Laser Direct Structuring Research Articles

Dielectric Properties of PEEK/PEI Blends as Substrate Material in High-Frequency Circuit Board Applications.

Substrate materials for printed circuit boards must meet ever-increasing requirements to keep up with electronics technology development. Especially in the field of high-frequency applications such as radar and cellular broadcasting, low permittivity and the dielectric loss factor are key material parameters. In this work, the dielectric properties of a high-temperature, thermoplastic PEEK/PEI blend system are investigated at frequencies of 5 and 10 GHz under dried and ambient conditions. This material blend, modified with a suitable filler system, is capable of being used in the laser direct structuring (LDS) process. It is revealed that the degree of crystallinity of neat PEEK has a notable influence on the dielectric properties, as well as the PEEK phase structure in the blend system developed through annealing. This phenomenon can in turn be exploited to minimize permittivity values at 30 to 40 wt.-% PEI in the blend, even taking into account the water uptake present in thermoplastics. The dielectric loss follows a linear mixing rule over the blend range, which proved to be true also for PEEK/PEI LDS compounds.

Laser-Induced Selective Metallization on Polyimide Films for Fabrication of Antennas

Laser direct structuring (LDS) technology used for polyimide (PI) films containing copper dichromate as the activator for antenna circuits was evaluated. The polyamic solutions containing various contents (1 to 10 phr) of copper dichromate were spread on a glass sheet. Then, the solvent was evaporated and imidated to obtain the PI films containing LDS activator. The near-infrared laser was used to scan the sieving and contrast patterns of the LDS PI films, and then the electrochemical copper was plated. The experiment results indicated that a copper dichromate content of 7 phr exhibited satisfactory quality for the fabrication of the antenna circuits. The comparison results of simulation and measurement from the constructed antenna circuits confirmed that the antenna circuits with the LDS PI film with a width of 50 μm operated appropriately.

Laser-Induced Selective Metallization on Aluminum Nitride for Fine Line Circuits

The technology of laser direct selective metallization on aluminum nitride substrate was investigated for manufacturing fine line circuits. In laser direct structuring, laser activation and electroless copper plating were used. The feasibility was evaluated by analyzing the sieving, contrast, and fine linewidth patterns, and the results showed that the width of the feasible fine reached 100 μm. A near-field communication (NFC) antenna circuit of the width of 400 μm was fabricated on the aluminum nitride substrate. The measured resonance frequency was close to the NFC specification frequency.

Photosensitive polyimide enabled simple, reversible and environmentally friendly selective metallization on diverse substrates

For breaking through the limitation of metal patterning in constructing conductive pathways of electronic devices, which is proposed to be an upgrade of traditional etching technique, a photo-reversible, transparent and soluble polyimide carrying spiropyran side chains is synthesized. The merocyanine isomers produced from the spiropyran moieties under digitalized ultraviolet irradiation bind the silver−histidine complexes in water, while the absorbed silver ions are in-situ reduced into metal silver seeds by the photo-generated radicals from histidine molecules, catalyzing copper deposition and yielding the desired copper patterns. Accordingly, the conventional time-consuming steps (like surface modification, immobilization of catalytic ions and (photo)chemical reduction) are simplified to a single one, and the environmentally unfriendly solvent and costly/toxic palladium reagent are excluded. The Cu patterns deposited on the polyimide show high resolution, electrical conductivity (1.785 × 10−6 Ω‧cm), adhesion strength (18.14 MPa) and fatigue resistance (> 2.6 million cycles of bending). Due to the absence of invasive surface treatments (e.g., surface hydrolysis and laser direct structuring), extremely low surface roughness of 7.83 nm is achieved. Besides, the developed strategy can be easily expanded to stretchable elastomer and other organic/inorganic substrates with irregular surfaces, showing promising application prospects. The solubility of the polyimide and ultraviolet/visible light triggered reversibility of the included spiropyran groups further enable repeated recycling, erasing and re-patterning.

