PCB Tech - Can the button metal reed be directly soldered to the PCB?

Can the metal-dome of the button be directly soldered to the circuit board? What are the advantages and disadvantages?

Although the current smart phones have rarely used physical numbers to press keyboard keys, some products and a few physical keys still exist. PCB products still use physical keys. The best touch and the longest life is the "metal dome", but the physical button has a very big disadvantage, that is, it takes up too much space on the circuit board, so the RD of the PCB company began to be wild. On a whim, the "metal dome" was directly soldered on the circuit board and used as a button, and it was also put into action. In this way, although the space for the use of electronic parts was increased, the tragedy was also Then it happened.

Generally speaking, the PCB space under the keys (keypad) is not likely to be used to place electronic parts. At present, most of the "metal reeds" on the market should be made of a whole piece of adhesive. Paste on physical buttons or circuit boards to save man-hours, and to ensure the accuracy and fixation of the "metal reed". Shenzhen Honglijie believes that there should be many aggressive R&D engineers who have thought about it. The "metal reed" can be directly soldered to the circuit board to increase a lot of usable space, but the general RD with a little experience is not afraid to steal space near the button after thinking about it, let alone solder it on the PCB .

Because there is no ready-made "metal reed" that can be directly soldered to the circuit board in the market, our RD decided to design a "metal reed" and have a pair of symmetrical places that can be flat-attached. The wings on the surface of the circuit board, and then of course the pair of wings are soldered on the circuit board with solder paste! After the finished product was made and sent to the life test of the button, it turned out that the button did not respond after only a little more than 100K times. After disassembly and inspection, it was found that the solder on the "metal reed" wings was actually A tin crack occurred, causing the "metal reed" to leave its original position on the circuit board.

At the beginning, RD came to Shenzhen Honglijie as usual. There was tin cracking. Can the factory strengthen the soldering tin? So we manually increased the amount of solder and covered the entire wings with tin. Of course, mass production was impossible because it was hand-soldered. Fortunately, we sent it to a life test later. It probably hit a little over 300K. "Metal spring The wings of "Pian" were actually broken, so I shouldn't even need to add "tin" now.

PCBA processing factory

What are the advantages of soldering the "metal reed" directly to the circuit board:

▪ It can save space on the circuit board, and parts can be arranged around the metal reed.

▪ Cost saving? Because the space of the circuit board is saved, the relative PCB cost can also be reduced.

What are the disadvantages of directly soldering the "metal reed" to the circuit board:

This shortcoming can be said to be a basket. In addition to the aforementioned "metal reed" wing fracture problem, there are many other engineering and quality problems waiting to be resolved.

▪ 「metal reed」float

Because solder paste is used for printing and soldering, there will be solder paste under the metal reeds. Depending on the amount of solder paste printed, there will be a problem that the reeds are lifted off the surface of the circuit board. What is the problem with the reed floating high? It will affect the pressing stroke of the reeds. The high-floating reeds need to be pressed harder or longer to trigger the button function. Inconsistent floating heights will cause each reed, that is, the pressing touch of the button to be inconsistent.

▪ "Metal reed" is tilted

Because the reed floats high, the opposite will cause the reed to tilt unilaterally on the surface of the circuit board. The larger the gap, the higher the risk of inhaling dust or foreign objects when pressing, and foreign objects will affect the button. The trigger mechanism causes the electronic signal to fail to respond when the button is pressed down.

Minor foreign matter contamination problems can be solved by using "metal reeds" with a dimple design, but not serious ones.

▪ The solder may climb to the inner wall of the "metal reed"

This is the most troublesome thing, and it is best to solve it by design. When the solder paste melts, the solder will flow in the direction where the tin can be eaten (the direction with smaller surface energy) and the "metal reed" will return. It will be directly heated during soldering, so the solder has a good chance to crawl along the inner wall of the reed. Climbing the inner wall of the tin will affect the touch feeling of pressing. When the solder climbs over the web area of the reed, it will make it difficult to press the button. Peak-Force (F1) will become larger, which will seriously affect the pressing force ratio of the button (click ratio), that is, the touch feeling of pressing will become worse. Different heights and thicknesses of the inner wall climbing tin will affect this force, which will cause inconsistent pressing feelings in the case.

postscript:

Why does the "metal reed" with wings welded to the circuit board will cause the problem of wing breakage? What do you think?

Of course, it is not the belief of PCB company RD to give up after encountering setbacks, so the story continues, and it seems that there is a turning point. As expected, failure is the mother of success! Have the courage to break through the shackles, but don’t know how much it will cost?

If you really want to increase the utilization of the circuit board under the button, someone will raise the button with mechanical parts, and then use a soft board to replace the original circuit board contact point, so that the original circuit board under the button can be used .