A flip-chip BGA device underfill adhesive detection method

Abstract

The application discloses a kind of detection methods of flip solder BGA device bottom filling glue, belong to component destructive physical analysis or failure analysis technical field, solve the problem of poor detection accuracy of flip solder BGA device bottom filling glue in prior art.The detection method comprises the following steps: once internal imaging is carried out to the device, the position of the heat-conducting silicone grease layer in the device is determined;The position of the heat-conducting silicone grease layer is used as the opening position to open, remove the heat dissipation cover plate, so that the chip is exposed, and a sample to be ground is obtained;The sample to be ground is subjected to secondary internal imaging, and the position of the defect in the bottom filling glue layer is determined;The sample to be ground is ground, so that the defect in the bottom filling glue layer is exposed;The defect in the bottom filling glue layer is observed and recorded.The method can be used for the detection of flip solder BGA device bottom filling glue.

Description

Technical Field

[0001] This invention belongs to the field of destructive physical analysis or failure analysis technology of electronic components, and particularly relates to a method for detecting the bottom filler of flip-chip BGA devices. Background Technology

[0002] With the continuous improvement of single-chip integration, the number of I / O pins has increased dramatically, leading to the emergence of flip-chip BGA (Ball Grid Array) devices, which have advantages such as high density, small size, high reliability and low signal delay.

[0003] BGA packaging technology began to develop rapidly in the early 1990s. Since the beginning of the 21st century, the application of BGA devices in electronic products has gradually increased, especially flip-chip BGA devices. Driven by the rapid development of the mobile communication market, they are gradually replacing QFN as the mainstream packaging for high I / O count ICs. The process flow of flip-chip BGA devices is roughly as follows: First, a chip with bumps and a multilayer substrate are fabricated and flip-chip bonded. Then, the interfaces are filled with underfill adhesive. Next, thermal grease is applied to the chip and the junction area between the substrate and the heat sink. Then, the heat sink is mounted onto the substrate. Finally, the solder balls on the substrate are attached.

[0004] Due to the complexity of the manufacturing process and the multiple heating processes involved in the packaging, the underfill often contains voids, gaps, uneven flow, or melted and then re-cured. This makes it difficult to pre-select defective products or batches using screening or destructive physical analysis after packaging. Consequently, failures in flip-chip BGA packages often occur at the underfill location.

[0005] Existing methods for detecting underfill adhesive typically involve directly preparing a cross-section of the device and grinding it. This indiscriminate grinding process can easily lead to missing potential defects in the underfill adhesive due to improper control or incorrect selection of the grinding location, resulting in poor detection accuracy. Summary of the Invention

[0006] Based on the above analysis, the present invention aims to provide a method for detecting the underfill adhesive of flip-chip BGA devices, which solves the problem of poor detection accuracy of underfill adhesive in existing technologies.

[0007] The objective of this invention is mainly achieved through the following technical solutions:

[0008] This invention provides a method for detecting the underfill adhesive of flip-chip BGA devices, comprising the following steps:

[0009] Step 1: Perform an internal imaging of the device to determine the location of the thermal grease layer within the device;

[0010] Step 2: Open the package by using the location of the thermal grease layer as the opening point, remove the heat sink cover to expose the chip, and obtain the sample to be polished;

[0011] Step 3: Perform secondary internal imaging on the sample to be ground to determine the location of defects in the bottom filler layer;

[0012] Step 4: Grind the sample to be ground to expose defects in the bottom filler layer;

[0013] Step 5: Observe and record the defects in the bottom filler layer.

[0014] Furthermore, step 2 includes the following steps:

[0015] Step 21: Using the location of the thermal grease layer as the opening position, cut along the edge of the device to cut open the thermal grease layer between the heat sink cover and the printed circuit board;

[0016] Step 22: Clamp one corner of the heat sink cover and slowly roll it up along the diagonal away from the printed circuit board until the thermal grease layer between the heat sink cover and the chip is completely peeled off. Remove the heat sink cover.

[0017] Step 23: Remove the residual thermal grease layer on the chip surface.

[0018] Furthermore, in step 21, the feed angle is 30–60°.

