PCB bonding sheet

1 Introduction

The rapid development of the modern communications industry has brought an unprecedented large market for the manufacture of high-frequency copper clad laminates. As one of the basic materials for the manufacture of high-frequency copper clad laminates, the bonding sheet material, its material composition and related performance indicators determine the realization and processability of the performance indicators of the final product designed.

In view of the increasing design and application of high-frequency polytetrafluoroethylene dielectric copper clad laminates, especially the increasing demand for the design of polytetrafluoroethylene dielectric high-frequency multilayer boards in recent years, it has brought unprecedented opportunities and challenges to the majority of printed circuit board manufacturers. At the same time, higher performance index requirements are put forward for the manufacture of high-frequency copper clad laminates of basic raw materials.

As we all know, for polytetrafluoroethylene high-frequency substrate materials, the performance indicators and processability of the bonding sheet material determine the application field of high-frequency copper clad laminates. In addition, the multi-layer manufacturing technology of high-frequency printed circuit boards, after focusing on solving the characteristic impedance control technology in the manufacturing technology of high-frequency multi-layer printed substrates, choose what kind of bonding sheet material system to realize the multi-layer manufacturing of high-frequency boards, which has become a thorny problem that every designer and process personnel must face.

2 Status of bonding sheet materials

Looking at the development history of the entire high-frequency copper clad laminate industry, the innovation of bonding sheet materials is gradually entering a new era under the requirements of meeting the performance

indicators of copper clad laminates.

There are two bonding sheet modes in terms of the resin system selected by the bonding sheet material. One is the thermoplastic resin system bonding sheet material; the other is the thermosetting resin bonding sheet material.

2.1 Thermoplastic film bonding material

For the market demand for high-frequency copper clad laminates, twenty years ago, it was in the manufacturing stage of single-sided and double-sided high-frequency boards. With the rapid development of modern communication technology, more and more high-frequency copper clad laminate materials are facing the development trend of multi-layer technology in design and processing, and the importance of bonding sheet materials is highlighted.

Looking back at the entire process of the generation and development of high-frequency multi-layer boards, thermoplastic film bonding materials will be a good choice whether from the design selection or the processing of RF multi-layer boards.

Usually, in the process of panel arrangement, the film is placed crosswise to achieve multi-layer clamping. Among them, what is often not known to people but needs to be paid attention to is that the selected thermoplastic film bonding material must meet the heating process in the lamination process. In other words, the melting point of this thermoplastic film adhesive material must be lower than the melting point of 327℃ (6200F) of the high-frequency copper-clad dielectric board – polytetrafluoroethylene resin.

As the lamination temperature rises, it exceeds the melting point of the thermoplastic film, and the adhesive film begins to flow. With the help of the uniform pressure applied by the lamination equipment on the clamping plate, it is filled between the copper layer circuits on the surface of the layer to be bonded.

Generally, thermoplastic film adhesive materials are roughly divided into two types according to the lamination temperature.

(1) 220℃ lamination temperature control

The application of this type of lower temperature thermoplastic film adhesive material is first recommended by Rogers’ 3001 (see Figure 1 for the lamination temperature rise control diagram of this product).

Figure 1 Rogers 3001 film lamination parameter control

Over the years, similar thermoplastic film adhesive materials, such as Nelco and Arlon’s honorable products FV6700 film (the product lamination temperature rise control diagram, see Figure 2) and Cuclad6700 film (the product lamination temperature rise control diagram, see Figure 3) adhesive materials are available on the market to provide multi-layer bonding for their respective customers.

(2) 290℃ lamination temperature control

Different from the above-mentioned lower temperature adhesive materials, there is also a thermoplastic film adhesive material with a higher lamination temperature that is widely used, which is DuPont’s honorable product – Cuclad6700.

Figure 2 Nelco’s FV6700 film lamination parameter control

Figure 3 Arlon’s Cuclad6700 film lamination parameter control

FEP adhesive material (the product lamination temperature rise control diagram, see Figure 4). How to choose often depends on the subsequent process route of multi-layer circuit board processing, including the thermal process experienced, the melting point of the film used for bonding, reliability requirements, etc. Of course, the process capability of multi-layer printed circuit boards is also an aspect that needs to be considered.

