Apr23
How many dimple need to be used for keypad metal dome?

metal_dome_5_dimplesLast article I mentioned that my company applied the metal dome without dimple as keypad contact solution just like the cell phone but we had trouble and met keypad press insensitive  issue. Now we solved the problem by adding the dimple back to the metal dome and still reach 1,000,000 (one millions) times of finger press cycle life.

We did some failure analysis and found the keypad insensitive defect was caused by two major issues.

  • One is the foreign contamination, include dust and fiber, that I mentioned at last article.
  • Another one is the oxidation show on the immersion gold finished surface on the ENIG board pads.

We found the oxidation particles look like the chemical of Phosphorus (P) exuded from base material of PCB. The P is one of the major element for the ENIG fabrication process. So we only can control less P but can’t remove it from the board.

All these kind of particles are nonconductors. They stop the metal dome from touch with PCB if we press the metal dome slightly and feel the click. Once we  give more force to force the metal dome down to the PCB then these particles been squelched and metal dome contact with PCB and active the keypad function.  

In order to solve the key insensitive defect, we start to clear the PCB gold finger and metal dome before assemble the keypad upon PCB. We did improve the yield rate but didn’t solve it completely.

3 dimples metal dome
3 dimples metal dome.

Later on we decide to phase in the dimple metal dome as keypad solution. We phase in 3 dimples metal to instead of common single dimple metal to increase the finger press life. Because we think 3 dimples metal dome will share the finger press force to prevent PCB gold pad from being damaged by shape dimple. The result was good and fix the key insensitive  issue. It also passed the environmental test compare with the original without dimple metal dome. The production line reported no key insensitive defect been found after implement the dimple metal dome. This 3 dimples metal dome also passed the reliability test and reached 1,000,000 times of finger press at least.

keypad on PCB was damaged by metal dome dimplekeypad on PCB was damaged by metal dome dimple
Gold pad after 1,000,000 cycles of finger press test with 3 dimples metal dome. Only a slightly mark on the gold pad and all function OK.

To Summary the 3 dimples metal dome as keypad solution fix below issues

  • No keypad press insensitive found in production line. The foreign contamination can be ignore.
  • It pass the environmental test and no keypad press insensitive found. The oxidization on the gold pad can be ignore.
  • Reach 1,000,000 times of key press cycles.

As research the dimple metal dome in the market, there are total 4 kind of dimple metal dome design.

  • One with only one dimple dome locates in the center of the metal dome.
  • 2nd one with 3 dimples locates as triangle on the metal dome.
    3 dimples metal dome
  • 3rd one with 4 dimples and 1 dimple locates in the center of metal dome and 3 dimples as triangle position.
    4 dimples metal dome
  • 4th one with 5 dimples and 1 dimple locates in the center of metal dome and 4 dimples as square position.
    5 dimples metal dome

All of them can be considering to solve the key insensitive issue.


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