Lead frame for a package for a semiconductor device, semiconductor device and process for manufacturing a semiconductor device

Abstract

A lead frame for an integrated electronic device includes a die pad made of a first metallic material. A top coating layer formed by a second metallic material is arranged on a top surface of the die pad. The second metallic material has an oxidation rate lower than the first metallic material. The top coating layer leaves exposed a number of corner portions of the top surface of the die pad. A subsequent heating operation, for example occurring in connection with wirebonding, causes an oxidized layer to form on the corner portions of the top surface of the die pad at a position in contact with the top coating layer.

Claims

1. A lead frame for an integrated electronic device, comprising: a die pad made of a first metallic material and including a top surface; and a top coating layer made of a second metallic material and arranged in contact with the top surface, wherein the second metallic material has an oxidation rate lower than an oxidation rate of the first metallic material; wherein said top coating layer covers all of the top surface except for corner portions of the top surface of the die pad that are not covered by said top coating layer and which are separate from each another; and an oxidized coating covering the top surface of the die pad at the corner portions that are not covered by said top coating layer, wherein the oxidized coating is laterally in contact with the top coating layer.

2. The lead frame according to claim 1, wherein the die pad includes a number of edges at locations where sides of the die pad intersect; and wherein each edge is perpendicular to a corresponding corner portion with which it is in direct contact.

3. The lead frame according to claim 1, wherein each corner portion has a squared shape.

4. The lead frame according to claim 1, wherein the die pad is supported by a main body, and wherein the die pad and main body form a single piece.

5. The lead frame according to claim 1, further comprising a plurality of pads having top surfaces coated by coating regions made of the second metallic material.

6. The lead frame according to claim 5, wherein the die pad and plurality of pads are supported by a main body, and wherein the die pad, plurality of pads and main body form a single piece.

7. A lead frame, comprising: a die pad made of a first metallic material and including a top surface; and a top coating layer made of a second metallic material and arranged in contact with the top surface, wherein the second metallic material has an oxidation rate lower than an oxidation rate of the first metallic material; wherein said top coating layer covers all of the top surface except for corner portions of the top surface of the die pad that are not covered by said top coating layer and which are separate from each another; a ground ring laterally surrounding the die pad and including a top surface, wherein the ground ring has a shape of a squared frame; and an additional top coating layer made of the second metallic material and arranged in contact with the top surface of the ground ring; and wherein corner portions of the top surface of the ground ring are not covered by said additional top coating layer.

8. The lead frame according to claim 7, wherein the die pad and the ground ring are supported by a main body, and wherein the die pad, ground ring and main body form a single piece.

9. The lead frame according to claim 7, further comprising an oxidized coating covering the top surface of the die pad at the corner portions that are not covered by said top coating layer and covering the top surface of the ground ring at the corner portions that are not covered by said additional top coating layer, wherein the oxidized coating is laterally in contact with the top coating layer and is laterally in contact with the additional top coating layer.

10. The lead frame according to claim 7, wherein the die pad includes a number of edges at locations where sides of the die pad intersect; and wherein each edge is perpendicular to a corresponding corner portion with which it is in direct contact.

11. The lead frame according to claim 7, wherein each corner portion has a squared shape.

12. The lead frame according to claim 7, wherein the die pad is supported by a main body, and wherein the die pad and main body form a single piece.

13. The lead frame according to claim 7, further comprising a plurality of pads having top surfaces coated by coating regions made of the second metallic material.

14. The lead frame according to claim 13, wherein the die pad and plurality of pads are supported by a main body, and wherein the die pad, plurality of pads and main body form a single piece.

15. The lead frame according to claim 7, further comprising an oxidized coating covering the top surface of the die pad at the corner portions that are not covered by said top coating layer, wherein the oxidized coating is laterally in contact with the top coating layer.

16. A lead frame, comprising: a die pad made of a first metallic material and including a top surface; and a top coating layer made of a second metallic material and arranged in contact with the top surface, wherein the second metallic material has an oxidation rate lower than an oxidation rate of the first metallic material; wherein said top coating layer covers all of the top surface except for corner portions of the top surface of the die pad that are not covered by said top coating layer and which are separate from each another; an inner frame which laterally surrounds the die pad; an outer frame which laterally surrounds the inner frame; a plurality of tie bars interposed between the outer frame and the inner frame; and a further top coating layer made of the second metallic material and arranged in contact with top surfaces of the inner frame and outer frame; wherein portions of the inner frame at locations where the inner frame is connected with the tie bars are not covered by said further top coating layer.

