A lead frame and packaging method

Abstract

This application provides a lead frame and a packaging method. The lead frame adopts a stacked structure in the bus area of ​​the cut track: the back side is an all-metal bus providing structural strength, while the front bus is formed with several holes through semi-etching to reduce the area of ​​tin-plated metal in advance by physically eliminating these gaps. In the packaging method, the package array is first semi-cut to remove the back bus and expose the front bus; then, a laser is used to pre-cut the front bus to form pre-cut grooves; subsequently, tin plating is performed; finally, a mechanical full cut is performed along the center of the pre-cut grooves to complete the separation of the package units. This invention, by setting "holes" on the front bus and combining them with the "pre-cutting" packaging process, significantly reduces the volume of metal and tin that can be squeezed during the full-cut stage, thereby reducing the tendency of tin to extend and bridge. While ensuring side solderability, it effectively solves the problem of metal bridging, significantly improving product yield and reliability.

Description

Technical Field

[0001] This application relates to the field of semiconductor chip packaging, and in particular to a lead frame and packaging method. Background Technology

[0002] In the field of electronic packaging, Quad Flat No-lead (QFN) packages are widely used in integrated circuit packaging due to their small size, good heat dissipation, and excellent electrical performance. To further meet the requirements for solder joint inspectability in high-reliability applications such as automotive electronics, the industry has developed Wettable QFN packaging technology. This technology forms a solderable layer on the cut side of the package, allowing surface mount solder joints to form a visible meniscus on the package sidewall. This supports automated optical inspection equipment for full inspection of solder joint quality, significantly improving assembly reliability.

[0003] Typically, the mainstream process route for achieving wettable sidewalls in QFN packages employs a "half-cut - tin plating - full-cut" process. First, a half-cut is made along the lead frame dicing groove, creating a trench of a certain depth and exposing the copper in the dicing area. Then, the entire substrate is tin-plated, depositing a tin layer on the sidewalls and bottom of the half-cut trench. Finally, a second full cut is made along the centerline of the trench, completely separating each package unit, thus obtaining a wettable tin layer on the chip's sidewalls.

[0004] However, because the etched surface width created by the semi-etching process is relatively large, the exposed copper surface area after the semi-etching is correspondingly large. Since copper itself has excellent ductility, the larger exposed copper surface area during tin plating leads to a larger area of ​​deposited tin layer. During full-cutting, the high-speed rotation of the dicing blade squeezes and drags the tin plating layer, causing the ductile tin to be pushed between adjacent pins. When the tin plating area is too large, the squeezed tin is very likely to form metal bridges between adjacent pins, directly causing short circuit failure and severely affecting product yield and reliability. See also Figure 1 As shown.

[0005] To address the aforementioned issues, some industry solutions involve design improvements to the lead frame, such as reducing the width of the metal conductor in the cut track, or introducing stepped structures or periodically retaining all-metal reinforcing ribs in the cut track. However, these designs often require a trade-off between structural strength and cutting processability, resulting in limited improvement in the solder bridging problem. Other solutions attempt to replace electroplating with chemical immersion tinning to reduce the solder layer thickness; however, an excessively thin solder layer may lead to insufficient side solderability, making it difficult to meet the reliability testing requirements for solder joints. Summary of the Invention

[0006] The purpose of this application is to address the shortcomings of existing packaging technologies that easily cause metal short circuits between pins, and to provide a lead frame and packaging method.

[0007] To achieve the above objectives, this application adopts the following technical solution:

[0008] This application provides a lead frame, including: multiple chip pads, several pins respectively surrounding each of the chip pads, several pin ribs connecting two adjacent package unit pins, and several dicing bus located in the center of the dicing channel and parallel to the dicing channel direction. Each of the pins, pin ribs, and dicing bus includes a front stack and a back stack. The front stack and the back stack are stacked in the thickness direction of the lead frame. The back stack of the dicing bus is a back bus, and the front stack of the dicing bus is a front bus. The back bus is an all-metal layer, and the upper half of the front bus is etched with several holes.

[0009] In one possible implementation, the front stack of the pin is a front pin, and the back stack of the pin is a back pin; the front stack of the pin connecting rib is a front connecting rib, and the back stack of the pin connecting rib is a back connecting rib, and the width of the front connecting rib is less than 50% of the width of the base of the back pin, and less than 50% of the width of the base of the front pin.

[0010] In one possible implementation, each of the aforementioned perforations is located at the intersection of the front connecting rib and the front bus.

[0011] In one possible implementation, the cross-section of the perforation is circular, polygonal, teardrop-shaped, or irregular.

[0012] In one possible implementation, the maximum outer diameter of the perforation is 30%-80% of the width of the front connecting rib.

[0013] On the other hand, this application also provides a packaging method for a wettable side-wing QFN chip, comprising the following steps:

[0014] A package array is provided having cleavage metal to be removed, the cleavage metal comprising a front bus and a back bus stacked along the thickness direction of the package array.

[0015] Along the cutting direction, the encapsulated package array is half-cut to remove the back bus metal and expose the front bus metal. The front bus has several perforations etched on it.

[0016] A non-contact energy beam is used to pre-cut the front bus, removing some of the metal material from the cut path to form a pre-cut groove slightly smaller than the width of the front bus.

[0017] A layer of tin is uniformly attached to the back of the lead frame;

[0018] The entire array is cut off, removing the pin ribs between the front bus and the pins of the two package units, thus dividing the package array into multiple package units.

[0019] In one possible implementation, during the pre-cutting process, 40%-80% of the metal material of the front bus is removed.

[0020] In one possible implementation, the width of the pre-cut groove is half the width of the front bus, the depth of the pre-cut groove is half the thickness of the remaining cutting track metal after half-cutting, and the depth of the pre-cutting is less than the thickness of the front bus. More specifically, the pre-cut groove width < front bus width < full cut width < half-cut width.

[0021] In one possible implementation, the depth of the half-cut is greater than the thickness of the back bus.

[0022] In one possible implementation, the non-contact energy beam is a laser beam, and the full cutting is performed using mechanical cutting.

[0023] The advantages of this application compared with the prior art are: this application can effectively reduce the amount of residual copper while ensuring structural strength and side solderability, thereby reducing the tin plating area after half-cutting, and thus effectively solving the problem of metal bridging between adjacent pins of the chip after full-cutting. Attached Figure Description

[0024] Figure 1 This is a magnified image of the pin portion after a QFN chip has been fully cut in the prior art.

[0025] Figure 2 This is a schematic diagram of the back side of the packaging array provided in one embodiment of this application.

[0026] Figure 3 This is a front view of the lead frame provided in one embodiment of this application.

[0027] Figure 4 This is a front view of the pin connections in one embodiment of this application.

[0028] Figure 5 This is a schematic diagram of the pin connections on the back of one embodiment of this application.

[0029] Figure 6 Before cutting, Figure 5 Cross-sectional view at point AA.

[0030] Figure 7 After being cut in half, Figure 5 Cross-sectional view at point AA.

[0031] Figure 8 For pre-cutting, Figure 5 Cross-sectional view at point AA.

[0032] Figure 9 After tin plating, Figure 5 Cross-sectional view at point AA.

[0033] Figure 10 After complete resection, Figure 5 Cross-sectional view at point AA.

[0034] Figure 11 A magnified photograph of a portion of the chip pins after a full cut, provided for one embodiment of this application.

[0035] Wherein: 100. Package array; 101. Package unit; 10. Lead frame; 11. Chip pad; 12. Pin; 121. Back pin; 122. Front pin; 13. Cutaway bus; 131. Back bus; 132. Front bus; 1321. Through hole; 14. Pin rib; 141. Back rib; 142. Front rib; 15. Half slot; 16. Pre-cut slot; 17. Full slot; 20. Encapsulant; 30. Tin layer; 31. Metal bridge. Detailed Implementation

[0036] To illustrate the technical content, structural features, achieved objectives, and effects of the invention in detail, the technical solutions of the embodiments of this application will be described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. In the following description, for illustrative purposes, numerous specific details are set forth to provide a detailed description of various exemplary embodiments or implementations of the invention. However, various exemplary embodiments may also be implemented independently without these specific details or in one or more equivalent arrangements. Furthermore, the various exemplary embodiments may differ, but are not necessarily exclusive. For example, the specific shape, construction, and characteristics of the exemplary embodiments may be used or implemented in another exemplary embodiment without departing from the inventive concept.

[0037] This embodiment provides a lead frame for a wettable side-wing QFN chip and its packaging method. The structure and packaging process of this lead frame are designed to solve the problem of metal bridging short circuits between pins during the final full cut due to excessive tin plating area.

