METHOD FOR OBTAINING INFORMATION ABOUT A LAYER OF AN ORGANIC SOLDERABILITY PRESERVATIVE ON A PRINTED CIRCUIT BOARD

Abstract

A method for obtaining information about a layer of an organic solderability preservative on a printed circuit board, the method including Providing or producing a printed circuit board (16) having a copper layer (18) covering a part of an area of the printed circuit board (16), Providing a fluorescence measuring system (10),
the method including following steps a) Obtaining (S2) information about the location of the openings (21) on the printed circuit board (16), b) Selecting (S3) at least one of the openings (21), thereby obtaining at least one selected opening, c) Moving (S4) the radiation source (11) and the printed circuit board (16) relatively to each other, d) Detecting (S5) the fluorescent radiation (23).

Claims

1. A method for obtaining information about a layer of an organic solderability preservative on a printed circuit board, the method comprising Providing or producing a printed circuit board (16) having a copper layer (18) covering a part of an area of the printed circuit board (16), wherein a solder resist (19) is placed on the copper layer (18), and the solder resist (19) has openings (21) wherein in the openings (21) a copper surface of the copper layer (18) is covered by a layer of an organic solderability preservative with an individual thickness (20), Providing a fluorescence measuring system (10), comprising a radiation source (11) suitable for emitting a radiation beam (22), a detection unit (12) for detecting fluorescent radiation (23), and a movement device (24) which is arranged to move the radiation source (11) and the printed circuit board (16) relatively to each other in at least one dimension, the method comprising following steps a) Obtaining (S2) information about the location of the openings (21) on the printed circuit board (16), b) Selecting (S3) at least one of the openings (21), thereby obtaining at least one selected opening, c) Moving (S4) the radiation source (11) and the printed circuit board (16) relatively to each other with the movement device (24) and placing the radiation source (11) at such position that the radiation beam (22) irradiates into the at least one selected opening (21), and on the layer of the organic solderability preservative (20) in the at least one selected opening, d) Detecting (S5) the fluorescent radiation (23) which is emitted from the organic solderability preservative (20) with the detection unit (12).

2. The method of claim 1, wherein the method is a method for determining an individual thickness of the layer of the organic solderability preservative, the method further comprising step e) Determining (S6) the individual thickness of the layer of the organic solderability preservative (20) in the at least one selected opening.

3. The method of claim 1, wherein in step c) the radiation beam (22) irradiates in such way that no solder resist (19) is irradiated.

4. The method of claim 1, comprising detecting a reference fluorescence radiation of different reference samples of a reference layer of the organic solderability preservative, wherein in each of the reference samples the reference layer has a known thickness, obtaining an assignment of the reference fluorescent radiation to the thickness of the layer of the organic solderability preservative.

5. The method of claim 4, wherein the individual thickness is determined on basis of the assignment.

6. The method of claim 2, comprising checking whether a signal or an intensity of the fluorescent radiation detected in step d), and/or the individual thickness of the layer of the organic solderability preservative determined in step e) is within a desired range or a rated range.

7. The method of claim 6, further comprising one or more of the following steps i) discarding the printed circuit board (16) ii) refinishing the printed circuit board (16) in order to change the individual thickness of the layer of the organic solderability preservative, iii) adapting a method of production of a layer of the organic solderability preservative on at least one further printed circuit board in order to obtain an individual thickness of a layer of the organic solderability preservative on the at least one further printed circuit board in the desired range or the rated range.

8. The method of claim 1, wherein the information about the location of the openings is comprised in a data set, and in step a) the information about the location of the openings is obtained from the data set.

9. The method of claim 1, further comprising providing information about the location of the at least one selected opening on the printed circuit board to a controller which controls the movement of the radiation source (11) and the printed circuit board (16) relatively to each other, wherein in step c) the moving of the radiation source (11) and the printed circuit board (16) relatively to each other and the placing of the radiation source is controlled by the controller.

10. The method of claim 2, wherein steps b) to d) are repeated once or more, wherein in each repetition in step b) a further opening is selected and wherein the thickness of the layer of the organic solderability preservative is determined at the respective location of irradiation in the further opening.

