WO2013037187A1

WO2013037187A1 - A pre-encapsulated lead frame structure with island and manufacturing method

Info

Publication number: WO2013037187A1
Application number: PCT/CN2012/001161
Authority: WO
Inventors: Xinchao Wang; Zhizhong Liang
Original assignee: Jiangsu Changjiang Electronics Technology Co Ltd
Current assignee: JCET Group Co Ltd
Priority date: 2011-09-13
Filing date: 2012-08-28
Publication date: 2013-03-21
Anticipated expiration: 2014-03-13
Also published as: CN102315191A

Abstract

A method is provided for manufacturing a lead frame structure (100) in a semiconductor packaging process. The method includes providing a metal substrate having a top surface and a back surface, forming a top surface etching pattern and a back surface etching pattern. The method also includes performing an etching process to form an island (1) and a plurality of leads (2). Outer leads of the plurality of leads (2) on the back surface extend to the proximity of the island (1) such that a length of the outer leads is greater than a length of inner leads of the plurality of leads (2) on the top surface by a predetermined amount. Further, the method includes encapsulating the etched metal substrate using a molding compound exposing top surfaces and back surfaces of the island (1) and the plurality of leads (2).

Description

A PRE-ENCAPSULATED LEAD FRAME STRUCTURE WITH ISLAND AND

MANUFACTURING METHOD

CROSS-REFERENCES TO RELATED APPLICATIONS

[0001 ] This application claims the priority of Chinese patent application

no.201110268347.0, filed on 13/09/2011 , the entire contents of which are incorporated herein by reference.

FIELD OF THE INVENTION

[0002] The present invention generally relates to the field of semiconductor assembly or packaging and, more particularly, to lead frame structures and related manufacturing processes.

BACKGROUND

[0003] In a conventional Quad Flat No-Lead (QFN) lead frame, it is generally required that the size of the chip to be mounted is less than the size of the island or die pad to avoid mounting glue (electrically conductive or non-conductive) contaminating the leads of the lead frame. For example, Fig. 1 shows a conventional QFN lead frame having an island 21 and leads 22. Fig. 2 shows a chip 23 is mounted on island 21 using mounting glue 24 and sealed by molding compound 25. As shown in Fig. 2, because the size of the chip 23 is larger than the size of the island 21, the chip 23 also touches the inner leads of the leads 22 and mounting glue 24 also touches or contaminates the inner leads of the leads 22. Therefore, to avoid such contamination, the size of the chip 23 to be mounted on the lead frame is limited, and the chip scale may be low. [0004] The disclosed methods and systems are directed to solve one or more problems set forth above and other problems.

BRIEF SUMMARY OF THE DISCLOSURE

[0005] One aspect of the present disclosure includes a method for manufacturing a lead frame structure in a semiconductor packaging process. The method includes providing a metal substrate having a top surface and a back surface, .forming a first photoresist film on the top surface of the metal substrate, and forming a top surface etching pattern in the first photoresist film using photolithography. The method also includes forming a second photoresist film on the back surface of the metal substrate, forming a back surface etching pattern in the second photoresist film using photolithography, and performing an etching process on the top surface and the back surface of the metal substrate using the top surface etching pattern and the back surface etching pattern as the respective masks to form an island and a plurality of leads. Outer leads of the plurality of leads on the back surface extend to the proximity of the island such that a length of the outer leads is greater than a length of inner leads of the plurality of leads on the top surface by a predetermined amount. Further, the method includes removing the first photoresist film and the second photoresist film, and encapsulating the etched metal substrate using a molding compound exposing top surfaces and back surfaces of the island and the plurality leads.

[0006] Another aspect of the present disclosure includes a lead frame structure for semiconductor packaging. The lead frame includes an island and a plurality of leads formed based on a metal substrate by an etching process, and a molding compound used to encapsulate the etched metal substrate with the island and plurality of leads. The etching process is performed by: providing the metal substrate; forming a first photoresist film on a top surface of the metal substrate; forming a top surface etching pattern in the first photoresist film using photolithography; forming a second photoresist film on a back surface of the metal substrate; forming a back surface etching pattern in the second photoresist film using photolithography; performing an etching process on the top surface and the back surface of the metal substrate using the top surface etching pattern and the back surface etching pattern as the respective masks to form an island and a plurality of leads, where outer leads of the plurality of leads on the back surface extend to the proximity of the island such that a length of the outer leads is greater than a length of inner leads of the plurality of leads on the top surface by a predetermined amount; and removing the first photoresist film and the second photoresist film.

