Manufacturing method of high-shielding embedded copper block circuit board

Abstract

The invention discloses a manufacturing method of a high-shielding embedded copper block circuit board, and the method comprises the steps: enabling a prefabricated windowing prepreg, a first core plate, a U-shaped copper block and a second core plate to be sequentially laminated and pressed from top to bottom according to corresponding patterns, and forming a pressed plate; the insulating dielectric layer of the first core board faces the through groove structure of the U-shaped copper block, and the second circuit pattern of the second core board faces the bottom surface of the U-shaped copper block; the laminated plate is sequentially polished twice and then electroplated to form an electroplated plate; making a surface circuit pattern on the electroplated plate to form a circuit pattern plate; pressing a surface prepreg on the surface of the copper block, and then polishing for the third time to form a high-shielding embedded copper block circuit board; a copper block is made into a U-shaped copper block, the U-shaped copper block and a multi-layer structure are pressed to form a copper block embedding state, then a surface copper layer is electroplated, a closed structure is formed, and the high-shielding signal transmission effect of a circuit is achieved; the flattening treatment is designed to be multiple times of polishing and multiple times of electroplating, effective cooperation is formed before and after the pressing process, and high reliability of installation and application is achieved.

Description

Technical Field

[0001] The invention relates to the field of circuit board manufacturing, and in particular to a method for manufacturing a high-shielding embedded copper block circuit board. Background Art

[0002] For a type of circuit board that needs to be used in high-precision communications or medical modules, it is usually required to have a high shielding structure to achieve high-fidelity and high-intensity signal transmission capabilities, that is, the circuit board has a high shielding area and a flat surface.

[0003] For this type of circuit board, the commonly used production method is: to form the circuit that needs to be shielded, and then use a shielding film or a shielding circuit graphic layer to form a shielding effect; the surface flattening production method is to scrape the surface solder mask or increase the pressing force to form a flat surface.

[0004] The shielding effect layer formed by the above-mentioned production method generally only achieves the shielding effect in the planar direction. In order to improve the line shielding effect, the method of increasing the line spacing or increasing the circuit board area is usually adopted, but it will lead to a decrease in the line density of the circuit board and an excessive size, which is not conducive to the processing requirements of high-precision and small-size circuit board products; and as for the production effect of surface flatness, it is easy to cause the solder mask to be recessed and the solder mask to cover the surface line, affecting the use of the surface line.

[0005] Therefore, in order to solve the problems raised by the above background technology, it is necessary to provide a method for manufacturing a high-shielding embedded copper block circuit board. Summary of the invention

[0006] The present invention aims to solve the problem that the flexible circuit board in the prior art requires high-fidelity signals, convenient processing and other comprehensive performances, and proposes a method for manufacturing a high-shielding embedded copper block circuit board, the manufacturing method comprising the following steps: S10: taking a copper block and processing it into a U-shaped copper block having a through groove structure; S20: taking a prepreg, making a prepreg window pattern, and forming a windowed prepreg, wherein the prepreg window pattern corresponds to the side wall top surface pattern of the U-shaped copper block; S30: taking a single-sided copper-clad laminate, making a first circuit pattern on its copper layer, and making a core board window pattern corresponding to the prepreg window pattern on its insulating dielectric layer, to form a first core board; S40: taking a double-sided copper-clad board and making a second circuit pattern on one copper layer of the double-sided copper-clad board to form a second core board; S50: stacking the windowed prepreg, the first core board, the U-shaped copper block and the second core board in sequence from top to bottom to form a stacked structure, and pressing them together to form a pressed board; the insulating dielectric layer of the first core board faces the through-groove structure of the U-shaped copper block, and the second circuit pattern of the second core board faces the bottom surface of the U-shaped copper block; the window pattern of the prepreg, the window pattern of the core board and the top surface of the side wall of the U-shaped copper block are stacked correspondingly; one side of the prepreg of the pressed board is the upper surface, and the other side is the lower surface; S60: performing a first grinding on the upper surface to expose the top surface of the side wall of the U-shaped copper block, then performing a first electroplating to form a first electroplated copper layer, and then performing a second grinding and a second electroplating process to form an electroplated plate; S70: forming a surface circuit pattern on the surface copper layer of the electroplating board to form a circuit pattern board; S80: pressing a surface prepreg onto the surface of the circuit pattern board, and then performing a third polishing to form the high-shielding embedded copper block circuit board.

[0007] Furthermore, the top surface of the side wall of the U-shaped copper block is a serrated edge.

[0008] Furthermore, the first grinding is to completely grind the serrated edge.

[0009] Furthermore, the outer surface of the U-shaped copper block is a continuous concave-convex pattern.

[0010] Furthermore, the semi-cured sheet is a second single-sided copper-clad laminate, and a circuit pattern is made on its copper layer to form a reinforced circuit, and the reinforced circuit is located within the range of the through-groove structure of the U-shaped copper block; the insulating dielectric layer of the second single-sided copper-clad laminate faces the first core board.

[0011] Furthermore, the stacked structure includes: a first release film, a coating film, a second release film and a first steel plate are sequentially arranged on the windowed semi-cured sheet to form an upper surface auxiliary pressing structure; a third release film and a second steel plate are sequentially arranged under the second core plate to form a lower surface auxiliary pressing structure.

[0012] Furthermore, the first electroplating is to electroplate the upper surface.

[0013] Furthermore, the thickness of the copper layer electroplated for the first time is 15 microns to 20 microns.

[0014] Furthermore, the second electroplating is whole-board electroplating.

[0015] Furthermore, the second polishing is for polishing the first electroplated copper layer, and the third polishing is for polishing the surface prepreg.

[0016] The technical solution of the present invention realizes the signal transmission effect of this type of circuit by forming a U-shaped copper block, pressing it with a multi-layer structure to form a buried copper block, and then electroplating a surface copper layer to form a closed structure, thereby solving the problem that the prior art uses a shielding film or a shielding circuit graphic layer to form a shielding effect, and improves the shielding performance by increasing the circuit gap or increasing the circuit board area; by designing the flattening process as multiple grinding and multiple electroplating methods, effective coordination is formed before and after the pressing process, and an overall effect is achieved, reducing the processing steps, and achieving high reliability performance during installation and application, solving the problem that the prior art uses scraping or increasing the pressing force to flatten the surface, resulting in a depression in the solder mask. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the structures shown in these drawings without paying creative work.

[0018] Figure 1 It is a key process flow chart of an embodiment of the present invention; Figure 2 Schematic diagram of the three-dimensional structure of a U-shaped copper block according to an embodiment of the present invention; Figure 3 It is a schematic diagram of the side structure of a U-shaped copper block with a serrated edge according to an embodiment of the present invention; Figure 4 This is a schematic diagram of the main structure of a U-shaped copper block with a concave-convex pattern according to an embodiment of the present invention; Figure 5 It is a schematic diagram of the typesetting structure of an embodiment of the present invention; Figure 6 is a schematic cross-sectional view of a pressed plate according to an embodiment of the present invention; Figure 7 is a schematic cross-sectional view of a pressed plate with a reinforced circuit according to an embodiment of the present invention; Figure 8 is a schematic cross-sectional view of a first grinding plate according to an embodiment of the present invention; Fig. 9 is a schematic cross-sectional view of a first electroplating plate according to an embodiment of the present invention; Fig.10 is a cross-sectional schematic diagram of a second electroplating plate according to an embodiment of the present invention; Fig.11 is a cross-sectional schematic diagram of a circuit pattern board according to an embodiment of the present invention; Fig.12It is a schematic diagram of the cross-sectional structure of a high-shielding embedded copper block circuit board according to an embodiment of the present invention.

[0019] Description of Figure Numbers:

[0020] The realization of the purpose, functional features and advantages of the present invention will be further explained in conjunction with embodiments and with reference to the accompanying drawings. DETAILED DESCRIPTION

[0021] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0022] It should be noted that all directional indications in the embodiments of the present invention (such as up, down, left, right, front, back, inside, outside...) are only used to explain the relative position relationship, movement status, etc. between the components in a certain specific posture (as shown in the accompanying drawings). If the specific posture changes, the directional indication will also change accordingly.