11. The method of claim 10, wherein the further opening, or in case of more repetitions the further openings, has/have a different area, the method further comprising Determining an assignment between the area of an opening and the thickness of the layer of the organic solderability preservative.

12. The method of claim 1, wherein steps b) to d) are repeated once or more, wherein in each repetition in step b) the same opening is selected and in step c) the radiation source is moved to a different position so that within the same opening a different location on the layer of the organic solderability preservative is irradiated.

13. The method of claim 1, wherein the organic solderability preservative comprises an imidazole compound and/or a benzimidazole compound.

14. The method of claim 1, wherein the organic solderability preservative comprises a imidazole-Cu(I) complex and/or a benzimidazole-Cu(I) complex wherein the layer of the organic solderability preservative is at least in part an organometallic layer.

15. The method of claim 1, wherein the radiation beam irradiates an area of 0.5-1 mm.sup.2 on the layer of the organic solderability preservative.

16. The method of claim 1, wherein manufacturing of the printed circuit board (16) comprises depositing (S0) a layer of the organic solderability preservative (20) in the openings (21) and on a copper surface of the copper layer (18) within the openings (21).

17. The method of claim 1, wherein the at least one selected opening (21) has an area of at least 1 mm.sup.2.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0123] FIG. 1 shows schematically the arrangement of the printed circuit board and the fluorescence measuring system utilized in the method of the present invention;

[0124] FIG. 2 shows the fluorescence measuring system, a printed circuit board, and the movement device in one particular embodiment of the method of the present invention;

[0125] FIG. 3 shows a calibration line of SITA CleanoSpector® versus layer thickness of OSP;

[0126] FIG. 4 shows a top view of a PCB and a number of measuring points thereon;

[0127] FIG. 5 shows an overview of the steps of the method of the invention.

EXAMPLES

[0128] The fluorescence measuring system was the CleanoSpector®, by SITA Company, a laser based Fluorescence measuring head.

[0129] Measuring Principle:

[0130] The principle is shown in FIG. 1. The SITA CleanoSpector® uses an excitation frequency of 365 nm and detects the emitted light from a respective substrate with a frequency of 460 nm. A beam guidance and various filters ensure that only the emitted light reaches the detector. It can record values between 0-2000 RFU (Relative Fluorescence Units).

[0131] With increasing layer thickness of the OSP, an increased RFU is recorded or output. The layer thickness of the OSP can be determined by means of standards and calibration lines.

[0132] In FIG. 1 following components are shown: [0133] 10 fluorescence measuring system [0134] 11 radiation source, emitting a radiation beam [0135] 12 detection unit, which preferably is an optical detection unit [0136] 13 low pass filter [0137] 14 high pass filter [0138] 15 semi permeable mirror [0139] 16 printed circuit board (PCB) [0140] 17 substrate for the copper layer, part of the PCB [0141] 18 copper layer, part of the PCB [0142] 19 solder resist, part of the PCB [0143] 20 organic solderability preservative (OSP) [0144] 21 openings (edge of opening in this cross sectional view) allowing access to a contact pad [0145] 22 radiation beam, UV light [0146] 23 fluorescent radiation

[0147] FIG. 2 shows the measuring system 10 and in addition the movement device 24. The movement device 24 comprises the holding device 25 to which the measuring system 10 is attached by fixing means. The holding device is movable along the guidance 26 in a direction Y and along another guidance (not shown) in a direction X, perpendicular to Y (coordinate system is shown). Movement is effected by a motor and gearing mechanism (not shown). The printed circuit board 16 is oriented in X-Y direction, so that the fluorescence measuring system 10 may be moved, by the movement device 24, at any place across and above the printed circuit board 16.

Example 1 (Calibration)

[0148] Four samples of FR4 base materials (5×5 cm) each with a 35 μm copper foil are deposited with a layer of organic solderability preservative Atotech OS Tech® with different OSP layer thicknesses as a result of different deposition times within a range from 15 seconds to 120 seconds. The main constituent of this OSP is 2-[(4-chlorophenyl) methyl]-1H-benzimidazole. Table 1 shows the results.

