Abstract: Ultra thick copper multilayer PCB is an integrated connecting device with strong and weak electrical connection functions. A study was conducted on the manufacturing process of ultra-thick copper multi-layer PCB boards with a thickness of 0.41 mm or more. Single sided copper clad plates+1.0 mm blackened copper clad plates+epoxy plates+single sided copper clad plates were used for lamination, and non flowing semi cured sheets were used for bonding between layers, effectively solving industry problems such as white spots and delamination in ultra-thick copper lamination.
preface
The special ultra-thick copper multi-layer PCB originated in North America, such as UPE Company in Canada. The company began developing and producing ultra-thick copper multi-layer PCBs in the 1990s, achieving good performance. This type of special PCB product is applied to components with strong current connection transmission and mixed strong and weak current connection. With the rapid development of automotive electronics and power communication modules in China, it has gradually become a special PCB with broad market prospects. According to market understanding, there is a demand for automotive electronics, IGBT assembly, wind power converters, ignition coils, and other aspects; On the other hand, with the widespread application of printed circuit boards in the electronic field, the functional requirements for them have become increasingly high. Printed circuit boards will not only provide necessary electrical connections and mechanical support for electronic components, but also gradually be endowed with more additional functions. Therefore, ultra thick copper multilayer PCBs that can integrate power supply, provide high current, and high reliability have gradually become new products developed in the PCB industry, with broad prospects, The profit margin is larger than traditional circuit boards and has great development value. Ultra thick copper multilayer PCBs will occupy an important position in the future high-end circuit connection market and will inevitably usher in a broader market prospect.
At present, the industry generally adopts the layering method of electroplating, depositing copper, gradually thickening, and multiple soldering printing assistance, or using ultra-thick copper foil to achieve the manufacturing of ultra-thick copper printed circuit boards. However, the thickness of copper in the above process can currently only reach a maximum of 0.41 mm (12 oz/ft ²）， It will become very difficult to process ultra thick copper multilayer boards beyond this copper thickness, and there is currently no technological breakthrough in this area. The difference between ordinary PCB and ultra thick copper PCB is shown in Table 1. This article mainly studies a new process method for manufacturing ultra-thick copper multi-layer PCBs. Drawing on the production process of stacked busbar, copper plate embedded pressing technology is adopted, and after process optimization, 0.5 mm (14 oz/ft) is achieved ²） The manufacturing of ultra-thick copper multi-layer printed boards mentioned above.
Manufacturing process of ultra-thick copper multi-layer PCB
Stacked structure
This article mainly studies an ultra-thick copper three-layer board, with an inner layer copper thickness of 1.0 mm, an outer layer copper thickness of 0.3 mm, and a minimum outer line width line spacing of 0.5 mm. The laminated structure is shown in Figure 1. The surface layer is made of FR4 copper clad plate (glass fiber epoxy resin copper clad plate) with a thickness of 0.3 mm, with single side etching treatment. The adhesive layer is made of non flowing PP sheet (semi cured sheet) with a thickness of 0.1 mm, and the ultra thick copper plate is embedded in the corresponding pore structure of FR-4 epoxy plate.
The processing process of ultra-thick copper PCB mainly involves surface and middle layer milling plates, and thick copper plate milling. After surface treatment, it is stacked in the overall mold for heating and pressing. After demolding, the finished product is produced according to the conventional PCB process.
Key Process Processing Methods
1 Super-thick copper inner layer lamination technology
Super-thick copper inner layer lamination: If copper foil is used for ultra-thick copper, it will be difficult to achieve this thickness. In this article, 1mm electrolytic copper plate is used for the ultra-thick copper inner layer, which is a conventional material that is easy to purchase and directly processed into shape by a milling machine; The outer contour of the copper plate in the inner layer is processed and formed using the same thickness of FR4 plate (glass fiber epoxy resin plate) as the overall filling. In order to facilitate lamination and ensure a close fit with the surrounding copper plate, the gap value between the two contours is controlled within 0~0.2 mm as shown in the structure in Figure 4. Under the filling effect of FR4 board, the copper thickness problem of ultra-thick copper plate has been solved, and the tight pressing and internal insulation problems after lamination have been ensured, so that the design of the inner layer copper thickness can be greater than 0.5 mm.
Ultra thick copper blackening technology
The surface of ultra thick copper needs to be blackened before lamination. Blackening of copper plates can increase the contact surface area between the copper surface and the resin, and increase the wettability of high-temperature flowing resin to copper, allowing the resin to penetrate into the pores of the oxide layer and exhibit strong adhesion after hardening, improving the bonding effect. Simultaneously improve the phenomenon of laminated white spots and problems such as whitening and bubbles on the board surface caused by baking tests (287 ℃± 6 ℃).