[0023] In addition, in the present invention, descriptions such as "first", "second", etc. are only used for descriptive purposes and cannot be understood as indicating or implying their relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" or "second" may explicitly or implicitly include at least one of the features. In the description of the present invention, the meaning of "plurality" is at least two, such as two, three, etc., unless otherwise clearly and specifically defined.

[0024] In addition, the technical solutions between the various embodiments of the present invention can be combined with each other, but it must be based on the fact that ordinary technicians in the field can implement it. When the combination of technical solutions is contradictory or cannot be implemented, it should be deemed that such combination of technical solutions does not exist and is not within the scope of protection required by the present invention.

[0025] See also Figure 1 , Figure 1 Schematic diagram of the process flow of an embodiment of the present invention.

[0026] The manufacturing process of the embodiment of the present invention includes using Figure 1 The following will implement the various steps in the process Figure 1 Each step in the process is further explained step by step.

[0027] See also Figure 2 , Figure 2It is a schematic diagram of the three-dimensional structure of a U-shaped copper block according to an embodiment of the present invention.

[0028] Step S10: A copper block is taken and processed to form a U-shaped copper block 100 with a through-slot structure. In this embodiment, the copper block is milled or etched to remove excess copper, thereby forming a U-shaped through-slot structure as a whole, which provides a spatial basis for subsequently burying the circuit inside the through-slot structure.

[0029] See also Figure 3 , Figure 3 It is a schematic diagram of the side structure of a U-shaped copper block with a serrated edge according to an embodiment of the present invention.

[0030] It is worth noting that the top surface of the side wall of the U-shaped copper block 100 is a serrated edge 110; by designing the edge of the copper block as a serrated edge 110, the contact area between the U-shaped copper block 100 and the insulating medium layer is increased, and the U-shaped copper block 100 has a certain pressure-bearing and buffering function during pressing, which is used to buffer the pressure it is subjected to during the pressing process.

[0031] See also Figure 4 , Figure 4 It is a schematic diagram of the main structure of a U-shaped copper block with a concave-convex pattern according to an embodiment of the present invention.

[0032] In one embodiment, the outer surface of the U-shaped copper block 100 is a continuous concave-convex pattern 120, such as a regular wavy pattern, which is similar to the design effect of the serrated edge 110 mentioned above. The edge of the concave-convex pattern 120 can increase the contact area between the outer surface of the U-shaped copper block 100 and the insulating dielectric layer, so that the U-shaped copper block 100 and the insulating dielectric layer can fit more closely together during pressing, avoiding deviation due to the side wall being too smooth during pressing, thereby improving the stability and firmness of the pressing.

[0033] See also Figure 5 , Figure 5 It is a schematic diagram of the typesetting structure of an implementation mode of the present invention.

[0034] Depend on Figure 5 It can be seen that the stacked structure composed of the prepreg, the first core board, and the second core board, as well as the auxiliary pressing structure on the upper and lower surfaces thereof.

[0035] Step S20: Take a semi-cured sheet and make a semi-cured sheet window pattern to form a windowed semi-cured sheet 200. The semi-cured sheet is a necessary layer for laminating multi-layer circuit boards. During lamination, the semi-cured sheet undergoes heat melting, flowing, and solidification in sequence, and finally the multi-layer boards are bonded together to form a reliable insulating layer. The window pattern of the windowed semi-cured sheet 200 corresponds to the side wall top surface pattern of the U-shaped copper block 100, which provides a lamination basis for the subsequent lamination of the windowed semi-cured sheet 200 and the U-shaped copper block 100 together without the windowed semi-cured sheet 200 covering the side wall top surface of the U-shaped copper block 100.

[0036] Step S30: A single-sided copper clad laminate is taken, and a first circuit pattern 3010 is made on its copper layer, and a core board window pattern 3020 corresponding to the semi-cured sheet window pattern is made on its insulating dielectric layer to form a first core board 300; in the present embodiment, the first core board 300 is a single-sided copper clad laminate. Considering the bonding effect between layers, subsequent pressing requires the insulating dielectric layer of the single-sided copper clad laminate to be laminated and pressed with the copper surface of the U-shaped copper block 100. If the other side is also set as a copper layer, then during pressing, the copper layer will contact the copper surface of the bottom surface of the U-shaped copper block 100, and no interlayer bonding effect can be formed. In addition, the first circuit pattern 3010 corresponding to the through-groove structure cannot form a high-shielding circuit subsequently.

