Why metal dome array doesn’t work with normal force?

Recently we received a customer complaint, he said some metal domes arrays couldn’t work with required (normal) force and need higher push force, and even more some metal domes are locked and no response.

The metal domes customer uses is S6F-160-D-Ni, mean F-leg, diameter 6mm, force 160gf, with a dimple in center and nickel plated. The normal force is 160gf before assemble, but it seemed force changed a lot after assemble on PCB, and need higher force.

Before seeing customer’s products, we also couldn’t find the root reason, we checked and tested our metal domes before assembly, it’s no problem, but after saw customer’s finished product, we understood why this issue happened, because customer’s design housing design is not suitable with metal domes array.

customer's original metal dome array

customer’s original metal dome array

 

 

 

 

 

 

Firstly, we checked customer’s assembled unit, obviously we found that four legs of metal domes are pressed and fixed by the front plate housing after assembly, so when people pressed the center of metal dome, the legs of metal dome can’t be extended peripherally as its original design, that means the front plate stop metal dome becoming collapse, so you have to press metal dome use much higher force than the designed force (160gf for that metal dome) to force it become collapsed.

metal dome with front plate

metal dome with front plate

 

 

 

 

 

 

 

But in most of cased, people don’t know that and still press metal dome use designed force (160gf) and then he will found out that metal dome will not work. And then he have to increase the force, then bigger force damage metal dome original structure, and then will change the force, and in most of case means reducing the clicking feeling. That’s also why we say metal dome can’t be soldered. (See more how metal domes work).

Second, we measured the diameter of plunger on front plate, it’s only 4mm, but the diameter of metal dome is 6mm, you can see, obviously front plate will press the inner function ring metal dome.

plunger of front plate

plunger of front plate

 

 

 

 

 

 

Third, we measured the diameter of rubber keypad, it’s 3mm, that’s too large, the larger stress area is, the higher push force will be needed. For 6mm metal dome, the recommend size is 1.5mm (25%-30%*diameter of metal dome).

Suggestions:

metal dome array assembly suggestion

metal dome array assembly suggestiong

  • Customer need to use front plate to fix the rubber keypad, but this front plate can’t lock metal dome, so bottom diameter of front plate should be more than 6mm, then metal dome will not be fixed by the front plate.
  • Use the suitable plunger to touch and press metal dome, 25%-30% of diameter of metal dome.

We hope above will be clear for people whom encountered the similar issues.

How to Assemble Metal Dome Array on PCB

Some people will ask us how to assemble metal dome array on PCB, and today I’d like to share some basic information for that. It’s a very simple to assemble our metal dome array on PCB, and here are some basic steps:

 

1)    Clear the PCB, make sure there’s no dirt, dust, or any other contamination;

metal dome array and PCB

metal dome array and PCB

 

 

 

 

 

 

 

 

 

 

 

metal dome array before assembly

metal dome array before assembly

2)    Peel off metal dome array from the release paper;

Peel Off dome array from release paper

Peel Off dome array from release paper

3)    Find a good way for alignment of dome array and PCB, normally by location holes, outline edge, silkscreen/legend printed on PCB;

metal dome array location

metal dome array location 1

4)    Put dome array on PCB by handle, or machine (using a vacuum nozzle)

metal dome array location

metal dome array location 2

 

5)    Press surface of dome array to make sure it adhere on PCB firmly

press dome array on PCB

press dome array on PCB

6) Finished the assembly

dome array on PCB

dome array on PCB

 

Please to be noted that the self-adhesive on metal dome array has a small initial bonding force, so that you can peel off it from the PCB easily after assembly if you want to adjust location of dome array.

 

But this will also cause a potential problem: Customers will find dome array does not stick to PCB so firmly and then they will be a little worry.

 

In fact, there’s another special characteristics for that glue: the bonding force will become more and more stronger after about 24 hours, without any movement, pressure, etc. It means if after 24 hours, you cannot peel off dome array from the PCB, without damage the dome array, because the peel strength at that time will be extreme big comparing to initial value.

 

Let’s know if has more questions for metal dome array assembly on PCB.

New SMD Metal Dome Switch

Are you trying to find out a metal dome which can be soldered on circuit board? I think so, because the normal metal dome are not recommended to be soldered on the circuit because it will reduce or damage function of metal dome.