17. The lead frame according to claim 16, wherein top surfaces of the tie bars are not covered by said further top coating layer.

18. The lead frame according to claim 16, wherein the inner frame and the outer frame have different heights, and wherein each tie bar includes a corresponding slanted portion to transition between said different heights.

19. The lead frame according to claim 16, further comprising: a bottom coating structure extending under the die pad and the inner frame; and an outer bottom coating region extending under the outer frame; and wherein portions of a bottom surface of the inner frame at locations where the inner frame is connected with the tie bars are not covered by said outer bottom coating region.

20. The lead frame according to claim 16, further comprising an oxidized coating covering the top surface of the die pad at the corner portions that are not covered by said top coating layer and covering the portions of the inner frame that are not covered by said further top coating layer, wherein the oxidized coating is laterally in contact with the top coating layer and is laterally in contact with the further top coating layer.

21. The lead frame according to claim 16, wherein the die pad includes a number of edges at locations where sides of the die pad intersect; and wherein each edge is perpendicular to a corresponding corner portion with which it is in direct contact.

22. The lead frame according to claim 16, wherein each corner portion has a squared shape.

23. The lead frame according to claim 16, wherein the die pad is supported by a main body, and wherein the die pad and main body form a single piece.

24. The lead frame according to claim 16, further comprising a plurality of pads having top surfaces coated by coating regions made of the second metallic material.

25. The lead frame according to claim 24, wherein the die pad and plurality of pads are supported by a main body, and wherein the die pad, plurality of pads and main body form a single piece.

26. The lead frame according to claim 16, further comprising an oxidized coating covering the top surface of the die pad at the corner portions that are not covered by said top coating layer, wherein the oxidized coating is laterally in contact with the top coating layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) For a better understanding of the present invention, preferred embodiments thereof are now described, purely by way of non-limiting example and with reference to the attached drawings, wherein:

(2) FIG. 1 schematically shows a perspective view of a lead frame;

(3) FIG. 2 schematically shows a cross-section of the lead frame shown in FIG. 1, taken along the line II-II shown in FIG. 1;

(4) FIG. 3 schematically shows a cross-section of a portion of a packaged device including the lead frame shown in FIGS. 1 and 2;

(5) FIG. 4 schematically shows a perspective view of a further lead frame;

(6) FIG. 5 schematically shows a perspective view of the present lead frame;

(7) FIG. 6 schematically shows a cross-section of the lead frame shown in FIG. 5, taken along the line VI-VI shown in FIG. 5;

(8) FIG. 7 schematically shows a cross-section of a portion of a packaged device including the lead frame shown in FIGS. 5 and 6, during a step of a manufacturing process;

(9) FIG. 8 schematically shows a cross-section of the packaged device shown in FIG. 7, during a following step of the manufacturing process;

(10) FIG. 9 schematically shows a perspective view of a variant of the present lead frame;

(11) FIG. 10 schematically shows a perspective view of a further variant of the present lead frame;

(12) FIG. 11 schematically shows an enlarged perspective view of a portion of the lead frame shown in FIG. 10;

(13) FIG. 12 schematically shows an enlarged perspective view, taken from bottom, of a portion of the lead frame shown in FIGS. 10 and 11;

(14) FIG. 13 schematically shows a cross-section of a packaged device including the lead frame shown in FIGS. 10-12, in a normal operative condition; and

(15) FIG. 14 schematically shows a cross-section of the packaged device shown in FIG. 13, when delamination occurs.

DETAILED DESCRIPTION

(16) The present lead frame stems from the idea of leaving exposed the corners of the die pad structure, so as to create sacrificial oxidized regions which allow to limit the delamination, as described here below, with reference to a lead frame of the type shown in FIG. 1, without this implying any limitation. Therefore, the following description will limit to the differences over the lead frame 1 shown in FIG. 1; in addition, the same reference signs will be used, unless stated otherwise.

