Hofmann Leiterplatten GmbH has developed a wide range of products during more than 25 years. New manufacturing know-how has been developed in fabricating organic PCB with embedded devices. A special name "AML (Active Multi Layer) was created to differentiate the Surface Mount Devices (SMD) and the Device Embedded Technology (DET) PCBs. Manufacturing processes that are not typical for standard PCB fabrication shops has been developed. The paper explains typical application where PCBs with embedded devices are used today. In addition, products with improved reliability will be shown and what are the benefits for the industry and the end user. The last section in the paper will focus on future trends and the option for systems in PCBs (SiPCB ). This enabling technology will open advanced production possibilities for improved reliability of high power electronics and harsh environment technology solutions. The paper will share specific knowledge applied in the production of products with embedded devices in PCBs more than 20 years. Important patents in Europe have expired. This will open a great opportunity for Device Embedded Technology in PCBs as an excellent future potential for Designers, PCB Fabricators, EMS and OEMs.
Thomas Hofmann founded the company Hofmann Leiterplatten GmbH in 1989. The objective was to provide special technology solutions based on Printed Circuit Board technology to the Electronics industry in the area of Regensburg in Germany. During this time period, many OEMs in Europe closed down their in-house PCB fabrication sites to streamline the electronic equipment fabrication process. PCBs became commodities like standard passive and active components. The goal of the newly formed PCB factory started by Thomas Hofmann was to create innovative fabrication support for electronic designers and development engineers for large OEMs and EMS companies like Siemens, Osram, Infineon and many others in the Regensburg area. PCB fabrication technology was selected because this includes the widest range of fabrication technologies. Here are some examples of typical processes: Mechanical drilling, mechanical and chemical milling, routing, photo imaging, screen printing, electroless and electro and electroless plating of different metals and etching. These are considered as the enabling steps to produce a wide range of products. As a result, many innovative solutions are offered for various industries such as automotive and industrial electronics, lighting technology, measurement and medical technology, as well as the home automation sector.
The following typical products are manufactured:
· Conventional Printed Circuit Boards (PCBs)
· Active Multi Layer (AML )
· Active and passive Front Plates
· Printed Board Assemblies
· Metal core substrates (MCPCB or IMS)
· Printed circuit boards with embedded components
· Mechanical Components and Mechatronics
· Conventional front panel applications, Front panels with integrated components (intelligent front panel)
The Trend in Device Embedding Technology The process for fabricating Device Embedding Technology PCBs Embedding of passive components has been used in ceramic hybrid circuits since this technology was introduced in the beginning of the 70s. Polymer Thick film Paste was used. OhmegaPly a resistor foil was available for PCBs as well. However, in mass production, only hybrid circuits are used. From the material point of view, this is an expensive technology selection. In cases where it is needed for reliability reasons, the higher cost has been justified. However, in mass production, the industry is looking for more cost-effective solution based on organic PCB technology. Here the PCB fabrication technology has a clear advantage. Large panel production and standard multi-layer processes offer an outstanding technology basis for embedding active and or passive components in the organic FR4 material. As technology has evolved, smaller components made it easier to embed active and or passive components inside of PCBs by using the standards soldering or other component attachment technologies like glueing, sintering or plating. In 2011, the integration of embedded connector was evaluated by Hofmann Leiterplatten in Germany and introduced in the standard production of the latest design for Device Embedded Technology. (Examples are shown in Fig. 16, 18 and 19)
The process for fabricating Device Embedding Technology PCBs
The surface mount device technology was an enabling way to place SMD parts on the surface of FR4 CCL material on solder and layer of the PCB. This was then the important step to take the Surface Mount Components to the inside of printed circuit boards (Fig. 3 Step 1) For this process, a thin core CCL Thinner FR4.0 and FR4.1 laminate can be used. In our company, we have experience with both brominated flame retardant that is used in FR4.0 and the different flame retardant that are used in FR4.1. We have gained experience with the resin flow, press condition and curing behavior of the important resin systems used in FR4.0 and FR4.1 CCL. By placing the components inside the PCB, the electronic components and the conductors are better protected against mechanical damage, shock, dust impact and the influence of humidity and of any liquids like water, cleaning agents, solvents, oil and others liquids that could damage components or would result in conductive surface contamination between the conductor and the solder lands. In addition compared to air, the Epoxy Glass reinforced CCL has a more than 10 times better heat dissipation. This allows the components to operate at lower temperatures.