[0037] Step S40: A double-sided copper clad laminate is taken, and a second circuit pattern 4010 is made on one side of the copper layer to form a second core board 400. In the present embodiment, the copper layers on both sides of the second core board 400 respectively play different roles, one side is used to make the second circuit pattern 4010, and the other side provides a basic copper layer for subsequent electroplating of copper layers and making surface circuit patterns, making the processing more convenient, and continuing electroplating on the original copper layer can ensure the flatness and uniformity of the electroplating.

[0038] See also Figure 6 and Figure 7 , Figure 6 is a schematic cross-sectional view of a pressed plate according to an embodiment of the present invention; Figure 7 It is a schematic cross-sectional view of a laminated plate with a reinforced circuit according to an embodiment of the present invention.

[0039] Step S50: The windowed semi-cured sheet 200, the first core board 300, the U-shaped copper block 100 and the second core board 400 are stacked in sequence from top to bottom to form a stacked structure 20, and pressed together to form a pressed board 30; the insulating medium layer of the first core board 300 faces the through-groove structure of the U-shaped copper block 100, and the second circuit pattern 4010 of the second core board 400 faces the bottom surface of the U-shaped copper block 100; the pattern of the windowed semi-cured sheet 200, the core board window pattern 3020 and the top surface of the side wall of the U-shaped copper block 100 are stacked correspondingly; one side of the semi-cured sheet of the pressed board 30 is the upper surface, and the other side is the lower surface.

[0040] In one embodiment, the semi-cured sheet is a second single-sided copper clad laminate, and a circuit pattern is made on its copper layer to form a reinforced circuit 210. After lamination, the reinforced circuit 210 is arranged within the range of the through-groove structure of the U-shaped copper block 100; that is, the reinforced circuit 210 is located above the middle area of ​​the through-groove structure, providing stronger copper surface adhesion for the subsequent first electroplating, ensuring that the electroplated copper layer within the surface range of the through-groove structure opening of the U-shaped copper block 100 has stronger adhesion, and preventing problems such as copper bubbles and falling off during subsequent polishing and circuit board application processing; at the same time, it is worth noting that the insulating dielectric layer of the second single-sided copper clad laminate faces the first core board 300 to avoid the failure to achieve the lamination effect when the two copper layers are relatively pressed together. Therefore, the insulating dielectric layer faces the circuit pattern of the first core board 300, so as to ensure that the copper layer and the insulating dielectric layer are stably pressed together.

[0041] The key feature of the present invention is that each layer is made separately, and the patterns of each layer are matched, and then the layers are laminated and pressed to form a pressed plate 30.

[0042] It is worth mentioning that the lamination process requires the participation of an auxiliary lamination structure to complete the process. The stacked structure 20 includes: a first release film 5010, a coating film 5020, a second release film 5030 and a first steel plate 5040 are sequentially arranged on the window semi-cured sheet 200 to form an upper surface auxiliary lamination structure 500; and a third release film 6010 and a second steel plate 6020 are sequentially arranged under the second core plate to form a lower surface auxiliary lamination structure 600.

[0043] In the present embodiment, the upper surface auxiliary pressing structure 500 adopts a structural setting with a coating layer, which is used to provide coating ability during the pressing process, ensure coating from the opening direction of the through-slot structure of the U-shaped copper block 100, and form coating filling between layers and between each layer and the U-shaped copper block 100; the lower surface auxiliary pressing structure 600 adopts a structural setting without a coating layer, so that the lower surface provides stronger flatness support, ensures the support of the U-shaped copper block 100 during pressing, ensures the flatness of the lower surface, and prevents the lower surface from being skewed and other problems caused by coating.

[0044] See also Figure 8 , Fig. 9 and Fig.10 , Figure 8 is a schematic cross-sectional view of a first grinding plate according to an embodiment of the present invention; Fig. 9 is a schematic cross-sectional view of a first electroplating plate according to an embodiment of the present invention; Fig.10 It is a cross-sectional schematic diagram of a second electroplating plate according to an embodiment of the present invention.

[0045] Step S60: The upper surface of the pressing plate 30 is first polished to expose the top surface of the side wall of the U-shaped copper block 100, which is a polished plate 40; then, the first electroplating is performed to form a first electroplated plate 50 having a first electroplated copper layer 510, and then the second polishing and second electroplating are performed to form an electroplated plate 60; It is worth noting that the formed pressing plate 30 has a pressing protrusion 310 at the pressing point with the top of the U-shaped copper block 100. The first grinding is to grind the pressing protrusion 310 flat to form a first grinding plane 410, exposing the top surface of the side wall of the U-shaped copper block 100.

[0046] When using the design of the serrated edge 110, the first grinding needs to completely grind the serrated edge 110 to form a first grinding plane 410 to avoid the failure to expose the top surface of the side wall of the U-shaped copper block 100 due to the uneven copper layer after grinding. Subsequent upper surface electroplating may cause failure to enclose the high shielding area and poor shielding of the circuit board.

[0047] In particular, the first electroplating is for electroplating the upper surface, and the thickness of the first electroplated copper layer 510 formed by the first electroplating is 15 microns to 20 microns, so that both the upper and lower surfaces of the press plate 30 have copper surfaces, providing a plating basis for the second whole-board electroplating.

[0048] In particular, the second polishing is to polish the first electroplated copper layer 510. Since the top surface of the side wall of the U-shaped copper block 100 is exposed after the first polishing, during the first electroplating, the copper coating speed of the top surface of the side wall of the U-shaped copper block 100 is faster than that of the copper-free area. Therefore, the electroplated copper thickness formed on the top surface of the side wall of the U-shaped copper block 100 is thicker than that of the copper-free area, and the surface is uneven. Therefore, a second polishing is required, and the second polishing polishes the first electroplated copper layer 510 to be consistent with the thickness of the lower surface copper layer 320 of the press plate 30, so that the upper and lower surface copper layers of the press plate 30 have the same thickness, and then the second electroplating is performed; the second electroplating is to electroplate the entire press plate 30, so that the second electroplated copper thickness 610 of the circuit board meets the requirements, providing a copper thickness foundation for subsequent surface circuit graphics.

[0049] See also Fig.11 , Fig.11 Schematic cross-sectional view of a circuit pattern board according to an embodiment of the present invention.

[0050] Step S70: A surface circuit pattern 620 is made on the surface copper layer of the electroplating board 60 to form a circuit pattern board 70; the surface opening range of the through-slot structure of the U-shaped copper block 100 forms a fully covered surface circuit pattern 620, and is formed into a whole with the top surface of the side wall of the U-shaped copper block 100 through electroplating. At this time, the first circuit pattern 3010 inside the through-slot structure of the U-shaped copper block 100 is completely wrapped by the copper layer on all four sides, that is, a closed shielding cage is formed, which effectively improves the shielding effect of the line and realizes the effective transmission of high-fidelity and high-intensity signals.

[0051] See also Fig.12 , Fig.12 It is a schematic diagram of the cross-sectional structure of a high-shielding embedded copper block circuit board according to an embodiment of the present invention.

[0052] Step S80: A surface semi-cured sheet 710 is pressed onto the surface of the circuit graphic board 70, and then a third grinding is performed to form a high-shielding embedded copper block circuit board 80; the surface semi-cured sheet 710 of this embodiment fills the gaps between the surface circuits, so that the surface circuits are "embedded" on the surface of the circuit board 70, and the third grinding is to grind the surface semi-cured sheet 710 to make the surface flat, forming a circuit board 80 with a high surface flatness, thereby improving the protection effect on the surface circuit, effectively preventing problems such as circuit bumps and oxidation, and avoiding the problem of solder mask depression and solder mask covering the surface circuit, which affects the use of the surface circuit.

[0053] In one embodiment (not shown in the drawings), resin ink is screen-printed on the surface of the circuit graphic board 70 and dried, and then polished for a third time to form a high-shielding embedded copper block circuit board 80; resin ink is used instead of the surface semi-cured sheet 710 to form an "embedded" surface circuit setting, and the resin ink can be more accurately screen-printed according to the distribution of the gaps between the surface circuit graphics, making the third polishing process easier and improving the processing accuracy.

