In experiments and daily life, when the ambient temperature is above 35 °C, some graphic overlays have local bulges. Most of symptoms are at or near the keys, causing the components to malfunction.

Since the temperature at which the bulging occurs is still far away from the nominal temperature of graphic overlay, we have conducted a detailed analysis. At first, I thought that the membrane panel has a feel type, and there are reeds in the buttons. When the temperature rises, the expansion coefficients of the metal material and the membrane panel material are inconsistent, resulting in the bulging.

As a result, I switched to a non-feeling type graphic overlay. When the temperature rises to a certain value, there is still a bulging phenomenon, and the temperature during the bulging has not reached the nominal temperature, so the idea that the bulge is caused by the presence of the reed is eliminated.

The box body of the equipment is sealed, so it is suspected that the internal pressure of the box body rises due to the increase in temperature. The internal pressure directly acts on the graphic overlay through the outlet hole and indicator light hole on the box body, causing the top layer to peel off, resulting in bulges. So we remove the sealing ring of the box. The sealing state was changed (to a non-sealed type).

And we arranged a temperature test. It was found that there was still the bulging issue. Therefore, it was determined that the root cause of the problem on the panel was not the box itself, but the sealing.

We analyzed the location of the bulges on the panel, found that those are mainly concentrated in areas where the keys are concentrated, so we suspected that the root cause of the bulges on the panel lies in the keys themselves. Because the buttons are relatively concentrated, and the spacing is small (3mm), that is, the width between the buttons for bonding is only 3mm, and the bonding area is small. If the adhesive layer is unevenly applied, the bonding area will be smaller. In extreme conditions, there will be no bonding surface.

Then, when the temperature rises, due to the thermal expansion and contraction of the material itself and the internal pressure inside the box caused by the temperature rise, the graphic overlay deforms outward at the key area due to the small bonding area and low bonding force.

In addition, since the key areas are embossed, there is internal stress in the graphic overlay, which contributes to the expansion of the film panel, thus increasing the degree of deformation.

With all these conclusions, the graphic overlay was improved in design:

1. Reduce the button size from 13mm×13mm to 11mm×11mm;

2. The spacing between the buttons is increased from 4mm to 6mm, and the bonding width between the buttons is increased by 4mm;

3. The buttons are changed from a touch-sensitive type to a non-touch type;

4. The panel and equipment leave a gap of 0.2mm - 0.5mm around the bonding surface. After implementing the above improvement measures, the bonding area between the keys has been increased, and the bonding force of the graphic overlay has been improved, which is enough to offset the deformation caused by the thermal expansion and contraction of the material, internal pressure, and the internal stress of the material.

The improved panel did not bulge after many high-temperature experiments, which proved the correctness of the improvement measures.