PCB News - Several possible reasons for PCB problem Key stuck

Several possible reasons for PCB problem Key stuck

The keyboard button [key] means that it cannot rebound after the button is pressed (the button is stuck), or it will not rebound to the normal position after a period of time. We sometimes call it [lazy key] ] Or call it [Key Stuck]. In English, most of us collectively refer to such bad keys as [Key Stuck] (English is being taught again!). Today I’m going to talk about the topic of key card keys. I mentioned it a little when I introduced the keys, but it’s not very complete. It just happened that the company had such problems recently, so let’s sort out this topic!

First think for yourself what is the basic reason for [Key Stuck]?

Experience tells us that more than 95% of the defects are caused by "the rebound force after the button is pressed is less than the friction between the button and the plastic hole wall". But my design will not let the button touch the hole wall of the plastic shell (the new cry in RD)? Yes, but there are tolerances in production. The button is not necessarily vertical when it is pressed. If the button is tilted when it is pressed, it may rub against the hole wall of the plastic shell.

On the basis of friction, many possible causes have developed.

1. The rebound force (F3) of the button is too small

This rebound force is directly related to the design of the button. Generally speaking, if F1 is larger, F3 will follow. Strictly speaking, F3 actually follows F2, but F2 is based on F1.

When the rebound force of the button is too small, the pressed button cannot be effectively lifted, and the chance of [Key Stuck] will greatly increase. In addition, most of the buttons are now made of rubber. If the design values are okay, I would suggest you to check the hardness of the rubber and the web (slanted wall) thickness of the buttons. These two values will directly affect the strength of the buttons.

2. The gap between the button and the plastic aperture is too small (it is recommended to leave more than 0.25mm on each side)

After working for so long, I often find that some institutional engineers naively think that as long as he draws out the pictures, the circuit boardmanufacturers must do it; in addition, I have also seen some ID (industrial design) engineers who insist on perfection and insist on pressing buttons and The gap between the plastic hole wall should be as small as possible, so a gap design of 0.1mm or less appeared.

In fact, if all the dimensions and tolerances can be controlled, the 0.1mm gap can indeed achieve mass production, but if you choose the Rubber Key, the general factory standard tolerance is +/-0.3mm. A better factory or a manufacturer who is willing to help you do the screening can achieve +/-0.15mm, which is great. At this time, the 0.1mm gap design should kill a lot of people, and the larger the amount, the more miserable it will be. Moreover, the wool comes out of the sheep, and the cost of the manufacturer's screening will be reflected in the price of the parts.

The overall size of the rubber button is easily enlarged or reduced due to the unstable control of the amount of the rubber material put in.

3. When the button is pressed to the end, it is lower than the surface of the case (it is recommended to exceed the surface of the case by more than 0.6mm)

I don’t know if you have ever pressed some buttons that are gradually lower than the surface of the case. What do you feel when you press it? You may not feel anything when you press it once or twice. But if you need to press a button frequently, your finger should be It hurts! Because the finger directly touched the case.

Moreover, if the button is lower than the surface of the case, it is more likely to cause [Key Stuck] phenomenon. Imagine that when you are sitting on a chair, there is someone next to you who presses your shoulders and presses your head. Assuming that this person uses the same force, Which is easier for you to stand up. If you have no experience, you can get experience by experimenting with friends around you.

Based on the above two reasons, the discomfort of the user and the risk of [Key Stuck], it is a better choice to design the button to be slightly higher than the surface of the case after the button is pressed to the end. Of course, if the button is big enough, it won't let your fingers touch the case.

4. The parting line of the plastic hole has too large burrs or is located at the outermost edge of the case

Generally speaking, if you want the key to be pressed smoothly, the plastic key hole wall should be as smooth as possible, so as to reduce the friction between the key and the coin. Unfortunately, there are so-called male and female molds when the plastic is injected. There will be parting line flash (flash, called "flash" in mainland China) where the mold is clamped, and it will appear on the inner side of the plastic hole. This parting line will become thicker as the number of shots increases, that is, The burrs will become larger, so the friction will become larger, and [Key Stuck] will happen.

In addition, if this parting line falls on the outermost edge of the case, it will form a hand pressure head. Therefore, it is generally required that the parting line be as close to the inside of the case as possible, but because of the requirements of plastic demolding, It is generally impossible to make the parting line too below. At this time, it becomes more important to give a certain gap between the button and the housing aperture. (Explained by circuit board manufacturer)