(17) In detail, FIGS. 5 and 6 show a lead frame (here designated by 101), in which the die pad (here designated by 110) has the same shape as the die pad 10 of FIGS. 1 and 2. In addition, the die pad 110 is delimited at top by a front surface 103. The top coating region (here designated by 112) extends on the front surface 103.

(18) In greater detail, the top coating region 112 leaves exposed four corner portions 117 of the die pad 110, which are delimited at top by corresponding portions of the top surface 103, hereinafter referred to as the top angular surface portions 117S.

(19) In particular, each corner portion 117 includes a corresponding edge (here designated by 111) where two adjacent sides of the die pad 110 intersect, perpendicular to the front surface 103 and in contact with this latter. In addition, without this implying any limitation, the four corner portions 117 have one and the same shape; in particular, in top plan view, the corner portions 117 have the shape of a square. Each top angular surface portion 117S extends from a corresponding edge 111 and is separate from the other top angular surface portions 117S; in top view, each top angular surface portion 117S extends from a corresponding vertex of the polygonal shape defined by the top coating region 112.

(20) During the manufacturing process, and in particular during the implementation of the wire bondings, the corresponding thermal treatment leads to the formation of a bottom oxidized layer 126, extending under the bottom surface 6b, and a top oxidized layer 127, as shown in FIG. 7. The bottom oxidized layer 126 and the top oxidized layer 127 may have a thickness comprised, as an example, in the range 100 nm-200 nm.

(21) The top oxidized layer 127 includes a main portion 128, which extends on the top surface 6a of the main body 2, as well as on the side walls of the pads 8 and the die pad 110; therefore, the main portion 128 of the top oxidized layer 127 coats the edges 111 of the die pad 110. In addition, the top oxidized layer 127 comprises four additional portions 129 (only one visible in FIG. 7), hereinafter referred to as the sacrificial portions 129.

(22) In particular, each sacrificial portion 129 extends on a corresponding corner portion 117 of the die pad 110, i.e. on a corresponding top angular surface portion 117S. In addition, each sacrificial portion 129 is laterally staggered with respect to the top coating region 112, with which it is in direct contact, thereby forming a corresponding interface IF.

(23) Referring as an example to a following manufacturing step (shown in FIG. 8) of forming the packaging region 25, such a step, per se known, includes a mold curing at (as an example) 170° C. The packaging region 25 extends, among other things, also on the sacrificial portions 129.

(24) The abovementioned curing step causes mechanical stresses due to CTE (coefficient of thermal expansion) mismatch which are unlikely to cause a delamination between the sacrificial portions 129 and the packaging region 25, for the following reasons.

(25) In detail, it is noted that the adhesion between the top oxidized layer 127 and the packaging region 25 is greater than the adhesion between the top oxidized layer 127 and the lead frame 101. In greater detail, at each corner of the die pad 110, the adhesion between the corresponding sacrificial portion 129 and the overlying portion of the packaging region 25 is greater than the adhesion between the sacrificial portion 129 and the underlying corner portion 117 of the die pad 110. Therefore, delamination may occur, at most, between the sacrificial portions 129 and the die pad 110; furthermore, such a delamination does not propagate beyond the interface IF between each sacrificial portion 129 and the top coating region 112, therefore it does not affect the adhesion between the packaging region 25 and the top coating region 112. Therefore, the packaging region 25 and the top coating region 112 continue to encapsulate the die, without exposing this latter to the external atmosphere. The reduced size of the top angular surface portion 117S with respect to the top coating region 112 contributes to achieving this result.

(26) FIG. 9 shows a further embodiment, which will be described hereinbelow with reference to the differences over the lead frame shown in FIG. 4.

(27) In detail, referring to the additional front surface 103′ to indicate the front surface of the ground ring 29, the additional coating region 12′ leaves exposed four corner portions 217 of the ground ring 29, which are delimited at top by corresponding portions of the additional front surface 103, hereinafter referred to as the additional top angular surface portions 217S.

(28) In particular, the corner portions 217 of the ground ring 29 include, each, a corresponding edge 211 of the lateral outer surface of the square frame forming the ground ring 29. Furthermore, in top plan view, each top angular surface portion 217S has the shape of an “L”.

(29) During the manufacturing process, each additional top angular surface portions 217S gets coated by a corresponding oxidized sacrificial region (not shown). Therefore, the embodiment shown in FIG. 9 guarantees the same advantages as the one shown in FIGS. 6-8.