[0149] Method 1 (not according to the present invention): Peeling off the entire OSP with 0.5% HCl and determining the concentration based on a UV measuring at 270 nm in a defined volume. As a result, an average layer thickness is calculated. This was used for establishing a calibration curve (FIG. 3)

[0150] Method 2 (not according to the present invention): Making a cross-section and determining the layer thickness by means of a scanning electron microscope (FIB).

[0151] Method 3 (according to the present invention): SITA CleanoSpector® (FIG. 1); correlating the results of method 1 with method 3.

TABLE-US-00001 TABLE 1 results Thickness Thickness OSP, OSP, Sample Method 1 Method 3 No. [μm] RFU [μm]* 1 0.17 4.0 0.17 2 0.24 7.4 0.23 3 0.25 8.0 0.26 4 0.28 9.2 0.29

[0152] FIG. 3 is a calibration line, setting the measured RFU into relation to the thickness measures according to method 1. * denotes additional results obtained by method 2.

[0153] Example 1 shows a very reliable correlation.

Example 2

[0154] The layer thickness distribution is determined on a printed circuit board by means of two methods. For this purpose, measurements are taken on different and similar openings, covered with OSP, on the printed circuit board.

[0155] FIG. 4 shows a top view of a PCB and the measuring points 1-9. Each of the measuring points is located in a different opening wherein no solder resist 19 is deposited, the openings designated as 1′-9′. Some of the openings differ in area 1′-9′. Parts of copper are covered by solder resist 19, such as a conductive path 27 which is covered.

TABLE-US-00002 TABLE 2 results Thickness Thickness Area of OSP, OSP, Measuring opening Method 2 Method 3 point mm.sup.2 [μm] RFU [μm] 1 1.56 0.34 24.7 0.34 2 1.56 — 28.0 0.37 3 1.56 — 32.5 0.42 4 4 — 33.8 0.43 5 4 0.26 15.6 0.25 6 9 0.18 10.8 0.20 7 9 — 12.3 0.21 8 25 0.14 7.9 0.17 9 49 0.19 10.2 0.19

[0156] Thickness according to method 3 was obtained with the calibration used in example 1 (FIG. 3).

[0157] It can be seen that thickness of the OSP layer in an opening and on a contact pad is dependent from the size of the opening. For example, thickness in the larger openings 8′ and 9′ (measuring points 8 and 9) is lower than in the smaller openings 1′-3′ (measuring points 1-3). Therefore, it is a great advantage of the method of the present invention that the individual thickness is reliably determined.

[0158] It can be seen that in the opening with the largest area, measuring point 9, the layer thickness is increased in comparison to the smaller opening with measuring point 8. This is an unexpected result but shows the advantage of the method of the invention which allows determining the individual thickness in an individual openings and allows determining such unexpected results, which can be used as a basis for adaptation of the production process of OSP layers on PCBs.

[0159] FIG. 5 shows an overview of the whole method of the invention, in a basic embodiment. It is referred to previous Figures, particularly FIGS. 1 and 2, and the previous examples.

[0160] S1: Placing the printed circuit board 16 adjacent to the movement device 24 of the fluorescence measuring system 10. This is shown in FIG. 2

[0161] S2: Obtaining information about the location of the openings on the printed circuit board 16. Here, data from a data set (Gerber data set) is preferably provided to a controller (not shown) which e.g. controls the movement of the movement device 24. Data are preferably selected by a program. As an alternative, information can be obtained by visual inspection of the PCB 16 with a camera.

[0162] S3: Selecting at least one of the openings 21. This selection is preferably done by individual decision or by a computer program.

[0163] S4: Moving preferably the radiation source 11 with the movement device 24 and placing the radiction source at such position that the radiation beam 22 irradiates into the at least one selected opening 21, and on the layer of organic solderability preservative 20 in the at least one selected opening, as shown in FIGS. 1 and 2.

[0164] S5: Detecting a fluorescence radiation 23 which is emitted from the organic solderability preservative 20 with the detection unit 12, as shown in FIG. 1.

[0165] S6: Determining the individual thickness of the layer of the organic solderability preservative 20 at the location of irradiation in the at least one selected opening 21. This was explained in above examples 1 and 2.

[0166] Steps S3-S6 are preferably repeated, for example in different openings and/or at different measuring points, e.g. 1-9, as shown in FIG. 4.