Test body for checking image quality, use of test body and performance evaluation method

Abstract

The invention relates to a test body for checking image quality during an X-ray test of a test object having a first layer of a test target to be checked and having an intermediate layer on which the test target is arranged, the test body having a first test layer and a solid separator arranged thereon, a thickness of the first test layer corresponds to a thickness of a test target in a test object, and a thickness of the separator corresponds to a thickness of the intermediate layer, in which a plurality of first holes having the same depth and diameter are formed in the first test layer; and a size of the first hole corresponds to a portion of a size of the assigned test target to be detected as a defect. The invention further relates to the use of a test body according to the invention for determining the accuracy of detection of defects and / or for determining the depth of field of an X-ray system, and to a method for determining the accuracy of detection of defects and / or for determining the depth of field of an X-ray system.

Description

Technical Field

[0001] The invention relates to a test body for checking the image quality during an X-ray examination of a test object, the test body having a plurality of layers of the test object to be examined, the test objects having the same dimensions as one another in each layer and having intermediate layers between the respective layers, and to the use of such a test body for determining the detection accuracy of errors and / or for determining the depth of field of an X-ray system. The invention also relates to a method for determining the detection accuracy of errors and / or for determining the depth of field of an X-ray system. Background Art

[0002] The field of application of the invention is X-ray-based material testing. The use of X-ray imaging provides the possibility of inspecting hidden structures without destroying the test object. The inspection is carried out in an X-ray system having an X-ray tube (hereinafter referred to as the tube) and an X-ray detector (hereinafter referred to as the detector) as an imaging system. In the meantime, the test object to be inspected is arranged on a manipulator. Depending on the X-ray system, some or all of the three components mentioned above can be moved translationally and / or rotationally. The entire device is located in a radiation protection cabin. In many systems, the test object is inspected manually. The operator must regularly check the image quality independently and ensure that the image chain (X-ray tube, manipulator, X-ray detector) can depict the required error size. For better reproducibility and the possibility of increasing the level of automation in the test process, the system must automatically check the image chain. A test object that can be regularly moved into the image is needed, and the system can evaluate on this test object whether the required defect size can still be depicted in the test object, or whether the system parameters need to be adjusted to restore the desired state. So far, such inspections have been carried out, for example, by inserting resolution samples and adjusting the focus of the X-ray tube according to the image.

[0003] The focal size is a parameter that affects the quality of a laminographic scan. However, there are other parameters such as manipulation accuracy, object movement or adjustments.

[0004] US 6,694,047 B1 and EP0874536A1 describe test bodies that are intended to cover many different applications in a single test body. Penetrometers with different filter thicknesses, line pairs and step wedges are placed on the test body. Depending on the application, the area on the test body to be used must be selected, and then the quality of the image at a certain position in the test volume can be evaluated, such as the signal-to-noise ratio (SNR) or the contrast-to-noise ratio (CNR). If one wants to do this for multiple areas of the test volume, one must measure them one by one and place the test body in different ways.

[0005] The ASTM E 2737 standard also describes the image quality at several points in the test chamber. Here too, a penetrometer and a step wedge are used to simulate real applications, such as evaluating SNR and CNR. An example is as follows: If the test object is made of aluminum and the maximum transmission length is 100 mm, 100 mm of aluminum will be used as the filter. If one wants to be able to see porosities of 1 mm in this aluminum, a penetrometer with the corresponding pore size can be placed on the filter. If pores can be detected in the X-ray image, it can be assumed that porosities of this magnitude can also be detected.

[0006] A major disadvantage of previous solutions is that the image quality is checked at only one point in the image. While it is possible to move the test volume to different points and check the quality one by one at multiple points, this is very time-consuming. The longer such an image quality check lasts, the less meaningful this check becomes, since the image quality can change due to, for example, thermal drift. Suppose there are three points in a test volume that need to be checked, the first two points are both OK and within specification, but the third point is not. The question now arises whether the first two points are still within specification or whether the system has changed and the first two points checked are now worse. They must be checked partially in order to be able to make reliable statements about these points. The advantage of checking all three points simultaneously is that different system states can be excluded as error factors. Summary of the invention

[0007] The object of the invention is to provide a test body with which statements can be made about the image quality of an X-ray process in all image regions of one slice or several slices.