UV Laser Copper Pad Surface Exposure for Laser Direct Structuring (LDS) of Interconnection

Microelectronics market demands strong miniaturization and high-quality packages solutions; therefore, companies and researchers start thinking about alternative methods to create more flexible and performing interconnections which are able to reduce package dimensions. Laser direct structuring (LDS) technology creates a conductive pattern on MID (Moulded Interconnected Device) with the combination of laser layout structuring and electroplating bath. LDS can be applied to leadframe based IC packages to realize interconnections in alternative to standard wires connection. Feasibility study on copper (Cu) pad covered by moulding compound is conducted varying laser parameters as power, frequency and scanning speed to obtain a 80μm diameter via on die. Die via is connected through a trace with a 200μm diameter via on lead to realize the interconnection path, filled with copper after plating process. UV 355nm wavelength laser is used to obtain a complete exposition of Cu surface without silicon damage. Visual inspection with optical microscope and SEM analysis method are performed to evaluate the integrity of metal pads and interconnection layout.

Selective Metallization of Thermoplastic Elastomers Based on Laser‐Induced Surface Activation

Laser direct structuring (LDS) technology successfully realized the selective metallization on thermoplastic elastomers. Polystyrene‐block‐poly(ethylene‐co‐propylene‐block‐polystyrene copolymer (SEPS) and polyether‐block‐amide (Pebax) containing 5 wt% Cu2(OH)PO4 are designed as LDS materials. Copper layers are obtained after NIR laser activation and metallization. The analysis of morphologies (optical microscope, scanning electron microscopy, confocal laser scanning microscopy), the chemical composition (X‐ray photoelectron spectroscopy [XPS], Raman, X‐ray diffraction), and the performance (scotch tape test, resistance) of materials before and after laser activation and metallization are carried out. The key to obtaining a high‐quality metal layer is as follows. 1) Cu2+ is reduced to copper (Cu0) with amorphous carbon acting as a reducing agent, and Cu0 acts as catalyst seeds in metallization. 2) The good adhesion between polymers and copper layers comes from the micropores etched by laser activation. In particular, XPS shows that 64.6% Cu2+ in SEPS/Cu2(OH)PO4 is reduced to Cu0, while only 18.4% Cu2+ in Pebax/Cu2(OH)PO4 is reduced after laser activation. Moreover, resistances of copper layers on SEPS/Cu2(OH)PO4 and Pebax/Cu2(OH)PO4 are 0.96 and 5.04 Ω, respectively. This study puts forward a way of fabricating flexible LDS materials. The prepared SEPS/Cu2(OH)PO4 composite is promising for shape variable decoration and flexible electronic devices.

Optimization of Laser Direct Structuring Process Parameters for Material Extrusion of Polycarbonate

Laser direct structuring (LDS) is critical in the integration of circuits onto 3D‐shaped plastic parts, such as antennas and radio frequency components. The LDS process encompasses three stages: deposition of 3D parts, laser structuring, and metallization. While laser‐direct structurable parts have been manufactured through plastic injection molding, material extrusion (MEX) is a favored additive manufacturing process for economic low‐volume production and fast prototyping advantages. Although injection‐molded LDS literature is available, 3D‐printed laser‐direct structured components merit further investigation. This study focuses on the MEX of catalyst‐loaded polycarbonate (PC) parts and its LDS process. The parameters from the nanosecond fiber laser, including scan speed, power, and frequency, are thoroughly analyzed to understand the surface property changes and metallization performance of the printed PC parts. The single scan track width, which corresponds to the accuracy of conductive path width and metallization thickness, is employed to elucidate the findings. A process map is built to keep the track width constant aimed at enhancing the uniform metallization of intricate components. Thresholds are established, identifying a minimum track width of 22.1 μm and metallization thickness of 2.5 μm. These delineate clusters of process parameters that yield conductivity levels suitable for various applications.

Laser-Induced Selective Metallization on Ceramic Substrate for Antenna Circuits

The technology of laser-induced selective metallization, or known as laser direct structuring (LDS), on a ceramic substrate was evaluated for the fabrication of antenna circuits. A sieving pattern was scanned by the near IR laser on the ceramic substrate, and electroless copper plating was carried out. Appropriate laser activation conditions were chosen to engrave a contrast pattern on ceramic sheet samples. A chosen condition was used to fabricate a line width evaluation pattern, and the results indicated that the feasible fine width could be as fine as 80 μm. This value was used as the design limit of the antenna circuits. The designed antenna circuit patterns were fabricated on the ceramic substrate, and the simulation and direct measurement of the antenna circuit were carried out. This evaluation indicates that the application of LDS technology on the ceramic substrate for the antenna circuits would be feasible.