[0038] The structure of the wettable side-wing QFN chip includes a lead frame, a chip soldered to the front of the lead frame, and an encapsulating colloid 20 for encapsulating the chip. Chip pads 11 and a number of pins 12 are formed on the back and sides of the QFN chip. A metal tin layer 30 is attached to the back of the chip pads, the back of the pins, and the sides.

[0039] Figure 2 The diagram shown is a schematic of the back side of an uncut package array 100. A package array 100 may contain several package units 101 with identical structures, i.e., QFN chips. In the latter half of the packaging process, the area between two adjacent package units 101 is fully cut to separate the package units 101. Figure 2 In the package array 100 shown, the dicing channels include two dicing channels along the X and Y directions, respectively.

[0040] Figure 3 As shown, a lead frame 10 provided in one embodiment of this application includes: a plurality of chip pads 11 for carrying chips, a plurality of pins 12 distributed around each chip pad 11, a plurality of pin ribs 14 connecting two adjacent package unit 101 pins 12, and a plurality of dicing busbars 13 located in the center of the dicing traverse and parallel to the dicing traverse direction. The lead frame 10 is typically made of copper or copper alloy.

[0041] The subsequent cutting and separation of the dicing channels is located between the pins 12 of two adjacent packaging units 101, including some pin connecting ribs 14 and the entire dicing bus 13. The pin connecting ribs 14 extend in a direction perpendicular to the dicing channel, directly connecting the corresponding pins 12 of the two adjacent packaging units 101. The dicing bus 13 is parallel to the direction of the dicing channel and extends along the center of the dicing channel. The dicing bus 13 intersects with all the pin connecting ribs 14 in the same row to form a supporting backbone. The dicing bus 13 must ensure a certain structural strength, be easy to cut, and reduce the amount of tin plating after cutting.

[0042] In one embodiment, the width of the slit bus 13 is 150 micrometers to 200 micrometers, and the width of the pin rib 14 is 100 micrometers to 140 micrometers. Preferably, the width of the slit bus 13 is 200 micrometers, and the width of the pin rib 14 is 120 micrometers.

[0043] In this application, each pin 12, pin rib 14 and cut-out bus 13 includes a front stack and a back stack, and the front stack and the back stack are stacked in the thickness direction of the lead frame 10.

[0044] Furthermore, the back stack of the dicing bus 13 is a back bus 131, and the front stack of the dicing bus is a front bus 132. In one embodiment of this application, the back bus 131 is an all-metal layer (retaining the full thickness of the base layer), while the front bus 132 is a half-etched layer (half-etched to form a pattern layer), with a plurality of holes 1321 etched on its upper half, the holes 1321 penetrating the front bus 132 in the thickness direction. In the dicing bus region, since the back stack retains the original full thickness of the lead frame, forming an all-metal layer, it provides the necessary structural strength for the entire strip in the early stages of the packaging process.

[0045] Preferably, the cutout 1321 on the front bus 132 is located at the intersection of each of the pin connecting ribs 14 and the front bus 132. Its cross-section can be circular, square, pentagonal, hexagonal, teardrop-shaped, or any irregular shape, and its maximum outer diameter is designed to be 30%-80% of the width of the front connecting rib 142. The function of this cutout is to reduce the area of ​​metal that can be tinned and stretched by extrusion through physical gaps in the subsequent tin plating process, thus forming a weak point.

[0046] See Figure 3 , 4 As shown, the front stack of pin 12 is front pin 122, and the back stack of pin 12 is back pin 121. The front stack of pin connecting rib 14 is front connecting rib 142, and the back stack of pin connecting rib 14 is back connecting rib 141. The width of the main body of the front pin 122 is greater than the width of the main body of the back pin 121. The front connecting rib 142 connects two front pins 122, and the width of the front connecting rib 142 is less than 50% of the width of the main body of the front pin 122. The back connecting rib 141 connects two back pins 121, and the width of the front connecting rib 142 is less than 50% of the width of the base of the back pin 121. Due to the reduction in the width of the connecting rib, to ensure support strength, the widths of the front connecting rib and the back connecting rib are the same, and both the front connecting rib and the back connecting rib are entirely metal.

[0047] The front bus 132 and the back bus 131 have the same width, and the cut-out bus 13 intersects with each pin connecting rib 14 in a cross shape. Each cutout 1321 is located at the intersection of the front connecting rib 142 and the front bus 132.