[0008] The object is then achieved using a test body having a structure similar to the test object to be inspected by the X-ray system. The test body according to the invention is used for the test object to be inspected, which has a first layer of the test target to be inspected, which has the same size as each other and is arranged on a solid partition. The test body has a test layer. The thickness of the test layer corresponds to the thickness of the test target in the test object. In order to achieve the best imitation of the test object, the test body also has a solid partition, i.e. completely composed of a material without holes or the like, which is arranged on the first test layer, wherein it has the same thickness as the intermediate layer and is made of a material with an absorption coefficient corresponding to the material of the intermediate layer. In order to evaluate whether the desired image quality is guaranteed over the entire surface of the test object, a plurality of first holes are formed in the first test layer, which first holes have the same depth and diameter dimensions and whose dimensions correspond to a part of the size of the assigned test target to be detected as a defect. The material made of the first test layer has an absorption coefficient corresponding to the absorption coefficient of the test target, which means that the absorption of the X-radiation is as strong as the absorption in the test object. When different points / areas in the overall X-ray beam are examined simultaneously through a plurality of first apertures, one advantage is that different system states can be excluded as error factors.

[0009] An advantageous development of the invention is that if the test object has a second layer of test objects to be checked, the following further features are present:

[0010] The test body has a second test layer, which is arranged on the side of the partition facing away from the first test layer, wherein the thickness of the second test layer corresponds to the thickness of the test object in the second layer and the second test layer is made of a material having an absorption coefficient corresponding to the material of the test object in the second layer, wherein a plurality of first holes are formed in the second test layer, which first holes have the same depth and diameter and whose size corresponds to a fraction of the size of the assigned test object to be detected as a defect. The quality of the depth of field, i.e., whether it is at different depths corresponding to the level of the two test layers, can be easily evaluated, because the combination of the first test layer, the partition and the second test layer of the test body in their respective thicknesses corresponds to the first layer, the intermediate layer and the second layer of the test object and is designed to be respectively made of a material having a corresponding absorption coefficient so that the same X-ray absorption occurs as in the test object, and the second test layer of the test body is arranged at the same position in the beam path as the layer having the test object to be inspected in the test object.

[0011] A further advantageous development of the invention provides that the test body has at least one further combination of a further web and a further test layer, wherein the further web and the further test layer correspond to the web and the second test layer according to the previous paragraph, respectively. This means that a test body with a large number of test layers can also be used to simulate a test object for testing, and it is also possible to use test objects with the same dimensions within a layer, but also with different dimensions in different layers.

[0012] A further advantageous development of the invention provides that a second hole is formed in each test layer, which hole has a diameter at least twice that of the first hole and the same depth as the first hole. The CNR can thus be determined in a simple manner.

[0013] Another advantageous development of the invention provides that a third hole is formed in each test layer, the third hole having a diameter at least four times that of the first hole, wherein the depth of the third hole is the same as the depth of the first hole. The CNR can thus likewise be determined in a simple manner.

[0014] A further advantageous development of the invention provides that the first holes and / or the second holes and / or the third holes are each arranged in at least one of the test layers in a matrix, the matrix extending over a large portion of the surface of the test layer, wherein in particular all holes are arranged in a common matrix. Due to the large distribution area of ​​the holes, the test body does not have to be positioned in multiple positions in order to determine the image quality of the surface of the test object, but rather can be positioned in a single position corresponding to the test object during the test. This saves time and avoids inaccuracies that could occur if the test body is positioned multiple times at different points in the beam path.

[0015] Another advantageous development of the invention provides that the test object is a printed circuit board and the test targets are solder balls. These are the most common applications within the scope of the invention.

[0016] Preferably in this case the test layer is made of a material with a relatively high atomic number (in the context of the present application this is understood to mean an atomic number greater than 22), such as nickel, tin or copper or an alloy containing at least one of these substances, and / or the separator is made of a material with a relatively low atomic number (in the context of the present application this is understood to mean an atomic number less than 14), preferably silicon. These are the materials of the test objects to be tested which occur in the most common applications within the scope of the present invention.

[0017] The object is also achieved by the use of a test body according to the invention for determining the detection accuracy of defects and / or for determining the depth of field of an X-ray system having a tube, a detector and a holder arranged therebetween for accommodating the test object.

[0018] Finally, the object is also achieved by a method for determining the defect detection accuracy and / or for determining the depth of field of an X-ray system having a tube, a detector and a holder arranged between them for accommodating a test object, wherein a single X-ray image of a test body according to the invention is recorded, which is located in the beam path instead of the test object at the point at which the test object is located during the test, and the X-ray image is then evaluated. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Further details and advantages of the invention will now be explained in more detail using the exemplary embodiments shown in the accompanying drawings. In the drawings:

[0020] Figure 1 A schematic top view showing a local detail of a test layer with holes;

[0021] Figure 2 shows a portion of a schematic longitudinal section through a test layer with holes;

[0022] Figure 3 Shown according to Figure 1 An isometric view of three test layers arranged one above the other;

[0023] Figure 4 shows a portion of a schematic longitudinal cross section of a test body having two test layers with a partition therebetween;

[0024] Figure 5 A comparison of a schematically represented test object's longitudinal section with a designated test body schematic longitudinal section is shown;

[0025] Figure 5a Shown according to Figure 5 Magnified image of the test body. DETAILED DESCRIPTION

[0026] Figures 1 to 4 A partial test body 1 and a complete test body 1 are shown for use with a test object 2, which has the following structure: It is a printed circuit board with two levels, in which the test object 10 to be tested is located. The first level has a ball grid array (BGA for short) with a ball (solder ball) size of 200 μm (diameter). The second level has a setting distance of 300 μm and consists of BGA with 20 μm solder balls. A substrate made of silicon is located between the two levels (corresponding to Figure 5 Each layer should be able to detect voids (air pockets in the ball) of 10% of the ball size.

[0027] Figure 1A schematic representation of a partial detail of a first test layer 3 according to the invention of a test body 1 according to the invention is shown in a top view (for a further exemplary embodiment see Figure 4 and Figure 5 and Figure 5a The first test layer 3 consists of a test object 2 to which a test body 1 is assigned (in Figure 5 1 and 2) are made of the same material as the test objects 10 of the first layer in the test body 1 (shown in FIG. 1 for another exemplary embodiment). This assignment means that the test body 1 is used to check the image quality during a series of tests of the same test object 2 and to simulate it structurally. In the present case, the first test layer 3 consists of tin, because the test objects 10 in the lower layer of the test object 2 are solder balls 10 made of tin.

[0028] Holes 5, 6, 7 are formed in the surface of the first test layer 3, and in the exemplary embodiment shown, the holes are arranged in a matrix 8. There are three different types of holes: first holes 5 (also called T1 holes), second holes 6 (also called T2 holes) and third holes 7 (also called T4 holes). Their respective diameters and depths depend on the size of the test target 10 to which they correspond and the size of any defects to be detected therein. The diameter of the first hole 5 is as large as the size of the defect to be detected. In this example, as described above, this is 10% of 200 μm, i.e. 20 μm. The depth of the first hole 5 is the same size as its diameter, i.e. also 20 μm. The assumption here is that if the T1 hole (first hole 5) is detectable in the image, then the 10% voids in the solder ball 10 (test target 10) will also be detectable.

[0029] The diameter of the second hole 6 (T2 hole) is twice the diameter of the first hole 5, i.e. 40 μm, and the diameter of the third hole 7 is four times the diameter of the first hole 5, i.e. 80 μm. The depth of these holes 6, 7 is the same as the depth of the first hole 5, i.e. 20 μm. These holes 6, 7 are used to determine the CNR.

[0030] In a row (in Figure 1The whole of the three adjacent holes 5, 6, 7 (taking the first row as an example) represents a penetrometer 16 known from the prior art. In the prior art (see also the above-mentioned US 6,694,047 B1 and EP 0874536A1), the test body 1 has only one penetrometer 16 or a small number of penetrometers 16, for example three. Since a large number of penetrometers 16 in the test body 1 according to the invention are located in a large number of rows of the matrix 8 (distributed over the entire surface of the first test layer 3), the image quality, SNR and CNR can be determined by taking an X-ray image only once at one position in the test body 1 (corresponding to the position of the test object 2 during the test). This significantly reduces the time required for the certification of the test space. This also applies to the positions in the optical path along the X-ray beam taking into account the following statements about the second test layer 4 and the partition 9 located between the two test layers 3, 4.