(30) FIGS. 10-12 show a further embodiment, in which the lead frame, here designated by 301, is suitable to form a package of the so-called thin quad flat pack (TQFP) type.

(31) In this case, the lead frame 301 includes a die pad 310 with a squared planar shape, delimited at top and at bottom, respectively, by the front surface, here designated by 303 (visible in FIG. 11), and by a rear surface 405. The top coating region, here designated by 312, leaves exposed the four top angular surface portions, here designated by 317S. Without this implying any limitation, the edges (here designated by 311) of the die pad 310 are beveled. Each edge 311 is still formed by a corresponding pair of adjacent side walls (here designated by 316) of the die pad 310.

(32) In addition, the rear surface 405 is coated by a bottom coating region 412, which is formed by the same material as the top coating region 312 (as an example, silver) and leaves exposed four corner portions 417S of the rear surface 405, hereinafter referred to as the bottom angular surface portions 417S.

(33) Without this implying any limitation, a pair of (optional) trenches with concentric squared annular shapes may be present. In particular, a first and a second trench T1, T2 may extend through the bottom coating region 412 and part of the die pad 310, with a rectangular cross-section. In greater detail, the second trench T2 surrounds, at distance, the first trench T1; furthermore, the first and second trenches T1, T2 do not completely cross the die pad 310 (i.e., they are blind) and extend in such a way that, as shown in FIG. 12, each bottom angular surface portion 417S is divided into a first, a second and a third exposed subportion 418A, 418B, 418C, the first trench T1 extending, in bottom view, between the first and the second exposed subportions 418A, 418B, the second trench T2 extending between the second and the third exposed subportions 418B, 418C. Therefore, the first exposed subportion 418A is arranged inwardly, whereas the third exposed subportion 418C is arranged outwardly, towards the corresponding edge 311; the second exposed subportion 418B is interposed between the first and the third exposed subportions 418A, 418C.

(34) In addition, the lead frame 310 comprises an inner frame 450 which is squared and planar and surrounds, at distance, the die pad 310, to which it is connected by means of a plurality of bridges 451. In particular, the die pad 310, the bridges 451 and the inner frame 450 form one single piece of metal (i.e., they are made up of one and the same material); in addition, the inner frame 450 may be vertically staggered with respect to the die pad 310, i.e. it may be arranged at an height (measured in a direction perpendicular to the die pad 310) higher than the die pad 310. In addition, the inner frame 450 is delimited at top and at bottom by, respectively, a top frame surface 452 and a bottom frame surface 453.

(35) The top coating region 312 extends also on the front surfaces of the bridges 451, as well as on the top frame surface 452, leaving exposed four angular portions 457S of this latter, hereinafter referred to as the top frame surface portions 457S. In top plan view, each top frame surface portions 457S has the shape of an “L”, whose concavity houses part of the corresponding top angular surface portion 317S.

(36) In addition, the bottom coating region 412 extends also on the rear surfaces of the bridges 451, as well as on the bottom frame surface 453, leaving exposed four angular portions 467S of this latter, hereinafter referred to as the bottom frame surface portions 467S. In bottom view, each bottom frame surface portion 467S has the shape of an “L”, whose concavity houses the corresponding third exposed subportion 418C. Each bottom frame surface portion 467S and the corresponding top frame surface portion 457S delimit, respectively at top and at bottom, a corresponding corner portion 499 of the inner frame 450.

(37) The lead frame 301 further comprises an outer frame 470, which is planar and squared and surrounds, at distance, the inner frame 450, and a plurality of cantilever leads 472, each cantilever lead 472 being fixed to the outer frame 470 and extending towards the inner frame 450. Without this implying any limitation, the outer frame 470 may be vertically staggered with respect to the inner frame 450, and in particular it may be arranged at an height higher than the inner frame 450. In this case, the die pad 310, the inner frame 450 and the outer frame 470 lie in corresponding planes which, though arranged at different heights, are parallel.

(38) In addition, the lead frame 301 comprises four connecting elements 474, hereinafter referred to as tie bars 474. Each tie bar 474 has an elongated shape and two ends, a first end being fixed to the inner frame 450, the second end being fixed to the outer frame 470. The die pad 310, the inner frame 450, the outer frame 470, the cantilever leads 472 and the tie bars 474 form a single metallic piece.

