Successful development of an IME product requires a steep learning curve. This is despite the fact that the individual processes used in IME are fairly standard. Indeed, a somewhat similar process, IMD or in-mold decoration, is already commercial. We seek to highlight challenges by contrasting IME with IMD in parts of this article.
The chart below shows the process steps involved in an IME production. In this article, we consider the differences between a conventional IMD and an IME process to highlight development challenges.
Printing on flat sheets: the material set is different between IME and IMD. In a conventional IMD, graphic inks are printed and the color quality is carefully controlled. In IME, functional inks, or stacks of functional materials, will need to be printed. This will require learning as the printing and drying conditions are different. The procedures might also be different depending on the specific set of materials used in the stack and the order in which the materials are arranged in the stack. The curing conditions, especially drying times and rates, are also different and require process as well as equipment adjustment. The printing will also require careful control of thickness so that the designed electrical properties are retained after the forming step. This step of deposition and curing functional materials on a flat sheet can reduce yield, e.g., 2% loss or so.
Pick-and-place: in IME, electronic devices will need to be placed. This will require a pick-and-place machine. It will also likely require the use of special dispensed conductive adhesives for surface mounting. In most cases, the SMDs are placed whilst the film is flat. This makes the pick-and-place easier. It will however require the development of design guidelines for part placement so that the subsequent 3D forming steps do not cause damage. Some have proposed doing the pick-and-place on an already-3D-shaped film. This will require special dispensing and pick-and-place machines able to manoeuvre in 3D spaces and will slow down the throughput. As such, this approach is not very attractive. The dispensed adhesives will also need to be cured without affecting the properties of previous layers.
Today there is some work to develop ultra-low-temperature solders compatible even with heat-stabilized PET substrates. The benefit of solder over conductive adhesives would be that (a) it would simplify part placement control due to their intrinsic orientational correction characteristics, and (b) it would provide a roadmap for supporting more complex ICs with higher I/O counts. For now, however, conductive adhesives dominate. Finally, note that the IC package is important. Since the conductive lines are screen printed the linewidths will be wide. As such, only packages with wide pin pads and pin spacings will be compatible with the process. This already points towards a further future evolution towards narrow metallization techniques. That would, however, require the development of appropriate ink sets compatible with IME processes and narrow linewidth printing techniques.
Trimming/cutting: the next step is to trim the film. Here, some die-cut process using part-specific tooling is carried out. One then proceeds to the connector assembly step. The connectors provide the connection to the outside world. The connectors are mounted, probably using an anisotropic conductive adhesive or film. This will require additional tools for applying the appropriate heat and pressure profile.
Injection molding: the film will be vacuum or mechanically held in place to allow injection-molding to take place. Depending on the application, the injection volume can be very large. The process also requires optimization, e.g., no air bubbles. This step takes place at a high temperature. As such, the printed circuitry and the formed material stacks will need to retain functionality after experiencing
high temperatures and after coming into contact with the molding resin chemicals. Note that the temperature will depend on the resin. For example, PC, PMMA, and TPU will require 260-340c, 240-280C, and 190-210C, respectively.
The molding process can also impact the commercial electronic packages used on the formed film. The high pressure (100MPa or so) can compress any gas trapped inside the package, thus deforming the package. The shear forces in the molding process can cause the component to detach. This risk is most acute when the polymer flow front arrives at the component site during the injection molding process. The change in temperature can also create stress via CTE mismatches, potentially deforming or detaching the SMEs. The selection, or development of, suitable packages can help here. Note that some firms are already responding to this market need.
Note that the products must remain functional in the field after deployment. This is particularly important given the industry’s past experiences and given the irrepairability of the structurally-embedded electronics. The reliability is closely linked to how the materials change their property as they experience heat, humidity, etc. It is also closely linked to the design and the material stack, and how that impacts and is impacted by moisture ingress.
In this article we mainly discussed the technical challenges, showing that, for example, an IMD maker can not overnight become an IME maker, and that it requires learning and significant adaptations. The process can be accelerated with the aid of strong technology and knowhow transfer. It should be noted that there are also many market challenges. We however postpone the discussion of these to another article.
