Why Metal Dome Force Changed After Assembly on PCB

Recently one customer complain that the metal dome force changed so much after assembly on PCB.

The metal dome used was “X6T-350-0.8*3.0”(A Triangle dome,  Diameter 6mm, force 350gf, leg: 0.8*3.0mm). Before assembly, force is in normal force range 350gf, but after assembly on PCB, force change to about 500gf and then make whole dome key hard to press.

After saw customer’s PCB samples, we understand why this happened.

What method used was method A we recommend in “How to Assemble Triangle series metal domes on PCB (2)” (Please click to see more details for that article: http://www.metal-domes.com/blog/2010/07/02/assemble-triangle-metal-dome-on-pcb-2/)

Method is okay, but the problem was related with PCB design of customer. For Diameter 6mm triangle metal dome, the distance between two legs is 6.0mm, tolerance +/-0.2mm, so the tooling hole on PCB should be designed following that value.

6mm Triangle metal dome

But when we measure the PCB, the distance between holes on PCB is only5.35mm. You can see a picture bellowing.

PCB Hole Distance

Because hole distance on PCB is too narrow, so it’s difficult to insert leg of metal dome into the PCB. You can see when put metal dome on the PCB holes, it will not go into the holes, but remain above the PCB.

When you press metal dome and leg can be inserted into the hole, but only one leg is properly installed, another one was shifted. You can see the red arrow.

Wrong PCB Hole Distance

When press metal dome, people have to add extra force to bend the edge metal dome toward to center, to make the diameter from current6.0mmto5.35mm, so that end of metal dome leg will be on the same level of PCB. That extra force will be at least 150gf, so final metal dome force become 500gf (350gf+150gf). This is the reason why metal dome force altered so much between before and after assembly.

After realizing the problem, customer re-designs PCB and problem solved. Metal dome force keeps the same before and after assembly on PCB.

Just remember to have the same hole distance like metal dome diameter. If dome used is6mm, then distance6.0mm, for8mm, use8.0mmhole distance.

If you are not sure about that, please contact us before you design the PCB. We will be glad to provide service.

How to Assemble Triangle Series Metal Dome on PCB (2)

See previous article here:

Question # 2: If soldering triangle dome is not recommended, then is there any better method to fix it?

Best Tech Answer:Of course. There’re three methods to make it:

A) For the triangle metal dome with leg, you can insert the leg directly to the PCB, and then curve the leg at back side, make sure it could buckle the PCB. That’s the common way.

Triangle metal dome with leg on PCB

Triangle metal dome with leg on PCB

But to be noted that the assembly speed is slow. So you need to calculate how much you need to pay for extra human handling fee.

To have two location holes on the PCB and choose the different leg length in accordance with thickness of PCB are two important things should be considered during your design.

PCB with triangle metal dome

PCB with triangle metal dome

Normally thickness of PCB which will use triangle metal dome are 0.6mm, 0.8mm, 1.0mm, 1.2mm, 1.6mm, 2.0mm, and the leg length of our triangle metal dome are 0.8mm, 1.5mm, 3.0mm, and a special one: 2.4mm. You need to choose a longer leg to insert into PCB and bend it. For examples, if you use 1.6mm dome, then our 3.0mm leg will be fine.

B) Directly using the adhesive tape to stick on the metal dome on PCB.

It will save little time compare with insert leg into hole of PCB (method 1).

But the stickiness of common transparent adhesive tape is not so well, metal dome was intended to be shifted or peeled off.

Also thickness of adhesive tape is too thin compare with height of metal dome, it’ll cause uneven of whole products, and too many bubble between PCB and adhesive tape.

C) Using metal dome array.

Triangle metal dome array on PCB

Triangle metal dome array on PCB

This is the most convenient and precision assembly method. Although the price is a little higher than other two methods mentioned before, but you will save a lot of time on inserting leg into hole of PCB or put adhesive tape on PCB, you only need to peel off dome array from liner paper and put whole products on PCB. In a word, this will increase assembly efficiency, precision (avoid potential alignment problem), and then reduce whole price.

According to different request of clicking feeling, you could use circle/triangle/four leg metal dome. If you want to get more information, please kindly visit our website for reference: http://www.metal-domes.com/cn/product-dome-array.htm

Ok, my friend, now do you have a idea to design and assemble your PCB with triangle metal dome? J

Let me know if you want to know more information on installing triangle metal dome, I will be happy to answer you anytime. We, Best Technology, are always be your best partner in metal dome and metal dome array in China.

How to Assemble Triangle Series Metal Dome on PCB

We have discussed assemble metal dome on PCB before, see “How to make the metal dome array to stick on the PCB firmly?”), but more and more customers asking for more details, plus pictures showing how to install various triangle metal dome, with and without legs. So we wrote an article plus some additional information and hope customers will understand the way more better.

Firstly, let us have a overlook at the specification of the triangle series metal dome. From the picture, you will see it includes four basic types: no leg, short leg, medium leg and long leg.

Triangle metal dome

The metal dome leg length divides into three basic series: 0.8mm, 1.5mm, and 3.0mm. We also have a special one which is 2.4mm. Of course, we can customize special leg length of metal dome, please contact us directly for more details.

Follows are popular questions & feedback from our customers and people in metla dome related forums, we summarize them and think you will understand more clearly.

Question #1: We want to solder a the triangle metal dome.

Best Tech Answer: For triangle metal dome, we don’t recommend solder it on FPC/PCB or membrane switch. When you press center of metal dome, the domes collapse and make contact with the secondary pathway, thereby closing the circuit. See more how metal domes work : . http://www.metal-domes.com/blog/2006/11/13/how-metal-dome-works/.

So if you solder it, three sides of metal dome cannot be extended as per designed, click feeling will be lessen, force always changed, and life cycles become low, and sometimes, dome will not return to its original position and become broken. We named it as “dead key”.

— to be continued: