PCB Tech - PCB process Precautions for using metal reeds as rubber keys

PCB process Precautions for using metal reeds as rubber keys

Earlier we introduced that most of the mobile phone keys use metal dome as the advantages and disadvantages of its key contact switch, also because most of the metal dome will be designed with a dimple in the middle. Contact point, so at the beginning our design unit was worried that all the force of the button would be added to this point. After a long time pressing, it would damage the gold finger of the PCB and cause poor contact. Therefore, our design unit decided The dimple was removed, and the reliability and life test proved that it can withstand pressing at least 1,000,000 times, but the good times are not very common. After the dimple is removed, the production line has successively reported that the buttons are not sensitive. The problem.

The so-called keypad insensitivity refers to the function that the key cannot be activated when the key is lightly pressed until there is a "click" sensation, and more effort is required to press it down to drive the key function. At the beginning, our design unit pointed the problem at the surface oxidation of PCB. We used ENIG (nickel immersion gold) board. If this board is not properly controlled in the manufacturing process of the PCB board factory, P (phosphorus) The density of black pad often occurs.

After the board below has been subjected to high and low temperature and humidity environmental cycle tests, some black spots appeared. After trying to wipe these spots with absolute alcohol, the problem of insensitive buttons disappeared, but after repeating the temperature and humidity cycle, The problem of insensitivity reappeared again and again, and this problem has always puzzled the design unit.

Later, we discovered that the problem of insensitive buttons is not just a possible "black pad" problem. As long as a little bit of dust or lint just falls on the position of the button, it will affect the function of the button. This can also be explained. Why there is no response when the button is pressed lightly, and the button must be pressed hard to act, because as long as there is any non-conductive dirt on the circuit of the button, it may block the contact with the PCB after the metal reed is pressed down. Apply more strength to squash the dirt or allow other parts of the metal reed to touch the circuit board to actuate the button. The generation of the black pad and the insensitivity of the buttons can also be regarded as the non-conductive material deposited on the PCB and the surface of the circuit board, which blocks the contact between the metal reed and the circuit board. As long as these non-conductive materials are eliminated, it can be used as Press the button.

Since these internal black pads and external dust and lint could not be effectively controlled, we began to think about how to connect the metal reed and the circuit board with these dirty things. Smart, you should have already thought of it. That's right, add back the dimples originally on the metal reeds. The dimples can effectively crush or penetrate these tiny dirt and achieve the purpose of contacting the circuit board. . But as a result, the effective life that we were worried about at the beginning would be shortened, so we started looking for metal reeds with multiple bumps. Sure enough, there are really 3, 4, and 5 bumps on the market. In mass production, others have already had the same problem, so these multi-bump metal reeds have been developed.

In fact, we tested all the multi-bump metal reeds, and later we chose a metal reed with 3 bumps, because this design can already meet our 1,000,000 finger pressing requirements, but the pressing force will change Big, but I personally feel that it feels better.

After 1,000,000 finger press life tests, only a small part of the button’s gold layer has a slight depression. Observation under the microscope also does not see the gold layer fracture, which shows that the ductility of gold is really good. .(PCB factory)