Packaging structure and preparation method thereof

Abstract

The invention discloses a packaging structure and a preparation method thereof, and the method comprises the steps: providing a substrate, and enabling the back surface of the substrate to be provided with a bonding pad; forming a first solder ball on the bonding pad; a first chip is mounted on the back face of the substrate, the first chip is electrically connected with the bonding pad, and the top face of the first welding ball is higher than the surface of the first chip; forming a first plastic package layer covering the first chip and the first solder balls on the back surface of the substrate; part of the first plastic package layer is removed, the end face of the first solder ball is exposed, and the end face of the first solder ball is flush with the surface of the first plastic package layer; the end face of the first solder ball is coated with scaling powder; and through a reflow soldering process, the first solder ball is melted and reflowed, the volume of the first solder ball is expanded under the action of the soldering flux to be reformed into a second solder ball, the top of the second solder ball protrudes out of the surface of the first plastic package layer, and the second solder ball is used for electrical lead-out of the package structure. The packaging steps of the back surface of the substrate in a packaging structure preparation process can be simplified, the packaging efficiency is improved, and the process and material cost is reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of semiconductor packaging, and more specifically, to a packaging structure and a preparation method thereof. Background Art

[0002] The existing process for processing DSM-BGA (Double Sided Molded Ball Grid Array) products will choose to plastic seal after the first ball planting, then grind the back of the substrate to expose the end face of the solder ball planted for the first time, and then use laser drilling technology to open holes at the ball planting position to further expose the solder ball. In order to ensure that the IO bumps protrude from the horizontal surface of the back side of the substrate, a secondary ball planting process is required, and the reflow process is used for curing and molding, such as Figure 1 As shown in FIG. 1 , the existing DSM-BGA product substrate backside packaging process includes: flux brushing - primary ball planting - surface mounting - primary reflow soldering - plasma treatment - plastic sealing - baking - grinding plastic sealing - laser opening - plasma cleaning opening - flux brushing to opening - low temperature reflow - secondary installation baking - secondary plasma cleaning treatment - secondary ball planting - secondary reflow soldering, wherein the main process steps of secondary ball planting on the back side of the substrate are as follows: Figure 2 As shown, including:

[0003] S01: one ball placement and one reflow;

[0004] S02: plastic sealing;

[0005] S03: Substrate grinding and laser drilling

[0006] S04: Secondary ball placement and secondary reflow.

[0007] It can be seen that the existing DSM-BGA product substrate backside packaging process requires two ball planting processes to meet the electrical lead-out design requirements of the packaging structure. There are problems such as complex packaging process steps, low production efficiency, and high process and material costs. Summary of the invention

[0008] The purpose of the present invention is to provide a packaging structure and a preparation method thereof, so as to simplify the packaging steps on the back side of the substrate in the preparation process of the packaging structure, improve the packaging efficiency and reduce the process and material costs.

[0009] To achieve the above objectives, in a first aspect, the present invention provides a method for preparing a packaging structure, comprising:

[0010] Providing a substrate, wherein a solder pad is provided on the back side of the substrate;

[0011] forming a first solder ball on the solder pad;

[0012] Mounting a first chip on the back side of the substrate, wherein the first chip is electrically connected to the pad, and the top surface of the first solder ball is higher than the surface of the first chip;

[0013] forming a first plastic packaging layer covering the first chip and the first solder ball on the back side of the substrate;

[0014] Removing a portion of the first plastic encapsulation layer to expose an end surface of the first solder ball, wherein the end surface of the first solder ball is flush with a surface of the first plastic encapsulation layer;

[0015] coating flux on the end surface of the first solder ball;

[0016] Through the reflow process, the first solder ball is melted and reflowed, and the volume expands and is reshaped into a second solder ball under the action of the flux. The top of the second solder ball protrudes from the surface of the first plastic packaging layer, and the second solder ball is used for electrical lead-out of the packaging structure.

