ADVANCED ANTENNAS FOR MOBILE PHONES Dipl.-Ing. Nils Heininger LPKF Laser & Electronics AG, 30827 Garbsen, Germany Tel: +49 5131 7095-0 Fax: +49 5131 7095-90 E-mail: [email protected]

Lic.Tech. Pauliina Mansikkamäki and Prof. Markku Kivikoski Tampere University of Technology, FI-33101 Tampere, Finland Tel: +358 3 3115 5337 Fax: +358 3 3115 3394 E-mail: [email protected]

Prof. Dr.-Ing. Young-Hyun Lee Kangnam University, Yonin-Shi, Kyunggi-Do, Korea Tel: +82 31 280-3497 Fax: +82 31 280-3807 E-mail: [email protected]

I. INTRODUCTION Diversification, styling, miniaturization and cost reduction are trends in the field of portable electronics. Nowadays the design is the one of the key features for any consumer product. End-users are expecting increasing amount of novel functions available inside trend-looking covers. An antenna makes no exception. In addition, new technologies directed into volume market products have to be flexibly modified into several products and cost competitively manufactured in global manufacturing networks. New innovative technologies are needed to develop in order to meet the market’s demands.[1] This paper introduces the LPKF-LDS (Laser Direct Structuring) technology for the manufacturing of 3D-MIDs (Moulded Interconnect Devices) that is predicted to have a successful future in electronics and mechatronics. The principle of the LPKF-LDS technology is the production of injection molded thermoplastic circuit carriers with laser treatment that is new means to incorporate electronic circuitry, connections, patterns, antennae, and many other features directly onto the 3D molded thermoplastic surface. The focus of this paper is in internal antenna applications for mobile phones and other portable devices made with Laser Direct Structuring method. A stamped metal sheet antenna technology is well known low cost technology. However, due to the stamping process there are limitations in the pattern geometry. Furthermore, tooling is expensive and an altering of a RF pattern causes high-priced and time-consuming modifications into tooling. The LPKF-LDS technology increases flexibility for design of a device, ease of modification of antenna patterns and production lines from product to product and allows multiple antennas integrated into the device’s mechanical structure. II. ANTENNA MARKET The wireless device market has exploded over the last decade. Worldwide there are almost a billion cellular/PCS subscriber. New product capabilities and applications such as short-range wireless connectivity (Bluetooth and WLAN), wideband data connectivity (2.5–3G services) and global connectivity (multi-band operation) are drivers for antenna technology development. Significant design challenges are needed to solve by product designers and manufacturers, since new feature set requirements do not map adequately into existing antenna solutions. Portable handheld devices are diversifying in future. New functions are required to add, but at the same time a size of a device should become smaller and a design should have a novelty appear. Even nowadays a mobile phone could have three or four radio units working in different frequency band and an increase of new functions means increasing number of antennas and multi-band solutions. These antennas must be placed inside continually shrinking device. As portable devices become smaller and have a need for multiple antennas, the footprint allowed for antennas is reducing. [1,2] Furthermore, a design is one of the key features of a portable device and end-users perceive external antennas old-fashioned. The integration of electronics and mechanics, like the 3D-

MID technology, is an innovative way to meet market’s demands by producing cost and space utilization efficient embedded antennas and offering increased design freedom for industrial designers.[2] Electronics is going towards modular or single-chip solutions. Many companies and research institutes have published System-inPackage (SiP) or System-on-Chip (SoC) solutions and the 3D-MID technology offers versatile and cost effective interconnection platforms for them. [3] III. MANUFACTURING TECHNOLOGY In the past, mechatronic products, also known as 3D-MIDs with real three-dimensional structures have primarily been produced using two-component injection molding (2-shot-molding) with subsequent chemical surface activation and selective metal coating – a method involving high initial costs and which is only economically viable for large production numbers. The 3D-MID production method using laser structuring throws out the 2shot-molding and enables MID blanks to be produced by single component injection molding. And compared to subtractive laser structuring, the additive LPKF-LDS technology also benefits from a very short process chain. The main process steps are shown in figure 1.

Figure 1: LPKF-LDS-process steps Compared to conventional methods, Laser Direct Structuring has additional advantages in the production of ultrafine tracks. Moreover, this method also gives high degrees of flexibility for the conductor layout because modifications can be easily incorporated by changing the structuring data. This enables subsequent changes to be made to the circuit layout without having to modify any tools. This degree of flexibility in particular enables users to harness the benefits of Laser Direct Structuring for their product development processes in the firm knowledge that this can simultaneously provide them with a series production process and therefore makes the cost and timeintensive transfer of prototype production to series production unnecessary. The application-oriented selection and industrial availability of the standard laser-activatable plastics used by the electronics industry is an important prerequisite for the use of the LDS method for series production. This availability is guaranteed by the intensive material development and corresponding licence contracts with the relevant plastic-makers: in this context, LPKF has already signed agreements with BASF AG, Bayer Material Science AG (since 01.07.04 LANXESS), Degussa AG and Ticona GmbH. A. Principle behind the method To circumvent the disadvantages of conventional 3D-MID methods [4-9], thermoplastic materials are modified so that an organo-metallic complex is dissolved or finely dispersed within the material. This special chemical compound can be modified by a laser beam in such a way that it catalyses selective metal precipitation in the lasered zones during the subsequent wet metallization process.

Figure 2: Basic structure of a metal complex Figure 2 shows the basic structure of a metal complex of this type. A favourable form is a chelate complex of a precious metal – either palladium (Pd2+) or copper (Cu2+) [10]. The organo-metallic complex has to fulfill a wide range of specifications. The thermal stability must comply with the processing temperature of the thermoplastic matrix and it has to be able to split in a defined way into the metal atom and the organic ligand fragments when hit by a high-energy laser beam. Other criterias are: -

good compatibility with the polymer matrix no diminution in the electrical properties adequate solubility or distribution in the matrix no catalytic activity leading to the formation of polymer chains extraction-resistant non-toxic.

An important parameter for qualifying electronic sub-assemblies is the adhesive force of the tracks. A high level of initial adhesive strength is specified to guarantee that the tracks properly adhere to the substrate even after undergoing an appropriate number of environmental cycles. The initial adhesive strength in PCB engineering is defined as between 0.6 – 1.1 N/mm in accordance with DIN IEC 326. Another important property of the laser activation also has benefits in this context: it is not only able to selectively and homolytically split the metal complex, the laser beam also ablates the polymer surface. This involves absorption of the laser energy by the polymer molecules which become excited and vibrate. Ideally, the bonds between the molecules would break with the input of a minimum amount of energy. In practice, in addition to pure photochemical ablation, the laser beam causes relaxation and associated thermal vaporisation of the material (figure 3). This sublimation process becomes predominant particularly when long wavelength laser light is used, e.g. an Nd:YAG laser (λ = 1064 nm).

Figure 3: Main laser ablation processes

By modifying a polymer in a suitable way with non-ablatable or only poorly ablatable fillers (mostly inorganic materials), lasering results in microscopically-small cavities and under-cuts which in themselves enable good adhesion between the plastic and the precipitating metal layer without the need for complicated extra treatment. The principle mode of action of the laser beam on the plastic surface is shown in figure 4.

Figure 4: Sketch showing the principles of laser direct structuring (LDS) B. Modular 3D laser system for the LDS method The LPKF MicroLine 3D Industrial is a specially designed laser system for the Laser Direct Structuring of injection-molded MIDs. One of the main advantages of processing materials with laser light is the feedback-free impact on the material combined with high processing speeds. In addition, the circuit layout can be created by the binary control of an optical processing head and is not predetermined by the geometry of a fixed tool (e.g. stamping dyes, 2-shot-tools). This gives shorter lead times and high levels of flexibility and economic efficiency. At the heart of each LPKF MicroLine 3D laser system is the above-mentioned processing head with three optical axes. Structures smaller than 100 µm can be produced on three-dimensional moldings thanks to the very rapid control and highly precise optics. The focused beam of a diode-pumped solid-state laser with a wavelength of 1064 nm is reflected with virtually no mass inertia by mirrors onto the surface of the MID. An f-Theta-lens focuses the beam onto the processing level, where a linear translator – a telescope with controllable lens – can shift the focal spot in a longitudinal direction by the specific defocusing of the telescope. The synchronous control of the telescope and the mirror-deflection unit guides the laser beam along complex three-dimensional surface topographies with high processing speeds of up to 4000 mm s-1 (figure 5).