Figure 4 Schematic diagram of temperature rise control of FEP lamination of DuPont products

2.2 Thermosetting adhesive material (prepreg)

The second bonding method requires the use of thermosetting adhesive material (commonly known as prepreg). Clamp the multi-layer board to be pressed filled with the thermosetting prepreg material, position and clamp, and then perform a program temperature rise operation.

Thermosetting prepregs often have a lower bonding temperature, which is lower than the melting point of 327 degrees of the high-frequency copper-clad polytetrafluoroethylene core board dielectric material.

Figure 5 Rogers RO4450B prepreg lamination parameter control

As the lamination temperature gradually increases, the prepreg resin will flow and fill between the copper circuit patterns with the help of uniform pressure attached to the multi-layer board to be pressed.

For the multi-layer mixed board structure where the traditional FR-4 copper-clad board dielectric material and the high-frequency copper-clad polytetrafluoroethylene dielectric laminate are bonded, according to experience, epoxy resin prepreg materials can usually be selected. However, when choosing epoxy resin prepreg, its impact on electrical properties should be carefully considered. Among Rogers’ traditional superior materials for thermosetting prepreg materials for high-frequency copper-clad laminates, there is RO4450B (the lamination temperature rise control diagram of this product is shown in Figure 5).

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3. Demand Outlook for Bonding Sheet Materials

With the rapid development of the communications industry and automotive electronic product access standards, the design and processability requirements are becoming increasingly urgent, and dissatisfaction with the insufficient performance of existing bonding sheet materials is becoming more and more prominent.

First, it is related to the realization of the performance indicators of high-quality high-frequency copper clad laminates; second, it directly affects the realization of multi-layer design and processing of high-frequency copper clad laminates.

In short, looking at the global high-frequency copper clad laminate companies, although the status of bonding sheet materials is greatly valued, the multi-layer and high-reliability metallized hole design and manufacturing of pure polytetrafluoroethylene dielectric high-frequency copper clad laminates are still under development

From the concept of similarity and compatibility in materials science, if you want to obtain excellent interlayer bonding, for high-frequency pure polytetrafluoroethylene copper clad laminate materials, thermoplastic bonding materials are preferred. However, for multiple lamination design and processing, thermosetting resin semi-cured sheets become a must, otherwise, multi-layer design and processing cannot be achieved.

As a world-class high-frequency copper clad laminate company, Rogers’ latest bonding sheet material, 2929Bondply (the product lamination temperature rise control diagram, see Figure 8) is first-class. However, on the one hand, its excellent blind hole filling performance brings about the problem of glue blocking in the window processing of high-frequency design;

on the other hand, this type of bonding sheet material belongs to the “MASSLAM” mode to meet large-scale industrial processing, and has the problem of poor machinability for multi-layer processing of pin positioning.

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With the advent of 5G base station construction, the “active module on the tower”, the demand for highly resistant substrates, and the manufacturing of high-layer high-frequency boards have brought new challenges to the majority of design and manufacturing companies.

The use of various types of high-frequency printed circuit boards for communications, especially polytetrafluoroethylene dielectric materials, is increasingly moving towards the manufacturing of high-frequency multi-layer circuit boards based on the original single-sided and double-sided manufacturing requirements for printed circuit boards. This high-frequency multi-layer printed circuit board is different from the traditional multi-layer printed circuit board.

Due to the particularity of its lamination manufacturing, it puts forward more stringent requirements on the accuracy of inter-layer overlap, the accuracy of graphic production, the consistency of the thickness of the inter-layer dielectric layer, the uniformity of the plating and the type of coating, as well as the inter-layer bonding force.

First and foremost, the thorny problem encountered by relevant knowledgeable people is that high-frequency copper clad laminate manufacturers provide effective targeted bonding sheet materials and manufacturing guide support for the manufacture of high-frequency, high-reliability, and high-layer printed circuit boards for the matching supply of high-reliability, high-tolerance, and processable bonding sheet materials.

Faced with the emergence of various new materials and new technologies, various problems are constantly testing the wisdom and ability of practitioners.

However, for the multi-layer realization of polytetrafluoroethylene dielectric high-frequency copper clad laminates, the focus on the manufacturing quality and reliability of the adhesive sheet materials used should always be the main basis for our decision-making.

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