[0017] Optionally, before applying soldering flux on the exposed end surface of the first solder ball, the method further comprises:

[0018] An opening is formed at the position of the first solder ball by using a laser drilling process, and the end surface of the first solder ball is exposed in the opening. The opening is used to expand the exposed area of ​​the end surface of the first solder ball.

[0019] Optionally, before forming the first solder ball on the solder pad, the method further includes:

[0020] Obtaining, through experiments, a corresponding relationship between the size of the first solder ball, the size of the solder pad, the size of the laser opening, and the amount of protrusion of the second solder ball from the surface of the first packaging layer;

[0021] The protrusion amount of the second solder ball from the surface of the first packaging layer is determined according to the process requirements, and the corresponding pad size, first solder ball size and laser opening size are set based on the determined protrusion amount and the corresponding relationship.

[0022] Optionally, removing part of the first plastic packaging layer to expose the end surface of the first solder ball includes:

[0023] The first plastic encapsulation layer is ground to remove a portion of the first plastic encapsulation layer to expose an end surface of the first solder ball.

[0024] Optionally, after forming an opening at the position of the first solder ball by using a laser drilling process, the method further includes:

[0025] The opening is plasma cleaned.

[0026] Optionally, forming a first solder ball on the solder pad includes:

[0027] Applying soldering flux on the pad;

[0028] Assembling solder balls on the pads by a ball planting process;

[0029] The solder ball is heated through a reflow process to make the solder ball melt and reflow to form the first solder ball.

[0030] Optionally, before forming a first solder ball on the pad, or before forming an opening at the position of the first solder ball by using a laser drilling process, the method further includes:

[0031] A second chip is mounted on the front surface of the substrate, and a second packaging layer covering the second chip is formed on the front surface of the substrate.

[0032] In a second aspect, the present invention further provides a packaging structure, which is formed by the method for preparing the packaging structure described in the first aspect.

[0033] The beneficial effects of the present invention are:

[0034] The packaging structure preparation method of the present invention first forms a first solder ball on the solder pad on the back of the substrate, then mounts the chip and forms a plastic sealing layer, then removes part of the first plastic sealing layer to expose the end face of the first solder ball, and then coats the end face of the solder ball with flux, and through a reflow process, the solder ball is melted and reflowed and reshaped under the action of the flux, and the top of the finally formed solder ball protrudes from the surface of the plastic sealing layer to achieve electrical lead-out of the packaging structure. Compared with the existing secondary ball planting process on the back of the substrate, the method of the present invention only needs to coat an appropriate amount of flux on the end face of the solder ball of the first ball planting before reflow soldering before the original secondary ball planting step. During the reflow soldering process, the solder ball melts and the volume expands with the flux to achieve the solder ball reshaping, thereby achieving the solder ball being higher than the surface of the back packaging layer, reaching a suitable protruding height to achieve the functional requirement of IO bonding (surface mounting) on ​​the subsequent product board, greatly simplifying the process steps of the second ball planting of the traditional preparation of DSM-BGA products based on the requirements of the two ball planting process, improving production efficiency and reducing process and material costs.

[0035] The system of the present invention has other characteristics and advantages, which will be apparent from the drawings incorporated herein and the following detailed description, or will be described in detail in the drawings incorporated herein and the following detailed description, which together serve to explain the specific principles of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The above and other objects, features and advantages of the present invention will become more apparent through a more detailed description of exemplary embodiments of the present invention in conjunction with the accompanying drawings, in which like reference numerals generally represent like components.

[0037] Figure 1 The figure shows the process flow chart of the back side packaging of the existing DSM-BGA product substrate.

[0038] Figure 2 The device structure diagram corresponding to the main process steps of secondary ball planting on the back side of the existing DSM-BGA product substrate is shown.

[0039] Figure 3 A step diagram of a method for preparing a packaging structure of the present invention is shown.

[0040] Figure 4 A complete process flow chart of a method for preparing a packaging structure according to an embodiment of the present invention is shown.

[0041] Figure 5 The device structure diagram corresponding to the main process steps of substrate ball implantation in a packaging structure preparation method according to an embodiment of the present invention is shown.

[0042] Figure 6 The actual structure diagram of the reshaped solder balls in a packaging structure preparation method according to an embodiment of the present invention is shown.

[0043] Figure 7 A top view of a ball grid array of reshaped solder balls in a method for preparing a packaging structure according to an embodiment of the present invention is shown.

[0044] Figure 8 The figure shows a fitting curve between the protrusion amount of the second solder ball and the size of the solder pad in a method for preparing a packaging structure according to an embodiment of the present invention. DETAILED DESCRIPTION

[0045] The present invention will be described in more detail below with reference to the accompanying drawings. Although preferred embodiments of the present invention are shown in the accompanying drawings, it should be understood that the present invention can be implemented in various forms and should not be limited by the embodiments set forth herein. On the contrary, these embodiments are provided to make the present invention more thorough and complete, and to fully convey the scope of the present invention to those skilled in the art.

[0046] Example 1

[0047] like Figure 3 As shown, the present invention provides a method for preparing a packaging structure, comprising:

[0048] S1: providing a substrate 1, wherein a pad 2 is provided on the back side of the substrate 1;

[0049] In this step, the provided substrate 1 can be any substrate 1 for making a double-sided molded ball grid array product, for example, the substrate 1 includes a redistribution layer, and the back of the substrate 1 can be used to install a first chip electrically connected to the redistribution layer, and the first chip can be a passive component chip, etc. The front of the substrate 1 can be used to install a second chip electrically connected to the redistribution layer, and the second chip can be a semiconductor chip, etc., and pads 2 are provided around the first chip on the back of the substrate 1, and the pads 2 can be a part of the redistribution layer, and the pads 2 are electrically connected to the first chip. The substrate 1 provided in this step is a substrate 1 whose back is to be processed to form an electrical lead-out structure (solder ball), and the related processes on the front of the substrate 1 will be described in subsequent steps.

[0050] Reference below Figure 4 and Figure 5 Perform the following process steps.

[0051] S2: forming a first solder ball 3 on the solder pad 2;

[0052] This step can be completed using existing ball placement and reflow soldering processes, specifically including:

[0053] S201: Apply flux on pad 2;

[0054] S202: assembling a solder ball on the pad 2 through a ball planting process; the main components of the solder ball are tin and lead;

[0055] S203 : heating the solder ball through a reflow process to melt the solder ball and reflow to form a first solder ball 3 .

[0056] S3: mounting a first chip on the back side of the substrate 1, the first chip is electrically connected to the pad 2, and the top surface of the first solder ball 3 is higher than the surface of the first chip;

[0057] In this step, the first chip can be mounted on the back of the substrate 1 by SMT (Surface Mounted Technology) process. Figure 4 As shown, this step can be completed between the above steps S202 and S203, that is, the first chip is mounted after the solder ball is assembled on the pad 2, and then the solder ball is subjected to a reflow process to form the first solder ball 3. It can also be completed after executing step S203, that is, the first chip is mounted on the back side of the substrate 1 after the first solder ball 3 is formed.

[0058] It should be noted that after completing step 203 to form the first solder ball 3 , it is necessary to perform a step of plasma treatment on the back side of the substrate 1 to remove residual solder particles on the back side of the substrate 1 .

[0059] S4: forming a first plastic packaging layer 4 covering the first chip and the first solder ball 3 on the back side of the substrate 1;

[0060] This step does not limit the plastic encapsulation process used to form the first plastic encapsulation layer 4. The first plastic encapsulation layer 4 can be formed on the back of the substrate 1 by a compression molding process, a transfer molding process, a liquid sealing molding process, a molded bottom filling process, a capillary bottom filling process, a vacuum lamination process or a spin coating process. In this embodiment, it is preferred to form the first plastic encapsulation layer 4 on the back of the substrate 1 by a molded bottom filling process. The advantage of using this plastic encapsulation process is that the plastic encapsulation material can be smoothly and quickly filled in the gap between the solder ball and the first chip, which can effectively avoid interface stratification, reduce the process difficulty, and can be used for a smaller connection gap, which is more suitable for a stacked structure. The material and specific layer structure of the first plastic encapsulation layer 4 can be selected according to actual process requirements. In one example, the first plastic encapsulation layer 4 can include a polyimide layer, a silicone layer, an epoxy resin layer, a curable polymer-based material layer or a curable resin-based material layer.

[0061] In this embodiment, after forming the first plastic sealing layer 4, a step of hardening the first plastic sealing layer 4 is required. The first plastic sealing layer 4 can be hardened and formed faster by baking and heating.

[0062] S5: removing a portion of the first plastic encapsulation layer 4 to expose the end surface of the first solder ball 3 , wherein the end surface of the first solder ball 3 is flush with the surface of the first plastic encapsulation layer 4 ;

[0063] Preferably, in this step, the first plastic encapsulation layer 4 can be ground and thinned by a grinding process to remove part of the first plastic encapsulation layer 4 and expose the end face of the first solder ball 3, so that the end face of the first solder ball 3 exposed in the first plastic encapsulation layer 4 is flush with the surface of the first plastic encapsulation layer 4.

[0064] like Figure 4 and Figure 5 As shown, in this embodiment, after the first plastic sealing layer 4 is ground and thinned in step S5, the method further includes:

[0065] The opening 5 is formed at the position of the first solder ball 3 by using a laser drilling process, and the end surface of the first solder ball 3 is exposed in the opening 5 . The opening 5 is used to expand the exposed area of ​​the end surface of the first solder ball 3 .

[0066] The first solder ball 3 can be further exposed through the laser opening 5, so that the first solder ball 3 can be re-molded by reflow soldering later. It should be noted that the laser opening 5 is formed by laser ablation of the first plastic sealing layer 4, and the formed opening 5 forms an annular groove around the first solder ball 3, thereby exposing the first solder ball 3.

[0067] Preferably, if Figure 4 As shown, after the opening 5 is formed at the position of the first solder ball 3 by using a laser drilling process, the method further includes:

[0068] The laser opening 5 is plasma cleaned to remove residual particle contamination generated by the laser opening 5 .

[0069] S6: coating flux on the end surface of the first solder ball 3;

[0070] In this step, the soldering flux used is a soldering flux commonly used in reflow soldering, such as rosin soldering flux, organic acid soldering flux or inorganic acid soldering flux.

[0071] In this step, a proper amount of solder flux may be brushed onto the exposed surface of the first solder ball 3 and the opening 5 around it. The solder flux may expand due to heating and generate volatile gas during the subsequent reflow process, thereby expanding the solder ball in a molten state.

[0072] S7: Through the reflow process, the first solder ball 3 is melted and reflowed, and the volume expands under the action of the flux and is reshaped into the second solder ball 6. The top of the second solder ball 6 protrudes from the surface of the first plastic packaging layer 4. The second solder ball 6 is used for electrical lead-out of the packaging structure.

[0073] In this step, during the reflow process, the first solder ball 3 is melted and then expands in volume under the action of the flux, so that the first solder ball 3 is reshaped into a second solder ball 6 with a larger volume, so that the reshaped second solder ball 6 is higher than the surface of the first plastic sealing layer 4, reaching the required protruding height to achieve the functional requirements of IO bonding (SMT) on the subsequent product. The actual structure of the solder ball (second solder ball 6) reshaped through the above steps in this embodiment is as follows Figure 6 As shown, the ball grid array formed on the back of the substrate 1 is as shown in FIG. Figure 7 shown.