EMC Hybrid Loop/Monopole LDS Antenna With Three-Sided Ground Walls for 2.4/5/6 GHz WLAN Operation

A multi-resonant, electromagnetic-compatible (EMC), hybrid laser- direct-structuring (LDS) antenna capable of operating in the 2.4, 5, and new 6 GHz wireless local area network (WLAN) bands is presented. The LDS metal patterns are formed on a holder with the volume of 8 mm × 10 mm × 30 mm and comprise an inner loop, a parasitic strip, and three-sided, vertical ground walls. The fundamental and the first higher-order resonance of the parasitic strip acting as a shorted monopole generate the 2460 and 6460 MHz modes. In addition, two loop modes formed by the inner loop with the parasitic strip and the inner loop only, respectively, contribute to the 5420 and 7020 MHz resonance. All of these together generate a tri-WLAN-band, 2.4/5/6 GHz operation. The vertical ground also functions as the shielding walls that assorted conductive components can be placed in the close vicinity of the design for compact integration. The proposed antenna applied to laptop computers for the most recent Wi-Fi 6E operation is demonstrated in this article.

Analysis of Tempering Effects on LDS-MID and PCB Substrates for HF Applications

Mechatronic Integrated Devices or Molded Interconnect Devices (MID) are three-dimensional (3D) circuit carriers. They are mainly fabricated by laser direct structuring (LDS) and subsequent electroless copper plating of an injection molded 3D substrate. Such LDS-MID are used in many applications today, especially antennas. However, in high frequency (HF) systems in 5G and radar applications, the demand on 3D circuit carriers and antennas increases. Electroless copper, widely used in MID, has significantly lower electrical conductivity compared to pure copper. Its lower conductivity increases electrical loss, especially at higher frequencies, where signal budget is critical. Heat treatment of electroless copper deposits can improve their conductivity and adhesion to the 3D substrates. This paper investigates the effects induced by tempering processes on the metallization of LDS-MID substrates. As a reference, HF Printed Circuit Boards (PCB) substrates are also considered. Adhesion strength and conductivity measurements, as well as permittivity and loss angle measurements up to 1 GHz, were carried out before and after tempering processes. The main influencing factors on the tempering results were found to be tempering temperature, atmosphere, and time. Process parameters like the heating rate or applied surface finishes had only a minor impact on the results. It was found that tempering LDS-MID substrates can improve the copper adhesion and lower their electrical resistance significantly, especially for plastics with a high melting temperature. Both improvements could improve the reliability of LDS-MID, especially in high frequency applications. Firstly, because increased copper adhesion can prevent delamination and, secondly, because the lowered electrical resistance indicates, in accordance with the available literature, a more ductile copper metallization and thus a lower risk of microcracks.

Bioinspired rapid self-replenishing liquid-infused surfaces for stable anti-adhesion of electrosurgical electrodes

Soft tissue adhesion on electrosurgical electrodes has been a significant concern in minimally invasive surgery. The liquid-infused surface (LIS) can effectively reduce the tissue adhesion on electrodes by constructing a lubricating interface. However, the tissue-cutting process involves a complex contact friction process and high temperature, resulting in lubricant depletion of LIS. Inspired by the rapid self-replenishment of liquid film on the peristome surface of Nepenthes, here, a novel self-replenishing liquid-infused surface (SR-LIS) is fabricated by the laser direct structuring method. SR-LIS consists of first-level microgrooves and second-level microcavities. The microcavities store lubricant for a long time and enhance the capillary force of microgrooves. The rapid self-replenishing lubricant of SR-LIS ensures that the cutting interface is composed of soft tissue-lubricant-steel, resulting in stable anti-adhesion. Soft tissue cutting experiments show that the SR-LIS could stably reduce tissue adhesion and tissue damage compared to LIS. The underlying mechanism is discussed, and further experiments conclude that the self-replenishing lubricant has the double action of lubrication and cooling. This novel structural design, preparation method, and the resulting surfaces have essential applications in electrosurgical electrodes and promote the development of slippery surfaces in various industries.