[0054] It is worth noting that the surface circuit pattern 620 can be subjected to surface treatments such as immersion gold, electroplated gold, immersion tin, etc. to form a corrosion-resistant protective layer on the surface. The above description is only a preferred embodiment of the present invention, and does not limit the patent scope of the present invention. All equivalent structural changes made by using the contents of the present invention specification and drawings under the inventive concept of the present invention, or directly / indirectly applied in other related technical fields are included in the patent protection scope of the present invention.

Claims

1. A method for manufacturing a high-shielding embedded copper block circuit board, characterized in that: S10: taking a copper block and processing it into a U-shaped copper block having a through groove structure; S20: taking a prepreg, making a prepreg window pattern, and forming a windowed prepreg, wherein the prepreg window pattern corresponds to the side wall top surface pattern of the U-shaped copper block; S30: taking a single-sided copper-clad laminate, making a first circuit pattern on its copper layer, and making a core board window pattern corresponding to the prepreg window pattern on its insulating dielectric layer, to form a first core board; S40: taking a double-sided copper-clad board and making a second circuit pattern on one copper layer of the double-sided copper-clad board to form a second core board; S50: stacking the window prepreg, the first core board, the U-shaped copper block and the second core board in order from top to bottom to form a stacked structure, and pressing them together to form a pressed board; The insulating medium layer of the first core board faces the through-groove structure of the U-shaped copper block, and the second circuit pattern of the second core board faces the bottom surface of the U-shaped copper block; The prepreg window pattern, the core board window pattern and the top surface of the side wall of the U-shaped copper block are stacked correspondingly; One side of the prepreg of the pressing plate is an upper surface, and the other side is a lower surface; S60: performing a first grinding on the upper surface to expose the top surface of the side wall of the U-shaped copper block, then performing a first electroplating to form a first electroplated copper layer, and then performing a second grinding and a second electroplating process to form an electroplated plate; S70: forming a surface circuit pattern on the surface copper layer of the electroplating board to form a circuit pattern board; S80: pressing a surface prepreg onto the surface of the circuit pattern board, and then performing a third polishing to form the high-shielding embedded copper block circuit board.

2. The method for manufacturing a high shielding embedded copper block circuit board according to claim 1, characterized in that: The top surface of the side wall of the U-shaped copper block is a serrated edge.

3. The method for manufacturing a high shielding embedded copper block circuit board according to claim 2, characterized in that: The first grinding is to completely grind the serrated edge.

4. A method for manufacturing a high shielding embedded copper block circuit board as claimed in claim 1 or 2, characterized in that: The outer surface of the U-shaped copper block is a continuous concave-convex pattern.

5. The method for manufacturing a high shielding embedded copper block circuit board according to claim 1, characterized in that: The prepreg is a second single-sided copper-clad laminate, and a circuit pattern is made on its copper layer to form a reinforced circuit, and the reinforced circuit is located within the range of the through-slot structure of the U-shaped copper block; The insulating dielectric layer of the second single-sided copper clad laminate faces the first core board.

6. The method for manufacturing a high shielding embedded copper block circuit board according to claim 1, characterized in that: The stacked structure comprises: A first release film, a coating film, a second release film and a first steel plate are sequentially arranged on the windowed prepreg to form an upper surface auxiliary pressing structure; A third release film and a second steel plate are sequentially arranged under the second core plate to form a lower surface auxiliary pressing structure.

7. The method for manufacturing a high shielding embedded copper block circuit board according to claim 1, characterized in that: The first electroplating is to electroplating the upper surface.

8. A method for manufacturing a high shielding embedded copper block circuit board as claimed in claim 1 or 7, characterized in that: The thickness of the copper layer electroplated for the first time is 15 microns to 20 microns.

9. The method for manufacturing a high shielding embedded copper block circuit board according to claim 1, characterized in that: The second electroplating is whole-board electroplating.

10. The method for manufacturing a high shielding embedded copper block circuit board according to claim 1, characterized in that: The second polishing is for polishing the first electroplated copper layer, and the third polishing is for polishing the surface prepreg.

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