: a die mounting location, a dummy pad, and a tie bar, the die mounting location being configured to have at least one semiconductor die disposed on the die mounting location, the tie bar extending between the die mounting location and the dummy pad; wherein the electrically insulating material penetrates into spaces between conductive formations in the pattern of conductive formations; and At least a portion of the tie bar extending between the die mounting location and the dummy pad is removed to electrically decouple the dummy pad from the die mounting location. 2 . The method of claim 1 , further comprising removing at least a portion of the tie bar extending between the die mounting location and the dummy pad via laser ablation. 3 . The method of claim 1 , further comprising removing at least a portion of the tie bar extending between the die mounting location and the dummy pad via etching.

4. The method according to claim 1, wherein: The pre-molded leadframe structure has a first thickness between the first opposing surface and the second opposing surface; and The tie bar has a second thickness less than the first thickness and is not covered by the electrically insulating material at the first surface of the pre-molded leadframe structure. 5 . The method of claim 4 , wherein removing at least a portion of the tie bar extending between the die mounting location and the dummy pad results in forming a recessed portion in the pre-molded leadframe structure.

6. The method according to claim 5, further comprising: mounting at least one semiconductor die on a corresponding die mounting location of the pre-molded leadframe structure; as well as molding a second electrically insulating material onto the pre-molded leadframe structure having the at least one semiconductor die mounted thereon; The second electrically insulating material penetrates into the recessed portion formed in the pre-molded lead frame structure.

7. A pre-molded lead frame, comprising: An embossed conductive leadframe structure comprising a pattern of conductive formations, wherein the conductive formations include: a die mounting location, a dummy pad, and a tie bar, the die mounting location being configured to have at least one semiconductor die disposed on the die mounting location, the tie bar extending between the die mounting location and the dummy pad; an electrically insulating material molded onto the embossed conductive leadframe structure; wherein the electrically insulating material penetrates into spaces between conductive structures in the pattern of conductive structures; and At least one gap in the tie bar extends between the die mounting location and the dummy pad to electrically decouple the dummy pad from the die mounting location.

8. The pre-molded lead frame according to claim 7, wherein: The pre-molded leadframe structure has a first thickness between the first opposing surface and the second opposing surface; and The tie bar has a second thickness less than the first thickness and is not covered by the electrically insulating material at the first surface of the pre-molded leadframe structure.

9. The pre-molded lead frame of claim 7, further comprising a recessed portion in the pre-molded lead frame structure at the location of the at least one gap.

10. A semiconductor device comprising: The pre-molded lead frame according to claim 7, and At least one semiconductor die is mounted on the die mounting location.

11. The semiconductor device according to claim 10, further comprising: a recessed portion in the pre-molded lead frame structure at the location of the at least one gap; as well as A second electrically insulating material is molded onto the pre-molded lead frame structure having the at least one semiconductor die mounted thereon at the die mounting location, wherein the second electrically insulating material penetrates into the recessed portion in the pre-molded lead frame structure.

Images