To achieve these benefits, an excellent cooperation between the printed circuit board fabricators, the electrical designer, the PC Board lay-out-engineer, the laminate fabricators and the component suppliers, as well as the assembly specialist and test engineers is mandatory to achieve high first pass yield. In addition, it is required to manage the supply chain and to get new components and known good bare dies. The total information sharing from design rules and expected innovative results may impact the total supply chain and will influence the miniaturization potential and the yield and reliability of the electronic devices with embedded device technology. To achieve these goals to our and the end user's satisfaction, it was required to establish our in-house assembly department with the capability of handling thin core PCBs and very small devices during the total fabrication process.
Thermal management Heat Dissipation
In-house test at Hofmann Leiterplatten factory has shown that the temperature at of MOSFET components assembled on the outside surface in standard technology on the surface of the PCB measured 188.5 C at the component hot spot. By placing the same MOSFET type components inside of the PCB construction, the temperature was reduced by 106 C to an operating temperature of 82.5 C. The thermal heat from the component was distributed over the total area of the PCB. For an electronic engineer, this could impact the total layout of a PCB and it could also help in miniaturisation of electronic devices and equipment. Cost reduction could also be achieved if no external cooling is needed. The photographs and the temperature measurements were taken by a thermal camera. The impact on heat conductivity factor of the epoxy resin (0.35 W(m.k) and the glass fibre (1.05 W(m.k) in the PCB material provides a more than 10 times improved thermal conductivity compared to air (0.024 W(m.k).
2. Resistance to water and other liquids
In figure 6 an example is shown that demonstrates LEDs embedded in a CCL material used in fabrication of PCBs. These materials are resistant to many liquids. These strips are connected to power and have been lighting partially in water and partially in air more than 5 years. In some application this CCL with embedded devices is used in hot oil and other hot liquids. Embedded electronic devices in CCL are also used in gearboxes to manage the electrical functionality at the gearbox temperature. The exposure to hot gearbox oil is of no issue for the device embedded technology PCBs.
These sensor modules (Fig 7) are placed in small areas. They have to be cost-effective in manufacturing. Embedding components is an outstanding way to miniaturise surface area of the unit through an effective design for manufacturing. Small unit designs manufactured using large panel processes offer an opportunity to lower manufacturing cost. At the same time, high product reliability is achieved as all electronic components have excellent protection against humidity, oil, shock, temperature and EMC impact by other uncontrolled radiation and frequencies.
Sharing experience in manufacturing PCBs with embedded devices
Over the last 20 years, a large number of tests have been conducted. This was needed to define reasonable design rules that will enable the use of standard packaged components as well as to allow sufficient epoxy resin for a reliable bond strength during the press cycle. In addition, air entrapments or voids must be avoided. This could have an impact on signal speed and insulation characteristics of the dielectric material.
Holes and vias
In the case of an AML PCB, it should be noted that no through-holes or vias are placed in the areas of the components. For vias or holes, a distance of at least 0.5 mm from the periphery of the drilling to the SMD land of the adjacent component should be maintained.
Distance from components and package density
When designing the layout, please note that the component areas, the surfaces intended for the inner layer assembly, occupy only a maximum of 40% of the circuit board area. The component area is defined by the component, including the land for the solder joints. A uniform distribution of these areas across the circuit board area is advantageous when embedding devices.
Thickness of the circuit board
The final thickness of the printed circuit board results essentially from the thickness of the embedded components. The maximum component height is decisive.
The Design for Manufacturing (DFM) strategy is key for the success of the device embedding technology. When this technology was started, design software did not exist. Today, many of the large CAD companies offer support. However, not many PCB fabricators have the experience to support the PCB fabrication for an effective design for manufacturing. At Hofmann Leiterplatten in Germany, we have learned that cooperation with the customer at the earliest possible time is needed to establish the foundation for a successful partnership and for a functional and innovative product. It is also the basis for cost- effective planning for a long-term strategy in the case
That the product shall be in mass production and shall be used in a global marketplace. The device embedding technology is driven by the costperformance relationship. However, the components define the functionality performance characteristics of the product. Also, the availability of these devices will be key for the development of the technology of device embedding PCBs. As indicated in Fig. 9 there are some limiting factors that are driven by the components, the manufacturing process and/ or the use of the materials that are not suitable for an effective device embedding fabrication process. New options have to be considered. Here, Metal Core PCBs (MC-PCB) with standard epoxy resins may be used. This could replace the need for thermal vias and/or expensive highly thermally conductive materials in conjunction with standards surface mount attachment technology PCBs With the new option of MC PCBs, relatively inexpensive constructions can be realised. This is possible by using a highly thermally conductive dielectric layer between the aluminium support and the conductor/components layers. From our experience, this build up construction has proven excellent heat dissipation capabilities for a cost-effective PCB used in conjunction with electronic components with high power application.
Each component has its own geometrical shape and dimension, each resin has its own flow characteristic and each multilayer press and press cycle has its own press profile and heat dissipation. Therefore it is necessary to understand the epoxy resin flow properties and the capability to encapsulate the components with a homogeneous epoxy embedding resin.
Projects utilized the AML device embedding technology
In this section of the paper, products that have been manufactured and are not under any non-disclosure agreement (NDA). Most of the present new developments are under NDAs with our key customers. As our company is focussing on new technology based on PCB fabrication technology, we have to get involved in many new projects at a very early stage of the product development process. Confidentiality is a key element in joint product development for the success of our customers and our company.As indicated the company Hofmann Leiterplatten GmbH started more than 20 years ago to fabricate the first PCBs with embedded devices. In the beginning, we very much depended on a friendly EMS fabricator to assemble our components to the thin core CCL inner layers. When components were large, this was still an option. However, when components became smaller and delivery times for development projects were under a tight time frame, we installed our own component assembly capability, as well as soldering and test, know-how in our factory in Regensburg. This enables our product development team to get involved in advanced products with unique product performance, product features and application at reasonable cost. Also, the time to market enabled our team to become a valuable resource for our European customers.In addition, the in-house manufacturing capabilities enabled us to participate in joint industry projects that are partially sponsored by funds provided by:
· Local authorities in Bavaria
· German Government and
· European Union funding from the EU commission in Brussels
Here are some of the projects as an example:
The ZIM 3D microsystem  (Fig. 12) was an innovative threedimensional stack of individual sensor modules that are connected by magnets. This was a model for the electronics industry to create technology demonstration modules that enable development engineers to test different functions and application in new product development laboratories. This enables development engineers, for example, to test out interface options in new product developments without soldering components to a test board. For the highly connected industry, this can be a helpful tool to shorten development time in automotive and industrial electronics.
In figure 13, a new project for the automotive industry in Europe is shown. Within the framework of a BMBF collaborative project, research will focus on the areas of performance, cooling, miniaturization and reliability in automotive and lighting applications. System integration technologies for highly integrated power logic modules using the example of drive and lighting technology in the project "ProPower" .
The next generation of electro mobility very much depends on cost- effective and high reliability solutions that meet the very high performance criteria of the automotive industry.
In this context is important to understand that flammability and electrical safety are key factors that will be considered in the projects as well. Automotive electronics is not made for a National or a European market. These standards have to be compliant with global safety specification and needs. Therefore it is well understood that the safety aspect, as well as technical functionality and cost-effectiveness, are key for these projects.
In figure 14, a framework of a BMBF collaborative project are in focus. Test methods for new materials and for different components are evaluated. Test samples will be made and new test methods will be established, as this has to do with the way how to manufacture cost-effective products that are operated at high temperatures. The important factor is that PCB material
is the first selected based on facts. In addition, global safety standards have to be fulfilled. An additional BMBF PCB 4.0  (Fig. 15) is targeted to the electronics that is needed for Industry 4.0. In this project, machines will talk to machines. The computer will take this input data and will analyse and store this information for future traceability. This is a requirement of the industry that has to be fulfilled as the requirements and the implementation of Industry 4.0. The need for documentation and sharing of process reliable data with the customers in the supply chain is the objective. This will be done with a new interfaces technology and sensor technics. This device MUST be reliable and highly flexible in terms of robustness against all kinds of environmental impacts like temperatures, vibration and mechanical shock, humidity, gases, solvents, light and others. The device embedding technology is an effective way of considering this as a way to provide effective industry solution. As the fabricators in Europe very much depend on innovation in conjunction with miniaturization, the BMBF 4.0 project is mandatory for future industry systems and robotics.
New Product and Process Developments in the device embedding process and application technology.
Embedding devices are mainly regarded as placing active and or passive components inside a PCB. However, processed digital or analogue signals will not stay forever inside the specific PCB. An interphase is needed to connect the data to the outside world. This can be wireless with Bluetooth, NFC or W-LAN or other standard interphases like USB, Thunderbolt or any other connector based technology. The company Hofmann Leiterplatten in Germany has developed a way to integrate the connector as a part of the technology of the embedded device that is used inside the PCB. (Fig. 16). The process is done simultaneously with embedding the other SMT devices
In the racing car, weight saving and improved functionality are key elements for a high chance to be first in class. The device embedding technology, using the AML technology processes, is used very successfully in this application. Opportunities for standard car application are under evaluation. As these are mainly cost driven functions, time will tell when high-end cars will install embedded devices for more robustness and higher functionality
Gold coins are one of the traditional methods to save money and to avoid devaluation of any currency. Gold coins are also traded when cash is needed. However, it is not easy to define whether the gold content is according to the book value of the coin. Many traditional test methods are taking the gold coins in water to define the volume. The volume of water and then multiplied with the specific gravity of gold. Although this is a traditional process, it takes time and experience of applying it.
A company in Germany has developed an electronic unit (Fig.18) to define how pure the gold content of a coin is. An inductive test method is used to define whether the coin is the type of gold it should have according to the books specification. This Gold test unit is made completely by using the AML technology to embed devices. Interferences with the calibration of the device will result in destroying the unit. This enables to minimise the risk for counterfeit measurements and wrong product definition.
Dividing voltage in a power electronic switching station requires often rewiring and or very expensive switching systems. The use of predefined templates, with spring loaded connections, allows a fast way change the way of dividing the flow of current (Fig. 19).
Specific for this product is the approval by interactional electrical and safety standards for global utilisation of the product with embedded connectors and devices. Therefore, the approvals for high voltage product application by the Underwriters Laboratories (UL) in the USA, were required. These also include the performance requirements in accordance with IEC regulation for high power application. The use of specific materials was needed to meet the safety standards of the customer, the industry segment. This was also certified by UL. High voltage strength, normally not needed for standard PCBs, has been fulfilled by using this technical solution.
Electronic devices as part of a decorative solution is also a new need by product designers and marketing experts. Besides functionality, aesthetic solutions are required by the industry that is specialised in high added value products. This is needed to satisfy the demand of a specific individual clientele group. Electronic functionality inside of wooden plates, or a plastic or an alumina-based housing are additional requirements by a specific end user. The device embedding technology meets many of these specific requirements by the electronics industry the OEMs or even by the individual consumer.
Summary and future opportunities
Over the last more than 20 years, the company Hofmann Leiterplatten GmbH has seen a lot of benefits for the total supply chain and the end-user for electronic equipment with embedded devices. However, to fully benefit from the features of the embedding device technology, the supply chain needs to be restructured.
Here are some of the key items:
· Product designer has to be involved at the concept stage of the new development.
· PCB fabricators must be capable of manufacturing device embedded PCBs.
· Assembly operation should be directly connected with the PCB fabricator. An inhouse assembly facility is beneficial.
· Solder less connection as attachment technologies shall be considered, for example glueing, plating or sintering.
· OEMs should understand the value of the miniaturization potential by using the device embedding technology in PCBs.
· Consider the cost-reduction potential offered through miniaturization and encapsulation of all components in a material combination of epoxy resin reinforced by woven glass structures.
· Avoiding special housings for the PCB with embedded devices opens additional potential for new costeffective products.
· Resistance to liquids and an excellent thermal dissipation offer new application in automotive, mining and other industrial environments fields.
The device embedding technology in PCBs should not be regarded as a disruptive PCB technology. It is an evolution of existing PCB fabrication technologies combined with new design, fabrication, assembly and testing methods
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