Super thick copper Multilayer PCB manufacturing process

1. Laminated structure

The main research of this paper is an ultra-thick copper three-layer board, the inner copper thickness is 1.0 mm, the outer copper thickness is 0.3 mm, and the minimum line width and line spacing of the outer layer is 0.5 mm. The laminated structure is shown in Figure 1. The surface layer is FR4 copper clad laminate (glass fiber epoxy copper clad laminate), with a thickness of 0.3 mm, single-sided etching treatment, and the adhesive layer is a non-flowing PP sheet (semi-cured sheet), with a thickness of 0.1 mm, super thick The copper plate is embedded in the corresponding hole structure of the FR-4 epoxy plate.

The process flow of ultra-thick copper PCB processing is shown in Figure 3. The main machining includes surface and middle layer milling, thick copper plate number milling. After surface treatment, it is stacked in the overall mold to heat up and press, and after demolding, follow the conventional PCB process The process completes the production of finished products.

2. Key technology processing methods

2.1 Ultra-thick copper inner lamination technology

Super-thick copper inner lamination: If copper foil is used for super-thick copper, it will be difficult to achieve this thickness. In this paper, the super-thick copper inner layer uses 1 mm electrolytic copper plate, which is easy to purchase for conventional materials and is directly processed by a milling machine; the outer contour of the inner copper plate The same thickness of FR4 board (glass fiber epoxy board) is used for processing and molding as the overall filling. In order to facilitate the lamination and ensure that it fits closely with the periphery of the copper plate, the gap value between the two contours as shown in the structure of Figure 4 is controlled at 0~0.2 Within mm. Under the filling effect of FR4 board, the copper thickness problem of the ultra-thick copper board is solved, and the tight pressing and internal insulation problems after lamination are ensured, so that the design of the inner copper thickness can be greater than 0.5 mm.

2.2 Super thick copper blackening technology

The surface of ultra-thick copper needs to be blackened before lamination. The blackening of the copper plate can increase the contact surface area between the copper surface and the resin, and increase the wettability of the high-temperature flow resin to the copper, so that the resin can penetrate into the oxide layer gap and show strong performance after hardening. The adhesion force improves the pressing effect. At the same time, it can improve the laminating white spot phenomenon and the whitening and bubbles caused by the baking test (287 ℃ ± 6 ℃). The specific blackening parameters are shown in Table 2.

2.3 Super thick copper PCB lamination technology

Due to the manufacturing errors in the thickness of the inner super-thick copper plate and the FR-4 plate used for the surrounding filling, the thickness can not be completely consistent. If the conventional lamination method is used for lamination, it is easy to produce lamination white spots, delamination and other defects, and the lamination is difficult. . In order to reduce the difficulty of pressing the ultra-thick copper layer and ensure the dimensional accuracy, it has been tested and verified to use an integral pressing mold structure. The upper and lower templates of the mold are made of steel molds, and the silicone cushion is used as the intermediate buffer layer. Process parameters such as temperature, pressure, and pressure holding time achieve the lamination effect, and also solve the technical problems of white spots and delamination of ultra-thick copper lamination, and meet the lamination requirements of ultra-thick copper PCB boards.

(1) Super thick copper PCB lamination method.

The stacking level of the product in the ultra-thick copper laminate mold is shown in Figure 5. Due to the low fluidity of non-flowable PP resin, if the conventional cladding material kraft paper is used, the PP sheet cannot be uniformly pressed, resulting in defects such as white spots and delamination after lamination. Thick copper PCB products need to be used in the lamination process As a key buffer layer, the silica gel pad plays a role in evenly distributing the pressure during pressing. In addition, in order to solve the pressing problem, the pressure parameter in the laminator was adjusted from 2.1 Mpa (22 kg/cm²) to 2.94 Mpa (30 kg/cm²), and the temperature was adjusted to the best fusion temperature according to the characteristics of the PP sheet 170°C.

(2) The lamination parameters of ultra-thick copper PCB are shown in Table 3.

(3) The effect of super thick copper PCB lamination.

After testing in accordance with Section 4.8.5.8.2 of GJB362B-2009, there should be no blistering and delamination that exceed Section 3.5.1.2.3 (under-surface defects) allowed when testing the PCB according to 4.8.2. The PCB sample meets the appearance and size requirements of 3.5.1, and is micro-sectioned and inspected according to 4.8.3, which meets the requirements of 3.5.2. The slicing effect is shown in Figure 6. Judging from the condition of the lamination slice, the line is fully filled and there are no micro-slit bubbles.

2.4 Super thick copper PCB flow glue control technology

Different from general PCB processing, its shape and device connection holes have been completed before lamination. If the glue flow is serious, it will affect the roundness and size of the connection, and the appearance and use will not meet the requirements; this process has also been tested in the process development. The process route of the shape milling after pressing, but the later shape milling requirements are strictly controlled, especially for the processing of the inner thick copper connection parts, the depth precision control is very strict, and the pass rate is extremely low.

Choosing suitable bonding materials and designing a reasonable device structure are one of the difficulties in the research. In order to solve the problem of the appearance of glue overflow caused by ordinary prepregs after lamination, prepregs with low fluidity (Benefits: SP120N) are used. The adhesive material has the characteristics of low resin fluidity, flexibility, excellent heat resistance and electrical properties, and According to the characteristics of glue overflow, the contour of the prepreg at a specific position is increased, and the contour of a specific shape is processed by cutting and drawing. At the same time, the process of forming first and then pressing is realized, and the shape is formed after pressing, without the need for CNC milling again. This solves the problem of glue flow after the PCB is laminated, and ensures that there is no glue on the connecting surface after the super-thick copper plate is laminated and the pressure is tight.

3. Finished effect of ultra-thick copper PCB

3.1 Ultra-thick copper PCB product specifications

Super-thick copper PCB product specification parameter table 4 and finished product effect are shown in figure 7.

3.2 Withstand voltage test

The poles in the ultra-thick copper PCB sample were tested for withstand voltage. The test voltage was AC1000V, and there was no strike or flashover in 1 min.

3.3 High current temperature rise test

Design the corresponding connecting copper plate to connect each pole of the ultra-thick copper PCB sample in series, connect it to the high current generator, and test separately according to the corresponding test current. The test results are shown in Table 5:

From the temperature rise in Table 5, the overall temperature rise of the ultra-thick copper PCB is relatively low, which can meet the actual use requirements (generally, the temperature rise requirements are below 30 K). The high current temperature rise of ultra-thick copper PCB is related to its structure, and the temperature rise of different thick copper structures will have certain differences.

3.4 Thermal stress test

Thermal stress test requirements: After thermal stress test on the sample according to the GJB362B-2009 General Specification for Rigid Printed Boards, visual inspection shows that there are no defects such as delamination, blistering, pad warping, and white spots.

After the appearance and size of the PCB sample meet the requirements, it should be microsectioned. Because the inner layer of copper of this sample is too thick to be metallographically sectioned, the sample is subjected to a thermal stress test at 287 ℃ ± 6 ℃, and only its appearance is inspected visually.

The test result is: no delamination, blistering, pad warping, white spots and other defects.

4. Summary

This article provides a manufacturing process method for ultra-thick copper multilayer PCB. Through technological innovation and process improvement, it effectively solves the current limit of copper thickness of ultra-thick copper multilayer PCB, and overcomes the common processing technical problems as follows:

(1) Ultra-thick copper inner lamination technology: It effectively solves the problem of ultra-thick copper material selection. The use of pre-milling processing does not require etching, which effectively avoids the technical problems of thick copper etching; the FR-4 filling technology ensures the pressure of the inner layer Close tightness and insulation problems;

(2) Ultra-thick copper PCB lamination technology: effectively solved the problem of white spots and delamination in lamination, and found a new pressing method and solution;

(3) Super-thick copper PCB flow glue control technology: It effectively solves the problem of glue flow after pressing, and ensures the implementation of the pre-milling shape and then pressing.