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Practical Experience Manufacturing PCBs With Embedded Active and Passive Devices
January 15, 2012 |Estimated reading time: 11 minutes
Editor's Note: This column originally appeared in the November 2011 issue of The PCB Magazine.
Introduction
Printed circuit boards with embedded devices have been used in the European PCB industry under development contracts for a number of years. Recently the embedding technology has gained momentum due to the availability of new smaller components, design software and PCB fabricators that are willing to step into this advanced technology. In addition, the sensor, industrial, medical and automotive industries have seen the features that embedding devices in PCB will offer, such as improved miniaturization and reliability of electronic equipment. Companies that are integrated forward and backward have already seen these as clear benefits to manufacture chip packages. In other sections, design engineers are trying to use the embedding for the next generation of electronic equipment. Europe has a strong position in developing new and advanced products. Advanced prototype PCBs are key for the success of the European electronics industry. This is based on new developments of advanced electronics in industrial manufactured products. Miniaturization, reliability and cost-effectiveness are vital for most of the new developments. The automotive industry has stated that more than 40% of the cost of a passenger car is related to electronic devices. The tendency will be even higher in the future when Hybrid or full electric cars are manufactured.
Topics of the Paper
- Background about Hofmann Leiterplatten (PCBs) in Germany and the AML® technology
- When was the patent filed and granted?
- What was the unique invention that made the process work?
- Practical examples
- Developments of standards for PCBs with embedded devices and its impact on PCBs
- How to manufacture PCBs with embedded components without infringing patents of Hofmann
- Outlook and future developments
Background about Hofmann Leiterplatten (PCBs) in Germany and the AML® Technology
The Company was founded in 1988 by Thomas Hofmann, whose visions for forming the company included: “Service the electronic industry through creativity, innovation and efficiency using PCB technology. The goal of the company is the support of the electronics industry in the regional area of Regensburg, Germany, and with special products support of automotive, medical and industrial electronics. Sensor technology and intelligent front panels with embedded devices are some of the products that manufactured today. In addition, fast and reliable PCB fabrication was needed to meet or even exceed industrial fabrication standards. Developments of innovative solutions for new products are some additional services the company provides. Managing energy resources to reduce the CO2 output of the company is also a key objective. The generation of solar energy and heat recovery system from processing equipment has resulted in a very effective energy efficiency of the company.
Figure 1. Factory of Hofmann Leiterplatten in Germany. Top left: office building with the fabrication hall in the back. Top right: Solar panels on the factory roof. Bottom left: Heat recovery system from cooling air used in the factory.
The development of embedded technology
Based on the needs of the industry and some of the key customers in the field of new product developments, the company started the development activities of embedded devices in PCBs in early 1993. The key designers of new products initiated these activities to achieve a higher miniaturization and improved reliability through an advanced thermal management by embedding devices in PCBs.
Figure 2. Thermal conductivity of different materials used in electronics.
In addition, an improved environmental protection against contamination from dust, humidity, chemicals and human impact, as well as an improved EMC shielding, was achieved. This work resulted in filing the patent in 1996. The patent was granted in Germany in 2004.
What was the unique invention that made the process work?
In the patent abstract of Germany: “DE 19627543 (A1) it was described that the substrate includes multiple surface-connected insulation layers with at least one inner insulation layer as a distance frame with a window (11). At least on one substrate surface side, and between adjacent insulation layers, are formed contact faces and/or conductive tracks (6-9). At least one inner insulation layer (3) forms a spacer frame with at least one window for an inner electric component (12). Both sides of the window are closed by further insulation layers (2, 4) adjacent to the spacer frame, whose thickness equals at least the height of the inner component. Preferably a separate window is provided for each electric component.”
The description of the patent stated that special cavities in the prepreg are used as a separator between the layers of a multilayer PCB. This enables the fabrication process to use standard PCB fabrication technologies and equipment without damaging the sensitive components during multilayer lamination and pressing. In the fabrication process, the components are placed on an inner layer of the multilayer. The connection between the components and the inner layer can be made either by soldering, gluing with conductive adhesive, by welding, riveting, bonding, sintering or plating. The selected attachment technology between the conductor lines and the components depends on the type of components and the requirement for electrical and/or thermal conductivity.
A specific window technology in the prepreg is used to allow for a stress-free embedding of different components in one pressing operation. The resin flow is sufficient and an additional underfill is not required. Here the correct opening in the spacer prepreg will provide sufficient resin to embed the components completely. A specific knowledge is needed to define the correct resin content, resin type and press cycle to avoid stress at the component packages.
Figure 3. Different components are embedded in PCBs without the need of underfill of extra resin. The resin provided by the prepreg is sufficient for encapsulation of components and connection to the PCB.
Without using the correct window and embedding technology, components have the risk of being damaged during the placement and multilayer press cycle. This will then result as the defect that is identified in final inspection. In addition, the defect cannot be repaired and the total electronic assemble will be rejected as 100% scrap. At this stage, all components are in place and the reject will be expensive. Using the correct window and prepreg technique ensures this defect is avoided.
By using the correct manufacturing technology, a cost-effective technology is available to manufacture electronic devices that provide outstanding protection against environmental and pollution influences. In addition, the hermetic encapsulation of the components provides an outstanding protection against moisture, water, dust, gases, chemicals and provides a reliable electrical insulation. Components are also well protected against vibration, shock, stress and strain or pressure. The reliable positioning and secure fixation of the components by embedding as well as the EMC protection using the Advanced Multilayer (AML®) PCB fabrication technique developed and patented by Hofmann Leiterplatten in Germany. The plating process of the completed module can be designed as a low-cost EMC shielding protection.
Figure 4. The embedding of components provides an excellent way to design electronic modules and assemblies with an outstanding electron magnetic compatibility (EMC) by reducing the radiation impact from outside and inside that could influence the performance.
The PCB technology with embedded devices has proven a superior thermal management of the assemblies. This resulted in a lower thermal stress of the components and an improved life expectation.
Figure 5. Embedding of components reduces the temperature at the component level on the above example to about 98°C to 90°C after 10 minutes of operation at 6 Watts. The outside temperature of the test sample changed from 188.5°C to 82.5°C.
Based on these results, a number of new products have been de- signed and manufactured that allow for improved reliability, miniaturization, as well as for potential cost reduction in the future.
In addition, the manufacturing process is using PCB fabrication processes that are tailored to use large-production panel fabrication. Lead-free solder technology and suitable laminate materials shall be used. In case that reflow soldering is used, the solder paste can be applied by using standard stencil printing or specific dispensing processes. The components are placed on the thin inner layer PCBs and connected using standard lead-free soldering or any other suitable attachment methods. After soldering, cleaning of the assembled inner layer is applied. This will avoid any ionic contamination that could lead to long-term defects based on migration, electrical conductivity or e-corrosion. Also, an AOI inspection with or without an electrical test will be possible. This is useful to define the position of the components, the solder fillets and the electrical functionality of the inner layer. Handling of such a thin inner layer needs some training and experience; however, in assembling flexible circuits, this is a common manufacturing process. The cost-effective fabrication and testing of semi-finished products allows for high yields in final testing of the finished assembled device. Here, the option for repair in the semi-finished state makes it possible to identify defect components, and to repair the replacement of the defect parts before the ML-PCB is pressed.
Figure 6: Large-panel production allows for cost-effective fabrication in PCB processes, in the assembly line as well as pretesting of submodules
Industry segments
The sensor and automotive industries have seen the potential of the embedding technology for future developments.
Figure 7: Sensor made by using embedded devices in PCBs. Three conductive layers are needed for connecting the components. The outside area is plated with copper and provides an outstanding shielding of the sensor electronic components.
LEDs embedded in PCBs are very well hermetically sealed so that water or any other environmental pollution could reduce functionality of the electronic devices. Figure 8. The above example shows how PCB technology supports the embedding of LEDs to improve long-term performance. In this example the assembled PCBs with LEDs are placed in water for more than 4 years and have still a useful life.
In automotive application, dashboard and lightening are key for safety, visibility and durability. 15 years of life expectation is typical for automotive electronics. Driving light, direction light and stoplight are factors for visibility of the vehicles. For the driver the dashboard is key to see the speed of the car and the functional performance of the engine. Good visibility of the instrumentation is vital for safe driving.
Figure 9. Cockpit display unite made by using embedded technology PCBs based on the AML technology developed by Hofmann Leiterplatten in Ger- many. Excellent readability, lightweight and well protected against shock and pollution are key factors for this construction.
Standardization of PCBs with embedded devices
For a wide utilisation of new technology, standardization is vital for a successful implementation. In 2009, the Japan Printed Circuit Association (JPCA) has submitted a standards proposal to the International Electrotechnical Commission. The JPCA Standard JPCA-EB01 – 2nd Edition is a standard on Device Embedded Substrate - Terminology / Reliability /Design Guide JPCA-EB01. These standards were submitted as an IEC Pas 62326-14 proposal in 2009. As the PCB industry in Japan has been using the technology for some years, it was logical to submit the standards proposal to have the technology recognized globally.
However, by comparing the standard proposal with the existing patented process of Hofmann in Germany, it turned out that some of the examples would infringe the patent “DE 19627543” of Mr. Hofmann.
Figure 10. Typically embedded device construction as included in the JPCA-EB01 that has been submitted as a pass proposal under the IEC Pas 62326-14 in 2009. Patent situation for embedded technology Standardization and patents shall not be in conflict. Under the regulation of most standard writing organisations—like the IPC or the common IEC, ISO, ITU Patent Rights Policy Administrative Circular AC/10/2007 of 2007-03-23 Disclosure of Patent rights—those participating in standards development work should draw the attention to relevant patents or pending patents as early as possible. At meetings, where appropriate, a TC/SC chairperson should remind participants of the above. This was done during the IEC meetings as well as at the IPC Meeting where the IPC-7092 standard for design and assembly process implementation for embedded components were reviewed. Members of the technical committees at the IEC and the IPC are well aware of the patent situation with embedded devices in PCBs in Germany.
Learn how to manufacture standardized PCBs with Embedded components without infringing patents of Hofmann
The IEC/ISO regulation is stating:
“A published document for which patent rights have been identified during the preparation thereof, shall include the following notice in the introduction:
“The International Organization for Standardization (ISO) [and/or] International Electrotechnical Commission (IEC) draws attention to the fact that it is claimed that compliance with this document may involve the use of a patent concerning embedding of passive and active divides in PCBs.
ISO [and/or] IEC takes] no position concerning the evidence, validity and scope of this patent right.
The holder of this patent right has assured the IEC that he is willing to negotiate licenses under reasonable and non-discriminatory terms and conditions with applicants throughout the world. In this respect, the statement of the holder of this patent right is registered with ISO [and/or] IEC.”
Embedding experience has been gained with the following parts:
- Chips in WCSP-technology (wafer chip size package)
- BGA components with 2.5 mm x 2.5 mm and 25 solder balls
- Component height of less than 0.5 mm have been used
Figure 11. The positioning light on the Audi R15 racing car indicates the car position one, two or three in the category (racing class). Visibility is important for the spectators at the race course.
Outlook
- Embedding of active and passive components allow for easy miniaturization and reliable electronic devices
- Improvement in EMC protection is also achieved
- The license of Hofmann Leiterplatten will be accessible on reasonable non-discriminating condition (as required by standard development groups)
- Global manufacturing and safety standards are in a process to be established (IEC / IPC / JPCA and others)
- Some of the standardised construction may lead to patent infringements in Germany
- Getting a license is an easy and cost-effective way to overcome manufacturing and patent issues.
Thomas Hofmann received his qualification in engineering of energy electronics, in business administration and management. He filed a patent for embedded devices in PCBs in 1996, and it was granted in 2004. In cooperation with the European PCB industry, he has developed a large number of PCBs with embedded devices for automotive, industrial, and sensor electronics. To contact Hofmann, click here.