[0007] Other aspects of the present disclosure can be understood by those skilled in the art in light of the description, the claims, and the drawings of the present disclosure.

BRIEF DESCRIPTION OF THE DRA WINGS [0008] Figure 1 shows a conventional QFN lead frame;

[0009] Figure 2 shows a large-size chip mounted on the conventional QFN lead frame with mounting glue contamination;

[0010] Figure 3 illustrates an exemplary pre-encapsulated lead frame structure consistent with the disclosed embodiments;

[0011] Figure 4 illustrates a large-size chip mounted on the pre-encapsulated lead frame structure; and

[0012] Figure 5 shows an exemplary packaging process based on the pre-encapsulated lead frame structure consistent with the disclosed embodiments. DETAILED DESCRIPTION

[0013] Reference will now be made in detail to exemplary embodiments of the invention, which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. [0014] Fig. 3 illustrates an exemplary pre-encapsulated lead frame structure 100 consistent with the disclosed embodiments. As shown in Fig. 3, lead frame 100 includes a die pad or island 1, a plurality of leads 2, and molding compound filled in the areas between the island 1 and the leads and the areas between the leads of the plurality of leads 2. Other components may also be included and certain components may be omitted. For example, lead frame 100 may also include a first metal layer (not shown) on the top surfaces of the island 1 and leads 2 and a second metal layer (not shown) on the back surfaces of the island 1 and leads 2.

[0015] The island 1 and leads 2 may be made from a metal substrate (unnumbered). The molding compound may be filled up to the top surface and the back surface of the metal substrate (e.g., island 1 and leads 2) such that both the top surface and the back surface of the metal substrate are flat.

[0016] Lead frame structure 100 may be formed as a part of a packaging process or as an independent lead frame manufacturing process. Fig. 5 illustrates an exemplary packaging process or lead frame manufacturing process 200 consistent with the disclosed embodiments. As shown in Fig. 5, at the beginning of the packaging process 200, a metal substrate is provided (202).

[0017] The metal substrate may have a top surface and a back surface, and may also have a desired thickness. The metal substrate may be made of any appropriate metal materials, such as copper, aluminum, iron, copper alloy, stainless steel, or nickel-iron alloy. The particular metal used for metal substrate may be determined based on the functionalities and characteristics of the chips to be packaged.

[0018] After the metal substrate is provided (202), adhesive films may be attached to the top surface and the back surface of the metal substrate (204). More particularly, a first photoresist film may be attached to the top surface of the metal substrate, and a second photoresist film may be attached to the back surface of the metal substrate. The first photoresist film and the second photoresist film may be attached using certain film equipment to protect the substrate from a later etching process. Further, the first photoresist film and second photoresist film can be used in a photolithographic process, and any of the first photoresist film and second photoresist film may be a dry photoresist film or a wet photoresist film. Other types of film may also be used.

[0019] Further, etching patterns may be formed based on the first photoresist film and second photoresist film (206). More specifically, a top surface etching pattern may be formed using the first photoresist film and a back surface etching pattern may be formed using the second photoresist film via photolithography. That is, the etching patterns may be formed using a photolithographic process. A top surface etching mask is used to expose the first photoresist film, which is followed by development, and certain parts of the first photoresist film are removed to form the top surface etching pattern to uncover the areas on the metal substrate that need to be etched. Similarly, a back surface etching mask is used to expose the second photoresist film, which is followed by development, and certain parts of the second photoresist film are removed to form the back surface etching pattern to uncover the areas on the metal substrate that need to be etched. [0020] The top surface etching pattern corresponds to inner contact surface of the island 1 and inner leads of the plurality of leads 2, and the back surface etching pattern corresponds to outer contact surface of the island 1 and outer leads of the plurality of leads 2. The top surface etching pattern is designed in a way that the distance between the inner contact surface of the island 1 and inner leads of the plurality of leads 2 is substantially extended to fit a large chip. That is, the distance between the island 1 and inner leads of the plurality of leads 2 is within a predetermined range such that a large chip can be mounted on the island 1 without touching the plurality of leads 2. A large chip may refer to a chip having a surface area larger than the surface area of a corresponding island. [0021 ] On the other hand, the back surface etching pattern is designed in a way that the outer leads of the plurality of leads 2 extends to the proximity of the island 1 such that the length of the outer leads is larger than the length of the inner leads.

[0022] Further, an etching process may be performed on the top surface and back surface of the metal substrate (208). That is, the etching process is performed on the exposed areas on the top surface and back surface of the metal substrate, which are exposed by removing parts of the first and second photoresist films. The top surface of metal substrate is etched using the top surface etching pattern as a mask, and the back surface of metal substrate is etched using the back surface etching pattern as a mask.

