A method for analyzing the unpacking failure of integrated circuits

Abstract

This invention discloses a method for analyzing the unpacking failure of integrated circuits, comprising the following steps: (a) identifying the specific location area requiring unpacking based on the internal wiring diagram or X-ray image of the integrated circuit to be unpacked; (b) placing the integrated circuit to be unpacked into a laser machine, importing the internal wiring diagram or X-ray image of the integrated circuit to be unpacked into the operating software of the laser machine, aligning it with the real-time image of the back side of the integrated circuit to be unpacked; (c) using a laser to etch a positioning integrated circuit groove on the back side of the integrated circuit to be unpacked; (d) milling away the base; (e) dripping hydrochloric acid or sulfuric acid into the integrated circuit groove where the base has been milled away, exposing the back side of the chip. This method not only overcomes the shortcomings of traditional chemical methods, such as difficulty in precisely controlling the corrosion range and easy damage to the external structure of the integrated circuit groove (e.g., solder joints), but also...

Description

Technical Field

[0001] This invention belongs to the field of semiconductor testing technology and relates to a failure analysis method, particularly a method for analyzing the unpacking failure of integrated circuits. Background Technology

[0002] With the development of science and technology, integrated circuits (ICs, also known as microcircuit, microchip, or chip) are increasingly widely used in electronic devices. The reliability of ICs has gradually become one of the key factors restricting product quality, and failure analysis of ICs is receiving increasing attention.

[0003] Integrated circuits are typically sealed within various materials to protect their complex internal circuitry. However, when a chip fails and requires in-depth analysis, the primary task is to break through this protective packaging, which necessitates decapsulation techniques. In short, decapsulation involves using specific processes to remove the chip's packaging, revealing its internal structure and paving the way for subsequent detailed testing and analysis.

[0004] Chinese invention patent application number 202510907005.0 discloses a method for unpacking and analyzing integrated circuits. Step one: Identifying the laser-etched area: The integrated circuit is placed in a laser machine, and the X-ray image is imported into the laser machine's operating software. The X-ray image is then dragged to align with the image on the back of the integrated circuit. Step two: Laser removal of the encapsulation: The area of ​​the integrated circuit to be laser-etched is located and laser-etched until the substrate is exposed, simultaneously forming an integrated circuit groove on the back of the integrated circuit. This application achieves precise positioning of the unpacked area on the back of the integrated circuit using a laser machine and sample X-ray images, effectively solving the problem of lost electrical connection of solder joints on the substrate caused by traditional methods of removing the chip substrate. However, it requires the use of aqua regia to remove the substrate; aqua regia, due to its extremely strong corrosiveness and toxicity, is explicitly classified as a hazardous chemical, affecting the safety and efficiency of integrated circuit unpacking. Summary of the Invention

[0005] In view of the shortcomings of existing technical approaches, the purpose of this invention is to provide a method for analyzing the failure of integrated circuits upon opening the packaging, which eliminates the need for aqua regia to improve the safety and efficiency of opening the packaging.

[0006] To achieve the above objectives, the present invention provides a method for analyzing the unpacking failure of integrated circuits, comprising the following steps: (a) Identify the specific locations and areas requiring unpacking based on the internal wire bonding diagram or X-ray image of the integrated circuit to be unpacked; (b) Place the integrated circuit to be opened into the laser machine, and import the internal wire drawing or X-ray image of the integrated circuit to be opened into the operating software of the laser machine so that it is aligned and overlapped with the real-time image of the back side of the integrated circuit to be opened; (c) Use a laser to etch a positioning groove for the integrated circuit on the back side of the integrated circuit to be unpacked; (d) Determine the thickness of the base based on the product appearance drawing, microscopic measurement drawing and / or X-ray measurement data of the integrated circuit to be unpacked, and mill the base away; (e) Drip hydrochloric acid or sulfuric acid into the integrated circuit groove where the base has been milled out, exposing the back of the chip.

[0007] Ideally, in step (a), before opening the integrated circuit slot, the packaging frame of the integrated circuit to be unpacked is calibrated using the operating software of the laser machine to ensure that the positioning laser is without deviation.

[0008] Optimally, in step (b), before placing the integrated circuit to be unpacked into the laser machine, the solder balls on the back of the integrated circuit to be unpacked are ground to the solder ball pads.

[0009] Ideally, in step (d), the integrated circuit to be unpacked is fixed on a chip milling and polishing machine, and the X and Y plane boundaries are precisely defined by the integrated circuit slots displayed on the chip milling and polishing machine's display screen.

[0010] Ideally, in step (e), the mass concentration of the hydrochloric acid is 36-38%.

[0011] Furthermore, the mass concentration of the sulfuric acid is 95-98%.

[0012] Furthermore, in step (e), after adding hydrochloric acid or sulfuric acid, the integrated circuit is moved onto the stage of the microscope, scraped along the edge of the integrated circuit groove with tweezers, and then the surface of the integrated circuit is rinsed with ultrapure water and dried.

[0013] Furthermore, the drying method involves blowing it with a compressed air gun and then letting it air dry naturally.

[0014] The present invention provides a method for analyzing the failure of integrated circuits after unpacking. By combining the step of milling off the base with the application of hydrochloric acid or sulfuric acid, it not only overcomes the shortcomings of traditional chemical methods, such as difficulty in accurately controlling the corrosion range and easy damage to the external structure of the integrated circuit (such as solder joints), but also precisely controls the cutting position and thickness without damaging the morphology of the die adhesive, thus laying the foundation for subsequent failure analysis (such as observation of the back of the chip, hot spot analysis, etc.). Attached Figure Description

[0015] Figure 1 This is a schematic diagram of the integrated circuit structure of the present invention; Figure 2 This is a schematic diagram of the overall structure of the chip milling and polishing machine in this invention; Figure 3 This is a schematic diagram of the overall structure of the chip milling and polishing machine of the present invention from another perspective; Figure 4 This is a schematic diagram showing the location of the circulating filter mechanism in the chip milling and polishing machine of the present invention; Figure 5 This is a schematic diagram of the circulating filtration mechanism in the chip milling and polishing machine of the present invention; Figure 6 This is a partial cross-sectional view of the annular cover and annular tube in the chip milling and polishing machine of the present invention; Figure 7 This is a schematic diagram of the moving mechanism in the chip milling and polishing machine of the present invention; Figure 8 This is a partial cross-sectional view of the filter box in the chip milling and polishing machine of the present invention; Figure 9 This is a schematic diagram of the scraper structure in the chip milling and polishing machine of the present invention. Detailed Implementation

[0016] To enable those skilled in the art to better understand the present invention, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art under the premise of equivalent changes and modifications should fall within the protection scope of the present invention. Example 1

[0017] like Figure 1 The integrated circuit to be unpacked shown includes a chip 1, a packaging layer 2 covering the surface of the chip 1, a die adhesive 3 disposed within the packaging layer 2 and formed on the back of the chip 1, and a base 4 disposed within the packaging layer 2 and formed on the back of the die adhesive 3.

[0018] The above-mentioned method for analyzing the unpacking failure of integrated circuits includes the following steps: (a) Based on the internal wire bonding diagram or X-ray image of the integrated circuit to be unpacked, identify the specific location area that needs to be unpacked; in step (a), before opening the integrated circuit slot 21, use the operating software of the laser machine to calibrate the packaging frame of the integrated circuit to be unpacked to ensure that the positioning laser is without deviation.

[0019] (b) Place the integrated circuit to be opened into the laser machine, and import the internal wire drawing or X-ray image of the integrated circuit to be opened into the operating software of the laser machine, so that it is aligned and overlapped with the real-time image of the back side of the integrated circuit to be opened; In step (b), before placing the integrated circuit to be unpacked into the laser machine, the solder balls on the back of the integrated circuit to be unpacked are ground until they are flush with the substrate.

[0020] (c) Use a laser to etch a positioning groove 21 on the back of the integrated circuit to be unpacked; (d) Determine the thickness of the base based on the product appearance drawing, microscope measurement drawing and / or X-ray measurement data of the integrated circuit to be unpacked, and mill off the base 4; In step (d), the integrated circuit to be unpacked is fixed on the chip milling and polishing machine, and the integrated circuit slots displayed on the chip milling and polishing machine screen are used to accurately define the boundaries of the X and Y planes.

