The nominal allowable maximum temperature of the film panel is 80°C for 500h. In experiments and actual use, when the ambient temperature is above 35°C, there are many cases of partial bulging of the film panel, and most of them are at or near the key position. , resulting in the failure of the key switch, affecting the use.
Since the temperature at which the bulging phenomenon occurs is still far from the nominal temperature of the film panel, we have carried out a careful and detailed analysis on it. At first, it was thought that since the membrane panel has a tactile feel and there are reeds in the keys, when the temperature rises, the expansion coefficients of the metal material and the membrane panel material are inconsistent, which leads to the bulge phenomenon, so the panel membrane switch with no tactile touch is used instead. , When the temperature rises to a certain value, there is still a bulging phenomenon, and the temperature when the bulging does not reach the nominal temperature of the panel, so the idea of the bulging phenomenon caused by the existence of the reed is ruled out.
Since the box body of the device is sealed, it is suspected whether the internal pressure of the box body rises due to the temperature rise, and the internal pressure directly acts on the film panel through the outlet holes and indicator holes on the box body, causing the panel PC layer to peel off, resulting in Drumming phenomenon. Therefore, the sealing ring of the box body was removed, its sealing state was changed (it became a non-sealing type), and a temperature test was carried out. It was found that there was still a bulging phenomenon. Therefore, it can be determined that the root cause of the bulging phenomenon on the panel is not the box itself but sealing problem.
Through careful analysis of the bulging position on the panel, the film panel factory found that the bulging area is mainly concentrated in the area where the buttons are concentrated, so it is suspected that the root of the bulging on the panel is the button itself. Because the positions of the keys are relatively concentrated and the spacing is small (3mm), that is, the width between the keys for bonding is only 3mm, and the bonding area is relatively small. If the adhesive layer is not uniformly coated, the bonding area will be smaller, or even 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 caused by the temperature rise inside the box , which makes the film panel bulge due to the small bonding area and small bonding force in the outward deformation of the key part. In addition, because the key part of the panel is subjected to convex treatment, the panel layer has internal stress, which will play a role in the expansion of the panel. The role of adding fuel to the flames, thereby increasing the degree of deformation.
Based on the above knowledge, the improved design of the film panel is made:
1. Reduce the button size from 13mm×13mm to 11mm×11mm;
2. The distance between the membrane key switches is increased from 4mm to 6mm, and the bonding width between the keys is increased by 4mm;
3. The keys are changed from tactile type to no tactile type;
4. There is a gap of 0.2mm ~ 0.5mm around the bonding surface of the membrane switch panel and the device.
After the above improvement measures are implemented, the bonding area between the keys is increased, the bonding force of the film panel is improved, and the deformation caused by the thermal expansion and contraction of the material, the internal pressure and the internal stress of the material can be offset enough. The improved film panel has no bulging phenomenon after many high temperature experiments, which proves the correctness of the improvement measures.
