PCB solder mask adhesion quality evaluation method based on slice analysis

Abstract

The invention discloses a PCB solder mask adhesion quality evaluation method based on slice analysis, and the method comprises the following steps: (1) PCB pretreatment: carrying out a thermal shock test on a PCB, and then carrying out a neutral salt spray test; (2) optical inspection: performing solder mask color difference inspection on the pretreated PCB by using a microscope, marking an area which is deeper than the surrounding color or has obvious change, and determining the area as a solder mask adhesion quality risk area; and (3) performing slice analysis: performing slice analysis on the solder mask adhesion quality risk area, and checking the bonding conditions of the solder mask, the copper foil circuit and the PCB substrate, thereby evaluating the solder mask adhesion quality.

Description

Technical Field

[0001] The invention belongs to the technical field of printed circuit boards, and in particular relates to a PCB solder mask attachment quality assessment method based on slice analysis. Background Art

[0002] Solder mask, also known as solder resist, is a protective film with a specific pattern formed on the surface of a rigid printed circuit board (hereinafter referred to as PCB) during the production process. Solder mask can prevent short circuits caused by tinning of non-welding parts during the welding process, save solder, prevent electrical short circuits between conductors due to moisture and chemical corrosion, and prevent electrical open circuits caused by scratches and scratches caused by various human factors during the manufacturing, welding and transportation of PCBs. It resists attacks and corrosion on PCBs in various harsh environments and ensures that the printed circuits function normally.

[0003] As PCBs develop towards higher speed and higher frequency, the diameter and spacing of copper-clad wires on PCBs are rapidly decreasing, and the requirements for the coating quality of solder mask on the PCB surface are becoming higher and higher; the reliability and safety of PCBs are closely related to the adhesion quality of solder mask. Insufficient adhesion of solder mask or straw-like gaps next to copper foil conductors will not only reduce the effective line spacing, but may also cause delamination of copper foil and solder mask, bringing reliability risks to the application of high-frequency and high-speed PCBs.

[0004] At present, the method used in the industry to test the adhesion of PCB solder mask is the tape method (also known as the grid method) described in Section 2.4.28.1 of the IPC-TM-650 test method book. The specific steps are as follows: ① Select a test area and ensure that the solder mask in the area is completely dry, clean and dust-free; ② Use a grid knife to cut a cross grid pattern in both the vertical and horizontal directions on the surface of the solder mask layer, and the depth of the incision reaches the surface of the PCB substrate or copper foil; ③ Use a special pressure-sensitive tape, such as 3M Scotch 600 tape, and cut the tape into a suitable size; ④ Stick the tape on the surface of the solder mask, and apply a certain amount of pressure on the tape with your fingers or a hook to ensure that the tape is tightly combined with the solder mask without obvious bubbles; ⑤ Let the tape stay on the solder mask for a while, and then quickly tear off the tape at a steady speed (usually 180° / s), usually at an angle of 90° or 180°; ⑥ Evaluate the adhesion of the solder mask based on the area of ​​solder mask shedding and the solder mask remaining on the tape.

[0005] However, the existing tape method cannot effectively reflect the adhesion quality of the solder mask. The main reasons are: ① The tape method is affected by human factors such as the maintenance and operation of the cutter. For example, the strength of scratching and tearing the tape will significantly affect the test results; ② The tape method does not clearly specify the area for PCB quality inspection, resulting in fluctuations in test results due to different inspection areas; ③ The surface unevenness caused by high-density wiring on the PCB surface and copper foil line distribution design factors will interfere with the scratching quality of the cutter, thereby affecting the stability of the test results. Therefore, the test process of the tape method is uncertain, and the test results are unstable, which is not conducive to the mutual recognition of PCB incoming material quality evaluation results between different testing agencies. According to production experience, even if the PCB materials are judged to have qualified adhesion quality through the tape method test, during the assembly and use process, due to the thermal stress and mechanical stress of the welding assembly process and the mechanical vibration and bending stress that may be experienced in the service environment, the solder mask in the stress concentration area is easy to be damaged, further weakening the adhesion of the solder mask and increasing the risk of corrosion of the circuit copper foil during service. Therefore, there is a problem that the PCB materials that are judged to have qualified adhesion quality through the tape method test cannot be effectively exposed to the circuit copper foil corrosion caused by insufficient adhesion of the solder mask during actual use. In addition, the surface unevenness caused by the high-density wiring on the PCB surface and the copper foil circuit distribution design factors will interfere with the engraving quality of the engraving knife, resulting in the risk of straw-like gaps in the solder mask that are not easy to find. Summary of the invention