[0021] Specifically, the integrated circuit to be opened is fixed on a chip milling and polishing machine with a processing accuracy of 1μm. This machine preferably has an automatic tilt adjustment function to ensure that the plane of the integrated circuit to be opened is parallel to the tool plane, thus removing only the base 4 of the target area, exposing the underlying mounting adhesive 3, while protecting the surrounding structure from damage. The equipment offers the following advantages: real-time video navigation for defining the X / Y milling area; closed-loop X and Y axis positioning with an accuracy of 1 micrometer; closed-loop Z-axis force control ranging from 0.5 to 10 Newtons; a full range of milling tool tip sizes (0.25 mm to 12 mm) suitable for various package types such as SOP, QFN, DFN, and BGA; and precise control of cutting position and thickness.

[0022] This chip milling and polishing machine, such as Figures 2 to 9 As shown, the device includes a worktable 101 and a cross slide 102. The cross slide 102 can move in the XZ directions. A Y-axis tooling slide 103 is provided on the worktable 101. The Y-axis tooling slide 103 can move in the Y direction, and a fixture 104 for positioning the chip is detachably connected to the Y-axis tooling slide 103. A spindle 105 is provided on the cross slide 102, and a cutting tool 106 is detachably connected to the spindle 105. These are all well-known technologies in this field and will not be described in detail here. A circulation filtration mechanism is provided at the bottom of the cross slide 102, and the circulation filtration mechanism is used to circulate and filter the abrasive suspension in the fixture 104.

[0023] The circulating filtration mechanism includes an annular cover 1101, which is fixed to the bottom of the cross slide 102 via a fixing rod 1102. A filter ring 1103 is slidably connected to the side wall of the annular cover 1101 via a telescopic mechanism. An annular rubber ring can be provided at the bottom of the filter ring 1103. A connecting plate 1105 is fixedly connected to the bottom of the cross slide 102. An annular tube 1104 is fixedly connected to the bottom of the connecting plate 1105. A circulating pump 1106 is connected to the top of the annular tube 1104. The circulating pump 1106 is connected to the annular cover 1101 via a circulating tube 1107. A liquid outlet mechanism is provided at the bottom of the annular tube 1104. A filtration mechanism is provided on the side wall of the circulating tube 1107. During the milling and polishing process, the abrasive suspension in the fixture 104 can be circulated and filtered to avoid chip surface scratches, processing size deviations, and other problems, thereby improving chip processing yield and the quality of chip milling and polishing.

[0024] The telescopic mechanism includes an annular plate 401 fixedly connected to the top of the filter ring 1103. A connecting block 402 is fixedly connected to the side wall of the annular cover 1101, and a first spring telescopic rod 403 is connected between the connecting block 402 and the annular plate 401. When the filter ring 1103 abuts against the top of the fixture 104, the first spring telescopic rod 403 can be gradually compressed.

[0025] The liquid outlet mechanism is rotatably connected to the rotating ring 201 at the bottom of the annular tube 1104, and the bottom of the rotating ring 201 is provided with a liquid outlet hole 202. The liquid outlet hole 202 is connected to the annular tube 1104, and the rotation of the rotating ring 201 is driven by a driving component. During milling and polishing, the circulation pump 1106 is started, and the abrasive suspension in the fixture 104 is absorbed through the annular cover 1101. Then, it enters the filtration mechanism through the circulation pipe 1107 for filtration. The filtered abrasive suspension enters the annular tube 1104 and is sprayed downward into the fixture 104 through the liquid outlet hole 202. It can also enter the inner side through the filter ring 1103.

[0026] The driving component includes multiple inclined blades 3 fixedly connected to the bottom of the rotating ring 201, and the blades 3 are located directly below the liquid outlet 202. When the abrasive suspension flows out through the liquid outlet 202, it can impact the blades 3, causing the rotating ring 201 to rotate. This makes the filtered abrasive suspension fall more evenly into the fixture 104, and the impact force of the abrasive suspension flow can be used as power to realize the automatic rotation of the rotating ring 201.

[0027] The filtration mechanism includes a filter box 501 fixedly inserted into the side wall of the circulation pipe 1107, and the filter box 501 is connected to the circulation pipe 1107. A filter screen 502 is provided at the bottom of the filter box 501, and a scraping mechanism for scraping and cleaning the filter screen 502 is provided inside the filter box 501. When the abrasive suspension enters the filter box 501, the abrasive suspension is filtered under the action of the filter screen 502.

[0028] The scraping mechanism includes a scraper 601 slidably connected within the filter box 501, and the scraper 601 is connected to the filter box 501 via a moving mechanism. A groove 603 is provided on the side wall of the filter box 501, and a filter plate 602 is inserted into the groove 603. The filter plate 602 is fixed to the side wall of the scraper 601, and multiple drainage holes 604 are provided on the side wall of the scraper 601. A one-way valve is installed in each drainage hole 604, and the one-way valve's conduction direction is from the side closest to the lifting plate 702 to the other side of the scraper 601. On the side, the bottom of the filter box 501 is provided with a collection mechanism for collecting scraped debris. The moving mechanism drives the scraper 601 to move closer to the lifting plate 702 inside the filter box 501. At the same time, the filter plate 602 moves to the bottom of the circulation pipe 1107 for temporary filtration. The scraper 601 can scrape and clean the debris on the surface of the filter screen 502 to prevent it from clogging, ensure the efficiency and effect of filtration, and push the debris to move, and use the collection mechanism to collect the debris.

[0029] The collection mechanism includes a collection box 701 inserted into the bottom of the filter box 501, and a mounting block 703 fixedly connected to the side wall of the collection box 701. The mounting block 703 is detachably connected to the bottom of the filter box 501, and a lifting plate 702 is provided through the top of the filter box 501. The lifting plate 702 is connected to the top of the filter box 501 through a reset mechanism. The lifting plate 702 is raised and lowered by a pushing mechanism, and the lifting plate 702 can movably seal the top of the collection box 701. When the scraper 601 pushes the debris to move, it abuts against the lifting plate 702, thereby squeezing the debris. The abrasive suspension can be discharged through the drain hole 604. After the extrusion is completed, the lifting plate 702 is moved upward by the pushing mechanism, which can gradually open the top of the collection box 701. When the lifting plate 702 moves upward, it can rub the debris. At the same time, the top of the filter box 501 can scrape and clean the surface of the lifting plate 702, so that the extruded debris can fall into the collection box 701 for collection. When the scraper 601 and the filter plate 602 move and reset, the filter plate 602 can move into the slide 603, which can scrape off the debris on the top of the filter plate 602 and fall above the filter screen 502.

[0030] The moving mechanism is inserted into the moving rod 801 on the side wall of the filter box 501. One end of the moving rod 801 is fixed to the scraper 601, and the other end of the moving rod 801 is connected to the moving block 802 through the second spring telescopic rod 803. The bottom of the moving block 802 is fixedly connected to the iron block 804. The side wall of the filter box 501 is fixedly connected to the electromagnet 805, and the moving block 802 is connected to the side wall of the filter box 501 through the third spring telescopic rod 806. When the electromagnet 805 is energized, it can attract the iron block 804, causing the moving block 802 to move closer to the filter box 501. At the same time, the third spring telescopic rod 806 is compressed, de-energizing the electromagnet 805. At this time, the moving block 802 can move and reset away from the filter box 501 under the action of the third spring telescopic rod 806. This process is repeated to make the moving block 802 move back and forth.

[0031] The reset mechanism includes a bracket 901 fixedly connected to the top of the filter box 501, and a fourth spring telescopic rod 902 is connected between the lifting plate 702 and the bracket 901, which guides and resets the lifting plate 702.

[0032] The pushing mechanism includes a mounting bracket 1002 fixedly connected to the top of the moving block 802, and a pushing plate 1003 fixedly connected to the side wall of the mounting bracket 1002. The side wall of the pushing plate 1003 is provided with an inclined surface 1004. The side wall of the lifting plate 702 is fixedly connected with a pushing pin 1001, and the pushing pin 1001 can slide on the inclined surface 1004. After the compression is completed, when the moving block 802 continues to move, the second spring telescopic rod 803 is gradually compressed. At the same time, the pushing plate 1003 can be moved by the mounting bracket 1002. When the inclined surface 1004 abuts against the pushing pin 1001, the lifting plate 702 can be pushed to move upward. At the same time, the fourth spring telescopic rod 902 is compressed.