Figure 5: Processing head with three optical axes / LPKF MicroLine 3D Industrial laser unit

In addition to the pure structuring time, the feed-in and feed-out of the components being structured determines the cycle time and thus the laser direct structuring throughput. Efficient component handling is therefore an important aspect of process design. Because injection-molded MIDs have a wide range of shapes and sizes, component handling is generally adapted to suit the geometry of the components being processed. This usually involves a rotary indexing table with a vision system for component location, as well as a version with a tool carrier system configured as a throughput system (figure 6).

Figure 6: Various handling versions of the LPKF MicroLine 3D Industrial C. Thermoplastic materials for series production A number of interesting industrial plastics are available which are used by the electronics industry. However, the range of plastics suitable for MIDs is limited. The main criteria they have to fulfill are the ability to be metallised, their adhesive strength, and their temperature resistance during SMD assembly. The range of thermoplastics is also often restricted further by the MID manufacturing methods: for instance, only thermoplastics with special rheological properties can be used for two-component injection molding (e.g. low molding viscosity to fill in fine cavities). The aforementioned restrictions do not generally apply to materials for Laser Direct Structuring. Production involves single component injection molding and therefore does not demand any special processing properties. The activation and subsequent adhesion of the metal coating is achieved by the surface treatment described in A. (interaction: material – laser). Only the temperature resistance required by many applications limits the range of available materials. However, if soldering is not required and if no high thermoforming resistance is needed, then every type of thermoplastic can in principle be modified for Laser Direct Structuring. The available materials with the appropriate qualifications are listed in table 1.

Property Polymertype Mold shrinkage

Unit

Ultramid®T 4381 LDS

%

Vectra® E820i LDS

PA6/6T

LCP

Vectra® E840i LDS

Pocan DP7102 LDS

LCP

parallel/normal 0,3 parallel/normal 0,1

constrained

0,47

/ 1,4

Pocan TP710-003

Pocan DPT7140 LDS

PBT

PBT

PET/PBT

parallel/normal

parallel/normal

parallel/normal

1,3/1,3

1,4 / 1,4

0,24 / 0,98

parallel/normal

parallel/normal

parallel/normal

0,1 / 0,3

0,3 / 0,3

0,05 / 0,22

/ 0,5

Post-shrinkage

%

-

-

-

Tensile strengh

Mpa

110

116

102

55

51

110

Tensile modulus

Mpa

9000

9000

9300

5600

5500

12000

Strain at break

%

2,1

4

3,4

2

3,3

1,5

Charpy impact strength

kJ/m²

40

19

19

25

40

25

Density

g/cm³

1,52

1,79

1,79

1,57

-

1,75

-

-

-

10

5

16 *

295

335

335

225

225

-

265 *²

220

220

115

110

250 *²

35

40

44

25

25

40

Melt volume-flow cm³/10min rate (MVR) Melting temp. °C DSC Deflection temp. °C (HDT/A) 1,8MPa Content of % material/filler * = 280°C/2,16 kg *² = 0,45MPa

Table 1: Comparison of material properties D. Metallization of laser-activated components The laser-assisted breakdown of metal complexes described in A. creates nuclei, which catalyse metal precipitation in the laser-activated zones. This process usually involves the use of chemical copper electrolytes which typically produce copper thicknesses of 5 ... 8 µm. The appropriate surface finish can then be applied. Table 3 shows a typical metallization sequence corresponding to a standard industrial process. Note the shorter process chain compared to standard plastic metallization. There is also no need for environmentally problematic etching and nucleation steps as required for all-over plastic metallization or two-component injection molding. Stage

Operation

Process

Temp

Time (minutes)

1 2

Ultrasonic cleaning Rinse

*Ronashed NT-10 Running water (city)

60oC Cold

7 1

3

Rinse

Running deionised water

Cold

1

o

4

Electroless Copper

Circuposit 3D-MID

55 C

30

5

Rinse

Running water (city)

Cold

1

6

Rinse

Running deionised water

Cold

1

7

Pd Catalyst

Ronamerse MID Catalyst

RT

1

8

Electroless Nickel

Ronamax MID

88oC

14

9

Rinse

Running water (city)

Cold

1

10

Rinse

Running deionised water

Cold

1

o

15

Electroless Gold

Aurolectroless MID

85 C

10

16

Rinse

Running water (city)

Cold

1

17

Rinse

Running deionised water

Cold

1

18

Dry

* Optional depending on the MID material to be processed.

Table 2: Metallisation process sequence

Metallization is typically carried out on a rack or using the barrel plating method. Integrating LDS metallization processes into installation technology suited for series production requires extensive know how in order to comply with given process windows and a technologically well-solved online analysis of the parameters of the metallization bath. Figure 7 shows a 500 liter rack plating line apparatus with integrated online bath monitoring for the control of the complete metallization process sequence.

Figure 7: Fully automated plating line with online-analysis (courtesy of I&T AG) IV. EXPERIMENTS A. Background The electrical and mechanical specifications are target values for typical mobile phones. Structures in which electronics functions are integrated directly onto the 3D surface of a thermoplastic part have no ready-made testing procedure. For integrated technologies a testing procedure is always application specific and drivers for testing come from several sources; development phase testing, obligatory, customer driven or product related, mass production related and problem solving. The evaluation procedure for an antenna performance of an internal antenna made with LPKF-LDS technology is based on usage environment conditions of targeted end product i.e. a mobile phone. The main target was to evaluate the LPKF-LDS technology’s capability for an internal antenna application directed into the mass volume market and this paper introduces the summary of these evaluation methods and results. In addition to RF properties also manufacturability, electrical, mechanical and environmental performances, cost efficiency of the LPKF-LDS technology were under the evaluation. B.

Mechanical performance of internal antenna

Peel strength is one of the most important characteristics of the 3D-MID technology. Peel strength is measured stress through peeling of a bonded surface resulting in a plating failure. The initial adhesive strength in PCB engineering is defined as between 0.6 – 1.1 N/mm in accordance with DIN IEC 326. As a supportive method a tape test and a cross hatch were included into the testing procedure. The tape test method uses pressure sensitive tape (3M Brand 600 ½ inch wide) to determine adhesion quality of plating, marking inks or paints, and other materials used in conjunction with printed boards. Cross Hatch adhesion test is used to test bonding strength of plating to plastic substrate. Using a sharp utility knife, score 4 parallel lines 3/16" apart. Score 4 additional lines in the same manner perpendicular to the first, creating a tic-tac-toe. Scoring should go through all layers of plating. Using a piece of clear cellophane tape, test the adhesion by removing the tape quickly. If no pieces of

plating are removed, the adhesion of the plating is sufficient. [11] According to this research the peel strength of metallization on LPKF-LDS substrates exceeds the defined peel strength value of a PCB. Every portable device will be exposed to vibration through its projected life. It will be shipped by truck, rail, ship or aircraft, all which involve varying vibration profiles. Customer will set it down on vibrating equipment such as vehicle seats. Mechanical stress testing was simulating real usage environmental mechanical stresses that a mobile phone is exposed to during its life cycle or stresses that an antenna module will be exposed to during a manufacturing process of a device. In the case on an antenna mechanical stresses can cause harmful defect into antenna pattern and disturb the antenna’s performance. Mechanical vibration testing was performed according to Random vibration, MIL-STD-202E, test method for electronic and electric components and parts. Test is carried out according to Method 214, test condition A (frequency range and shape of spectrum changed, but spectral acceleration about same). The test level: − frequency range 20-1000 Hz − PSD-level +3 dB/oct 20-50 Hz − PSD-level 0,04 g2/Hz, 50-500 Hz − PSD-level –6 dB/oct 500-1000Hz − total spectral acceleration 5,37 grms − test duration 60 min in the perpendicular direction of the layer − uncertainty of measurements ± 5% In Figure 8 is presented the test setup for mechanical vibration and shock testing.

Figure 8: The test setup for mechanical tests The samples are mounted to the test fixture from the normal mounting points of the casing. The test level is controlled with an accelerometer attached to the test fixture.