[0019] Further, step 1 includes the following steps:

[0020] An acoustic scanning microscope was used to perform internal imaging, with the imaging focal plane aligned with the thermal grease layer to determine the location of the thermal grease layer in the device.

[0021] Furthermore, in step 1, the acoustic scanning microscope is equipped with a mid-frequency probe with a frequency of 50MHz, and the scanning rate of the mid-frequency probe does not exceed 200mm / s.

[0022] Furthermore, step 3 includes the following steps:

[0023] An acoustic scanning microscope was used for internal imaging. The imaging focal plane was aligned with the underfill adhesive layer between the chip and the printed circuit board to obtain a complete image of the underfill adhesive and determine the location of defects in the underfill adhesive layer.

[0024] Furthermore, in step 3, a high-frequency probe with a frequency of 230MHz is installed on the acoustic scanning microscope, and the scanning rate of the high-frequency probe does not exceed 30mm / s.

[0025] Furthermore, in step 4, grinding is performed using a planar sample preparation method.

[0026] Furthermore, step 4 includes the following steps:

[0027] Step 41: Determine whether the size of the sample to be ground exceeds the mold size of the polishing machine. If so, use a cutting machine to cut the printed circuit board part of the sample to be ground away from the chip so that the size of the sample to be ground is smaller than the mold size of the polishing machine, and proceed to step 42. If not, proceed directly to step 42.

[0028] Step 42: Place the sample to be ground at the bottom of the mold, and perform casting, vacuuming and curing in sequence to fix the sample to be ground to the mold.

[0029] Step 43: Use a polishing machine to grind the sample to be ground in the mold from the solder ball side until the solder balls and printed circuit board on the sample to be ground are completely ground away, revealing the shape of the bottom filler glue covering the bump layer.

[0030] Furthermore, in step 43, as the grinding continues, the sandpaper is replaced, with the grit size of the next sandpaper being smaller than that of the previous sandpaper.

[0031] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0032] A) The method for detecting underfill adhesive of flip-chip BGA devices provided by the present invention helps to improve the positioning accuracy and efficiency of underfill adhesive defects, and can solve the problem of poor detection accuracy of underfill adhesive of flip-chip BGA devices in the prior art.

[0033] B) The method for detecting the underfill adhesive of flip-chip BGA devices provided by the present invention, on the one hand, determines the position of the thermal grease layer by first internal imaging, accurately locates the opening position, which can effectively improve the opening efficiency and reduce the damage to the chip caused by mechanical stress; on the other hand, after removing the heat sink cover, a second internal imaging can accurately locate the position of defects in the underfill adhesive, which serves as the basis for subsequent polishing.

[0034] C) The method for detecting the bottom filler of flip-chip BGA devices provided by this invention, in step 2 above, firstly, a thin blade is used to cut open the thermal grease layer between the heat sink cover and the printed circuit board. The blade is inserted from the edge and the angle is controlled, which effectively improves the separation efficiency of the thermal grease layer between the heat sink cover and the printed circuit board while ensuring the integrity of both. Secondly, when separating the heat sink cover from the chip, a rolling method is used to roll up the heat sink cover to separate it from the chip. This method effectively reduces stress during separation and ensures the integrity of the chip. It should be noted that in the prior art, the success rate of opening the package in this step is usually only 30-40%, and most chips will be damaged. However, using the separation method of this embodiment, the success rate of opening the package can be increased to over 95%, and in practical applications, it can basically guarantee a 100% success rate.

[0035] D) The method for detecting the underfill adhesive of flip-chip BGA devices provided by the present invention uses planar sample preparation, which can obtain the overall morphology of the underfill adhesive layer, making it convenient for subsequent analysts to observe and record the quality of the underfill adhesive layer.

[0036] Other features and advantages of the invention will be set forth in the description which follows, and will be apparent in part from the description, or may be learned by practicing the invention. The objects and other advantages of the invention may be realized and obtained by means of the structures particularly pointed out in the written description and the accompanying drawings. Attached Figure Description

[0037] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts.

[0038] Figure 1 This is a schematic diagram of the structure of the flip-chip BGA device provided in Embodiment 1 of the present invention;

[0039] Figure 2 This is an exploded view of the flip-chip BGA device provided in Embodiment 1 of the present invention;

[0040] Figure 3 This is a schematic diagram of the infeed process in step 2 of the method for detecting the bottom filler of a flip-chip BGA device provided in Embodiment 1 of the present invention.

[0041] Figure 4 The sample to be ground is obtained after opening the package in step 2 of the detection method for the bottom filler of the flip-chip BGA device provided in Embodiment 1 of the present invention.

[0042] Figure 5 This is a schematic diagram of the structure of the manual grinding bracket in step 4 of the detection method for the bottom filler of the flip-chip BGA device provided in Embodiment 1 of the present invention.

[0043] Figure 6 This is a physical image of the underfill adhesive layer obtained by using the detection method for underfill adhesive of flip-chip BGA devices provided in Embodiment 1 of the present invention.

[0044] Figure label:

[0045] 1-Heat dissipation cover; 2-First thermal grease layer; 3-Second thermal grease layer; 4-Chip; 5-Bottom filler layer; 6-Bump layer; 7-Printed circuit board; 8-Solder ball; 9-Transparent frame; 10-Vertical adjustment ruler; 11-Horizontal indicator strip; 12-Fixing rod; 13-Thin blade. Detailed Implementation

[0046] Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which form part of the present invention and, together with the embodiments of the present invention, serve to illustrate the principles of the present invention.

[0047] Example 1

[0048] First, the structure of flip-chip BGA devices is described:

[0049] In this embodiment, the flip-chip BGA device is referred to Figures 1 to 2 It includes a heat dissipation cover plate 1, a first thermally conductive silicone grease layer 2, a second thermally conductive silicone grease layer 3, a chip 4, an underfill adhesive layer 5, a bump layer 5, a printed circuit board 7, and solder balls 8. The heat dissipation cover plate 1 and the printed circuit board 7 are stacked. The side of the heat dissipation cover plate 1 facing the printed circuit board 7 is provided with a groove. The groove and the printed circuit board 7 form an accommodating space. Along the direction from the bottom of the groove to the printed circuit board 7, the first thermally conductive silicone grease layer 2, the chip 4, the underfill adhesive and the bump layer 5 are stacked in sequence in the accommodating space. The second thermally conductive silicone grease layer 3 is located between the heat dissipation cover plate 1 and the printed circuit board 7. One side of the second thermally conductive silicone grease layer 3 is in direct contact with the heat dissipation cover plate 1, and the other side of the second thermally conductive silicone grease layer 3 is in direct contact with the printed circuit board 7. The solder balls 8 are located on the side of the printed circuit board 7 away from the heat dissipation cover plate 1.

[0050] This embodiment provides a method for detecting the underfill adhesive of flip-chip BGA devices. See [link to relevant documentation]. Figures 1 to 6 It includes the following steps:

[0051] Step 1: Perform an internal imaging of the device to determine the location of the thermal grease layer within the device;

[0052] Step 2: Open the package by taking the position of the thermal grease layer as the opening position, remove the heat dissipation cover 1, so that the chip 4 is exposed, and obtain the sample to be polished;

[0053] Step 3: Perform secondary internal imaging on the sample to be ground to determine the location of defects in the bottom filling adhesive layer 5. It should be noted that the defects mainly include voids or missing parts in the bottom filling adhesive layer 5 and displacement or bridging of protrusions in the protrusion layer 6 wrapped by the bottom filling adhesive layer 5.

[0054] Step 4: Grind the sample to be ground to expose the defects in the bottom filling adhesive layer 5;

[0055] Step 5: Use a metallographic microscope to observe and record the defects in the bottom filling adhesive layer 5.

[0056] Compared with existing technologies, the detection method for underfill adhesive of flip-chip BGA devices provided in this embodiment helps to improve the positioning accuracy and efficiency of underfill adhesive defects, and can solve the problem of poor detection accuracy of underfill adhesive in existing technologies. On the one hand, by determining the position of the thermal grease layer through the first internal imaging, the opening position can be accurately located, which can effectively improve the opening efficiency and reduce the damage to the chip 4 caused by mechanical stress. On the other hand, performing secondary internal imaging after removing the heat sink cover 1 can accurately locate the position of defects in the underfill adhesive, providing a basis for subsequent polishing.

[0057] In order to accurately locate the position of the thermal grease layer in the device, step 1 above includes the following steps:

[0058] An acoustic scanning microscope was used, with a mid-frequency probe (50MHz) installed. The internal imaging of the device was performed using the intensity information of the sound waves. The imaging focal plane was aligned with the thermal grease layer, and the probe scanning rate did not exceed 200mm / s to determine the location of the thermal grease layer in the device.

[0059] It is worth noting that in the prior art, chip 4 is easily damaged during the unpacking process. In order to obtain a structurally intact chip 4, this embodiment improves the unpacking method. Specifically, step 2 above includes the following steps:

[0060] Step 21: Using the location of the thermal grease layer as the opening position, use a thin blade 13 to cut along the edge of the device at an angle of 30 to 60° (e.g., 45°) to cut open the thermal grease layer (i.e., the second thermal grease layer 3) between the heat sink cover 1 and the printed circuit board 7.

[0061] Step 22: Use diagonal pliers to clamp one corner of the heat sink cover 1, and slowly roll up the heat sink cover 1 along the diagonal direction away from the printed circuit board 7 until the thermal grease layer (i.e. the first thermal grease layer 2) between the heat sink cover 1 and the chip 4 is completely peeled off, and the heat sink cover 1 can be completely removed.

[0062] Step 3: Use a scriber to remove the residual thermal grease layer on the surface of chip 4.

[0063] In step 2 above, on the one hand, a thin blade is first used to cut open the thermal grease layer between the heat sink cover 1 and the printed circuit board 7. The blade enters from the corner and the angle is controlled, which effectively improves the separation efficiency of the thermal grease layer between the heat sink cover 1 and the printed circuit board 7 while ensuring the integrity of both. On the other hand, when separating the heat sink cover 1 from the chip 4, a rolling method is used to roll up the heat sink cover 1 to separate it from the chip 4. This method effectively reduces stress during separation and ensures the integrity of the chip 4. It should be noted that in the prior art, the success rate of this opening step is usually only 30-40%, and most chips 4 are damaged. However, using the separation method of this embodiment, the success rate can be increased to over 95%, and in practical applications, it can basically guarantee a 100% success rate.

[0064] In order to accurately locate the defects in the bottom filler layer 5 of the device, step 3 above includes the following steps:

[0065] An acoustic scanning microscope was used, with a high-frequency probe (frequency of 230MHz) installed. The intensity information of the sound waves was used to perform internal imaging of the sample to be ground. The imaging focal plane was aligned with the bottom filler layer 5 between the chip 4 and the printed circuit board 7. The probe scanning rate did not exceed 30mm / s to obtain a complete image of the bottom filler layer and determine the location of defects in the bottom filler layer 5.

[0066] To obtain a comprehensive view of the defects in the bottom filler layer 5, step 4 above employs a planar sample preparation method for grinding. Planar sample preparation refers to grinding the sample along a direction perpendicular to its thickness. It should be noted that in existing technologies, cross-sectional sample preparation is typically used, i.e., grinding along the thickness direction of the sample. This only yields multiple cross-sections of the bottom filler layer 5, which is not very intuitive for analysts. This embodiment uses planar sample preparation, which allows for obtaining the overall morphology of the bottom filler layer 5, facilitating subsequent observation and recording of its quality by analysts.

[0067] Specifically, step 4 above includes the following steps:

[0068] Step 41: Determine whether the size of the sample to be ground exceeds the mold size of the polishing machine. If so, use a cutting machine to cut the printed circuit board 7 part of the sample to be ground that is far away from the chip 4, so that the size of the sample to be ground is smaller than the mold size of the polishing machine, and proceed to step 42. If not, proceed directly to step 42.

[0069] Step 42: Place the sample to be ground at the bottom of the mold, and perform casting, vacuuming and curing in sequence to fix the sample to be ground to the mold.