[0033] Working principle: When milling and polishing a chip is required, the chip is positioned using a jig 104 and the jig 104 is fixed on the Y-axis tooling slide 103. Then, the jig 104 is filled with abrasive suspension, and the tool 106 is mounted on the spindle 105. During milling and polishing, the tool 106 is moved in the XZ direction by the cross slide 102, and the jig 104 and the chip are moved in the Y direction by the Y-axis tooling slide 103. When the cross slide 102 moves downward, it can drive the annular cover 1101, the filter ring 1103 and the annular tube 1104 to move downward synchronously.

[0034] When the filter ring 1103 abuts against the top of the fixture 104, the first spring telescopic rod 403 can be gradually compressed. At the same time, the annular cover 1101 is immersed in the milling abrasive suspension in the fixture 104. During the milling and polishing process, the filter ring 1103 can slide on the top of the fixture 104. Under the action of the filter ring 1103, the debris generated by milling and polishing can be blocked inside.

[0035] Meanwhile, during milling and polishing, the circulation pump 1106 is started, and the abrasive suspension in the fixture 104 is absorbed through the annular cover 1101. Then, it enters the filtration mechanism through the circulation pipe 1107 for filtration. The filtered abrasive suspension enters the annular pipe 1104 and is sprayed downward into the fixture 104 through the outlet hole 202. It can also enter the inner side through the filter ring 1103, so that the abrasive suspension can be circulated and filtered during milling and polishing to ensure the quality of milling and polishing. Furthermore, when the abrasive suspension flows out through the outlet hole 202, it can impact the blade 3, causing the rotating ring 201 to rotate, so that the filtered abrasive suspension falls more evenly into the fixture 104.

[0036] When the abrasive suspension enters the filter box 501, it is filtered by the filter screen 502. At the same time, the electromagnet 805 is energized, which attracts the iron block 804, causing the moving block 802 to move closer to the filter box 501. Meanwhile, the third spring telescopic rod 806 is compressed, de-energizing the electromagnet 805. At this time, the moving block 802 can move away from the filter box 501 and reset under the action of the third spring telescopic rod 806. This process is repeated to make the moving block 802 move back and forth.

[0037] When the moving block 802 moves toward the filter box 501, it can drive the moving rod 801 to move via the second spring telescopic rod 803, and drive the scraper 601 to move toward the lifting plate 702 inside the filter box 501. At the same time, the filter plate 602 moves to the bottom of the circulation pipe 1107 for temporary filtration. The scraper 601 can scrape and clean the debris on the surface of the filter screen 502 to prevent it from clogging and ensure the efficiency and effect of filtration. It also pushes the debris against the lifting plate 702, thereby squeezing the debris. Meanwhile, the abrasive suspension can be discharged through the drain hole 604.

[0038] After the compression is complete, as the moving block 802 continues to move, the second spring telescopic rod 803 is gradually compressed. At the same time, the mounting bracket 1002 drives the push plate 1003 to move. When the inclined surface 1004 abuts against the push pin 1001, it can push the lifting plate 702 to move upward. At the same time, the fourth spring telescopic rod 902 is compressed. At this time, the top of the collection box 701 can be gradually opened. When the lifting plate 702 moves upward, it can rub the debris. At the same time, the top of the filter box 501 can scrape and clean the surface of the lifting plate 702, so that the compressed debris can fall into the collection box 701 for collection.

[0039] When the scraper 601 and filter plate 602 move and reset, the filter plate 602 can move into the chute 603, scraping off the debris on the top of the filter plate 602 and letting it fall above the filter screen 502. Furthermore, the blocking effect of the scraper 601 prevents the abrasive suspension from entering the collection box 701. When it is necessary to process the collected debris, the mounting block 703 can be disassembled and the lifting plate 702 can be removed. This facilitates the filtration and collection of debris in the abrasive suspension and allows for the compression of the debris, which is not only more convenient and faster but also avoids the waste of the abrasive suspension.

[0040] (e) Hydrochloric acid is dripped into the integrated circuit groove where the base has been milled out to remove the die adhesive 3 (Henkel QM519 adhesive in this application), exposing the back of the chip. In this embodiment, the mass concentration of hydrochloric acid is 36-38%.

