Publication date: 29 April 2008
Instead of replacing existing technologies, MID much more lends itself to open new application sectors which would entail excessive overhead or which would be totally impossible using existing technologies.
MID (Moulded Interconnect Device) is a technology enabling a three-dimensional implementation of electronic circuits. It enables the fabrication of conductive traces on injection-moulded parts.
Due to the use of highly temperature-resistant plastic, it is possible to solder components onto the traces or to further process the completed assemblies using soldering technologies.
The technology is also attractive for connector applications due to its ability to integrate mechanical functions. Along with the high degree of freedom offered by injection-moulded components, the inherent flexibility of direct laser structuring and subsequent surface coating result in a large variety of options for implementing connectors.
Significant reductions of size and component count can be achieved by unifying mechanical and electrical functions.
For instance, if the circuitry is placed on the inner surface of a device’s enclosure, no printed circuit board is required because the electronic components can be soldered or glued directly inside the enclosure, making MID particularly attractive for sensor applications.
Additional applications can be identified in medical equipment and the telecom and automotive sector. Several MID processes are now successfully used in high-volume applications.
The two-shot injection moulding process is an example of the ‘classical’ MID processes. Initial MID parts were manufactured using this technology back in the early nineties. Although the process has been refined and diversified in the meantime, the basic steps have remained the same:
The first injection step (shot) is made using a plateable, non-conductive plastic. In this step, embossed structures must be formed for the future conductive traces.
In a second shot, a non plateable plastic can laterally surround of the traces. Depending on the desired geometry, the non plateable plastic may also be injected first.
In any case, it is important that the structure to be metallised be visible on the part’s surface. The electrical connections themselves are subsequently created by galvanic plating.
The plastic part must pass different plating baths for this purpose. Before metallising the plateable plastic, the surface must be activated by etching.
Next, different layers of copper, nickel, gold etc. can be deposited. As these plating steps are usually done without applying an external current, the thickness of the layers exclusively depends on the time the part is left in the galvanising bath.
As a recent alternative to the 2-shot process, Laser Direct Structuring (a process developed by LPKF Laser & Electronics AG) enables the low-cost and flexible production of MID parts. The laser-structured parts are produced using a single shot injection-moulding process, followed by the activation of the surface using a focused laser beam and the subsequent metallization in a chemical bath.
This method is highly flexible because the circuit pattern stored in the computer is directly transferred to the injection-moulded part by the laser beam without any additional tools or masks. Structuring is carried out exclusively using the existing CAD data (Source: http://www.lpkf.de).
In many applications, an electronic system must be connected to the outside world for energy supply or data transmission purposes.
This is often done using connectors. In general terms, a connector is a high-tech element used for transferring signals, light, energy or other media.
The application described here provides two contacting options. At the front side, double-sided spring pins touch the gold-plated MID traces. The spring pins are guided by an additional enclosure.
A high-density pitch is achievable due to the interlaced placement of the traces on the front and back sides of the MID wafer.
Multiple wafers are stringed together. The other end carries galvanised cylindrical pins that can be soldered to a printed circuit board in a vapour phase or lead-free reflow process.
Several requirements must be met to make the MID technology suitable for connector manufacturing.
Apart from mechanical stability, these requirements include the surface conditions of the contact pads which largely determine the number of insertion cycles supported by the product.
In order to obtain good results in this discipline, the layer composition and the materials used in the galvanising steps must be defined very carefully.
Electroplating should also be considered if the trace geometries tolerate this.
Using this technology, it will be possible to produce much thicker layers in less time, resulting in less conduction resistance and higher current capability.
As the technology is restricted to plastic parts, common connection technologies including crimping, welding and insulation-displacement solutions cannot be used with MID components.
Instead, alternative methods including soldering, conductive glues or spring contacts must be used.
Bonding technology is also being explored as a possible option to better exploit the advantages of integrating electronics and mechanics.
This, however, will result in special requirements regarding the surface roughness of the contact pads and the thermal expansion characteristics of the carrier materials.
Further size reductions could be achieved by directly inserting the die into the sensor package and connecting it to the relevant circuitry using wire-bond technology. In addition, a connection to the outside world will be required here as well.
New processing technologies, high-performance plastics and short processing chains have made MID technologies a profitable option for volume manufacturing. More and more companies are considering MID in their feasibility studies. MID offers the potential to overcome the restrictions of conventional techniques and to open new applications.
Exploring new paths, however, requires courage and the willingness to invest in new technologies.
