Effect of flow lines on the metallization of laser-structured polymer parts

Transcription

1 Effect of flow lines on the metallization of laser-structured polymer parts Andreas J. Fischer and Dietmar Drummer, Institute of Polymer Technology, Friedrich-Alexander-University Erlangen-Nürnberg (FAU), Am Weichselgarten 9, 9158 Erlangen, Germany Thomas Kuhn and Jörg Franke Institute for Factory Automation and Production Systems, Friedrich-Alexander-University Erlangen- Nürnberg (FAU), Egerlandstraße 7, 9158 Erlangen, Germany Hagen Müller and André Zimmermann Hahn-Schickard, Stuttgart, Allmandring 9B, 7569 Stuttgart, Germany Abstract Molded interconnect devices (MID) offer great potential for circuit board applications, especially regarding three-dimensional shaping and functional integration. Applying circuits to polymer substrates can be performed by means of laser patterning methods. In these, the matrix polymer is filled with a special metal additive, enabling laser activation and subsequent metallization. Important effects emerge during processing of the matrix polymer. In this work, the influence of the geometry and the resulting flow behavior during injection molding on the quality of the metallization is investid. Introduction Thermoplastic materials become increasingly more relevant as substrates for circuit boards in electronics and electrical engineering with various advantages. These plastics allow for a shaping of the circuit carrier in three dimensions in addition to functional integration, e. g. introduction of additional fastening elements as well as further plastic specific benefits such as savings in weight and resistance against corrosion. Therefore, the circuitry is applied onto the polymer substrate. This is reflected in the term molded interconnect device (MID). [1] For the integration of interconnect structures onto injection-molded circuit carriers, laser direct structuring in accordance with the LPKF-LDS process has firmly established itself in the market, mainly in the field of telecommunications. Therefore, the polymer matrix is mixed with a metal additive which is cleaved upon exposure to the laser beam and thus activated. The matrix is partially ablated which gives rise to a rugged surface. A subsequent electro-less metallization leads to the growth of interconnect structures starting at the activated nuclei. [2] [3] Compared to full surface metallization processes of polymers (e. g. physical vapor deposition), active catalytic germs enable a selective metallization [4]. In addition to the LDS additive, high contents of mineral fillers (e. g. glass fibers talc) are added to the materials to reduce thermomechanical stresses between polymer and metal layer as well as to improve the mechanical properties [5] [6] [7]. The influence of fillers on the metallization quality has already been investid elsewhere [8] [9]. The advantages of this technology are a flexible change in pattern design by changing the CAD data, a short process chain, high precision with fine pitches down to 5 µm, the possibility of mass production and almost unlimited freedom of shape. Challenges of the process are the economic limits of the metal layer s thickness of about 1 15 µm and small parts due to high plating times. In addition, mechanisms and interactions of light-off are yet to be fully understood, in particular where the substrate material is concerned. [1] Thus, the metallization quality of LDS-parts is mainly qualified at the end of the process chain. As such, before defective metallization can be detected, the component must pass through the entire process chain from material preparation, molding, cleaning, laser structuring to metallization which is quite inefficient. [1] To increase the reliability of the process chain, the effect of substrate material processing must be understood in detail with particular reference to injection molding. Caused by the velocity profile in the melt flow, anisometric fillers such as fibers or platelets are oriented during injection molding. Simplified, a three-layer structure is formed, consisting of fillers oriented in flow direction (boundary layers) and fillers oriented perpendicular to the flow direction (core layer). [11] [12] The different layers affect the mechanical and thermomechanical properties, e. g. the coefficient of thermal expansion, and cause different values in the three spatial directions [13]. Weld lines are generally defined as a stop after the clash of two melt fronts. In this case, they are more specifically called cold or stagnating weld lines. If the melt fronts continue their flow after unification, the weld lines are called hot or flow lines ( flowing weld lines). Triggers for weld or flow lines in injection-molded parts are obstacles in the flow path, multiple s, different wall thicknesses or jets and can hardly be SPE ANTEC Anaheim 217 / 819