[0041] Specifically, the hydrochloric acid is dripped in to completely cover the die adhesive 3, and the mixture is left to soak at room temperature for 10 minutes to allow the die adhesive 3 to dissolve and separate from the back of the chip. The integrated circuit is then transferred to the microscope stage, and tweezers are used to scrape along the edge of the integrated circuit groove. The surface of the integrated circuit is then rinsed with ultrapure water and dried (by blowing it with a compressed air gun and allowing it to air dry naturally). Example 2

[0042] This embodiment provides a method for analyzing the failure of integrated circuits upon opening the package. It is basically the same as that in Embodiment 1, except that in step (e), sulfuric acid is added with a mass concentration of 95-98%; the removal effect of the substrate adhesive 3 is similar to that in Embodiment 1.

[0043] Comparative Example 1 This example provides a method for analyzing the failure of integrated circuits upon unpacking. It is basically the same as that in Example 1, except that in step (e), acetone is dripped in, which can only cause the die adhesive 3 to swell, but cannot completely remove the die adhesive 3.

[0044] Comparative Example 2 This example provides a method for analyzing the failure of an integrated circuit after unpacking. It is basically the same as that in Example 1, except that in step (d), aqua regia is dripped in to remove the base 4, which causes corrosion at the connection edge between the encapsulation layer 2 and the base 4.

[0045] Comparative Example 3 This example provides a method for analyzing the unpacking failure of integrated circuits (i.e., the unpacking method of BGA devices), which is carried out according to the method of Embodiment 2 in Chinese Invention Patent Application No. 202011642638.7; the operation steps of this method require heating the BGA device first, otherwise the unpacking cannot be performed.

[0046] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of protection of the present invention. At the same time, those skilled in the art should understand and implement the above description. Therefore, any equivalent changes or modifications made without departing from the concept disclosed in the present invention should be covered within the scope of protection of the present invention.

Claims

1. A method for analyzing the unpacking failure of integrated circuits, characterized in that, Includes the following steps: (a) Identify the specific locations and areas requiring unpacking based on the internal wire bonding diagram or X-ray image of the integrated circuit to be unpacked; (b) Place the integrated circuit to be opened into the laser machine, and import the internal wire drawing or X-ray image of the integrated circuit to be opened into the operating software of the laser machine so that it is aligned and overlapped with the real-time image of the back side of the integrated circuit to be opened; (c) Use a laser to etch a positioning groove for the integrated circuit on the back side of the integrated circuit to be unpacked; (d) Determine the thickness of the base based on the product appearance drawing, microscopic measurement drawing and / or X-ray measurement data of the integrated circuit to be unpacked, and mill the base away; (e) Drip hydrochloric acid or sulfuric acid into the integrated circuit groove where the base has been milled out, exposing the back of the chip.

2. The method for analyzing the unpacking failure of integrated circuits according to claim 1, characterized in that: In step (a), before opening the integrated circuit slot, the packaging frame of the integrated circuit to be opened is calibrated using the operating software of the laser machine to ensure that the positioning laser is without deviation.

3. The method for analyzing the unpacking failure of integrated circuits according to claim 1, characterized in that: In step (b), before placing the integrated circuit to be unpacked into the laser machine, the solder balls on the back of the integrated circuit to be unpacked are ground to the solder ball pads.

4. The method for analyzing the unpacking failure of integrated circuits according to claim 1, characterized in that: In step (d), the integrated circuit to be unpacked is fixed on the chip milling and polishing machine, and the X and Y plane boundaries are precisely defined by the integrated circuit slots displayed on the display screen of the chip milling and polishing machine.

5. The method for analyzing the unpacking failure of integrated circuits according to claim 1, characterized in that: In step (e), the mass concentration of the hydrochloric acid is 36-38%.

6. The method for analyzing the unpacking failure of integrated circuits according to claim 1 or 5, characterized in that: The sulfuric acid has a mass concentration of 95-98%.

7. The method for analyzing the unpacking failure of integrated circuits according to claim 6, characterized in that: In step (e), after adding hydrochloric acid or sulfuric acid, the integrated circuit is moved onto the stage of the microscope, scraped along the edge of the integrated circuit groove with tweezers, and then the surface of the integrated circuit is rinsed with ultrapure water and dried.

8. The method for analyzing the unpacking failure of integrated circuits according to claim 7, characterized in that: The drying method involves blowing the material with a compressed air gun and then allowing it to air dry naturally.

Images