Mechanical shock serves to simulate the forces involved in situations like when a car strikes a pothole or a device falls from a desktop. The applications are numerous. Drop test for small portable electronic device is typically done from height of 0,5 to 3m to surfaces such as concrete, wood and steel. Some specifications call out precision drops to a combination of planes, edges and corners and others random drop by hand suffice. For the drop test a total design is needed to know. In this work device design is not defined, so only mechanical shock test has been performed. According to MIL-STD-202E, Test method for electronic and electrical parts, Method 213, Condition A. Tests were carried out in both directions of perpendicular axis of the conductive pattern. The pulse sine was half sine and the number of pulses was 10 in each direction. [12] The test level: − Peak acceleration 500m/s2 (50 gn) − Pulse duration 11 ms − Uncertainty of measurements ± 5 % The pulse shape was measured with accelerometer attached on the test fixture. The mechanical vibration and shock tests were performed with and without attached component. The component attachment methods were a soldering and a conductive clue. Structural characterizations and electrical measurements were performed before and after the mechanical vibration and shock testing. No signs of deterioration of the adhesion were observed on the test models and the electrical properties were similar before and after the mechanical vibration and shock testing. C. Electrical performance of integral antenna The electrical specifications were target values for typical mobile phones. The electrical performance of internal MID antennas was evaluated by the comparison between a commercial antenna model and similar MID antenna. The measurement included the full 3D radiation patterns in free space and return loss measurements. The pattern and dimensions were similar between the commercial antennas and the MID antennas. In Figure 9 is presented a part of the return loss measurement results for test antennas. The antenna test pattern was designed to simulate a not optimized antenna in order to find out the technology’s capability to produce an internal antenna. Returnloss Measurement

-3

23

23

37 ,5

-1

43 ,8 23 50 23 56 ,3 23 62 ,5 23 68 ,8 23 75 23 81 ,3 23 87 ,5 23 93 ,8 24 00 24 06 ,3 24 12 ,5 24 18 ,8 24 25 24 31 ,3

1

dB

-5 -7 -9 -11 -13 -15 MHz

Figure 9: Return loss measurement graph

The return loss measurement results show that with the LPKF-LDS technology it is possible to fabricate functional internal antennas. The 3D free space radiation patterns were very similar between commercial stamped metal antennas and the MID antennas manufactured by Laser Direct Structuring Moisture and Insulation resistance measurements were performed according to IPC-6018, Paragraph 3.9.3 and IPC-TM-650, Method 2.6.3. Test samples are conditioned for 24 hours at 50°C after specimens are cooled initial insulation resistance measurements are measured. Five hundred VDC is applied to the test samples for one minute prior to taking each insulation resistance measurement. Test specimens are placed in a vertical position in a temperature humidity chamber. A polarizing voltage of 100±10 VDC is applied during the entire test. Samples are exposed to 20 cycles of temperature and humidity. The humidity chamber is cycled from temperature of 25°C and relative humidity 90% to +65°C and relative humidity 90%. Insulation resistance measurements were made after one hour and before two hours stabilization at ambient.[13] The results didn’t show any unwanted phenomena. D. Environmental characteristics Technologies that integrate electrical and mechanical functions into one coherent unit have to be able to resist environmental stresses critical for both mechanical and electrical performances. The approach for environmental testing was based on the usage environment condition of a mobile phone. Climatic exposures are used to simulate the conditions to which the electrical devices are exposed during transportation, operation or while in warehouse. The objective is to check whether such conditions can harm the device - does the device after the exposure still meet the requirements of the safety standards or still operate normally. The testing procedure was consists of thermal cycling, damp heat and salt mist tests.[14] Humidity testing is typically done at extremes e.g. high temperature/high humidity and low temperature/low humidity. The upper extreme tests to see if moisture can be forced into tested structure resulting in undesirable parameter variances or corrosion. The lower extreme tests factors like the vulnerability of materials that might dry out and crack. [15] The Damp Heat test was carried out according to IEC 60068-2-30, Test Db. Conditions were: High Temperature +55°C ± 2°C (9 hours) Low Temperature +25°C ± 2°C (9 hours) Temperature change 2 * 3hours Humidity 90% ± 5% Cycle 6