[0070] Step 43: Use a polishing machine to grind the sample to be ground in the mold from the side of the solder ball 8. As the grinding proceeds, change the sandpaper. The grit size of the next sandpaper is smaller than that of the previous sandpaper. The grit size of the sandpaper used decreases from large to small until the solder ball 8 and the printed circuit board 7 on the sample to be ground are completely ground away, revealing the shape of the bottom filling adhesive wrapped bump layer 6.

[0071] It should be noted that in step 43 above, when the sample to be ground is ground using a polishing machine, the height of the solder balls 8 may be inconsistent, which may cause the grinding direction to deviate. Therefore, during the grinding process, the analyst should visually judge whether the grinding direction has deviated. If deviation occurs, the following steps should be taken:

[0072] The sample to be ground was manually corrected using sandpaper.

[0073] During manual correction, the analyst needs to observe the degree of grinding in real time, which involves a large subjective error. Therefore, the sample to be ground can be placed on a manual grinding stand for correction.

[0074] For example, the aforementioned manual grinding support includes a transparent frame 9, a vertical adjustment scale 10, a horizontal indicator strip 11, and a fixing rod 12. There are two vertical adjustment scales 10, which are vertically aligned and slidably fixed to the transparent frame 9. One end of the horizontal indicator strip 11 is fixedly connected to one of the vertical adjustment scales 10, and the other end is fixedly connected to the other vertical adjustment scale 10. The fixing rod 12 is slidably fixed to the transparent frame 9. The sample to be ground is located within the space enclosed by the transparent frame 9. The lower end of the vertical adjustment scale 10 and one end of the fixing rod 12 within the transparent frame 9 abut against the sample to be ground, while the horizontal indicator strip 11 contacts the upper surface of the sample to be ground.

[0075] The following steps are involved in correcting the deviation of the sample to be ground using a manual grinding stand:

[0076] Step a: Place the sample to be ground in the transparent frame 9, and adjust the fixing rod 12 to initially fix the sample to be ground so that the lower end of the short side of the sample to be ground is flush with the bottom of the transparent frame 9.

[0077] Step b: Adjust the position of the two vertical adjustment marks so that the intersection of the two vertical adjustment scales 10 and the transparent frame 9 is at the same scale.

[0078] Step c: Determine whether the horizontal marking strip 11 is parallel to the side of the printed circuit board 7 closest to the chip 4. If it is parallel, proceed to step d. If it is not parallel, it indicates that there is an error in the manual polishing bracket and it needs to be inspected.

[0079] Step d: Tighten the fixing rod 12 to finally fix the sample to be ground;

[0080] Step e: Grind the part of the sample that protrudes from the transparent frame 9 until the part that protrudes from the transparent frame 9 completely disappears, thus completing the correction.

[0081] In this way, by using a manual grinding support, the grinding direction of the sample to be ground can be determined by both the vertical adjustment scale 10 and the horizontal marking strip 11. This ensures that after manual correction, the grinding direction is completely perpendicular to the thickness direction of the sample to be ground, thus avoiding errors caused by the analyst's subjective judgment of the manual grinding direction.

[0082] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for detecting the underfill adhesive of flip-chip BGA devices, characterized in that, The process includes the following steps: Step 1: Perform an internal imaging of the device to determine the location of the thermal grease layer; Step 2: Deseal the device using the location of the thermal grease layer as the opening point, remove the heat sink cover, and expose the chip to obtain the sample to be polished; Step 3: Perform a second internal imaging of the sample to be polished to determine the location of defects in the bottom filler layer; Step 4: Polish the sample to be polished to expose the defects in the bottom filler layer; Step 5: Observe and record the defects in the bottom filler layer. During the grinding process, the analyst visually checks for any deviation in the grinding direction. If deviation occurs, the following steps are taken: the sample to be ground is manually corrected using sandpaper and a manual grinding stand. The manual grinding stand includes a transparent frame, vertical adjustment scales, horizontal marking strips, and a fixing rod. There are two vertical adjustment scales, which are set vertically and slidably fixed to the transparent frame. One end of the horizontal marking strip is fixedly connected to one of the vertical adjustment scales, and the other end is fixedly connected to the other vertical adjustment scale. The fixing rod is slidably fixed to the transparent frame. The sample to be ground is located in the space enclosed by the transparent frame. The lower end of the vertical adjustment scale and the end of the fixing rod located in the transparent frame abut against the sample to be ground. The horizontal marking strip is also fixed to the sample. The upper surface of the grinding sample contacts the surface; the correction includes the following steps: Step a: Place the sample to be ground in the transparent frame, and adjust the fixing rod to initially fix the sample so that the lower end of the short side of the sample is flush with the bottom of the transparent frame; Step b: Adjust the position of the two vertical adjustment marks so that the intersection of the two vertical adjustment scales and the transparent frame is at the same scale; Step c: Determine whether the horizontal mark is parallel to the side of the printed circuit board closest to the chip. If it is parallel, proceed to step d. If it is not parallel, it indicates that there is an error in the manual grinding bracket, which needs to be inspected; Step d: Tighten the fixing rod to finally fix the sample to be ground; Step e: Grind the part of the sample that protrudes from the transparent frame until the part that protrudes from the transparent frame completely disappears, completing the correction.