[0031] Since the size (diameter) of the second layer of the test object 2 of the solder ball to be tested is 40μm, the material thickness of the second test layer 4 is also 40μm. Since a 10% error is to be detected, the diameter of the first hole 5 is 4μm; accordingly, the diameter of the second hole 6 is 8μm, and the diameter of the third hole 7 is 16μm. The hole depth of all holes 5, 6, and 7 is 4μm. The second test layer 4 is separated from the first test layer 3 by a partition 9 with a thickness of 50μm. In order to be as close to actual application as possible, the partition 9 is made of a 50μm thick silicon disk. In principle, the partition 9 can be made of any material close to the application, but is not limited to this; if possible, it should have the same absorption coefficient as the corresponding material of the test object 2, or be thick enough so that its absorption in the light beam path passing through it approximately corresponds to the absorption in the light beam path between the two layers of the test object 2. A partial longitudinal cross-sectional schematic diagram of the entire structure is shown in FIG. Figure 4 shown.

[0032] By stacking and adjusting the actual distance of the layers and components of the test object 2 (here the BGA) to the focus of the X-ray tube, the depth of field of the system can be checked. This is not constant for the test space. Therefore, it is important to understand the structure of the printed circuit board in order to be able to check whether the error size can still be detected in the actual distance.

[0033] Since the dimensions of the holes 5, 6, 7 and the arrangement of the test layers 3, 4, 14 must be able to be flexibly adapted to the application, i.e. the test object 2 to be tested, a general description of the test body 1 is difficult. This was explained above using an example application. Figure 5 , Figure 5a A further exemplary embodiment of the present invention explains the structure of a test body 1 according to the invention. Figure 5 In FIG. 1 , the left side shows a test object 2 to be tested, and the right side shows a test body 1 which is a model of the test object 2. Figure 5aIn the figure, the magnified image is shown according to Figure 5 The test body 1 is adjusted so that in particular the holes 5 , 6 , 7 can be seen better.

[0034] The exemplary test object 2 is a printed circuit board with different layers, in which different components are present. The structure thereof is discussed from bottom to top below, wherein only the components necessary for the present invention are discussed.

[0035] On the bottom layer, as a test target 10, there are a large number of solder balls 10 in the form of packaged balls 11 with a diameter of 200 μm, which are usually composed of tin or an alloy with a high tin content. An intermediate layer 15 is arranged above it. Next is another level, in which the test target 10 is in the form of a large number of solder balls 10 (here: C4 bumps 12, which are usually made of tin or a high-tin alloy and have a diameter of 40 μm). Another intermediate layer 15 is attached to it. On this intermediate layer 15, there is another level of test targets 10 in the form of a large number of solder balls 10 (here: μ bumps 13, which are usually also made of tin or an alloy with a high tin content and have a diameter of 10 μm). The next layer is again the intermediate layer 15. Above this is another structure, which consists of the above two levels: μ bumps 13 and the intermediate layer 15 located thereon.

[0036] As part of a non-destructive X-ray analysis of such a circuit board, the quality of the soldered connections between the functional components of the circuit board and the carrier component, i.e. the test object 10, is (in particular) checked. This is usually done using tomographic methods which are well known in the art. It is necessary to regularly check whether the image chain is able to achieve a sufficiently high image quality of the X-ray images taken using the tomographic process. This is also crucial whether defects in the test object 10 can be detected whose dimensions exceed a predeterminable level (which would result in the printed circuit board having to be removed from the production process). In order to be able to carry out such an assessment of the image quality in the context of a series test of printed circuit boards designed in this way, a test body 1 according to the invention is used which mimics the X-ray properties of the test object 2 as accurately as possible, as described above in another embodiment. Figures 1 to 4 The illustrated embodiment has been described. Figure 5 and Figure 5a The test body 1 according to the present invention shown on the right side of Figure 5 The test object 2 shown on the left of FIG. 1 and just described in more detail is held in the beam path of the X-ray system at the point where the test object 2 was located during the test, and a single recording of the test body 1 is made. In this way, the image quality can be evaluated over the entire test area, as already mentioned above with respect to a further exemplary embodiment, both in a plane perpendicular to the X-ray beam and in the direction of the X-ray beam (i.e. in depth).