[0074] In order to ensure that the protrusion of the second solder ball 6 from the surface of the first plastic sealing layer 4 after molding meets the subsequent IO bonding requirements, it is necessary to design appropriate sizes of the first solder ball 3 (including the lateral diameter and longitudinal height of the solder ball), the size of the solder pad 2, and the size of the laser opening 5 according to the required protrusion. Therefore, before forming the first solder ball 3 on the solder pad 2, the following steps are also included:

[0075] The corresponding relationship among the size of the first solder ball 3, the size of the solder pad 2, the size of the laser opening 5, and the protrusion amount of the second solder ball 6 protruding from the surface of the first packaging layer is obtained through experiments;

[0076] The protrusion amount of the second solder ball 6 from the surface of the first packaging layer is determined according to the process requirements. Based on the determined protrusion amount and the above correspondence, the corresponding pad 2 size, first solder ball 3 size and laser opening 5 size are set during the process.

[0077] Among them, the selection of the size of the first solder ball 3 during the first ball planting needs to be based on the size of the laser opening 5 after the first ball planting and the height of the final solder ball above the EMC part to select the appropriate solder ball size. The design of the size of the substrate 1 pad 2 needs to be based on the selection of the solder ball size, and the appropriate pad 2 size needs to be designed to match the appropriate opening 5 size and the effect of the later solder ball forming, to ensure the protrusion after the solder ball is formed. The drilling shape and size design of the laser drilling process needs to be designed in accordance with the solder ball diameter and related tolerances.

[0078] Specifically, the height increase data of the first solder balls 3 of different sizes after being remolded into the second solder balls 6 can be obtained through multiple experiments, and the height increase data is the protrusion amount of the second solder balls 6 protruding from the surface of the first packaging layer; then the pad 2 size data corresponding to each first solder ball 3 when the first solder balls 3 of different sizes are formed in the experiment is obtained, and the relevant data are shown in the following table. In the table, the solder ball diameter is the lateral diameter of the first solder ball 3, the solder ball height is the height from the bottom to the top of the first solder ball 3, and the protrusion height is the height of the second solder ball 6 protruding from the surface of the first plastic packaging layer 4, that is, the protrusion amount.

[0079] Relationship table between substrate pad and final solder ball height:

[0080]

[0081] Afterwards, according to the pad 2 size data corresponding to the first solder balls 3 of different sizes and the protrusion height (protrusion amount) data after being reshaped into the second solder balls 6, a fitting curve (binary function) between the protrusion amount of the second solder balls 6 and the size of the pad 2 can be obtained by a binary fitting method, such as Figure 8 As shown;

[0082] Then, based on the protrusion amount of the second solder ball 6 from the surface of the first plastic packaging layer 4 required by the process, the size of the solder pad 2 required by the process is determined according to the obtained binary function.

[0083] Then, the size of the first solder ball 3 and the size of the laser opening 5 can be selected according to the size of the solder pad 2, so as to ensure that the protrusion height of the first solder ball 3 protruding from the first plastic packaging layer 4 after being reshaped into the second solder ball 6 meets the design requirements.

[0084] In this embodiment, the steps of mounting a second chip on the front side of the substrate 1 and forming a first plastic packaging layer 4 are also included to form a double-sided packaging structure. The processing steps on the front side of the substrate 1 can be performed before step S2 (i.e., before forming the first solder ball 3 on the pad 2), or before the step of forming an opening 5 at the position of the first solder ball 3 using a laser drilling process. The packaging process on the front side of the substrate 1 includes: installing a second chip on the front side of the substrate 1, and forming a second packaging layer covering the second chip on the front side of the substrate 1. The process for forming the second packaging layer and the material of the second packaging layer can refer to the related processes of the first packaging layer mentioned above, and will not be repeated here.