Fabrication of metallic patterns on ordinary polymer substrates by laser direct activation and electroless plating

In recent years, the fabrication of metallic patterns on polymer substrates has been given increasing attention because of its wide and important applications in many fields. This paper reported a selective metallization technology on ordinary polymer substrates basing on laser direct activation of copper hydroxyl phosphate [Cu2(OH)PO4] and electroless deposition of copper. Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were utilized to characterize the interaction mechanism between nanosecond-pulsed fiber laser (1064 nm wavelength) and Cu2(OH)PO4. It was found that after laser modification and activation by appropriate parameters, the special surface microstructure (e.g., porous sponge-like) appeared and explained the good adhesion strength (the highest grade of 5B) between the copper layer and substrate. At the same time, Cu+ was generated from the Cu2(OH)PO4, adsorbed on the microstructure, and acted as catalytic active centers to realize the selective copper deposition in the laser-activated zone. In addition, the copper layer possessed good selectivity (< 3% error of line space and width) and electrical conductivity (10−6 Ω·cm volume electrical resistivity). By comparison with laser direct structuring (LDS) technology and with palladium chloride as laser sensitive material, this was a more universal and less-cost technical pathway to fabricate planar and/or three dimensional (3D) metallic patterns on more kinds of ordinary polymer substrates.

Laser Direct Structured 3D Circuits on Silicone.

Silicone rubber is a biocompatible elastomeric polymer, with great potential for mechanical and biologic sensing applications, if electrical circuits can be reliably integrated. Laser direct structuring is a bottom-up circuit fabrication process, whereby copper is chemically grown on laser exposed regions of a modified substrate, promoting adhesion by laser roughening the circuit tracks. In this Research Article, we successfully demonstrate this process using superflexible biocompatible silicone (30 hardness on Shore 00) with copper chromite additive, cast into both 2D planar and 3D contour substrates. A horseshoe pattern circuit, meander and Hilbert fractal inductors, and a 3D hemispherical helix trace are fabricated and tested. The range of laser power and copper chromite concentration are explored. Mechanical testing is performed to determine breakage strain and elastic modulus. Material stiffness and trace peel strength are shown to increase with copper chromite concentration. Peel strength is measured to be very high, from approximately 1 to 5 kN/m, depending on dopant loading. With high adhesion and conductivity, the simple laser-writing process presented here enables high-quality circuit integration into elastomeric silicone.

Flexural Fatigue Test—A Proposed Method to Characterize the Lifetime of Conductor Tracks on Polymeric Substrates

High quality and long product life are two fundamental requirements for all circuit carriers, including molded interconnect devices (MID), to find application in various fields, such as automotive, sensor technology, medical technology, and communication technology. When developing a MID for a certain application, not only the design, but also the choice of material as well as the process parameters need to be carefully considered. A well-established method to evaluate the lifetime of such MID, respective of their conductor tracks, is the thermal shock test, which induces thermomechanical stresses upon cycling. Even though this method has numerous advantages, one major disadvantage is its long testing time, which impedes rapid developments. Addressing this disadvantage, this study focuses on the laser direct structuring of thermoplastic LCP Vectra E840i LDS substrates and the subsequent electroless metallization of the commonly used layer system Cu/Ni/Au to force differences in the conductor tracks’ structure and composition. Performing standardized thermal shock tests alongside with flexural fatigue tests, using a customized setup, allows comparison of both methods. Moreover, corresponding thermomechanical simulations provide a direct correlation. The flexural fatigue tests induce equivalent or even higher mechanical stresses at a much higher cycling rate, thus drastically shorten the testing time.

Patent Watch February 2022

Several noteworthy patents, related to polymer additives, were published worldwide last month. As usual, many of these new patents involve flame retardants, plasticizers and stabilizers. But there are also compositions to address laser direct structuring and 3D printing problems, and more. The full patents can be read by entering the patent number into a dedicated patent search engine (like https://patents.google.com/ or https://patentscope.wipo.int ).

Characterization of Wire-Bonding on LDS Materials and HF-PCBs for High-Frequency Applications

Thermosonic wire bonding is a well-established process. However, when working on advanced substrate materials and the associated required metallization processes to realize innovative applications, multiple factors impede the straightforward utilization of the known process. Most prominently, the surface roughness was investigated regarding bond quality in the past. The practical application of wire bonding on difficult-to-bond substrates showed inhomogeneous results regarding this quality characteristic. This study describes investigations on the correlation among the surface roughness, profile peak density and bonding quality of Au wire bonds on thermoplastic and thermoset-based substrates used for high-frequency (HF) applications and other high-end applications. FR4 PCB (printed circuit board flame resitant class 4) were used as references and compared to HF-PCBs based on thermoset substrates with glass fabric and ceramic filler as well as technical thermoplastic materials qualified for laser direct structuring (LDS), namely LCP (liquid crystal polymer), PEEK (polyether ether ketone) and PTFE (polytetrafluoroethylene). These LDS materials for HF applications were metallized using autocatalytic metal deposition to enable three-dimensional structuring, eventually. For that purpose, bond parameters were investigated on the mentioned test substrates and compared with state-of-the-art wire bonding on FR4 substrates as used for HF applications. Due to the challenges of the limited thermal conductivity and softening of such materials under thermal load, the surface temperatures were matched up by thermography and the adaptation of thermal input. Pull tests were carried out to determine the bond quality with regard to surface roughness. Furthermore, strategies to increase reliability by the stitch-on-ball method were successfully applied.

