Ultra-thick copper multilayer PCB originated from North America, such as UPE Company in Canada. In the 1990s, the company began to develop and produce ultra-thick copper multilayer PCB, and achieved good results. This kind of special board product is applied to the components connected with strong current and mixed connection of strong and weak current. With the rapid development of automotive electronics and power communication modules in China, it has gradually become a kind of special PCB with broad market prospects. According to the market, there are demands in automotive electronics, IGBT assembly, wind power converter, ignition coil, etc. On the other hand, with the wide application of printed circuit boards in the electronic field, the requirements for their functions are getting higher and higher. Printed circuit boards will not only provide the necessary electrical connection and mechanical support for electronic components, but also gradually be given more additional functions. Therefore, the ultra-thick copper multilayer PCB, which can integrate power supply, provide high current and high reliability, has gradually become a new product developed by PCB industry, with broad prospects, larger profit margin than traditional circuit boards, and great development value. Thick copper multilayer PCB will occupy an important position in the future high-end circuit connection market, and it is bound to usher in a broader market prospect.
At present, in the industry, it is widely used to manufacture ultra-thick copper printed circuit boards by means of lamination assisted by multiple solder resist printing or ultra-thick copper foil. However, at present, the thickness of copper in the above process can only reach 0.41 mm (12 oz/ft) at most, and it will be very difficult to process extra-thick copper multilayer boards beyond this thickness. At present, there is no technical breakthrough in this area. The difference between ordinary PCB and extra-thick copper PCB is shown in Table 1. In this paper, a new technology for manufacturing ultra-thick copper multilayer PCB is mainly studied. By using the laminated bus production technology for reference and adopting the embedded pressing technology of copper plate, the ultra-thick copper multilayer PCB with a thickness of more than 0.5 mm (14 oz/ft) is manufactured through process optimization.
1. Manufacturing process of ultra-thick copper multilayer PCB
1.1 laminated structure
This paper mainly studies an ultra-thick copper three-layer board. The inner copper layer is 1.0 mm thick, the outer copper layer is 0.3 mm thick, and the minimum line width and line spacing of the outer layer is 0.5mm. The laminated structure is shown in Figure 1. Among them, the surface layer is FR4 copper clad laminate (glass fiber epoxy resin copper clad laminate) with a thickness of 0.3 mm, which is etched on one side, and the adhesive layer is non-flowable PP sheet (prepreg) with a thickness of 0.1mm. The extra-thick copper plate is embedded in the hole structure corresponding to FR-4 epoxy plate, as shown in Figure 2.
1.2 process flow
The processing process of ultra-thick copper PCB is shown in Figure 3, which mainly includes surface and middle milling plates and thick copper plate milling. After surface treatment, it is stacked in the overall mold for heating and pressing, and after demoulding, the finished product is manufactured according to the conventional PCB process.
1.3 key processing methods
1.3.1 Superthick copper inner layer lamination technology
Super-thick copper inner layer lamination: Super-thick copper will be difficult to reach this thickness if copper foil is used. In this paper, 1 mm electrolytic copper plate is used for super-thick copper inner layer, which is easy to purchase as a conventional material and directly processed and formed by milling machine. The outer contour of the inner copper plate is filled with FR4 plate (glass fiber epoxy resin plate) with the same thickness. In order to facilitate lamination and ensure its close fit with the periphery of the copper plate, two rounds are shown in the structure of Figure 4. The clearance value of profile shall be controlled within 0 ~ 0.2 mm Under the filling effect of FR4 plate, the problem of copper thickness of super-thick copper plate is solved, and the problems of tight pressing and internal insulation after lamination are ensured, so that the design of inner copper thickness can be greater than 0.5mm
1.3.2 blackening technology of ultra-thick copper
The surface of super-thick copper needs blackening treatment before lamination. The blackening of copper plate can increase the contact surface area between copper surface and resin, and increase the wettability of high-temperature flowing resin to copper, so that the resin can penetrate into the gap of oxide layer, show strong adhesion after hardening, and improve the lamination effect. At the same time, it can improve the white spot phenomenon of laminate and the whitening and bubbles of the board surface caused by baking test (287℃ 6℃). Specific blackening parameters are shown in Table 2.
1.3.3 Ultra-thick copper PCB lamination technology
Because of the manufacturing error between the thickness of the inner super-thick copper plate and the FR-4 plate used for peripheral filling, the thickness can’t be completely consistent. If the conventional lamination method is adopted for lamination, it is easy to produce lamination white spots, delamination and other defects, and the lamination is difficult. In order to reduce the pressing difficulty and ensure the dimensional accuracy of the extra-thick copper plate, it is verified by experiments that the laminated effect is achieved by using the integral pressing die structure, the upper and lower templates of the die are made of steel molds, and the silica gel pad is used as the intermediate buffer layer. The technical problems such as white spots and delamination of the extra-thick copper plate are solved, and the pressing requirements of the extra-thick copper PCB plate are met.
