Substrate having stiffener fabrication method

Abstract

An integral plated semiconductor package substrate stiffener provides a low-cost and space-efficient mechanism for maintaining substrate planarity during the manufacturing process. By patterning and plating the stiffener along with the other substrate fabrication process steps, the difficulty of attaching a separate stiffener is averted. Also, the stiffener pattern can be provided around other substrate elements such as the circuit patterns and terminals, while maintaining requisite spacing. The stiffener is a two-layer metal structure, the first layer is a thin film metal layer on which a thicker outer metal layer is plated up. The two metal layers may be of different metals or alloys and the thin film metal layer may be the same layer plane that provides one of the substrate interconnect layers or may be the metal layer removed from other areas of the substrate during isolation of an embedded circuit layer.

Description

FIELD OF THE INVENTION

The present invention relates generally to semiconductor packaging, and more specifically, to a semiconductor package substrate having an integral plated stiffener that is produced during the substrate fabrication process.

BACKGROUND OF THE INVENTION

Semiconductors and other electronic and opto-electronic assemblies are fabricated in groups on a wafer. Known as “dies”, the individual devices are cut from the wafer and are then bonded to a carrier. The dies must be mechanically mounted and electrically connected to a circuit. For this purpose, many types of packaging have been developed, including “flip-chip”, ball grid array (BGA) and leaded grid array (LGA) among other mounting configurations. These configurations typically use a planar printed circuit etched on the substrate with bonding pads and the connections to the die are made by either wire bonding or direct solder connection to the die.

As the overall semiconductor package height is decreased, the thickness of the substrate has likewise been decreased and typical substrates for BGA/LGA packages today are thin film circuits fabricated on KAPTON or other film material, so that the substrates are thin, but with sufficient strength and thermal stability to handle the thermal cycles and handling during the manufacturing process. However, the application of thin films as substrates have led to the need for a stiffener that supports the substrate during the manufacturing process so that the substrate is maintained in proper shape during encapsulation or other final packaging steps.

Typical stiffeners are pre-formed metal strips or rings that are bonded onto the surface of the substrate film after etching/plating of the interconnect circuit patterns and any laminating of multiple substrate layers. Since the stiffeners are added in a separate process after circuit formation, clearances between the stiffener and features of the substrate must be carefully maintained during the bonding process and are generally limited to the periphery of the substrate outside of circuit pattern, wafer bonding, and semiconductor package terminal areas.

SUMMARY OF THE INVENTION

A semiconductor package substrate having an integral plated stiffener and a process for forming the stiffener on the substrate generate a metal stiffener structure of arbitrary shape on one or both sides of the substrate by plating. A metal film layer that covers a dielectric layer of the substrate is plated up in areas to form the stiffener and then exposed portions are removed via etching. The resulting stiffener is a two-level metal structure that may be of differing metals in each level.

The outer metal layer can be made by using a plating resist material that is then laser-ablated or patterned via a photo-lithographic process, yielding a negative stiffener image. The regions between the ablated resist are filled by plating up metal and the resist is removed to yield the stiffener.

The outer metal layer is generally much thicker than the metal film layer, and while the metal film layer will generally be copper from which circuit patterns are formed either within or atop the dielectric layer, the outer metal can be made of a much harder material for stiffness, such as brass or other copper alloy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1N are pictorial diagrams depicting cross-sectional views of various stages in the preparation of a substrate in accordance with an embodiment of the present invention;

FIGS. 2A and 2B are pictorial diagrams depicting semiconductor packages in accordance with embodiments of the present invention; and

FIG. 3 is a pictorial diagram depicting a plan view of a substrate including a stiffener in accordance with an embodiment of the present invention.

The invention, as well as a preferred mode of use and advantages thereof, will best be understood by reference to the following detailed description of illustrative embodiments when read in conjunction with the accompanying drawings, wherein like reference numerals indicate like parts throughout.