Minimally Invasive Sensors for Transurethral Impedance Spectroscopy

Bladder tumor recurrence is a serious problem in uro-oncology, as it increases the risk of fatal progression and makes further surgeries necessary. The possibility of intraoperative tissue differentiation can help for diagnostic purposes as well as for ensuring that the entirety of the tumor is removed. The latter not only differ visually from healthy tissue, but also in their mechanical and electrical properties due to an altered physiology. A possible base for intraoperative tissue differentiation are impedance measurements, which are, however, difficult to obtain passing through the limited space of the urethra. This work presents a comparative study of two impedance sensors, which are obtained by laser direct structuring and rely on a four terminal measurement. The miniaturized sensors are suitable for minimally invasive usage. Circular electrodes are placed in a square for a first sensor, whereas a novel combination of one circular electrode and three concentric rings is designed for the second sensor. For each sensor, the specific geometry factor is determined and the impact of sensitivity towards small conductivity changes inside the tissue investigated. Furthermore, multiple <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ex vivo</i> measurements on fresh pig bladders are carried out with each sensor. It is shown that both sensors reliably determine tissue impedance data that fit empirical tissue models. Conductivity values close to listed data of human urinary bladder are obtained. Ultimately, the novel ring sensor configuration turns out more favorable in terms of sensitivity distribution and yields more reproducible results than the square electrode arrangement.

A Dual-Band Low-Power-Consumption Active RFID Tag Based on a Meander FPCB Antenna for Subway Vehicle Management

A meander microstrip antenna was designed to operate in the UHF and microwave bands. Laser direct structuring (LDS) and the flexible printed circuit board (FPCB) method were used to fabricate the antenna, and their results were compared. A dual-band low-power-consumption active radio frequency identification (RFID) tag based on the present antenna was developed and fabricated for an automated subway vehicle maintenance system. The recognition distance of the active tag is more than 20 m at 900 MHz and more than 170 m at 2.4 GHz, while the recognition rate is &gt;95% at 900 MHz and 100% at 2.4 GHz. The 100% recognition rate was obtained when the RFID readers were placed in a line parallel to the subway track. Therefore, we have shown that our active RFID tag can be used in a subway vehicle management system.

Compact Dual-Polarized Base Station Antenna Array Using Laser Direct Structuring Technique

In this letter, a compact ±45° polarized base station antenna array fabricated using laser direct structuring (LDS) technique is presented. The proposed antenna array operates in fifth-generation (5G) NR band 41 (2496–2690 MHz). The simple antenna elements of the array consist of a modified patch radiator fed by Y -shaped probes realized by injection molding, which are amenable to efficient mass production. An antenna unit (AU) is investigated to meet the specification requirements in practical base-station applications. A prototype of the AU was manufactured and measured, and the results showed that it is able to fulfill the design requirements within the operating band. These features also show that this LDS-based antenna array would be a promising solution for 5G base station applications.

Toroidal Ferrite Transformers With Conformal Laser Direct Structured Windings

Many applications in communications and power electronics demand inductive components that operate at high frequency or a broad range of frequencies. For applications requiring miniature devices with large inductance values, most often the components are manufactured imprecisely in the traditional method where magnetic cores are wound with a fine gauge wire. To achieve the best high-frequency performance, there is a desire to use a practical manufacturing method that removes the imprecision of wire winding. In this article, a unique 3-D laser exposure system is used to direct-write conformal winding patterns on the surface of MnZn ferrite toroids. The toroidal helix windings were fabricated by both an additive laser direct structuring (LDS) process and a subtractive laser-written mask-and-etch process. Both methods were performed on both enamel-coated ferrites and bare ferrites. The performance and equivalent circuit model parameters of the transformers were measured up to 1.5 GHz and will be presented. The transformers fabricated on the enamel-coated cores were found to have comparable or better inductances and broadband coupling than wire-wound transformers of matching construction.