[0023] The etching process may be a half etching and may be performed on both the top surface and the back surface simultaneously. After the etching process, the island 1, the plurality of leads 2, and certain etched areas are formed on the metal substrate. The plurality of leads 2 have smaller length on top surface of the metal substrate and larger length on back surface of the metal substrate. That is, the length of the inner leads is less than the length of the outer leads by a predetermined amount. The amount may be determined based on a particular application of the lead frame 100. For example, the lead frame 100 may have a maximum chip size that can be fitted into the lead frame 100 or onto the island 1, and the amount may be determined based on the maximum chip size. [0024] More specifically, the inner leads may retreat from the island 1 by a first value, and the outer leads may extend towards the island 1 by a second value such that the length of the outer leads is greater than the length of the inner leads based on the first value and the second value. Thus, with same size as conventional lead frames, such small-top-size and large-bottom- size lead structure can fit larger chip without causing any extra problems, such as mounting glue contamination, i.e., support a larger maximum chip size. Other types of etching, such as a full etching, may also be used.

[0025] After the etching process, the remaining adhesive films on the surfaces of metal substrate are removed (210). The films may be removed by mechanical means or chemical means. Further, the etched metal substrate is encapsulated in a molding process or a pre- encapsulating process (212). That is, molding compound 5 is filled into the etched metal substrate using a mold. The term "pre-encapsulation" refers to the encapsulation or molding process that is performed during the lead frame manufacturing to encapsulate the lead frame before other subsequent packaging processes, such as chip mounting.

[0026] After the molding compound is injected in the metal substrate, i.e., the lead frame is pre-encapsulated, the pre-encapsulated lead frame has a flat top surface comprising the exposed top surfaces of the island 1 and leads 2 and the molding compound, and a flat back surface comprising the exposed back surfaces of the island 1 and leads 2 and the molding compound. [0027] Optionally, if the lead frame 100 includes the first metal layer on the top surface of the metal substrate and the second metal layer on the back surface of the metal substrate, a plating process may be performed on the pre-encapsulated lead frame to form the first metal layer and the second metal layer. The plating process may include any appropriate type of electrical or chemical plating process, such as a multi-layer electrical plating process. The plating material may include gold, nickel-gold, nickel-palladium-gold, or silver. Other materials may also be used.

[0028] The first metal layer may be formed on the top surfaces of the island 1 and leads 2 to provide the inner contact surface for the island 1 and inner leads for leads 2. The second metal layer may be formed on back surfaces of the island 1 and leads 2 to provide the outer contact surface for the island and outer leads for leads 2. Alternatively, the first metal layer and the second metal layer may be formed on the top surface and the back surface of the metal substrate, respectively, before the etching process.

[0029] The pre-encapsulated lead frame 100 may then be used in other subsequent processes in the packaging process 200 (214). For example, as shown in Fig. 4, a die 3 may be attached on the island 1 using conductive or non-conductive adhesive material 4 in a die attaching process. Although the size of die 3 may be larger than the size of the island 1, the adhesive material 4 does not contaminate the inner leads of the plurality of leads 2. That is, even when the area of the die 3 is outside the island, the die 3 does not touch or is above any of the plurality of leads 2.

[0030] Further, the top surface of die 3 and the top surface of the inner leads of the plurality of leads 2 are connected with metal wires in a wire bonding process. The inner leads, the die 3, and the metal wires are then encapsulated using encapsulation material 5. For example, molding equipment may be used to seal or encapsulate the packaging structure by the molding compound 5. Post-molding curing may also be performed such that the molding compound 5 or other plastic encapsulation materials may also be cured.

INDUSTRIAL APPLICABILITY AND ADVANTAGEOUS EFFECTS

[0031 ] Without limiting the scope of any claim and/or the specification, examples of industrial applicability and certain advantageous effects of the disclosed embodiments are listed for illustrative purposes. Various alternations, modifications, or equivalents to the technical solutions of the disclosed embodiments can be obvious to those skilled in the art.

[0032] The disclosed lead frame structures and manufacturing methods may be applied in a variety of semiconductor packaging applications for different carriers with different substrates and filling materials. For example, the disclosed structures and methods may be used to package plastic leaded chip carrier (PLCC), plastic quad flat pack (PQFP), ball grid array (BGA), system in package (SiP) or 3D ICs, and multi-chip module, etc.