DETAILED DESCRIPTION

The present invention concerns a process for making a semiconductor package substrate having an integral plated stiffener. A metal structure is built up on one or both surfaces of the substrate by plating up a metal that may be the same as the metal used to form interconnect surface patterns, may be an alloy of the circuit pattern metal or-may be a differing metal. The stiffener is provided to aid in maintaining planarity of the substrate during the manufacturing process, and will also enhance the stiffness of the final semiconductor package, which may or may not be encapsulated.

The stiffener may be fabricated in conjunction with processes and structures such as those described in the above-incorporated parent U.S. Patent Application, in which a semi-additive process is used to form a plated metal-on-foil structure above a preexisting metal foil. Alternatively, the stiffener may be fabricated in conjunction with the seed plating techniques used in the laser embedded circuit pattern substrates described in the above-incorporated grandparent U.S. Patent Applications.

Referring now to the figures and in particular to FIGS. 1A-1N, cross-sectional views illustrate a substrate manufacturing process in accordance with an embodiment of the present invention. FIG. 1A shows a clad dielectric laminate 10A, as is commonly used for forming patterned circuit substrates via an etching process. Laminate 10A includes a copper film layer 8 bonded to each side of a dielectric sheet 12. In the present invention, laminate 10A is used to provide only dielectric sheet 12 and as shown in FIG. 1B, copper film layer 8 is removed from each side of dielectric sheet 12 via an etching process to yield a very uniform substrate form 10B comprising denuded dielectric sheet 12.

Next, dielectric sheet 12 is laser-ablated to form channels 14A and cavities/thru-vias 14B for various features that will be embedded as circuit patterns or vias, yielding modified dielectric sheet 12A of substrate step 10C. Then, a seed plating 16 (generally copper) is applied via an electro-less process to coat dielectric sheet 12A, forming substrate step 10D of FIG. 1D. After seed plating, a thicker layer of metal is plated up on each side of substrate step 10D via a controlled plating process that yields a very planar coating of metal 18 (generally copper again) on each side of a substrate step 10E as shown in FIG. 1E.

As shown in FIG. 1F, a substrate step 10F is provided by coating both sides of substrate step 10E with an etch resist material 20A, 20B. Etch resist material 20A, 20B is generally photosensitive and is imaged to leave resist material 20C, 20D as shown in substrate step 10G of FIG. 1G for the formation of ball-grid array (BGA) lands and semiconductor die interconnect terminals. Next, as shown in FIG. 1H, substrate step 10H is formed by etching metal 18 to leave only a thin metal layer 18A, except in the regions still covered by etch resist material 20C, 20D. Then etch resist material 20C, 20D is removed to expose BGA land areas 22A, 22B.

Now, as shown in FIG. 1J, the formation of the stiffener on one or both sides of substrate 10I is commenced by coating each side of substrate step 10J with a plating resist material 30A, 30B. Then plating resist 30A, 30B is laser-ablated in the desired stiffener shape to form voids 32A, 32B, which are generally channels running parallel to the surfaces of substrate step 10K as shown in FIG. 1K. The shape of the stiffener is therefore arbitrary and is constrained only by the stiffener requirements and the areas of the substrate that must be avoided (such as BGA land areas and the circuit channels exposed at the surface). The precision with which the stiffener is plated (rather than placed) makes it possible to locate the stiffener very close to circuit features without making electrical contact.

A stiffener 34A, 34B is formed as shown in substrate step 10L of FIG. 1L, by plating a metal within voids 32A, 32B in plating resist material 30C, 30D using an electroplating process that uses metal layer 18A as an electrode. The plated metal may be the same or may differ. In general, copper is used for metal layer 18A, as conductive patterns are formed from the same metal and stiffener 34A, 34B is formed from a plating-compatible metal or alloy that is less ductile, such as Nickel or Tin, or a copper alloy such as Brass. After formation of stiffener 34A, 34B, remaining plating resist material 30C, 30D is removed to form substrate step 10M of FIG. 1M.