[0006] The object of the present invention is to provide a PCB solder mask adhesion quality assessment method based on slice analysis, the test assessment result of the method has high stability and effectively reflects the adhesion quality of the solder mask.

[0007] To achieve the above objectives, the present invention adopts the following technical solutions.

[0008] A PCB solder mask attachment quality assessment method based on slice analysis includes the following steps:

[0009] (1) PCB pretreatment: thermal shock test and neutral salt spray test are performed on the PCB;

[0010] (2) Optical inspection: Use a microscope to inspect the color difference of the solder mask on the pre-treated PCB, mark the area that is darker than the surrounding color or has obvious changes, and identify it as the risk area for solder mask adhesion quality;

[0011] (3) Slice analysis: Slice the risk area of ​​solder mask adhesion quality to check the bonding between the solder mask and the copper foil circuit and PCB substrate, thereby evaluating the adhesion quality of the solder mask.

[0012] The present invention firstly generates thermal stress on the PCB through a thermal shock test to stimulate the potential risk of solder mask adhesion quality, and then uses a neutral salt spray test to remove the free Cl - The H2O penetrates into the surface of the PCB substrate or circuit copper foil where the solder mask is poorly bonded, and the redox reaction with Cu and the color change caused by the increase of local water residue in the solder mask are used to expose the area with poor solder mask adhesion quality, thereby determining the risk area of ​​solder mask adhesion quality, thereby clarifying the inspection area of ​​solder mask adhesion quality; specifically, after the neutral salt spray test, due to the invasion of water molecules and the aggregation at the solder mask / substrate interface, in the risk area of ​​solder mask adhesion quality, the copper foil along the circuit appears inconsistent with the surrounding color and discontinuous color, such as the green solder mask, the surface color generally changes from green to dark green, and even becomes dark brown due to copper foil corrosion. Therefore, the present invention effectively solves the problem of unclear solder mask adhesion quality inspection area in the prior art, and also conducts service environment adaptability assessment on the quality of PCB incoming materials.

[0013] Furthermore, the present invention adopts slice analysis to evaluate the adhesion quality of the PCB solder mask, which effectively avoids the influence of human factors in the prior art, as well as the influence of the surface unevenness caused by the wiring density and copper foil line distribution design of the PCB board surface on the solder mask adhesion quality evaluation results.

[0014] The method of the present invention can be applied to the solder mask adhesion quality assessment of different rigid PCB products, including:

[0015] ① Single-sided and double-sided printed boards with or without plated-through holes (PTH);

[0016] ②Multilayer printed boards with plated-through holes (PTH) with or without buried / blind vias / microvias;

[0017] ③ Embedded active or passive circuit printed boards with discrete capacitor layers and / or embedded capacitance or resistance components;

[0018] ④ Metal core printed circuit boards with or without external metal heat dissipation frames (active or passive).

[0019] As a preferred embodiment of the present invention, the PCB is dried before the thermal shock test at a temperature of 105-125° C. for 6 hours.

[0020] The thermal shock test can adopt a conventional PCB thermal shock test method. As a specific embodiment of the present invention, the PCB thermal shock test method disclosed in Article 2.6.7.2 of the IPC-TM-650 test method manual can be used to perform the thermal shock test, wherein the temperature change rate of the thermal shock test is required to be no less than 10°C / min, and a total of 100 cycles are performed.