But now there’s a good news for you: We’ve release a new type of SMD dome switch today and you can choose suitable dome switch you like to assemble them on any of your board, without worrying anything!

SMD Dome Switch is a type of new metal dome package which designed to be soldered on PCB/FPC/MCPCB, or any other circuits, by directly SMT/SMD reflow soldering technology. The circuit board for soldering is on the bottom of each dome, and on top of metal dome, there’s a layer of yellowish Polyimide with a Plunger & Actuator in center area, to seal the dome and add the clicking feeling.

SMD dome switch from Best Tech

SMD dome switch from Best Tech

Please visit our website for more details: http://www.metal-domes.com/SMD-dome-switch.htm

Do you like that dome switch? If you have any idea or comments for that, please leave a message here, or write email to us. Any words on SMD dome switch will be appreciated.

 

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.

PCB PAD Finishing for Metal Dome

If it is the first time you try to use metal dome or metal dome array on circuit board (PCB or FPC), then probably you want to know what kind of surface finishing of circuit board should be used if metal dome will be assembled on.

Basically there’re two types of surface finishing can be chosen: Gold (immersion or plating) and HASL (Tin). In most of time, we suggest using Gold. Why?

As you know, function of metal dome or metal dome arrayis to act like a switch, to make circuit to be closed or opened. And low contact resistance will be helpful for smooth, stable currency. All of us know Gold bas much better performance than Tin on electric conductivity. This is why we recommend Gold.

Gold-plating-PAD-on_PCB

 

 

 

 

 

 

Whatever metal dome will be assembled on Rigid PCB, or flex circuit, gold is primary choice. Here are some pictures.

Gold-plating-PAD-on_FPC

 

 

 

 

 

 

 

But when you need lower price, does not need strict stable currency or don’t care too much on switch on/off time, and then you can choose HAL (Tin). For most of circuit board used in ESU pencil are of HAL (LF). Here are pictures.

HAL-finishing-PAD-on_PCB

 

 

 

 

HAL-finishing-PAD-on_PCB

I hope above is clear for everybody. If still have question, leave your comments or send an email, we will reply very soon.

Four-Leg Dome with Legs

As we know, there’re already four legs for our Four-leg series metal dome, and this is why it named four-leg dome, or cross metal dome. So what do you means “four-leg dome with legs”?

Let me firstly see the looking of a normal four-leg  (Cross) metal dome.

Normal four leg metal dome

We can see there’s no extra legs except these four legs which will be put on contact PAD of PCB, FPC, or membrane.

As we don’t recommend soldering metal domes on the board, so how do I adhere the dome, except normal “dome array“, “single key dome array“, or in the socket? Can I fix it on like triangle metal dome using two legs insert into the board hole? That’s the reason whey there’re extra legs for F series (cross) dome. You can see pictures.

four-leg dome with legs

four-leg dome with legs

Now is everything clear? There’re two extra leg, and you can insert the into the holes of board like triangle metal dome do. Currently on our Dia 11.4mm four-leg dome, these extra legs length is 1.2mm and width 0.9mm.

If you need more information about this special four-leg (Cross) dome with legs, contact BEST and we will give details to you. We can also make new tooling to make new diameter four-leg metal domes.

Circle Metal Dome with Leg

Previous we have discuss how to assemble triangle metal dome, some people told me they also want a circle dome with a leg. But for most of circle (round) series metal dome, there’s no leg.

So when assemble with a circle metal dome dome, you need to put it on a printed circuit board, or a flex circuit, by a adhesive tape,  or put it in a socket.

For 1st method, you can use our “standard single key dome array“; for 2nd way, just buy dome in bulk. it’s easy, but if you also want to fix circle dome in the socket, then you will hope this circle dome with a leg. Am I right? Hooo, dream comes true. Here are some pictures for them.

Circle dome with leg details 1

circle dome with leg details 2

Currently, there’re few circle domes has leg, because this kind of requirement can be replaced by triangle dome. So now only Diameter 5mm is available in the market now. Force 160gf, 180gf  are standard, 130gf and 200gf also no problem to be made.

If  you need other legged circle metal dome, we can set up new tooling to make the special dome. Contact BEST if you interest in this circle metal dome with leg.

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: