Fluorosilicone membrane keypad design guidelines

One of our customer is developing a medical device probe with a handpiece, that has a small silicone keypad.He send us the design drawing for evaluation and quotation.

This keypad is used for medical equipment which has strict requirements.So the product went through several design stages before success.

First design stage.

Silicone keypad is designed with metal dome carrier.

Second design stage.

After we provide samples to customer to test, we received his feedback.

This keypad is part of a microscope system and during testing, customer found that the metal dome mechanical click causes vibration in the microscope image.

They would like to test a keypad with conductive carbon pills instead of the metal dome.

Customer wants to do change on the current keypad mold.But the current keypad design is metal dome structure without key webbing design.So the only way is to remake a new keypad mold.

Our engineer update the keypad design with key webbing structure.There are following changes:

• Add the key webbing.

The key webbing is the key factor to get the actuation force and rebound of keys. Only when the key webbing is well designed, can the keypad get a good tactile feeling.

Generally, the key webbing thickness is about 0.2mm.But should depend on the keypad actuation force.We will adjust it according to the actual application of keypad.

• Conductive carbon pill design.

Our customer require 5mm carbon pill, but after analysis by our engineer, the maximum carbon pill size should be 3mm.If 5mm carbon pill, it is too close to key webbing, which will cause broken key webbing when processing.

Customer approved our design and decide to proceed the new keypad mold.

Third design stage.

Some time later, we heard back from the customer:

        We have been doing validation testing with the silicone keypad. This included intermittent soak testing in IPA and Oxivir (Hydrogen Peroxide disinfectant). During the testing the Keypad leaked causing the PCB contacts to fail.The initial leaks were around the air channels (See yellow in attached drawing) and possibly through the key kebbing membrane section (See blue in attached drawing).We are looking at getting another revision to avoid the leaks.

          To stop any diffusion through the Key Webbing, Can the key webbing be made:

          1. Thicker  

          2. OR a different grade of Silicone

          3. OR another type of silicone such as Fluorosilicone

          4. OR a different process such as higher pressure

The Air channels will need to be filled, we will add breather holes to the PCB, under the Buttons to allow air to escape.Can you consider the above and offer any other suggestions; this could involve changing the shape or size of the keypad.

Silicone Keypad were Deformed after Chemicals Soaking Test

According to the customer's description, the liquid gets inside the keypad, causing the PCB to malfunction.Our engineer do the detailed analysis of the keypad, and provide the following suggestions:

Q1:We have been doing validation testing with the Silicone Keypad. This included intermittent soak testing in IPA and Oxivir (Hydrogen Peroxide disinfectant). During the testing the Keypad leaked causing the PCB contacts to fail.The initial leaks were around the Air Channels (See yellow in attached drawing) and possibly through the Key Webbing membrane section (See blue in attached drawing).

-Solution

Q2:After the soak testing failed another new Keypad was tested and this showed the webbing section leaking like a sieve, so it is assumed the IPA & Oxivir diffused thru the webbing section.

-Solution:

The current keypad material is ordinary silicone rubber,which is not resistant to chemical agents. If the keypad needs chemicalresistance, we suggest to change to fluorosilicone keypad.

Fluorosilicone is good at oil resistance and chemical resistance.

Customer think it is a good ides to add pre-pressure seals around the screw holes & removing air channel.But he also think if the key webbing thickness can be increased so as to avoid leaking.

As mentioned above, the key webbing is the key factor of  keypad actuation force.

If adding double thickness of the keypad webbing, the tactile feel will be changed. The actuation force will be increased a lot.

So we suggest customer to do the changes of the pre-pressure seals around the screw holes, removing air channel and also change material to fluorosilicone first.

If these changes are still not workable, then we can find another solution.

Forth design stage.

After test, the fluorosilicone keypad has been an improvement but the PCB pads still being degraded over time due to leaks. These tiny leaks are in other parts of the Probe, so the Fluorosilicone material is good.

But customer still try to seal the PCB.

According to customer's requirements, we have done the following changes of the keypad:

· Reduced buttons to allow improved sealing around the Screw Hole

· Enlarged pressure Seals to improve sealing around the Screw Holes

· Changed light guide area (still translucent rectangle but no rod extrusion) to improve sealing around it

· Added bottom lip to form a seal around the PCB

· Add air channel for button

Now, the silicone keypad works well.