[0021] The neutral salt spray test can adopt the existing neutral salt spray test method with a test period of 48 hours; a 24-hour test period can also be used to shorten the test time. As a specific embodiment of the present invention, a neutral salt spray test is carried out according to the method disclosed in GB / T10125-2021 "Artificial atmosphere corrosion test salt spray test".

[0022] As a preferred embodiment of the present invention, in step (2), a 20X to 50X magnifying glass or an ultra-depth optical microscope is used to check the color change of the solder mask on the surface of the PCB, and the area with a darker color than the surrounding area or a significant change is marked, and the representative area is determined as the solder mask attachment quality risk area; the representative area is selected from at least one of the following: the side of the PCB panel, the 5 to 15 mm area around the non-metallized via hole at the corner of the PCB, the center of the PCB board, and the intersection area of ​​the short side and the long side. Preferably, in step (2), the color change of the solder mask in the high-density wiring area and the high-stress strain area of ​​the PCB is checked in particular, and the inspection can be carried out according to the principle of first checking the high-stress strain area and then the high-density wiring area.

[0023] As a specific embodiment of the present invention, in step (3), a slice sample is cut in a radial direction perpendicular to the copper foil circuit in the solder mask adhesion quality risk area, and a metallographic sample is prepared, and a microscope is used to check the bonding between the solder mask and the copper foil circuit and the PCB substrate perpendicular to the radial direction of the copper foil circuit in the metallographic sample. The steps of preparing the metallographic sample are cold mounting, grinding, and polishing in sequence. The microscope can clearly show the bonding between the solder mask and the copper foil and the PCB substrate in the metallographic sample, and can be specifically selected from an electron microscope, an ultra-depth of field microscope, or a metallographic microscope.

[0024] As a specific embodiment of the present invention, in step (3), a microscope is used to check whether there is blistering or delamination between the solder mask film and the copper foil circuit and the PCB substrate perpendicular to the radial direction of the copper foil circuit in the metallographic sample, thereby evaluating the adhesion quality of the solder mask film. The specific standard for evaluating the adhesion quality of the solder mask film can be based on the provisions of clause 3.7.2 of IPC-6012 "Qualification and Performance Specification for Rigid Printed Boards", or can also be based on relevant regulations of the industry or enterprise.

[0025] As a further embodiment of the present invention, the present invention can also inspect the straw-shaped gap problem of the solder resist film in the metallographic sample during the slice analysis, and measure the thickness of the solder resist film.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] The PCB solder mask adhesion quality assessment method based on slice analysis provided by the present invention clarifies the quality inspection area, avoids the influence of human factors and the surface unevenness problem caused by the wiring density on the surface of the PCB board and the copper foil line distribution design, improves the effectiveness and stability of the assessment result, and is conducive to the mutual recognition of the quality assessment results of PCB incoming materials among different testing institutions.

[0028] Among them, the present invention performs PCB pretreatment before slicing analysis, which not only clarifies the quality inspection area, but also takes into account the influence of PCB assembly process and the influence of subsequent service scenarios on the adhesion quality of solder mask, so that the evaluation results can more effectively reflect the adhesion quality of solder mask during the service process. In addition, it also realizes the evaluation of the service environment adaptability of PCB.

[0029] During the application test, the qualified PCBs evaluated by the present invention showed no obvious copper foil circuit corrosion, which greatly reduced the reliability and safety risks of PCB products caused by the quality problems of solder mask adhesion. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 Schematic diagram of the solder mask attachment quality risk area on the PCB surface after pretreatment;

[0031] Figure 2 A schematic diagram of the slicing position within the solder mask attachment quality risk area;

[0032] Figure 3 for Figure 2 Electron microscope image of metallographic sample 1 at the position indicated by the middle dotted line 1;

[0033] Figure 4 for Figure 2 Electron microscope image of metallographic sample 2 at the position marked by the middle dotted line 2. DETAILED DESCRIPTION