2 Flow direction avoided in technical parts. [14] In filled polymers, the orientation of anisometric fillers is affected in the vicinity of weld or flow lines. Here, flakes and fibers are oriented perpendicular to the flow. However, this affects the mechanical properties in these regions. [15] A systematic analysis of the influence of flow lines on the metallization quality in laser-structured polymer parts, especially under increased temperatures, represents a severe gap in research within this field. The aim of this paper is the examination of the influence of flow lines during injection molding of highly filled polymers on the metallization quality and their behavior under temperature load. Therefore, the types and dimensions are varied. The metallization quality is evaluated electrically and optically. Furthermore, the thermo-mechanical behavior is characterized. Experimental Material`s selection and preparation A PA1T-based copolyamide Vestamid HTplus M331 (Evonik Industries AG, Marl, Germany) with a melting point of 285 C was used for the investigations. As MID assemblies are typically exposed to high temperatures (e. g. lead-free soldering ~ 27 C) LDSmodified plastics have mainly been derived from the field of such high-performance plastics [5]. This material is already equipped for laser structuring and contains approximately 3 wt% talc in addition to 1 wt% glass fibers and a small amount of LDS additive. [16] The material was injection molded into plates of 5 mm x 5 mm with a thickness of 2 mm (Ergotech 25/28, Sumitomo (Shi) Demag Plastics Machinery GmbH, Schwaig, Germany). The thicknesses of the used s are.5 and. Furthermore a was used (see also Figure 2). The processing parameters are given in Table 1. Table 1. Processing conditions of the PA1T. Specimen thickness Gating system Injection speed v in 2 mm.5 mm / 12 mm/s Mass temperature T M 34 C Tool temperature T T 14 C The processing temperatures are based on the values specified by the manufacturer. Partly, the neat material Vestamid HTplus M3 was used for reference. Optical Analysis Polished sections of the injection-molded plates parallel and perpendicular to the flow direction were embedded in epoxy resin and evaluated using a light microscope (Axio Imager.M2m, Carl Zeiss AG, Oberkochen, Germany). Metallization The structuring of the specimens was carried out using a Nd:YAG-laser (wavelength: 164 nm) from LPKF Laser & Electronics AG, Garbsen, Germany (ML16i). The feeding rate was 4 mm/s with a laser hatch of 45 µm. Three samples of each setting were then plated in a copper bath (Kupfer 4 SC, Enthone, GmbH, Langenfeld, Germany) for 4 min. The layout is shown schematically in Figure 1, the middle of the part is covered by a meander structure representing a circuit. The area was chosen as the flow lines appear there. Gate Meander structure 5 µm Figure 1. Schematic illustration of the metallized structure. Characterization of metallization To qualify the metallization the metal layer thickness was measured using X-ray fluorescence with a FISCHERSCOPE X-RAY (Helmut Fischer GmbH, Sindelfingen, Germany) and a measuring surface of 33 μm 1 μm on the small circles in the middle of each specimen (Red arrow in Figure 1). For electrical investigations, the resistance of the meander structure was measured on the three plated samples. This was carried out directly after metallization and in a second step after applying 185 C for five minutes to the sample. Optical investigations of the metallization were based on polished section cuts in epoxy resin perpendicular to the flow direction in the middle of the specimens under a light microscope (Axio Imager.M2m, Carl Zeiss AG, Oberkochen, Germany) and a field emission scanning electron microscope (SEM) Ultra Plus (Carl Zeiss AG, Oberkochen, Germany). Furthermore, the metallization was characterized on topview by a stereomicroscope (SteREO Discovery.V12, Carl Zeiss AG, Oberkochen, Germany). SPE ANTEC Anaheim 217 / 82

3 Perpendicular to flow Parallel to flow Thermomechanical analysis To determine the coefficient of thermal expansion, (CTE) a thermomechanical analysis on a TMA 294 (TA-Instruments, Inc., New Castle, USA) was carried out according to DIN EN ISO 11359, 1-3. Therefore, three samples of 7 mm x 7 mm were cut from the center regions of the plates and tested parallel (x) and perpendicular (y) to the flow direction. For this investigation the samples were not plated. The first and the second heating in the range between -4 to 22 C were evaluated. The CTE values below the glass transition temperature (T G) were taken between -2 and 6 C, above T G between 185 and 22 C. Results Considering the short shots of the samples, a significant influence of varying dimensions on the flow behavior of the filled material can be observed (Figure 2). In contrast to the unfilled material, the melt stream is divided, resulting in a "wavy" flow front when using the with a thickness of.5 mm. This effect disappears when increasing the thickness to. The flow front develops symmetrically. This phenomenon could be caused by the temperature distribution in the flow profile of the plastic melt. At low thicknesses the shear increases, especially for highly filled plastics. This might reinforce the temperature gradient between mold wall and component interior and cause a proceeding of the melt into the outer area. This flow inhomogeneity results in clearly visible flow lines on the parts with the.5 mm for the filled material. The samples manufactured with a do not show this behavior. The use of a also results in flow lines on the part surface caused by the unification of the divided melt flow. Unfilled material Filled material.5 mm Figure 2: Different flow profiles in short shots of the neat and filled material caused by different types and dimensions. Optical Analysis The filler orientations of the samples can be visualized with section polishes in and perpendicular to the flow direction (Figure 3). Talc platelets and glass fibers are oriented in the typical three layer structure reported in literature, e. g. [12]. Optical analysis does not show any significant influence of the thickness on the filler orientation and the layer thickness. The sample molded with a, in contrast, shows a thinner middle layer in flow direction which cannot be seen on the sample perpendicular to the flow direction..5 mm 6 µm 6 µm 6 µm 6 µm 6 µm 6 µm Figure 3. Section polishes of the samples in and perpendicular to the flow direction with different gating systems. SPE ANTEC Anaheim 217 / 821