[0048] In one embodiment of this application, a packaging method for a wettable side-wing QFN chip is also disclosed, comprising the following steps:

[0049] Step 1: Provide a package array 100, such as Figure 1 , 6 As shown, the package array 100 includes multiple package units 101 that have undergone chip mounting, wire bonding, and encapsulation with encapsulating adhesive 20. The lead frame 10 structure of the package array 100 is as described above.

[0050] Multiple package units 101 have cleavage metal to be removed: including cleavage bus 13 and partial pin ribs 14, the cleavage bus 13 including front bus 132 and back bus 131 stacked along the thickness direction of the package array.

[0051] Step 2, as follows Figure 5 , 7 As shown, the encapsulated package array 100 is half-cut along the cutting direction, forming a half-cut groove 15. The half-cut can be performed using a mechanical dicing blade, and the width of the half-cut groove 15 is defined as the half-cut width.

[0052] The depth of the half-cut is greater than the thickness of the back bus 131 to remove the metal of the back bus 131 and the back connecting rib 141, and to expose the metal of the front bus 132 and the front connecting rib 142. Because the half-cut removes part of the thickness of the front bus, the front pins can form a stepped structure, allowing the full cut to occur at a thinner position.

[0053] Step 3: Using a non-contact energy beam, pre-cut the front bus 132 to further remove some of the metal material on the front bus 132, forming a pre-cut groove 16 slightly smaller than the width of the front bus 132, such as... Figure 5 , 8 As shown.

[0054] The non-contact energy beam can be a laser beam or a plasma beam. In this embodiment, laser beam cutting is preferred. The laser beam scans along the centerline of the cutting path, vaporizing and removing a portion of the metal material on the front bus. The laser energy can be selected as 100W, the laser wavelength can be green light, and the spot size can be selected as 1 / 2 of the width of the cutting path bus (front bus).

[0055] The width of the pre-cut groove 16 must be smaller than the width of the front bus 132 and also smaller than the half-cut width, and the pre-cut depth must be smaller than the thickness of the front bus. In other words, the laser does not cut away the entire front bus 132, but rather creates a narrower groove in the middle to remove most of the metal material. Preferably, during the pre-cutting process, 40%-80% of the metal material of the front bus is removed. These remaining extremely narrow metal strips become the final connection points that need to be cut off during the subsequent mechanical full cut. Before the full cut, using a laser to pre-cut a narrow groove in the cutting path to remove most of the extended metal can reduce the amount of subsequent tin plating. Since laser processing has no mechanical stress, it will not produce extrusion deformation or burrs.

[0056] Preferably, the width of the pre-cut groove is 1 / 2 of the width of the cutting track bus, and the depth of the pre-cut groove is 1 / 2 of the thickness of the remaining cutting track bus after half-cutting.

[0057] Step 4: Apply a uniform layer of tin 30 to the back of the lead frame.

[0058] like Figure 9 As shown, a layer of tin 30 is uniformly attached to the exposed surfaces of the entire lead frame, including the back and sidewalls of the back pins 121, the back and sidewalls of the front connecting ribs 142, and the sidewalls of the pre-cut grooves 16.

[0059] Since the pre-cutting has removed most of the metal material of the front bus 132, leaving only two extremely narrow metal strips as conduction paths, the area of ​​the tin layer that can be squeezed by the subsequent full cutter on these two narrow surfaces is minimized after tin plating.

[0060] This step can be achieved using either chemical immersion tinning or electroplating. In chemical immersion tinning, a uniform pure tin layer of 7-13 μm is deposited through a displacement reaction. The tin layer produced by immersion tinning is uniform and thin, which can significantly reduce the amount of tin squeezed out during full cutting.

[0061] Step 5: Using a mechanical dicing blade, perform a full cut along the center line of the pre-cut groove 16 to remove the front bus 132 and the front connecting rib 142 between the two front pins 122, dividing the package array 100 into multiple package units 101a and 101b, as shown below. Figure 10 As shown.

[0062] The width of the full cutter is greater than the width of the pre-cut slot 16, but less than the width of the half-cut slot 15. The full cutter only needs to cut the extremely narrow metal material with a thin tin layer on both sides of the pre-cut slot 16, thereby completely separating the adjacent package units 101.

