If you think of the evolution of electronic devices, you will most likely envisage ever-increasing functionality along with a continuous reduction in size. This functionality can be attributed to certain advancements in technology, which include but are not limited to, advancements in materials science, sensor technology, and CPU processing power.

A device can only be functional if it can effectively operate within a range of different environments. These environments have become increasingly similar to the environments that their human user inhabits. Wearables, particularly medical device wearables, allow for the monitoring of the advanced ecosystem that is the human body. Such devices have the capability of monitoring vital signs including blood pressure, measuring blood glucose levels, or helping in the diagnosis of conditions such as sleep apnea.

 

Environment & Reliability:

Living on the skin of the user exposes the device to a surprising number of harsh environmental conditions. These conditions if not accounted for in the development stage will result in a loss of critical functionality. An average user may shower or bathe once a day. Some will engage in physical activity several times a week. Many will walk up continuous flights of stairs or run for a bus. Users quite often apply lotions, fragrances, or cosmetics to their skin. Perfume and sunscreen are examples.

Resistance to these chemicals can be challenging. Hazardous chemicals can often cause damage to the bond. However, the most common environmental hazard that a device can endure is excess moisture.

Bond Line Exposure:

Moisture can pose different types of hazards to such devices. Immersion in water can apply excess pressure on the adhesive bond line, especially at an above ambient temperature. This is where thermal stresses are induced onto the assembly. If the materials in combination do not have adequate adhesive strength, adhesive failure may propagate along this bond line. This can also be viewed as the path of least resistance.

Such risks regarding areas of possible stress concentration can be mitigated by ensuring the adhesive and material combination will have adequate bond strength, usually quantified by OLS (Overlap Shear) testing along with design considerations for an adequate bond line thickness. The repeatability of this thickness can be assured with correctly calibrated automated dispensing equipment.

Moisture Vapor Transmission:

With adequate development and testing to validate such assumptions an engineer can conclude that their bond line will be impenetrable to such hydrostatic pressure with very low failure rates. However, moisture does not only pose a threat to the functionality of the device when in fluid form. Users can live in a variety of climates. For example, relative humidity in parts of southeast Asia is often in excess of 80%. When exercising, a user will often sweat. Because these devices can be very close or even fixed to the skin, it is vulnerable to such perspiration. Water in its gaseous form, known as vapor, is another threat to the reliability of the device.

If we turn to the example of a user exercising, we can practically relate to the principle of Moisture Vapor Transmission. Modern sportswear is frequently advertised as “breathable”. There has been a divergence from more traditional fabrics such as cotton which tends to absorb moisture to materials such as polyester which is water repellent. This allows the user to perspire without the inconvenience of their clothes becoming soaked in sweat.

Comparing cotton with polyester, we can say that the breathable polyester has a high Moisture Vapor Transmission Rate, and the absorbable cotton has a low Moisture Vapor Transmission Rate. This metric can be described as the amount of moisture that will pass through a material in a given period. It is usually expressed by the following equation:

                                                                   MVTR = g/m^2/day

The following schematic shows how a basic Moisture Vapor Transmission Rate test is carried out. A known geometry of material is positioned as a divider between two chambers. One chamber contains a known temperature and percentage of relative humidity. As the moisture vapor is transmitted through the material, it is measured on the opposite end.

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                                         This schematic is representative of ASTME 96/E96M-16.

Material Selection & Risk:

When an electronic device is assembled, it will have a minimum of two materials exposed to the external environment. For example, a phone or wearable device could contain an outer casing of two components manufactured from a polymer and metal. In this case, an adhesive would be used to bond these dissimilar materials. This outer assembly’s primary function is to safeguard the complex and sensitive components housed inside.

Metals & Plastics:

When integrating metal components into a housing, moisture vapor will not be an issue. Oxides may form on the surface of these materials but the process of surface treatment such as the anodizing of aluminium prevents this. Phones or high-value wearables such as watches are normally manufactured from such aluminium or stainless-steel housings.

The construction of medical device wearables differs. Such devices are often only designed for short-term usage that may be limited to several days or weeks. The short lifespan of these devices is a massive driver when it comes to the selection of materials. Materials such as Polycarbonate or Nylon are often chosen due to their robustness. They will also require chemical resistance and mechanical strength. However, these materials are very cost-effective which ensure manufacturers maintain a low unit cost. Polymers, unlike metals, absorb moisture. MVTR must be considered when specifying these materials. However, the greatest material risk involves the adhesive selection.

Oxidation & Corrosion:

Oxidation of metals causes a surface change on the material. The level of change will depend on the type of metal present. Electronic assemblies often include Printed Circuit Boards, traditional PCBs are manufactured using an array of metal components such as aluminium, copper, and plated nickel. Oxidation can in turn cause the corrosion of their surface. This type of oxidation will propagate through the surface of the component and deteriorate the mechanical properties. Fortunately, metals used on PCBs are not likely to suffer this type of damage.

However, when oxidation is present, highly conductive materials used in these assemblies, such as copper, can develop a lower retention of its electrical conductivity. This is a major issue for PCBs, the reliability and in turn the functionality of the whole device.

Often electronic components are plated to ensure corrosion resistance, particularly in connector type applications. Noble metals such as gold and silver provide excellent corrosion resistance. This process, as robust as the results may be, is very costly to a manufacturer and cannot be implemented on board level.

There are many different forms of corrosion; galvanic corrosion or bimetallic corrosion can occur when dissimilar materials are in electrical contact and are exposed to a corrosive electrolyte. Water, although one of the weaker substances, contains electrolytes.

The most common type of oxidation is atmospheric corrosion. Atmospheric corrosion occurs when moisture from the air meets metallic components. The moisture vapor condenses onto the component which in turns forms the oxidative layer. This is one of the most challenging aspects of developing a reliable electronic device.

Polymers as a material are inherently permeable. If they are to be utilized in the design of electronic devices, there must be an acceptance that some moisture will pass through the protective casing onto the electronic components. As electronic assemblies become more complex in their design and functionality, researchers may discover other ways that moisture can damage these assemblies apart from the more well-known types of corrosion.

MVTR:

Engineers will not stop in their efforts to continuously try and reduce the risk of board level failures. To ensure the greatest possible reliability of the device, engineers must ensure that their material selection has the best possible barrier properties. The Moisture Vapor Transmission Rate or MVTR is widely regarded as the best measure of a materials ability to resist the absorption and transmission of moisture through its wall thickness.

Adhesives are widely regarded as the weak point of an electronic housing. Whether applied to adhere the components of a housing or utilized to encapsulate an assembly, a low MTVR is critical to safeguard against the permeation of the adhesive bond line or encapsulation area.

Adhesives are generally differentiated from sealants by their ability to offer a high level of structural adhesion while sealants are characterized by their ability to seal against such hazards as chemicals and moisture.

A Novel Solution:

Krylex’s KH9005 is a Polyurethane Reactive (PUR) adhesive that offers the lowest possible MVTR while ensuring good chemical resistance and excellent adhesion to common electronic device materials. KH9005 is unique as it offers the performance of a structural bonder alongside the properties of a robust sealant.

Krylex’s KH9005 also offers the following features that are important in the areas of electronic device and wearables:

• Excellent biocompatibility, ISO-10993-5 certification

• Excellent impact performance

• Low, stable dielectric constant

• Excellent green strength/ fixture time for high volume production