Is My Mechanical Stack Up Important For Illuminated HMI And Display

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The Problem

A Human to Machine Interface may seem simple. This is because it is designed to be user friendly and easy to understand as a touch interface or display indicator. Underneath the front graphic/facia lies unique and important layers that all contribute form and functionality towards the overall mechanical assembly. So why are these layers important and what kind of role does this play in these

three areas: Look & Function, Lighting, and Assembly & Quality?

Issues from poor design of mechanical stack-up:

-Cap touch inconsistent

-Lighting – low uniformity & low intensity

-Witness lines and delamination

-Mechanical misalignment and low-quality manufacturability

The Solution

The following overview will show solutions for each problem point and the relationship between each layer regarding Look & Function, Lighting, and Assembly & Quality. The balance of these three attributes play important roles in each mechanical layer. (See diagram)


The only visible layer that’s responsible for the direct connection from human to machine is the graphic/facia. To achieve your desired look & function, it is important to take into consideration lighting solutions early on. If your lighting solutions are an afterthought, this will increase the risk of low light intensity, low light uniformity (dark or bright spots) and ultimately an undesirable look and/or underperforming functionality.

For a mechanical assembly to have high qualitylow cost and long-term reliability, it is important to not have components from other layers protruding into your graphic. This will result in witness lines and delamination of the graphic layer leading to a low-quality product. Protruding components can be prevented by establishing hard stop mechanical tolerances. Establishing mechanical tolerances and mechanical measurement hard stops ensures long term reliability and high quality.

Keep in mind the application of your HMI/Display before choosing a graphic/facia material. For example, using a dead-front facia for an outdoor machine interface may not be a wise choice as dead front interface will greatly reduce illumination transmittance and make it difficult to view in direct sunlight.

Whether using membrane, dome or capacitive touch technology, another key aspect for the graphic/facia is the viewing angle and light transmission. To achieve proper light uniformity, intensity, and consistency (as noted in the diagram) it is important to use a front graphic with the proper diffusion properties to accommodate light transmission requirements.  The application of this front “A” surface layer specific to the integration with the underlying light panel should not be overlooked in the early design stage to optimize the desired illumination results.

Light Panel

The second key layer for the mechanical stack up is the light panel sometimes referred to as the light guide or light pipe. This is the thin layer in which light enters the system horizontally to precisely deliver the light to more efficiently achieve a desired illumination using custom optical engineering. In an ideal design process, the light panel would be taken into consideration early on, because the light intensity and uniformity is strongly dependent on the light panel design as optical features interfaces with the other layers.

In order to keep the designed-in look & function through the manufacturing process it is important to handle the light panel with PPE, preventing fingerprints, debris and dirt. This level of quality & assembly ensures the light intensitylight uniformity is not tampered with from unwanted blockages.


In order to illuminate the display, there needs to supporting circuitry to power the side fire LED(s) allowing light to enter the light panel system horizontally. The LED location on the PCB/light panel directly affect the light intensitylight uniformity and desired look. Once the LED(s) location is determined through design, fine tune adjustments are applied using optical engineering and tooling.

Also note, there are many options for PCBs depending on what is needed, including ultra-thin flex PCBs, rigid PCBs, etc.

The solder mask of the PCB is important for lightingquality and desired look & function. Normally for a display application the solder mask is white. This allows light that escapes the system to reflect directly towards the user to optimize light intensity.

Bonding the Layers

Bonding between layers is one of the most important aspect of long-lasting quality. Many low-cost solutions will include options such as heat stakes and adhesive. The quantity volume of production and manufacturability is important to considering when choosing the means to bond the mechanical assembly together. In an ideal manufacturing process, adhesive sheets would be applied using a vacuum table with registration edges to accommodate for many mechanical assemblies at once.

Mechanical tolerance and design of the mechanical assembly is important. Eliminating variability is key. The cap touch can be tuned to overcome the various layers and even air-gaps provided the layering design and tolerancing is consistent part to part.  With this in mind, for example, it is important to the note adhesive applied to the B-side of the light panel will extract light away from the system and reduce desired light intensity and uniformity if located directly underneath the light extraction (illuminated graphic) zones or within the LED light entrance region. Some designers have expressed concerns that the resulting airgap in these areas will negative affect the capacitive performance, but that will not be the case when properly accounted.  This type of design when integrated with a light panel has been successful executed on millions for production HMI products.

Miscellaneous Layers

Some unique design scenarios require creative solutions to enhance the desired look & function. An example may be adding in capability for the system to illuminate graphic regions independently of one another.  This dynamic functionality can be made possible while still maintaining uniform aesthetics with the option of imbedded, light-blocking gasketing layer.

The Conclusion

It is important to take all layers into consideration when initiating a design for HMI/display. They play important roles and the relationship between them are important in regard to achieving Look & Function, Lighting, and Assembly & Quality objectives.