Miniaturization of electronic components: A new manufacturing process for 3D-MIDS

A large step towards a significant miniaturization of electronic components is the integration of electrical and mechanical functions as implemented in 3D-MID (Mould Interconnect Device) technology. Here a housing can serve as a three-dimensional circuit board. A more rational fabrication, 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 SD-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 organometallic complex.

2) The surface of the thermoplastic part is partially activated by laser irradiation.

3) Circuit tracks are selectively deposited on activated areas using an additive electro-less plating process.

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 pm, 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 electrically 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 dispersive organometallic complexes into the matrix, which are designed in a way that they can be activated by the laser irradiation.

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 supported by incorporating laser irradiation resistant filler particles, which protrude on the surface after the laser treatment.

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") and/or copper (Cull). 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 transported. 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.

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.