MANUFACTURING
3M, Bedford, UK
Multifunctional adhesives
A primary technology trend in adhesives for medical electronics is the evolution of multifunctional adhesives that perform more than one task. For example, as medical device parts get smaller, adhesives must provide adhesion and durability on ever‑shrinking surfaces areas. They must also be able to join materials that have different thermal expansion rates that put additional stress on bonded joints. Ultrasound probes are one example where different materials are bonded in a challenging structure and must survive arduous conditions. Adhesives that are ideal for these conditions include epoxy resins that have high adhesion and durability, combined with low out-gassing.
The rapid growth of networking and wireless communications is another area where adhesives perform multiple functions. Ambient electromagnetic interference (EMI) can be high in medical environments and electrically conductive adhesives, including transfer tapes and EMI shielding sheets and films, enable precise and secure positioning of shielding materials while protecting against EMI leakage. These adhesives can also be used to ground electronic devices. Examples of these products include adhesive-backed polyurethane sheets filled with metal flakes for EMI absorption; or copper, aluminium or metallic foil with a conductive or nonconductive acrylic adhesive for shielding.
Conductivity
Another area of interest is adhesives that are actively thermally conductive, particularly tapes. Materials used for these kinds of adhesives can include thermal interface components consisting of ceramic or metal particles suspended in an organic polymer. The degree of thermal conductivity increases with the density of the filler. When used to bond heat-producing devices to heat-dissipating structures, thermally conductive adhesives maximise the transfer of heat away from the active components. This type of adhesive has been used in light emitting diodes to transfer heat away from the diode and light emission, thus preventing over-heating.
Anisotropic conductive films provide an electrical connection and a secure bond between circuits. These heat-bondable conductive films consist of thermoplastic and thermoset adhesives that are randomly loaded with conductive particles. The particles allow interconnection of circuit lines through the adhesive thickness, but are spaced far enough apart for the product to be electrically insulated along the plane of the adhesive. These films are ideal for permanently connecting flex circuits in a confined space.
Sealing
Adhesives are being used to seal as well as bond to make products more streamlined. This is of growing importance with regard to disinfection and decontamination once medical devices are in use. Many devices that were initially designed to be simply wiped over with a disinfecting cloth have had to be redesigned to withstand being soaked with disinfectant cleansers. For example, traditional screws and catches are not watertight, unlike many adhesives.
It is essential to identify the points of moisture ingress on medical electronic devices. By understanding the range of potential solutions and the trade-offs for each point of ingress, the right solution can be identified. One example of a common point of ingress is around the perimeter of a two-part casing. To solve this, an elastomer form-in-place gasket (FIPG) can be used if the part needs to be easily re-enterable. This kind of gasket provides a watertight seal and is created by dispensing a liquid FIPG adhesive onto one side of the casing; when it cures, it bonds to the plastic and is flexible yet solid. Thus, when the two parts of the casing are pressed together, it is a compressive yet re-enterable seal that also provides vibration damping and can include conductive particles for EMI shielding.
Alternatively, a bonding film may be preferred to seal a casing if the device needs to be re-enterable, but this requires a high level of skill and knowledge to re-heat the adhesive. For permanent sealing, a conformable closed-cell foam tape with a high bond adhesive that is environmentally stable and moisture resistant is ideal. When deciding which adhesive product to use, the product designer has to balance the relative importance of the criteria, some of which may include re-enterability, durability, ease of application and flexibility to seal around odd-shaped parts.
Water contact indicating tapes are another sealing aid. These tapes do not prevent the ingress of moisture, but do indicate exposure to moisture. These tapes can help to indicate or expose over-aggressive cleaning techniques or immersion in fluid, and can also help the service technician to diagnose the point of entry of water getting into a device.
Increasing adhesive performance
It is also important to note that more effective cleaning of parts prior to bonding will typically mean better performance from the adhesive. By removing undesirable materials that inhibit bonding, precision cleaning of a plastic or metal surface can increase adhesion and the effective area of the bond. This would involve adding a cleaning step to the manufacturing process such as immersion in a fluid bath or using an aerosol spray. Modern cleaning products are often more effective than traditional water or alcohol washes, but the solvents used must comply with health and safety requirements, which will vary from country to country. For example, in Europe hydrochlorofluorocarbons, hydrofluorocarbons, n-propyl bromide and hazardous air pollutants are subject to increasing legislation, which in many cases prohibits their use and is the reason why companies are turning to hydrofluoroether solvents.
Clearly, today’s adhesives can do much more that just bond and there are often multiple adhesive types to consider to overcome any single challenge. To find the best solution for a particular application, it is advisable to collaborate with adhesive manufacturers through bench-to-bench engagement. By understanding the design constraints and goals of the product designer, the adhesive manufacturer can help product designers identify creative solutions that address their challenges.
Phil Jones is a Business Development Manager for 3M United Kingdom plc, tel. +44 1234 229 463, Hudson Road, Bedford MK41 0HR, UK,e-mail: pjones2@mmm.com, www.3m.com/uk/electronics.