2. The method for detecting the bottom filler of flip-chip BGA devices according to claim 1, characterized in that, Step 2 includes the following steps: Step 21: Using the location of the thermal grease layer as the opening position, cut along the edge of the device to cut open the thermal grease layer between the heat sink cover and the printed circuit board; Step 22: Clamp one corner of the heat sink cover and slowly roll it up along the diagonal away from the printed circuit board until the thermal grease layer between the heat sink cover and the chip is completely peeled off. Remove the heat sink cover. Step 23: Remove the residual thermal grease layer on the chip surface.

3. The method for detecting the bottom filler of flip-chip BGA devices according to claim 2, characterized in that, In step 21, the feed angle is 30~60°.

4. The method for detecting the bottom filler of flip-chip BGA devices according to claim 1, characterized in that, Step 1 includes the following steps: An acoustic scanning microscope was used to perform internal imaging, with the imaging focal plane aligned with the thermal grease layer to determine the location of the thermal grease layer in the device.

5. The method for detecting the bottom filler of a flip-chip BGA device according to claim 4, characterized in that, In step 1, the acoustic scanning microscope is equipped with a mid-frequency probe with a frequency of 50MHz, and the scanning rate of the mid-frequency probe does not exceed 200mm / s.

6. The method for detecting the underfill adhesive of flip-chip BGA devices according to claim 1, characterized in that, Step 3 includes the following steps: An acoustic scanning microscope was used for internal imaging. The imaging focal plane was aligned with the underfill adhesive layer between the chip and the printed circuit board to obtain a complete image of the underfill adhesive and determine the location of defects in the underfill adhesive layer.

7. The method for detecting the underfill adhesive of flip-chip BGA devices according to claim 6, characterized in that, In step 3, the acoustic scanning microscope is equipped with a high-frequency probe with a frequency of 230MHz and a scanning rate of no more than 30mm / s.

8. The method for detecting the underfill adhesive of a flip-chip BGA device according to claim 6, characterized in that, In step 4, grinding is performed using a planar sample preparation method.

9. The method for detecting the underfill adhesive of flip-chip BGA devices according to claims 1 to 8, characterized in that, Step 4 includes the following steps: Step 41: Determine whether the size of the sample to be ground exceeds the mold size of the polishing machine. If so, use a cutting machine to cut the printed circuit board part of the sample to be ground away from the chip so that the size of the sample to be ground is smaller than the mold size of the polishing machine, and proceed to step 42. If not, proceed directly to step 42. Step 42: Place the sample to be ground at the bottom of the mold, and perform casting, vacuuming and curing in sequence to fix the sample to be ground to the mold. Step 43: Use a polishing machine to grind the sample to be ground in the mold from the solder ball side until the solder balls and printed circuit board on the sample to be ground are completely ground away, revealing the shape of the bottom filler glue covering the bump layer.

10. The method for detecting the underfill adhesive of a flip-chip BGA device according to claim 9, characterized in that, In step 43, as the grinding proceeds, the sandpaper is replaced, and the particle size of the next sandpaper is smaller than that of the previous sandpaper.

Images