Finally, a controlled etching process is used to remove the remaining excess metal 18A and the seed layer 16, leaving only the circuit pattern metal 18A (and seed layer 16) previously deposited within circuit pattern channels and the metal 18B, 18C and seed layer that connect stiffener portions 34A, 34B to the dielectric layer 12A as shown in the resulting substrate 10N. As such, two or three distinct crystalline structures of a single metal, or two distinct structures of the first metal (e.g., Copper) and a distinct structure of stiffener portions 34A, 34B can be observed in substrate 10N via microscopy or other metallurgic examination techniques.

While the illustrative embodiment shows a stiffener formed on both sides of a substrate, it should be understood that the present invention provides for plated stiffeners on one or both sides of the substrate, and that the composition of the metals is not a limitation as pointed out above. The shape of the stiffener can be arbitrary, or can be patterned to form traditional shapes such as a boundary ring and the stiffener can further be used for attachment of a metal lid.

FIG. 2A shows a semiconductor package 40A in accordance with an embodiment of the present invention. A semiconductor die 44A is attached to substrate 10N by flip-chip terminals 46A or posts. Solder balls 42 are added to provide a mechanism for attaching semiconductor package 40A to external devices. The resulting package 40A may be encapsulated and a plating step can be optionally applied to terminal mounting areas prior to attachment of terminals 46A and solder balls 42.

While die 44A is depicted as mounted above substrate 10N, a die mounting recess may also be laser-ablated or otherwise provided in substrate 10N, reducing the package height.

FIG. 2B shows a semiconductor package 40B in accordance with an embodiment of the present invention. A semiconductor die 44B is attached to substrate 10N by bond wires 46B. Solder balls 42 are added to provide a mechanism for attaching semiconductor package 40B to external devices. The resulting package 40B may be encapsulated and a plating step can be optionally applied to-wire bond and-terminal mounting areas prior to attachment of wire 46B and solder balls 42. While die 44B depicted as mounted above substrate 10N, a die mounting recess may also be laser-ablated or otherwise provided in substrate 10N, reducing the package height.

Referring now to FIG. 3, a plan view of a substrate 10 including a plated stiffener 34 in accordance with an embodiment of the invention is depicted. As is shown, portions of stiffener can be located in the interconnect metal 18A and terminal 22 areas of substrate 10 without shorting. A ring-shaped portion of stiffener 54 can provide for solder or welding attachment of a lid to the final semiconductor package.

The above description of embodiments of the invention is intended to be illustrative and not limiting. Other embodiments of this invention will be obvious to those skilled in the art in view of the above disclosure and fall within the scope of the present invention.

Claims

1-17. (canceled)

18. A method of making a substrate for a semiconductor package, the method comprising: providing a substrate form comprising a substantially planar thin foil metal layer bonded to a dielectric layer; applying a plating resist material to a surface of the thin foil metal layer; laser-ablating the plating resist material to form a plating resist structure defining voids in a shape of a stiffener for the substrate; and plating an outer metal layer within the voids to form the stiffener atop a first surface of the planar thin foil metal layer.

19. The method of claim 18, further comprising etching the thin foil metal layer to leave only regions corresponding to the stiffener.

20. The method of claim 19, wherein the providing a substrate form provides the substantially planar thin foil metal layer that has extensions into the dielectric layer filling channels extending within the dielectric layer in a plane parallel to primary surfaces of the dielectric layer, and wherein the etching separates the extensions of the thin foil metal layer to form a circuit pattern within the channels within the dielectric layer.

21. A method of making a substrate for a semiconductor package, the method comprising: forming a metal layer over a first surface of a dielectric layer; forming a plating resist material on the metal layer; laser-ablating the plating resist material to form voids in the plating resist material; and plating an outer metal layer using the metal layer as an electrode within the voids to form a stiffener.

22. The method of claim 21 further comprising forming a seed layer on the dielectric layer.

23. The method of claim 22 wherein the metal layer is plated atop the seed layer.

24. The method of claim 21 further comprising: forming an etch resist material on the metal layer; patterning the etch resist material to remove the etch resist material except where lands and terminals are to be formed.

25. The method of claim 24 further comprising etching the metal layer except in regions covered by the etch resist material.