Figure 10: LPKF-LDS antenna after the damp heat test

Salt mist tests were performed according to ASTM B117 (5% NaCl at 35°C) with an exposure time of 48 hours. The objects are positioned in the salt spray chamber in such a way that the inside is directed towards the spray nozzle. After completed exposures testing the objects are rinsed with deionised water, to remove traces of sodium chloride, and thereafter dried and wrapped in sealed polyethylene bags. ASTM B117 Salt Mist test is aggressive test method and literature has reported several salt mist test method with smaller per sentage of NaCl.[16] Temperature cycling is the process of cycling through two temperature extremes, sometimes at relatively high rates of change. It is used to age products through fatigue. As components heat up and cool down, they expand and contract. This causes failure over time. In addition to evaluating designs, it is also used in manufacturing to screen products to catch early-term, latent defects. In this work the temperature cycling test temperatures are taken from the handheld device specification that is commonly used in the telecom market. The lower temperature is -30° ± 2°C, the upper temperature is +60° ± 2°C and the rate of change of temperature is -7°C/min. The test duration is 50 test cycles, which is approximately six days. [17] Thermal shock is an extreme application of rapid temperature change. It compares to moving from the desert to the arctic in few seconds. Improperly formulated or manufactured materials can have undesirable internal stresses that can lead to failures. Thermal shock provides a stimulus to relieve these stresses in the form of cracks, warping, etc. The scope of this test method is to determine the physical endurance of conformal coating/printed boards to sudden changes of high and low temperature excursions that cause physical fatigue. Samples are subjected to one hundred cycles of thermal shock between temperatures of –40°C and +85°C. Dwell times at each temperature extreme is 15 minutes and transition time between extremes is less than two minutes. [18, 19] After the climatic tests electrical properties and the adhesion quality of the conductive pattern was verified. As a summary can be concluded that the MIDs manufactured in LPKF-LDS technology passed all operational environment testing. There was no evidence of cracking, separation, blistering, delamination, deterioration or change in resistance and no conductors lost continuity. E. Mass production capability The capability evaluation of a technology’s manufacturability is always application and technology specific. In this work the manufacturability evaluation is focused on a mobile phone’s internal antenna applications made with the LPKF-LDS technology and targeted at the volume production. In the case on an internal antenna the position accuracy and repeatability during mass production were the most important characteristics under evaluation. The manufacturability evaluation is needed to perform through the entire manufacturing network. Targets of evaluation were: − − − − − −

Process repeatability Process stability Bottle-necks on process Cost structure Improvements needed Know-how needed

Features like wirings, antenna patterns and connections integrated directly into a plastic part’s surface require high pattern’s shape repeatability and positioning accuracy due to automated assembly processes in a mass production. According to the manufacturability evaluation results Laser Direct structured MIDs have very good pattern’s shape repeatability and positioning accuracy. The LPKF-LDS process is highly flexible, cost effective and viable technology for internal antenna applications.

V. SUMMARY OF RESULTS This paper introduces a sweeping summary of the evaluation results due to the confidentiality of measurement data related to the commercial product or companies’ strategic know-how. The evaluation of a technology that integrates electrical and mechanical functions into one coherent unit has no ready-made testing procedures. The evaluation of LPKF-LDS was based on the operational environment conditions of portable electronic devices i.e. mobile phones directed into the volume market. Although, the focus of this paper is in LPKF-LDS’s capability for internal antenna applications the goal of the evaluation was more extensive. The goal was the overall feasibility evaluation of the LPKF-LDS technology. Results proved that LPKF-LDS technology could be used in antennas electronic circuitries, connections and many other features in portable devices. VI. CONCLUSIONS It can be concluded that the LPKF-LDS technology is reliable and flexible 3D MID technology for internal antennas of mobile phone applications. Furthermore, the LPKF-LDS technology offers a low cost manufacturing process and offers increased design freedom for product designers. ACKNOWLEDGMENT The authors would like to acknowledge Perlos Oyj with whom the technical evaluation was carried out. REFERENCES [1]

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[4]

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