[0034] The technical solution of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0035] Example 1

[0036] (1) PCB pretreatment: Place the PCB in a temperature controlled box at 105-125°C and dry for 6 hours;

[0037] First, a thermal shock test is performed according to the PCB thermal shock test method disclosed in Article 2.6.7.2 of the IPC-TM-650 test method manual to stimulate the quality risk of solder mask adhesion; the specific steps are: ① According to the type of PCB sample board, refer to Table 6-1 to select the temperature range; ② The extreme dwell time should ensure that the temperature of the sample reaches temperature stability under the extreme temperature conditions, and this embodiment sets it to 15min; ③ The temperature change rate, the temperature change rate of the middle 60% stage of each conversion process from hot to cold and from cold to hot is at least 10℃ / min; ④ After determining the above test parameters, carry out a 100cycles thermal shock test on the PCB sample.

[0038] Then, the PCB is subjected to a neutral salt spray test according to the method of GB / T 10125-2021 "Artificial Atmosphere Corrosion Test Salt Spray Test"; the specific steps are as follows: ① At a temperature of 25℃±2℃, dissolve NaCl in distilled water or deionized water with a conductivity of no more than 20μS / cm to prepare a NaCl solution with a concentration of 50g / L and a pH value of 6.5-7.2; ② Adjust the pressure of the spray box, and control the spray pressure in the range of 70kPa-170kPa, specifically 98kPa±10kPa; ③ Set the temperature of the test box, and the temperature of the neutral salt spray test box is 35±2℃; ④ Place the PCB sample after thermal shock in the salt spray box, and the surface of the sample to be tested is 15°~25° to the vertical direction, and as much as possible 20°, and the distance between the samples should not affect the free fall of salt spray on the test surface. ⑤ The test cycle is set to 48h; ⑥ After the neutral salt spray test, take out the sample for optical inspection.

[0039] (2) Optical inspection: Use a 20X to 50X magnifying glass or an ultra-depth of field optical microscope to inspect the color difference of the solder mask on the pre-treated PCB. There is no obvious color change in the area with good solder mask adhesion quality. Taking the green solder mask as an example, the area with risk of solder mask adhesion quality will change from the original green to dark green or even brown.

[0040] Then mark the area on the PCB surface where the color of the solder mask is darker than the surrounding area or there is a significant change. Take at least one of the following areas as representative areas: the PCB panel side, the 5-15mm area around the non-metallized vias at the corners of the PCB, the center of the PCB board, and the intersection of the short side and the long side, and determine it as the solder mask attachment quality risk area, such as Figure 1 As shown in the red box area;

[0041] (3) Slice analysis: Cut perpendicular to the copper foil line radial direction in the solder mask adhesion quality risk area, such as Figure 2 As shown, cutting is performed according to the indication of the dotted line 1 to obtain a slice sample 1;

[0042] In this embodiment, the non-solder mask adhesion quality risk area is also Figure 2 As shown, the sample 2 is cut according to the dotted line 2 to obtain the sliced ​​sample 2, which is compared with the sliced ​​sample 1 in the solder mask adhesion quality risk area;

[0043] The sliced ​​samples 1 and 2 were made into metallographic samples 1 and 2 respectively by cold mounting, grinding and polishing; the combination of the solder mask perpendicular to the radial direction of the copper foil circuit in the metallographic samples with the copper foil circuit and the PCB substrate was checked by electron microscope to check whether there was blistering or delamination of the solder mask and straw-shaped gaps. Figure 3 and 4 .

[0044] This embodiment specifically performs identification and evaluation in accordance with the provisions of clause 3.7.2 of IPC-6012E “Identification and Performance Specification for Rigid Printed Boards”.

[0045] like Figure 3 As shown, there is delamination between the solder mask and the substrate of metallographic sample 1, which exceeds the requirements of the standard, so the evaluation result is that the solder mask adhesion quality is unacceptable, that is, the PCB sample is unqualified.

