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A large step towards a significant miniaturization of electronic components is the integration of electrical and mechanical functions as implemented in 3D-MID (Mould Inter-connect Device) technology. Here a housing can serve as a three-dimensional circuit board. A more rational fab-rication, design flexibility, and shorter process chains are the main advantages of this technology, which is of growing interest to the circuit board industry.

A new production process — LPKF-LDS (Laser Direct Structuring) — for 3D-MIDs has been developed and comprises merely three steps (see figure 1):

1. A thermoplastic part is injection molded based on a granule modified with an organ-ometallic complex.
2. The surface of the thermo-plastic part is partially activated by laser irradiation.
3. Circuit tracks are selectively deposited on activated areas using an additive electro-less plating process.

(Figure 1 - Main technology steps)

In addition to being highly flexible relating to changes in the electrical circuit design, the LPKF-LDS process provides high throughput lines and spaces down to 20 µm,
and is above all an environmentally friendly technology. This method is now ready-for-market and is currently being adapted for several materials of interest to the electronics industry. This article further describes the process, system technology, and results.

Granule & Organometallic Complexes

The fundamental concept of the LPKF-LDS process is to modify an elec-trically isolating polymer matrix while maintaining its non-conductive property and to set free seeds on the surface of the polymer via laser irradiation of a certain energy density level. These seeds enable a selective wet-chemical reductive metal precipitation. The polymer is modified by incorporating dis-persive organometallic complexes into the matrix, which are designed in a way that they can be activated by the laser ir-radiation.

On one hand laser irradiation induces a physio-chemical reaction, specifically the cracking of chemical bonds. On the other hand it makes a strong adhesion of the forming metal layer possible by ablating polymer material, i. e. roughening the surface and thus providing an effective anchoring for the forming metal layer. Optimum cavities are produced, providing a mechanical anchoring for the metal plating (see figure 2). This effect is sup-ported by incorporating laser irradiation resistant filler particles, which protrude on the surface after the laser treatment.

(Figure 2 - Surface after laser structuring and after beginning metallization)

The starting point of the LPKF-LDS process has been the development of organometallic complexes with the following characteristics:

• Electrically non-conducting
• Visual-light-resistant
• Sufficient soluble and/or colloidal dispersible in the polymer matrix
• Good compatibility in the polymer filler material system
• No catalytic activity
• Separable in metal seeds and organic residuals by laser irradiation
• High thermal resistance
• Little toxicity
• Low costs

The organometallic complexes are based on palladium (Pd 2+ ) and/or cop-per
(Cu 2+). Due to high palladium prices alternative systems of different transition metals like copper are preferable. The developed organometallic complexes are of an exceptionally high stability.

Pulverized organometallic complexes, inorganic filler materials, the polymer as well as further additives are processed in a heating-cooling mixer combination (fluid mixer, see figure 3) to a homogeneous agglomerate. The next step is the compounding. In an extruder this agglomerate is molten and trans-ported. The result is a homogenized modified thermoplastic. After cooling the extruded thermoplastic is crushed in a granulator to a conventional granule, which is outstandingly well suited for molding three-dimensional parts using conventional injection molding technology.

(Figure 3 - LPKF system: Heating-cooling mixer combination, extruder, cooler, and granulator at FH Lippe, from right to left)

Laser Radiation & Electro-Less Plating

A string of different technologies is available for coating a plastic surface with a metallic layer, for example PVD coating, laminating with metal foils, spray coating, and electroplating methods. The latter are especially suited for metallizing three dimensional pieces. When using an electroplating technique, plastic parts are usually metallized in a multi-stage process where the surface is first cleaned and roughened, then given a catalytical nucleation, and finally coated with metal using a chemical and/or electroplating method. In the field of plastics metallization, creating a plastic surface that catalyses a chemical metallization process is called activation.

Selective activation followed by selective metal deposition is an especially promising approach to the problem of metallizing only partial areas of three-dimensional plastic surfaces (e. g. in MID production). When using special substrate materials, laser irradiation can directly trigger such a selective activation. Indirect activation by a laser is possible as well. Here the catalytic plastic surface is not directly created by laser irradiation but rather by deposition of a catalyst in the irradiated areas.

Surface activation and roughening is achieved by using a UV-laser. With regard to the surface structuring of the thermoplastic, the necessary laser radiation demonstrates high-resolution ablation of material, especially applying to organic materials, and induction of a physio-chemical reaction. The conductor lines are selectively built up on the thermoplastic in the areas activated by the laser in a following electroless plating process.

Conventional metallization baths (e. g. Shipley, MacDermid) can be used. An economical plating thickness lies in the range of 5 µm copper, a succeeding Ni-layer of 5 µm, and 0.1 µm gold finish. To achieve a higher copper plating thickness, the part can be placed in an electrolytic bath afterwards.