4 Characterization of metallization Comparing the plating thicknesses (Figure 4) of the different samples, no significant change can be detected. All samples achieve values of approximately 7.5 µm. An exposition of the samples to a temperature of 185 C for five minutes leads to failures only in samples with flow lines. The conductor paths do not fail for the specimen molded with the (Figure 6) and still achieve the lowest value for the resistance min@185 C Plating thickness [µm] Electrical resistance [ ] failure 1 failure mm Figure 4. Effect of different types and dimensions on the plating thickness. The electrical characterization after metallization as well does not show a significant negative influence of the flow lines (.5 mm, ) on the electrical resistance compared to the part with the (Figure 5). However, the resistance of the shows a slightly lower value. Electrical resistance [ ] mm Figure 6. Effect of different types and dimensions on the electrical resistance after thermal treatment (185 C for 5 min). In the top views of the paths recorded by stereomicroscopy after exposition to temperature, cracks and delaminations can be detected (Figure 7). Cracks can be found throughout all the samples investid, as well in samples with a thickness of. Interestingly, this does not inevitably lead to a failure of the circuit. Delaminations only occur on the sample with the. These delaminations can already be detected on cross sections of the samples with the.5 mm and the after metallization without thermal treatment under the SEM (Figure 8). On the samples with the they cannot be detected. This does probably originale in the irregular orientation of the fillers in the vicinity of the flow lines. However, an influence of the specimen preparation cannot be excluded completely. For this reason, the coefficients of thermal expansion of the specimen were also measured..5 mm Figure 5. Effect of different types and dimensions on the electrical resistance before thermal treatment. SPE ANTEC Anaheim 217 / 822

5 Crack.5 mm 15 µm.5 mm 5 µm Crack 15 µm 5 µm Delaminations 5 µm 15 µm Figure 7. Exemplary stereomicroscopic images of the meander structure for different types and dimension after thermal treatment. 5 µm Figure 8. Exemplary SEM images of section polishes of the metallized structures before thermal treatment. SPE ANTEC Anaheim 217 / 823

6 Thermomechanical analysis The coefficients of thermal expansion are strongly affected by the flow lines. In all samples the values perpendicular to the flow direction are higher than those in flow direction, especially for temperatures above T G (Figure 9). Furthermore, they increase for the. Coefficient of thermal expansion [µm/m C] mm x-direction y-direction > T G 1 st heating 2 nd heating.5 mm Figure 9. Effect of different types and dimensions on the coefficient of thermal expansion above T G. For temperatures below T G, only a small variation between flow direction and perpendicular direction can be detected (Figure 1). Coefficient of thermal expansion [µm/m C] mm x-direction y-direction < T G 1 st heating.5 mm 2 nd heating Figure 1. Effect of different types and dimensions on the coefficient of thermal expansion below T G. The increase for the is also visible. This effect applies for the first and second heating of the sample as well. The first heating is important, since components are often not annealed before soldering. In the second heating, the thermal history of the material is eliminated. The deviation of the CTE perpendicular to the flow direction can be attributed to the small middle layers of glass fibers suppressing the thermal expansion. In flow direction, the glass fibers in the big boundary layers prevent the thermal expansion. The strong increase of the coefficients of thermal expansion for the samples with flow lines is caused by the very small middle layer, combined with the irregular orientation of the fillers in these areas. This can also be seen on the section polishes. Conclusions In the present work, the influence of different types and dimensions in injection molding on the metallization quality of laser-structured polymer parts has been investid. A thickness of.5 mm was found to change the flow profile of filled materials compared to the with a thickness of. Also a was used resulting in flow lines. Cross sections show no change in the three layer filler structure for.5 mm and. The middle layer nearly disappears for the. Neither the plating thickness nor the electrical resistance of the metallization is significantly affected by the gating system and resulting flow behavior. A thermal impact leads to failures in the conductor paths for the.5 mm and, caused by cracks and delaminations. For the, cracks can be detected, but no failure is caused by this. The probable reason for these effects are higher coefficients of thermal expansion especially perpendicular to the flow direction. Furthermore, it is possible that degradation products of the polymer are formed in the area of flow and weld lines and accumulate on the part surface. This can also lead to a decrease of metallization quality and metal adhesion but has to be further investid. However, a metallization in the area of flow lines should be avoided. Failures cannot be detected directly after the metallization but develop due to thermal impact as during the use. SPE ANTEC Anaheim 217 / 824