[0037] The following describes the assignment Figure 5The test body 1 of the test object 2 is designed so that the aforementioned image quality check can be performed as well as possible with as little effort as possible.

[0038] The bottom layer, the first test layer 3, corresponds to the level of the package balls 11 of the test object 2 and is composed of copper with a thickness of 200 μm. There are three types of holes 5, 6, 7 in their surface (as described above for Figures 1 to 4 Another exemplary embodiment of the invention has already been described): a first hole 5 (T1 hole) having a diameter of 20 μm, a second hole 6 (T2 hole) having twice the diameter and two third holes 7 (T4 holes) having four times the diameter; the depth of all holes 5, 6, 7 is the same, corresponding to the diameter of the first hole 5, 20 μm.

[0039] A spacer 9 made of silicon with a thickness of 50 μm is arranged on the first test layer 3 , which corresponds to the intermediate layer 15 of the test object 2 .

[0040] A second test layer 4 is arranged thereon, made of copper and 40 μm thick, which corresponds to the level of the C4 bump 12 of the test object 2. There are also three types of holes 5, 6, 7 in its surface (as described above for Figures 1 to 4 Another exemplary embodiment of the present exemplary embodiment and the first test layer 3 has already been described: the first hole 5 has a diameter of 4 μm, the second hole 6 has twice the diameter and the third hole 7 has four times the diameter; all holes 5, 6, 7 have the same depth, corresponding to the diameter of the first hole 5, 4 μm. By arranging the holes 5, 6, 7 in a matrix 8 (only in top view, as shown Figure 1 and Figure 3 A large number of penetration gauges 16 are formed, each of which is formed by three different holes 5, 6, 7 adjacent to each other (see also Figure 1 and Figure 4 ).

[0041] A further spacer 9 made of silicon with a thickness of 20 μm is arranged on the second test layer 4 , which corresponds to the intermediate layer 15 of the test object 2 .

[0042] Arranged thereon is a third test layer 14 made of copper and having a thickness of 10 μm, which corresponds to the level of the μ-bumps 13 of the test object 2. Its surface also has three types of holes 5, 6, 7 (as already described above for the first test layer 3 and the second test layer 4 of the present exemplary embodiment): a first hole 5 with a diameter of 1 μm, a second hole 6 with twice the diameter and a third hole 7 with four times the diameter; all holes 5, 6, 7 have the same depth, which corresponds to the diameter of the first hole 5, 1 μm. These holes 5, 6, 7 are also arranged in a matrix 8, thereby forming a large number of penetration gauges 16.

[0043] A further spacer 9 made of silicon with a thickness of 20 μm is arranged on the third test layer 14 , which corresponds to the intermediate layer 15 of the test object 2 .

[0044] Arranged thereon is a fourth test layer 14 made of copper and having a thickness of 10 μm, which corresponds to the level of the μ-bumps 13 of the test object 2. Its surface also has three types of holes 5, 6, 7 (as already described above for the first test layer 3 and the second test layer 4 of the present exemplary embodiment): a first hole 5 with a diameter of 1 μm, a second hole 6 with twice the diameter and a third hole 7 with four times the diameter; all holes 5, 6, 7 have the same depth, which corresponds to the diameter of the first hole 5, 1 μm. These holes 5, 6, 7 are also arranged in a matrix 8, thereby forming a large number of penetration gauges 16.

[0045] A further spacer 9 made of silicon with a thickness of 20 μm is arranged on the fourth test layer 14 , which corresponds to the intermediate layer 15 of the test object 2 .

[0046] With the present test body 1 , a device is thus obtained which corresponds exactly to the test object 2 , with which a check of the image quality during a series of checks of such test objects 2 can be easily and quickly carried out.

