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How to Properly Select LEDs for Your Keypad - Q&A

Epec Engineered Technologies
Written by Epec Engineered Technologies
Posted on November 29, 2021 at 9:51 AM

At the conclusion of our webinar, How to Properly Select LEDs for Your Keypad, we had several questions submitted to our presenter, Steven J. Goodman, User Interface and Cable Assembly Product Manager at Epec. We have compiled these questions into a readable format on our blog.

Q&A From Our Live User Interface Webinar

Quick Links:

Watch the Recording Below:

Free Download: 5 Mounting Panel Design Tips for Membrane Switches

Question: How can you reduce risk of bad LED contact on PET circuits due to conductive adhesive cracks?

Answer: Open circuits due to a breakage of the electrical connection between the LED and PET circuit are not unheard of. Our primary recommendation is to understand the true failure mode and root cause that is creating your anomaly. Typically for a conductive adhesive to “break”, there is a mechanical stress that is imparted upon the joint. This can be stress due to bending, vibration, thermal cycling, or potentially a workmanship/assembly issue. Our recommendation is to perform a detailed study to determine if you can identify what is causing the break and figure out a way to repeat it through test. If you are still having challenges, perform in-process electrical testing to see if the point at which the failure occurs is isolated.

Question: Is there a brand of LED or LED manufacturer you can recommend?

Answer: We would actually prefer not to specify a specific LED manufacturer unless there's something especially unique about the LED. I don't think it's best to recommend an LED manufacturer and put that on the drawing. We strongly suggest leaving it at the high-level requirements.

Question: How is the future for smaller LEDs? Is there a limitation?

Answer: As with most manufacturing techniques, when assembly risk increases so do the likelihood of fallout, the time to assemble, and the overall direct cost to assemble and troubleshoot. Typically, 0603 package size LEDs are common enough through electronics distribution and easy enough to assemble that they are the preferred small-packaged size LED. Smaller package size LEDs are possible to assemble, but this gives way to traditional pick ‘n’ place machinery and automated assembly techniques. Still, their small size makes them difficult to test and rework if changes are needed. These SMT-type machinery must use rigid FR4 PCB or a polyimide flex FPC as the base circuit. Low-cost conductive ink and PET circuits are usually assembled by hand-making smaller package size LEDs difficult to assemble. If you need an ultrasmall LED, go with an FPC or rigid PCB circuit.

Question: How do you attach LEDs to a PET film?

Answer: PET film circuits use a conductive ink that is silkscreened atop the insulating PET film to create the necessary circuit traces. Since there is no etched copper, like with a traditional FR4 rigid PCB, components cannot be soldered or “welded” to the circuit. Instead, conductive epoxies are used to create the mechanical and electrical bond to the circuit. While the bond is not as strong as a soldered joint, and the conductive epoxy has a higher electrical resistance than a soldered connection, this method to attach LEDs to a PET circuit is especially common. Most LEDs are hand populated on these low complexity circuits so paying attention to a complicated LED form factor (such as multicolor LEDs and small package sizes) is critical.

Question: We are having issues sourcing an NVIS compliant film. The supplier has completely ghosted us. We aren't sure how to proceed. What advice can Epec pass along?

Answer: It's probably best to try to find another vendor depending on what your issue is and what you're trying to address. There may be an option, whether it's finding an off-the-shelf LED or trying to find an alternate film and pairing that with a white or other color LED.

Question: Looking for guidance. Our EE spec’d in an LED that's barely able to be seen on our keypad that's used in direct sunlight.

Answer: This is actually something we talked about here earlier in the presentation. Assuming this question is based on a yellow LED because we run into these a lot and if that's the case, we'd strongly suggest an amber LED or to try and find a higher intensity LED. But ultimately, it sounds like a careful study needs to be done of how that LED's placed and what's above it that's potentially reducing the overall brightness.

Question: My company has a board respin and we're considering moving the LED's current limiting resistor locally to the membrane switch. How much does this cost and how long do you think it will take?

Answer: YIf it’s only a change to the circuit, that's a pretty quick change. It may just be modifying the trace, adding additional locations to locate a service-mounted resistor. This would be a few week’s type of an impact and ideally less than $1,000 of NRE, but every design is different. We would need more information to be able to better quantify a timeline and cost.

Question: My marketing department isn't thrilled with our backlighting and wants to illuminate each button, but we can't find a snap dome that can be used in our application.

Answer: If a dome switch is getting in the way of the LED and impacting the backlighting performance, there are a couple of things we could look at, one of which is a light guide film. That definitely can add some complexity but may not be a bad consideration. Another option is actually to use a silicone rubber keypad with a conductive carbon pill. The carbon pill can be placed on the rear of the keypad and actually, an LED can be placed adjacent to that pad location. And if the silicone specified is translucent, I think that that could be a good solution to eliminate the need for a dome switch.

Topics: User Interfaces

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