Design for Reliability and Best Practices – The Path to a Robust HMI

The Philosophy of Design for Reliability

Design for Reliability is more than a checklist—it is a mindset. It begins with the realization that reliability cannot be introduced into a product through inspection or testing but must be built in during the design phase. DfR is a proactive, data-driven process that identifies potential failure points before the first product is manufactured and systematically takes measures to eliminate or mitigate them.

The DfR process begins with the definition of measurable reliability goals. These are derived from customer requirements, regulatory guidelines, and business objectives. For example, a heavy-duty HMI for use in construction machinery might require an MTTF of at least 100,000 operating hours, a survival rate of 95% after ten years, and a failure rate of less than 1% in the first two years. These goals are non-negotiable — they define success.

During the concept phase, an FMEA is conducted to identify and prioritize potential failure modes. Simulations—thermal, mechanical, and optical—help predict the system’s behavior under various conditions. In the detailed design phase, materials are selected based on environmental conditions and lifespan requirements. Prototyping and testing validate the design, and the results inform design iterations. The process does not end after production approval — field monitoring and a continuous feedback loop ensures that each generation becomes more reliable than the previous one.

DfR is not a linear process, but a cycle of continuous improvement based on data, analysis, and systematic learning.

Edge Sealing: The Critical Weak Point

Edge sealing is the most underestimated aspect of a bonded HMI—and at the same time one of the most critical. It is the primary point of entry for moisture and thus the first barrier against delamination caused by hydrolysis.

Effective edge sealing must meet several requirements. It must form a moisture barrier that prevents water vapor from penetrating for years. It must be flexible enough to withstand thermal expansion and contraction without cracking. And it must be chemically compatible with the optical adhesive to prevent degradation.

Silicone-based sealants are a popular choice due to their high flexibility and good adhesion to glass and metal. Polyurethane sealants offer higher mechanical strength but are less flexible. Epoxy sealants form an excellent barrier but are brittle and can fail under thermal cycling. The choice depends on specific requirements, but for heavy-duty applications with large temperature cycles, silicone is often the best option.

The geometry of the edge seal is just as important as the material. The seal must extend at least beyond the edge of the optical adhesive to ensure complete coverage. The thickness is selected according to the geometric requirements and the bonding layer thickness to achieve a sufficient barrier effect. Rounded edges prevent stress concentrations that could lead to cracks. The application must be complete and uniform — a single gap of just a few micrometers can lead to moisture ingress and delamination over the years.

Edge sealing is not an optional feature, but an absolute necessity for any bonded HMI intended to survive in the field for years.

Mechanical Design: Shock and Vibration Resistance

Heavy-duty HMIs must withstand mechanical stresses that would destroy consumer devices in seconds. Mechanical design is therefore just as important as optical performance.

Optical bonding itself is already a major advantage for shock resistance. Full-surface bonding distributes the energy of an impact across the entire surface of the display, rather than concentrating it at a few mounting points. This reduces the peak stress on the glass and increases break resistance by a factor of 2 to 3. Sufficient cover glass thickness is still required—glass that is too thin will break despite bonding.

The housing design plays a crucial role. A frame surrounding the display can absorb impact energy before it reaches the glass. Rubber buffers or elastic seals between the display and the housing dampen vibrations and prevent high-frequency oscillations from being transmitted directly to the display. The mounting must be secure to avoid resonances that can lead to fatigue fractures.

The elastic adhesive in the optically bonded display also acts as a vibration damper. It absorbs high-frequency vibrations that could otherwise lead to microcracks or delamination. This is another, often overlooked advantage of optical bonding.

Design Checklist: From Requirements to Delivery

A systematic design process requires a structured checklist to ensure that no critical aspect is overlooked. This checklist should cover the following areas:

Environmental Analysis: Definition of the temperature range, evaluation of humidity conditions, quantification of UV exposure, specification of mechanical stresses, and identification of chemical exposures.

Material Selection: Selection of the adhesive based on the environment, specification of the cover glass, qualification of the display module, and selection of the touch technology.

Process Planning: Definition of cleanroom conditions, specification of surface preparation, determination of lamination parameters, design of edge sealing, and planning of quality control.

Validation: Conducting and documenting the FMEA, defining reliability tests, establishing acceptance criteria, testing prototypes, and optimizing the design based on the results.

Production and Field: Qualification of the production process, implementation of HASS screening, planning of field monitoring, establishment of a feedback mechanism, and institutionalization of continuous improvement.