3 Ultra thick copper PCB lamination technology
Due to manufacturing errors in the thickness of the inner ultra-thick copper plate and the FR-4 plate used for surrounding filling, the thickness cannot be completely consistent. If conventional lamination is used for lamination, it is easy to produce defects such as lamination white spots and delamination, making lamination difficult. In order to reduce the difficulty of laminating the ultra-thick copper plate layer and ensure dimensional accuracy, through experimental verification, an integral lamination mold structure was used. The upper and lower templates of the mold were made of steel molds, and silicone pads were used as intermediate buffer layers. By setting appropriate lamination temperature, pressure, holding time and other process parameters, the lamination effect was achieved, and technical problems such as white spots and layering in ultra-thick copper lamination were also solved, meeting the lamination requirements of ultra-thick copper PCB boards.
4 Ultra thick copper PCB lamination method.
The stacking level of products inside the ultra thick copper lamination mold. Due to the low fluidity of the non flowing PP resin, if the conventional covering material Kraft paper is used, the PP sheet cannot be pressed evenly, resulting in white spots, delamination and other defects after pressing. The thick copper PCB products need to use the silicone pad as the key buffer layer in the lamination process, which plays a role in evenly distributing the pressure in the pressing. In addition, to solve the pressing problem, the pressure parameter in the laminator is adjusted from 2.1 Mpa (22 kg/cm ²） Set to 2.94 Mpa (30 kg/cm ²）， And adjust the temperature to the optimal fusion temperature of 170 ℃ based on the characteristics of the PP sheet.
After conducting the test according to section 4.8.5.8.2 of GJB362B-2009, there should be no blistering or delamination exceeding the allowable values in section 3.5.1.2.3 (subsurface defects) when inspecting the PCB according to section 4.8.2. The PCB sample meets the appearance and size requirements of 3.5.1, and is subjected to microsection and inspection according to 4.8.3, meeting the requirements of 3.5.2. From the perspective of laminated slicing, the line is fully filled with no micro seam bubbles.
5 Ultra thick copper PCB flow control technology
Unlike general PCB processing, its appearance and device connection holes are already completed before lamination. If the adhesive flow is severe, it will affect the roundness and size of the connection, 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 strict control is required for the later milling of the shape, especially for the processing of thick copper connection parts in the inner layer. The depth accuracy control is very strict, and the qualification rate is extremely low.
Choosing appropriate bonding materials and designing reasonable device structures is one of the difficulties in research. To solve the appearance problem of adhesive overflow caused by ordinary semi cured sheets after lamination, a semi cured sheet with low flowability (beneficial: SP120N) is used. This bonding material has characteristics such as low resin flowability, flexibility, heat resistance, and excellent electrical properties. Based on the adhesive overflow characteristics, the contour of the semi cured sheet at a specific position is measured and machined into a specific shape contour through cutting and drawing. At the same time, the process of forming first and then pressing is achieved, and the shape is formed after pressing, without the need for CNC milling of the shape again. This solves the problem of adhesive flow on the PCB after pressing, ensuring that there is no adhesive flow on the connecting surface of the ultra-thick copper plate after lamination and the pressing is tight.
Product specifications for ultra thick copper PCB
Voltage withstand test
Perform a voltage withstand test on each pole in the ultra-thick copper PCB sample, with a test voltage of AC1000V and no impact or flashover for 1 minute.
High current temperature rise test
Design corresponding connecting copper plates to connect the poles of the ultra-thick copper PCB sample in series, connect them to a high current generator, and test them according to the corresponding test current.
From the temperature rise situation, the overall temperature rise of ultra-thick copper PCB is relatively low, which can meet the actual usage requirements (generally the temperature rise requirement is below 30 K). The high current temperature rise of ultra thick copper PCB is related to its structure, and different thick copper structures will have certain differences in temperature rise.
Thermal stress testing
Requirements for thermal stress test: After conducting thermal stress test on the sample in accordance with the GJB362B-2009 General Specification for Rigid Printed Boards, there shall be no defects such as delamination, blistering, solder pad warping, and white spots upon visual inspection.
After the appearance and size of the PCB sample meet the requirements, it needs to be micro sectioned. Because the inner layer of copper in this sample is too thick to undergo metallographic sectioning, only visual inspection of its appearance will be conducted after thermal stress testing at 287 ℃± 6 ℃.
The test results show that there are no defects such as delamination, blistering, solder pad warping, and white spots.
summary
This article provides a manufacturing process method for ultra-thick copper multi-layer PCBs. Through technological innovation and process improvement, it effectively solves the current limitation of copper thickness in ultra-thick copper multi-layer PCBs and overcomes common processing technical difficulties as follows:
(1) Ultra thick copper inner layer lamination technology: effectively solves the problem of selecting ultra thick copper materials, and adopts pre milling forming processing without etching, effectively avoiding the problem of thick copper plate etching technology; The FR-4 filling technology ensures tight compression and insulation of the inner layer;
(2) Ultra thick copper PCB lamination technology: effectively solves the problems of lamination white spots and delamination, and finds a new pressing method and solution;
(3) Ultra thick copper PCB glue flow control technology: effectively solves the glue flow problem after pressing, ensuring the implementation of the pre milling shape re pressing process.