[0046] from Figure 4 It can be seen that the metallographic sample 2 in the non-solder mask adhesion quality risk area meets the requirements of level 2 and 3 products; in other words, if the sample is taken in the non-solder mask adhesion quality risk area, the evaluation result cannot effectively reflect the actual solder mask adhesion quality. The above results show that the present invention is scientific and reasonable to determine the solder mask adhesion quality inspection area through PCB pretreatment and optical inspection, and effectively reflects the adhesion quality of the solder mask.

[0047] Figure 3 and 4 It also shows that the PCB sample does not have the straw-like gap problem.

Claims

1. A PCB solder mask adhesion quality assessment method based on slice analysis, characterized in that: The following steps are involved: (1) PCB pretreatment: thermal shock test and neutral salt spray test are performed on the PCB; (2) Optical inspection: Use a microscope to inspect the color difference of the solder mask on the pre-treated PCB, mark the area that is darker than the surrounding color or has obvious changes, and identify it as the risk area for solder mask adhesion quality; (3) Slice analysis: Slice the risk area of ​​solder mask adhesion quality to check the bonding between the solder mask and the copper foil circuit and PCB substrate, thereby evaluating the solder mask adhesion quality.

2. The PCB solder mask adhesion quality assessment method based on slice analysis according to claim 1 is characterized in that: The PCB is dried before the thermal shock test; the drying temperature is 105-125° C. and the drying time is 6 hours.

3. The PCB solder mask adhesion quality assessment method based on slice analysis according to claim 2 is characterized in that: In step (2), a 20X to 50X magnifying glass or an ultra-depth-of-field optical microscope is used to check the color change of the solder mask on the surface of the PCB, and the area that is darker than the surrounding color or has obvious changes is marked, and the representative area is determined as the solder mask adhesion quality risk area; the representative area is selected from at least one of the following: the side of the PCB panel, the 5-15mm area around the non-metallized via hole at the corner of the PCB, the center of the PCB board, and the intersection area of ​​the short side and the long side.

4. The PCB solder mask adhesion quality assessment method based on slice analysis according to claim 3 is characterized in that: In step (2), the color change of the solder mask in the high-density wiring area and high-stress and strain area of ​​the PCB is checked in particular, and the inspection is carried out according to the principle of first checking the high-stress and strain area and then the high-density wiring area.

5. The PCB solder mask adhesion quality assessment method based on slice analysis according to claim 4 is characterized in that: In step (3), a slice sample is cut in a radial direction perpendicular to the copper foil circuit in the solder mask adhesion quality risk area, and a metallographic sample is prepared. The bonding between the solder mask and the copper foil circuit and the PCB substrate perpendicular to the radial direction of the copper foil circuit in the metallographic sample is examined using a microscope; the microscope is selected from an ultra-depth of field microscope, an electron microscope or a metallographic microscope.

6. The PCB solder mask adhesion quality assessment method based on slice analysis according to claim 5 is characterized in that In step (3), a microscope is used to check whether there is blistering or delamination between the solder mask film and the copper foil circuit and the PCB substrate perpendicular to the radial direction of the copper foil circuit in the metallographic sample, thereby evaluating the adhesion quality of the solder mask film.

7. The PCB solder mask adhesion quality assessment method based on slice analysis according to claim 6 is characterized in that: During the section analysis, the metallographic samples were checked for the problem of straw-like voids in the solder mask.

8. The PCB solder mask adhesion quality assessment method based on slice analysis according to claim 7 is characterized in that: The thermal shock test is performed using the PCB thermal shock test method in Section 2.6.7.2 of the IPC-TM-650 test method manual.

9. The PCB solder mask adhesion quality assessment method based on slice analysis according to claim 8 is characterized in that: The neutral salt spray test is carried out according to the method of GB / T 10125-2021 "Artificial atmosphere corrosion test salt spray test", where the test cycle is 48h.

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