26. The method of claim 25 further comprising removing the etch resist material to expose the lands and terminals.

27. The method of claim 21 wherein the voids comprise channels parallel to the first surface of the dielectric layer.

28. The method of claim 21 wherein the outer metal layer is selected from the group consisting of nickel, tin, copper, a copper alloy, and brass.

29. The method of claim 21 further comprising: forming another metal layer over a second surface of the dielectric layer; forming another plating resist material on the another metal layer; laser-ablating the another plating resist material to form voids in the another plating resist material; and plating another outer metal layer using the another metal layer as an electrode within the voids of the another plating resist material to form another stiffener

30. A method of making a substrate for a semiconductor package, the method comprising: laser-ablating a dielectric layer to form channels and via apertures in the dielectric layer; forming a seed layer on the dielectric layer; forming a metal layer on the seed layer, wherein the seed layer and metal layer fill the channels and via apertures; forming an etch resist material on the metal layer; patterning the etch resist material to remove the etch resist material except where lands and terminals are to be formed; etching the metal layer except in regions covered by the etch resist material; removing the etch resist material to expose the lands and terminals; forming a plating resist material on the metal layer; laser-ablating the plating resist material to form voids in the plating resist material; and plating an outer metal layer using the metal layer as an electrode within the voids to form stiffeners on both sides of the substrate.

31. The method of claim 30 further comprising removing the plating resist material.

32. The method of claim 31 further comprising etching the metal layer and the seed layer to form a circuit pattern within the channels and via apertures.

33. The method of claim 32 wherein after the etching the metal layer and the seed layer, portions of the metal layer and seed layer remain within the channels and via apertures, the portions forming the circuit pattern.

34. The method of claim 32 wherein after the etching the metal layer and the seed layer, portions of the metal layer and the seed layer connect the outer metal layer to the dielectric layer.

35. The method of claim 34 wherein the stiffeners comprise: the portions of the seed layer on first and second surfaces of the dielectric layer; the portions of the metal layer on the portions of the seed layer; and the outer metal layer on the portions of the metal layer.

36. The method of claim 35 wherein the stiffeners comprise three distinct crystalline structures.

37. The method of claim 30 wherein the stiffeners are ring shaped.

38. A method of making a substrate for a semiconductor package, the method comprising: laser-ablating a dielectric layer to form channels and via apertures in the dielectric layer; forming a seed layer on the dielectric layer; forming a metal layer on the seed layer, wherein the seed layer and metal layer fill the channels and via apertures; forming an etch resist material on the metal layer; patterning the etch resist material to remove the etch resist material except where lands and terminals are to be formed; etching the metal layer except in regions covered by the etch resist material; removing the etch resist material to expose the lands and terminals; forming a plating resist material on the metal layer; laser-ablating the plating resist material to form voids in the plating resist material; and plating an outer metal layer using the metal layer as an electrode within the voids to form a stiffener on the substrate.

39. The method of claim 38 further comprising: removing the plating resist material; and etching the metal layer and the seed layer such that portions of the metal layer and the seed layer connect the outer metal layer to the dielectric layer.

40. The method of claim 39 wherein the stiffener comprises: the portions of the seed layer on the dielectric layer; the portions of the metal layer on the portions of the seed layer; and the outer metal layer on the portions of the metal layer.

41. The method of claim 40 wherein the stiffener comprises three distinct crystalline structures.

42. A method of making a substrate for a semiconductor package, the method comprising: laser-ablating a dielectric layer to form channels and via apertures in the dielectric layer; forming a seed layer on the dielectric layer; forming a metal layer on the seed layer, wherein the seed layer and metal layer fill the channels and via apertures; forming an etch resist material on the metal layer; patterning the etch resist material to remove the etch resist material except where lands and terminals are to be formed; etching the metal layer except in regions covered by the etch resist material; removing the etch resist material to expose the lands and terminals; forming a plating resist material on the metal layer; laser-ablating the plating resist material to form voids in the plating resist material; plating an outer metal layer using the metal layer as an electrode within the voids to form a stiffener on the substrate; removing the plating resist material; and etching the metal layer and the seed layer such that portions of the metal layer and seed layer remain within the channels and via apertures, the portions forming a circuit pattern.