[0063] In a preferred embodiment, the full-cutting step also includes a cooling process to reduce the ductility of the tin. Using an ice water machine to spray water for cooling can promptly remove chips, cool the blades, and effectively prevent secondary adhesion of the tin.

[0064] Since the area of ​​the metal to be cut and the area of ​​the tin layer have been reduced by 40%-80% through pre-cutting, the squeezing and dragging effect of the mechanical blade on the tin during the full cut is greatly reduced. The small amount of tin squeezed out cannot form a bridge that can cross the pin gap, thus effectively solving the bridging problem, while ensuring that the package side has good solderability due to the integrity of the tin plating layer.

[0065] After the above steps, the fully cut packaging unit is obtained. Figure 11 The photo shows the chip pins, and it can be seen that there is no metal residue between the pins.

[0066] Step 6: Perform plasma cleaning on the separated packaging unit 101 to effectively remove the tiny solder beads remaining in the cutting channel and eliminate bridging risks.

[0067] In some embodiments, the cleaned packaging unit may be further buffed to remove residual copper or tin burrs, further reducing the risk of short circuits and having little impact on the plating quality.

[0068] Step 7: Detect solder bridging defects online using automated optical inspection (AOI) equipment to achieve a zero-defect quality control closed loop.

[0069] In this application, the structure and total amount of metal in the cutting path are changed by "drilling" and "pre-cutting" to reduce the amount of tin plating, thereby avoiding the formation of tin bridges between pins. The resulting wettable sidewall QFN chip has continuous and full sidewall solder joints, which can significantly improve production yield and product manufacturing efficiency through rigorous AOI inspection.

[0070] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope. The scope of protection of the present invention is defined by the appended claims, specification, and their equivalents.

Claims

1. A lead frame, comprising: The package includes multiple chip pads, several pins surrounding each chip pad, several pin ribs connecting two adjacent package unit pins, and several dicing busbars located in the center of the dicing channel and parallel to the dicing channel direction. The pins, pin ribs, and dicing busbars each include a front stack and a back stack, the front stack and back stack being stacked in the thickness direction of the lead frame. The back stack of the dicing busbar is a back busbar, and the front stack of the dicing busbar is a front busbar. The back busbar is an all-metal layer, and the upper half of the front busbar is etched with several perforations.

2. The lead frame according to claim 1, characterized in that: The front stack of the pin is a front pin, and the back stack of the pin is a back pin; the front stack of the pin connecting rib is a front connecting rib, and the back stack of the pin connecting rib is a back connecting rib, and the width of the front connecting rib is less than 50% of the width of the base of the back pin, and less than 50% of the width of the base of the front pin.

3. The lead frame according to claim 2, characterized in that: Each of the aforementioned perforations is located at the intersection of the front connecting rib and the front bus.

4. The lead frame according to claim 1 or 3, characterized in that: The cross-section of the perforation can be circular, polygonal, teardrop-shaped, or irregular.

5. The lead frame according to claim 4, characterized in that: The maximum outer diameter of the perforation is 30%-80% of the width of the front connecting rib.

6. A packaging method for a wettable side-wing QFN chip, characterized in that, Includes the following steps: A package array is provided having cleavage metal to be removed, the cleavage metal comprising a front bus and a back bus stacked along the thickness direction of the package array. Along the cutting direction, the encapsulated package array is half-cut to remove the back bus metal and expose the front bus metal. The front bus has several perforations etched on it. A non-contact energy beam is used to pre-cut the front bus, removing some of the metal material from the cut path to form a pre-cut groove slightly smaller than the width of the front bus. A layer of tin is uniformly attached to the back of the lead frame; The entire array is cut off, removing the pin ribs between the front bus and the pins of the two package units, thus dividing the package array into multiple package units.

7. The packaging method according to claim 6, characterized in that: During the pre-cutting process, 40%-80% of the metal material of the front bus is removed.

8. The packaging method according to claim 6, characterized in that: The width of the pre-cut groove is half the width of the front bus, and the depth of the pre-cut groove is half the thickness of the remaining cutting track metal after half-cutting.

9. The packaging method according to claim 6, characterized in that: The depth of the half-cut is greater than the thickness of the back bus.

10. The packaging method according to claim 6, characterized in that: The non-contact energy beam is a laser beam, and the full cutting is performed using mechanical cutting.

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