[0047] Reference numerals list

[0048] 1 Test Body

[0049] 2 Test subjects

[0050] 3First test layer

[0051] 4 Second test layer

[0052] 5First hole

[0053] 6 Second hole

[0054] 7Third hole

[0055] 8 Matrix

[0056] 9 Partition

[0057] 10 Test targets, especially solder balls

[0058] 11 Package Ball

[0059] 12C4 Bump

[0060] 13 micro bumps

[0061] 14 Another test layer

[0062] 15 Middle Layer

[0063] 16-hole penetration gauge

Claims

1. A test body (1) for checking image quality during an X-ray test of a test object (2), the test object having a first layer of a test object (10) to be checked and the test object having an intermediate layer (15), the test object (10) being arranged on the intermediate layer (15), in, The test body (1) comprises a first test layer (3) and a solid partition (9) arranged on the first test layer. wherein the thickness of the first test layer (3) corresponds to the thickness of the test target (10) in the test object (2), and the thickness of the partition (9) corresponds to the thickness of the intermediate layer (15), wherein a plurality of first holes (5) are formed in the first test layer (3), the depth and diameter of the first holes being the same, and the size of the first holes corresponding to a portion of the size of the assigned test object (10) to be detected as a defect, wherein the material of which the first test layer (3) is made has an absorption coefficient corresponding to the absorption coefficient of the assigned test object (10), The material of which the partition (9) is made has an absorption coefficient corresponding to the absorption coefficient of the intermediate layer (15).

2. The test body (1) according to claim 1, which is assigned to a test object (2) having a second layer of a test object (10) to be checked, the second layer being arranged on a side of the intermediate layer (15) facing away from the first layer, in, The test body (1) has a second test layer (4) arranged on a side of the partition (9) facing away from the first test layer (3). wherein the thickness of the second test layer (4) corresponds to the thickness of the test object (10) in the second layer, and the second test layer (4) is made of a material having an absorption coefficient corresponding to the material of the test object (10) in the second layer, A plurality of first holes (5) are formed in the second test layer (4), the depth and diameter of the first holes being the same, and the size of the first holes corresponding to a portion of the size of the assigned test object (10) to be detected as a defect.

3. The test body (1) according to claim 2, wherein: The test body has at least one other combination of a further separator (9) and a further test layer (14), wherein the further separator (9) and the further test layer (14) correspond to the separator (9) and the second test layer (4), respectively.

4. The test body (1) according to claim 3, wherein: A second hole (6) is formed in each test layer (3, 4, 14), the second hole (6) having a diameter at least twice that of the first hole (5), wherein the depth of the second hole (6) is the same as the depth of the first hole (5).

5. The test body (1) according to claim 4, wherein: A third hole (7) is formed in each test layer (3, 4, 14), the third hole (7) having a diameter at least four times that of the first hole (5), wherein the depth of the third hole (7) is the same as the depth of the first hole (5).

6. The test body (1) according to claim 5, wherein: The first holes (5) and / or the second holes (6) and / or the third holes (7) are respectively arranged in the form of a matrix (8) in at least one test layer (3, 4, 14), and the matrix extends to cover most of the surface of the test layer.

7. The test body (1) according to claim 6, wherein: All holes (5, 6, 7) are arranged in a common matrix (8).

8. The test body (1) according to claim 1, wherein: The test object (2) is a printed circuit board and the test target (10) is a solder ball.

9. The test body (1) according to claim 3, wherein: The test layers (3, 4, 14) are made of a material with an atomic number greater than 22, and / or the partition (9) is made of a material with an atomic number less than 14.

10. The test body (1) according to claim 9, wherein: The test layer (3, 4, 14) is made of nickel, tin or copper or an alloy containing at least these elements, and / or the separator (9) is made of silicon.

11. Use of a test body (1) according to any of the preceding claims 1 to 10 for determining the detection accuracy of defects and / or for determining the depth of field of an X-ray system, the X-ray system having a tube, a detector and a holder arranged therebetween for accommodating the test object (2).

12. A method for determining the detection accuracy of a defect and / or for determining the depth of field of an X-ray system, the X-ray system having a tube, a detector and a holder arranged between them for accommodating a test object (2), in, A single X-ray image of a test body (1) according to any one of claims 1 to 10 is recorded, the test body being located in the beam path at the point where the test subject (2) is located during the test, and the X-ray image is evaluated.

Images