7 References [1] Franke, J. (ed.).: Three-Dimensional Molded Interconnect Devices (3D-MID) - Materials, Manufacturing, Assembly, and Application for Injection Molded Circuit Carriers, 2 nd ed., Carl Hanser Verlag: München, 214. [2] Naundorf, G., Wißbrock, H.: A fundamentally new mechanism for additive metallization of polymeric substrates in ultra fine line technology illustrated for 3D-MIDs, Galvanotechnik 91 (2). [3] Wißbrock, H.: A New Addition to MID Technologies: Laser Direct Structuring of Plastics, Kunststoffe 92 (22). [4] Rytlewski, P.: "Fundamentals of Laser-Polymer Interactions and their Relevance to Polymer Metallization," in Laser Surface Modification and Adhesion, K. L. Mittal, Bahners, T., Ed., ed Beverly, MA: Scrivener Publishing, 215. [5] Strohm, V.: Looking for the Right Polymer, Kunststoffe 11 (211). [6] N.N.: Evonik Industries AG: Vestamid HTplus - Classification, 212. [7] N.N.: Ticona GmbH: Materialdatenblatt Vectra E84i LDS, 211. [8] Fischer, A. J. Meister, S., Drummer, D.: Effect of fillers on the metallization of laserstructured polymer parts, Journal of Polymer Engineering (216). [9] Fischer, A. J., Drummer, D.: "Polymer films for laser-structured circuit carriers," in 12th International Congress Molded Interconnect Devices (MID), 216. [1] Fischer, A., Kuhn, T., Müller, H.: "Reliability of LDS Conductor Paths - Influencing Factors and Characterization Methods," in Jörg Franke; Thomas Kuhn; Albert Birkicht; Andreas Pojtinger (Hrsg.): 11th international congress molded interconnect devices: Scientific proceedings : Selected, peer reviewed papers from the 11th International Congress Molded Interconnect Devices (MID), 214. [11] Menges, G., Geisbüsch, P.: Die Glasfaserorientierung und ihr Einfluß auf die mechanischen Eigenschaften thermoplastischer Spritzgießteile - Eine Abschätzmethode, Colloid and Polymer Science 26 (1982). [12] Heinle, C., Amesöder, S., Brocka, Z., Schmachtenberg, E.: "Product developement with thermal conductive polymers and the need for integrated use of simulation." The Polymer Processing Society 23rd Annual Meeting, 27. [13] Heinle, M., Drummer, D.: Temperaturedependent coefficient of thermal expansion (CTE) of injection molded, short-glass-fiberreinforced polymers, Polymer Engineering & Science 55 (215) 11. [14] Nguyen-Chung, T.: Flow analysis of the weld line formation during injection mold filling of thermoplastics, Rheologica Acta 43 (24) 3. [15] Fisa, B., Dufour, J., Vu-Khanh, T.: Weldline integrity of reinforced plastics: Effect of filler shape, Polymer Composites 8 (1987) 6. [16] N.N.: Evonik Industries AG: Vestamid HTplus - PA 6T/X and PA 1T/X product range and main properties, 212. Acknowledgements Parts of this research were carried out within the project N from Research Association Molded Interconnect Devices 3-D MID e. V., which was funded by AiF within the program for sponsorship by Industrial Joint Research (IGF) of the German Federal Ministry of Economic Affairs and Energy based on enactment provision of the German parliament. Furthermore the support of Evonik Industries AG, Marl, Germany is greatly acknowledged for supplying the materials. SPE ANTEC Anaheim 217 / 825