(1) Lamination method of ultra-thick copper PCB.
The stacking level of products in the super-thick copper lamination mold is shown in Figure 5. Due to the low fluidity of non-fluidity PP resin, if conventional coated kraft paper is used, the PP sheet can’t be uniformly pressed, resulting in defects such as white spots and delamination after lamination. In the lamination process of thick copper plate PCB products, the silica gel pad needs to be used as the key buffer layer, which plays a role in evenly distributing the pressure during lamination. In addition, in order to solve the pressing problem, the pressure parameter in the laminator was adjusted from 2.1 MPa (22 kg/cm) to 2.94 MPa (30 kg/cm), and the temperature was adjusted to the optimal fusion temperature of 170℃ according to the characteristics of PP sheets.
(2) Lamination parameters of ultra-thick copper PCB are shown in Table 3.
(3) Lamination effect of super-thick copper PCB.
After the test according to section 4.8.5.8.2 in GJB362B-2009, the PCB shall be inspected according to section 4.8.2, and there shall be no blistering and delamination beyond that allowed by section 3.5.1.2.3 (subsurface defects). The PCB sample meets the appearance and size requirements of 3.5.1, and it is sectioned microscopically, and inspected according to 4.8.3, meeting the requirements of 3.5.2. The slicing effect is shown in Figure 6. Judging from the laminated slice condition, the circuit is full of filling, and there are no micro-cracks and bubbles.
1.3.4 control technology of super-thick copper PCB glue flow
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 joint, and the appearance and use cannot meet the requirements. In this process development, the process route of first pressing and then milling the shape has also been tested, but the requirements for the later milling of the shape are strictly controlled, especially for the processing of the inner thick copper joint. The depth accuracy control is very strict, and the qualification rate is extremely low.
It is one of the difficulties to choose suitable bonding materials and design reasonable device structures. In order to solve the appearance problem of adhesive overflow caused by common prepreg after lamination, a prepreg with low fluidity (Shengyi: SP120N) was used. The adhesive material has the characteristics of low fluidity, flexibility, excellent heat resistance and electrical properties, etc. According to the characteristics of adhesive overflow, the contour of the prepreg at a specific position was increased, and the contour of a specific shape was processed by cutting and drawing. At the same time, the technological process of first forming and then pressing is realized, and the shape is formed after pressing, so there is no need to process the shape by numerical control milling again. Therefore, the glue flowing phenomenon of the PCB after lamination is solved, and the connection surface of the super-thick copper plate after lamination has no glue flowing and the lamination is tight.
2. Finished product effect of ultra-thick copper PCB
2.2.1 withstand voltage test
The withstand voltage of each pole in the ultra-thick copper PCB sample was tested. The test voltage was AC1000V, and there was no striking or flashover phenomenon in 1 min.
2.2.2 High current temperature rise test
A corresponding connecting copper plate is designed to connect the poles of the ultra-thick copper PCB sample in series, connect them to a large current generator, and test them according to the corresponding test current. The test results are shown in Table 5:
According to 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 is 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 be different.
2.2.3 thermal stress test
Thermal stress test requirements: After thermal stress test of the sample according to 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 meet the requirements, it should be sectioned microscopically. Because the inner layer of this sample is too thick for metallographic section, after thermal stress test at 287℃ 6℃, only the appearance of the sample is visually inspected.
The test results show that there are no defects such as delamination, blistering, pad warping and white spots.
3. Summary
In this paper, a manufacturing process of ultra-thick copper multilayer PCB is provided. Through technical innovation and process improvement, the current limitation of copper thickness of ultra-thick copper multilayer PCB is effectively solved, and the common processing technical problems are broken through as follows:
(1) Ultra-thick copper inner layer laminating technology: effectively solves the problem of selecting ultra-thick copper materials, and pre-milling and forming processing do not need etching, thus effectively avoiding the technical problem of etching thick copper plates; FR-4 filling technology ensures the tight pressing of the inner layer and the difficult problem of insulation;
(2) Ultra-thick copper PCB lamination technology: effectively solved the problems of white spots and delamination, and found a new lamination method and solution;
(3) Glue flowing control technology of ultra-thick copper PCB: effectively solved the glue flowing problem after pressing, and ensured the process implementation of pre-milling and re-pressing.