(39) Each tie bar 474 is delimited at top and at bottom by, respectively, a respective top surface 493 and a respective bottom surface 495. Furthermore, each tie bar 474 comprises a slanted portion 475A and a planar portion 475B. The planar portion 475B connects to the outer frame 470. The slanted portion 475A is interposed between the inner frame 450 and the planar portion 475B; in particular, the slanted portion 475A connects to a corresponding corner portion 499 of the inner frame 450. In addition, either the top surface 493 and the bottom surface 495 have the shape of a pair of plane surfaces connected to one another (i.e, one slanted plane surface and an horizontal plane surface, connected to one another).

(40) In addition, the upper sides of the cantilever leads 472 and of the outer frame 470 are coated by an outer top coating region 512; the lower sides of the cantilever leads 472 and of the outer frame 470 are coated by an outer bottom coating region 613. The outer top coating region 512 and the outer bottom coating region 613 may be formed by the same metallic material as the top coating region 312 and the bottom coating region 412.

(41) The outer top coating region 512 leaves exposed a portion 517S of the top surface 493 of each tie bar 474, hereinafter referred to as the exposed top elongated surface 517S. Each exposed top elongated surface 517S connects to a corresponding top frame surface portion 457S.

(42) The outer bottom coating region 613 leaves exposed a portion 617S of the bottom surface 493 of each tie bar 474, hereinafter referred to as the exposed bottom elongated surface 617S. Each exposed bottom elongated surface 617S connects to a corresponding bottom frame surface portion 467S.

(43) As shown in FIG. 13, the lead frame 301 may be used to manufacture an integrated electronic device 700, which includes a die 502, fixed on a portion of the top coating region 312 covering the die pad 310, by means of an interposed layer 503 of epoxy conductive glue, silver filled. The packaging region 25 englobes the lead frame 301, which has previously undergone a cutting process which, in a per se know manner, led to the removal of the outer frame 470, as well as of parts of the cantilever leads 472 and of the tie bars 474 connected thereto.

(44) The packaging region 25 leaves exposed the bottom coating region 412 and part of each tie bar 474. In particular, for each tie bar 474, outer parts of the exposed top elongated surface 517S and of the exposed bottom elongated surface 617S extend outside the packaging region 25; inner parts of the exposed top elongated surface 517S and of the exposed bottom elongated surface 617S extend inside the packaging region 25.

(45) In addition, parts of the tie bar 474 covered, on opposite sides, by the outer top coating region 512 and the outer bottom coating region 613 extend outside the packaging region 25.

(46) As previously explained, the exposed parts of the lead frame 301 are covered by a metal oxide layer. In particular, each exposed top elongated surface 517S and the corresponding top frame surface portion 457S are coated by a corresponding top oxidized region 800; each exposed bottom elongated surface 617S and the corresponding bottom frame surface portion 467S are coated by a bottom oxidized region 802, forming one single oxidized region with the top oxidized region 800.

(47) It is noted that delamination is more likely to occur between the top/bottom oxidized region 800/802 and the lead frame 301, than between the top/bottom oxidized region 800/802 and the packaging region 25. As an example, FIG. 14 shows a delamination occurring between the top oxidized region 800 and the lead frame 301 and propagating only up to the interface (still designated by IF) between the top oxidized region 800 and the top coating region 312, without affecting the adhesion between the top coating region 312 and the die pad 310.

(48) In greater detail, the interface IF between the top oxidized region 800 and the part of the top coating region 312 adjacent (i.e., in direct contact) to the top oxidized region 800 is overlaid by the packaging region 25 and acts as stop point to the propagation of a delamination between the top oxidized region 800 and the lead frame 301. Therefore, such a delamination does not cause, as an example, the detachment of the packaging region 25 from the top coating region 312.

(49) From what has been described and illustrated previously, the advantages that the present solution affords emerge clearly.

(50) In particular, the present lead frame represents a cost-effective solution which allows to reduce the occurrence, in packaged integrated electronic devices, of the delamination between the molded packaging region and the metallic coating regions of the lead frame. In conclusion, it is clear that modifications and variations may be made to what has been described and illustrated so far, without thereby departing from the scope of the present invention, as defined in the annexed claims.

(51) For example, the lead frame may have a shape different from the ones previously shown.