Basically the process can be applied to many thermoplastics. In a first step polypropylene (PP) was chosen to demonstrate the feasibility of the process. PP is a demanding and within the frame of industrial applications a proven thermoplastic with low temperature characteristics. Its extremely difficult metallization behavior increases the requirement profile for this technology. More recent developments have concentrated on the adoption of the complexes for incorporation into the high perfor mance thermoplastic polybutylene-terephtalate (PBT) and partly aromatic polyamid (PA6/6T MID), which is well suited for microinjection molding.

Laser Technology

The laser system used in this process is comprised of five mechanical and three optical axes. The lateral moving range of the xy-table comprises 200 x 200 mm with an absolute position accuracy of ±4 µm. The z-axis has a positioning range of 300 mm with an accuracy of ±3 µm. An axis of rotation (360°, accuracy ±10°) is mounted on a swivel axis (±90°, 10°) to allow an in-feed of parts with multiple planes to be structured.

The initial laser beam is focused, positioned, and deflected within a scanning volume. Three optical axes make it possible to guide the laser focus spot within a plane as well as along complex three-dimensional contours. The laser beam is moved relative to the work piece with two deflecting mirrors in lateral direction. Moving mirrors are synonymous to low moving masses. The result is an extremely high working speed and thus an economical throughput. The mirrors are mounted on a rotational axis of special motors (gal-vanometer scanner). In addition to high positioning speeds, high accuracies can be provided.

The third optical (a Kepler telescope) axis provides a shift in longitudinal direction. This shift is achieved by moving one of the lenses mounted on a linear translator. At present a scanning volume of 200 x 200 x 50 mm3 can be reached. Basically the system can be operated with an ultraviolet (= 355 nm) or infrared (= 1064 nm) laser. Up to now only UV laser radiation has been used for the activation of the thermoplastics. The laser source that has been used for the laser activation process is a frequency- tripled Nd:YAG laser (= 355 nm). Increasing absorption characteristics of polymer materials as well as decreasing spot diameters with smaller wavelengths enable the polymer ablation with the desired characteristics of cracking the organometallic complexes and roughening the surface for a metallization of strong adhesion. This is supported by operating the laser in Q-switch mode producing extremely short pulse durations combined with extremely high pulse powers. The small spot sizes also make it possible to achieve finest lines down to approximately 20 µm.

The laser beam is homogenized in order to achieve a constant intensity distribution over a defined cross section. On the other hand a typical laser beam intensity profile is Gaussian. Therefore, less energy is incorporated into the surface of the thermoplastic at the edges of the laser-structured track than within its center. As a result a melting zone is formed at the edges of the track, which should be avoided in terms of a strong metallization adhesion. Thus a clearly defined beam profile with a homogeneous intensity distribution is required. This kind of “Top Hat” beam can be implemented, using fibers for example, a pipe with mirror-glass on the inside or an individual diffractive component.

Current research focuses on a circular Top-Hat intensity profile that is generated using diffractive lenses. At present, diffractive optical elements (DOE) are used in laser technology, fiber technology, communications technology, projection lens technology, and sensor technology.

Material Properties

In connection with the investigations on the characteristics of the developed thermoplastics, measurements of the viscosity in dependence on the shear speed are very important. The dependence does not show any significant difference to standard PP. This proves that high quality components can be fabricated in standard injection molding processes with doped PP. This also applies to PBT and PA6/6T MID.

Apart from impressing electrical and mechanical properties of doped PP and PBT, the exceptional adhesion strength of the plating with values of 13 N/cm for PP-MID, 8 N/cm for PBT-MID and 10N/cm for PA6/6T MID, is important for the circuit board manufacturing industry. Figure 4 shows a complete fabrication of a 3D-MID part, from the molded part to a complete circuit with components.

(Figure 4 - Complete chain for manufacturing a 3D-MID: molding, activation, plating, and component )

Conclusion and Outlook

Laser supported additive metallization of different thermoplastic materials for 3D MIDs is an environmentally friendly technology for structuring fine lines down to approximately 20 µm for microelectronic applications with a high throughput. At present three modified thermoplastic materials are available that can be injection molded in standard processes with exceptional mechanical and electrical properties. Applications are foreseen within the areas of telecommunication, sensors, and automotive systems.

This article was composed by M. Hüske and J. Kickelhain of LPKF Laser & Electronics AG, Germany; J. Müller the Chair of Manufacturing Technology, University of Erlangen- Nuremberg, Germany; and G. Eßer of the Bavarian Laser Center, Germany. For more information contact Stephan Schmidt of LPKF Laser & Electronics, 28220 SW Boberg Rd., Wilsonville, OR 97070 at (503) 454-4000 or sschmidt@lpkfusa.com.

Visit LPKF online at www.lpkfusa.com