[0033] By using the disclosed methods and structures, a lead frame with island structure can be created using etching and pre-encapsulation. Such lead frame structures and methods can be suitable for mounting large-size chips. The outer leads extends to the proximity of the island such that the length of the outer leads is bigger than the length of the inner leads. That is, the plurality of leads have smaller length on top surface of the metal substrate and larger length on back surface of the metal substrate. Such small-top-size and large-bottom-size lead structure can fit larger chip without causing mounting glue contamination and the chip scale can also be increased. Reference Signs List

Island 21

Leads 22

Die 23

Adhesive material 24

Molding compound 25

Lead frame structure 100

Island 1

Leads 2

Die 3

Adhesive material 4

Molding compound 5

Packaging process 200

Providing a metal substrate 202

Attaching adhesive films 204

Forming etching patterns 206

Performing an etching process 208

Removing adhesive films 210

Pre-encapsulating the lead frame 212

Performing other packaging processes 214

Claims

What is claimed is:

1. A method for manufacturing a lead frame structure in a semiconductor packaging

process, comprising: providing a metal substrate having a top surface and a back surface; forming a first photoresist film on the top surface of the metal substrate; forming a top surface etching pattern in the first photoresist film using photolithography; forming a second photoresist film on the back surface of the metal substrate; forming a back surface etching pattern in the second photoresist film using

photolithography; performing an etching process on the top surface and the back surface of the metal substrate using the top surface etching pattern and the back surface etching pattern as the respective masks to form an island and a plurality of leads, wherein outer leads of the plurality of leads on the back surface extend to the proximity of the island such that a length of the outer leads is greater than a length of inner leads of the plurality of leads on the top surface by a predetermined amount; removing the first photoresist film and the second photoresist film; and

encapsulating the etched metal substrate using a molding compound exposing top surfaces and back surfaces of the island and the plurality leads.

2. The method according to claim 1, wherein: the etching process is a half etching process.

3. The method according to claim 2, wherein: the etching process is performed on the top surface and the back surface of the metal substrate simultaneously using the top surface etching pattern and the back surface etching pattern as the respective masks.

4. The method according to claim 1, further including: performing a plating process on the metal substrate to form a first metal layer on the top surface of the metal substrate and a second metal layer on the back surface of the metal substrate, wherein the first metal layer contains an inner surface of the island and the inner leads of the plurality of leads, and the second metal layer contains an outer surface of the island and the outer leads of the plurality of leads.

5. The method according to claim 1, further including: attaching a die on the island using an adhesive material in a die attaching process, wherein a substantial area of the die is outside the island without touching the plurality of leads; connecting a top surface of the die and top surfaces of the inner leads of the plurality of leads with metal wires in a wire bonding process; and encapsulating the inner leads, the die, and the metal wires using a molding compound.

6. A lead frame structure for semiconductor packaging, comprising: an island and a plurality of leads formed based on a metal substrate by an etching process; and

a molding compound used to encapsulate the etched metal substrate with the island and plurality of leads, wherein the etching process is performed by: providing the metal substrate;

forming a first photoresist film on a top surface of the metal substrate; forming a top surface etching pattern in the first photoresist film using

photolithography; forming a second photoresist film on a back surface of the metal substrate; forming a back surface etching pattern in the second photoresist film using photolithography; performing an etching process on the top surface and the back surface of the metal substrate using the top surface etching pattern and the back surface etching pattern as the respective masks to form an island and a plurality of leads, outer leads of the plurality of leads on the back surface extending to the proximity of the island such that a length of the outer leads is greater than a length of inner leads of the plurality of leads on the top surface by a predetermined amount; and removing the first photoresist film and the second photoresist film.

7. The lead frame structure according to claim 6, wherein: the etching process is a half etching process.

8. The lead frame structure according to claim 7, wherein: the etching process is performed on the top surface and the back surface of the metal substrate simultaneously using the top surface etching pattern and the back surface etching pattern as the respective masks.

9. The lead frame structure according to claim 6, further including: a first metal layer formed on the top surface of the metal substrate and a second metal layer formed on the back surface of the metal substrate by a plating process performed on the metal substrate, wherein the first metal layer contains an inner surface of the island and the inner leads of the plurality of leads, and the second metal layer contains an outer surface of the island and the outer leads of the plurality of leads.

10. The lead frame structure according to claim 6, further including: a die attached on the island using an adhesive material by a die attaching process, wherein a substantial area of the die is outside the island without touching the plurality of leads; metal wires connecting a top surface of the die and top surfaces of the inner leads of the plurality of leads by a wire bonding process; and a molding compound encapsulating the inner leads, the die, and the metal wires.