[0085] Based on the above, the packaging structure preparation method of this embodiment only needs to apply a proper amount of flux on the end surface of the solder ball of the primary ball planting before the reflow soldering before the original secondary ball planting step in the traditional DSM-BGA product manufacturing process. During the reflow soldering process, the solder ball melts and the volume of the flux expands to achieve the solder ball re-molding, so that the solder ball is higher than the surface of the back packaging layer, reaching a suitable protruding height to achieve the functional requirements of IO bonding (surface mounting) on ​​the subsequent product board, thereby Figure 1 and Figure 4 By comparison, it can be seen that the present method eliminates multiple process steps related to the secondary ball planting process, namely, secondary mounting baking - secondary plasma cleaning - secondary ball planting - secondary reflow soldering, which greatly simplifies the process steps of the second ball planting of traditional DSM-BGA products based on the requirements of the two ball planting process, improves production efficiency and reduces process and material costs.

[0086] Example 2

[0087] This embodiment provides a packaging structure, and the packaging structure is formed by the preparation method of the packaging structure described in Embodiment 1.

[0088] Compared with the traditional DSM-BGA product, the packaging structure of this embodiment has higher production efficiency and lower process cost and material cost because the manufacturing process omits the traditional secondary ball planting related steps.

[0089] The embodiments of the present invention have been described above, and the above description is exemplary, not exhaustive, and is not limited to the disclosed embodiments. Many modifications and changes will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments.

Claims

1. A method for preparing a packaging structure, characterized in that: include: Providing a substrate, wherein a solder pad is provided on the back side of the substrate; forming a first solder ball on the solder pad; Mounting a first chip on the back side of the substrate, wherein the first chip is electrically connected to the pad, and the top surface of the first solder ball is higher than the surface of the first chip; forming a first plastic packaging layer covering the first chip and the first solder ball on the back side of the substrate; Removing a portion of the first plastic encapsulation layer to expose an end surface of the first solder ball, wherein the end surface of the first solder ball is flush with a surface of the first plastic encapsulation layer; coating flux on the end surface of the first solder ball; Through the reflow process, the first solder ball is melted and reflowed, and the volume expands and is reshaped into a second solder ball under the action of the flux. The top of the second solder ball protrudes from the surface of the first plastic packaging layer, and the second solder ball is used for electrical lead-out of the packaging structure.

2. The method for preparing a packaging structure according to claim 1, characterized in that: Before applying soldering flux on the exposed end surface of the first solder ball, the method further comprises: An opening is formed at the position of the first solder ball by using a laser drilling process, and the end surface of the first solder ball is exposed in the opening. The opening is used to expand the exposed area of ​​the end surface of the first solder ball.

3. The method for preparing a packaging structure according to claim 2, characterized in that: Before forming the first solder ball on the solder pad, the method further includes: Obtaining, through experiments, a corresponding relationship between the size of the first solder ball, the size of the solder pad, the size of the laser opening, and the amount of protrusion of the second solder ball from the surface of the first packaging layer; The protrusion amount of the second solder ball from the surface of the first packaging layer is determined according to the process requirements, and the corresponding pad size, first solder ball size and laser opening size are set based on the determined protrusion amount and the corresponding relationship.

4. The method for preparing a packaging structure according to claim 1, characterized in that: The step of removing a portion of the first plastic encapsulation layer to expose the end surface of the first solder ball includes: The first plastic encapsulation layer is ground to remove a portion of the first plastic encapsulation layer to expose an end surface of the first solder ball.

5. The method for preparing a packaging structure according to claim 2, characterized in that: After forming an opening at the position of the first solder ball by using a laser drilling process, the method further includes: The opening is plasma cleaned.

6. The method for preparing a packaging structure according to claim 1, characterized in that: Forming a first solder ball on the solder pad includes: Applying soldering flux on the pad; Assembling solder balls on the pads by a ball planting process; The solder ball is heated through a reflow process to make the solder ball melt and reflow to form the first solder ball.

7. The method for preparing a packaging structure according to claim 2, characterized in that: Before forming a first solder ball on the pad, or before forming an opening at the position of the first solder ball by using a laser drilling process, the method further includes: A second chip is mounted on the front surface of the substrate, and a second packaging layer covering the second chip is formed on the front surface of the substrate.

8. A packaging structure, characterized in that: The packaging structure is formed by the method for preparing a packaging structure according to any one of claims 1 to 7.

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