9 Replies Latest reply on Nov 15, 2017 12:26 PM by Keitarusm

    Flexible PCB Strip Design

    Keitarusm

      Hey folks. I've been having a lot of trouble finding any information on the topic so I want to throw this out as a long shot. I'm looking to do a flexible strip, basically what I want to design is a cheap LED strip like you might buy for cheap on Amazon.

       

      Of course the obvious response is, 'That's dumb. Just buy an LED strip.' but I do promise I have a good reason for even considering this. I want to make a Digital Thermometer Chain, essentially this looks very much like an LED strip, but with digital thermometers instead of LEDs. Aside from your personal thoughts on the value of this endeavor, I have a few questions for anyone who might know anything about doing this.

       

      What sort of special considerations need to be made for this type of PCB? Remember I'm looking for the extremely flexible, think cheapest of the cheap LED strips, not a much stiffer type typically found in rigid-flex pcbs.

       

      What is normally required of the design? One segment of a repeating pattern? Do I need to layout a 5 meter long board?

       

      Finally, the biggest one I have is, where on earth do I begin to search for someone who can make these?

       

      Thanks in advance for your time. I really appreciate any advice you might have.

       

      V/r

      James Whitlock

        • Re: Flexible PCB Strip Design
          shabaz

          Hi James,

           

          This is quite specialist if you want a 5m long flex PCB, it could be worthwhile directly e-mailing or telephoning and talking with a PCB manufacturer.

          An example would be Newbury Electronics | PCB Manufacture & PCB Assembly since they have 60 years of experience in PCB manufacture.

          I'm assuming you want this in large quantities, because I can't imagine a 1-off being in anything other than in the thousands of $. Smaller flex pieces joined together with a connector would be far cheaper. Anyway, the PCB manufacturers will provide more accurate information.

          2 of 2 people found this helpful
            • Re: Flexible PCB Strip Design
              Keitarusm

              Hi Shabaz,

               

              I'm well aware of the cost unfortunately. I didn't include all of the boring background info, but this is a scientific instrument, and there are a whole host of reasons why connected segments don't work. The short version of all of that is that we've been deploying them in the arctic for about 3 years now and have seen many different failure modes. While I'm sure 'mastering' that technology has some merit, we're looking to try something new. I haven't tried Newbury yet, but every other prototyping board house I've contacted always wants us to do something that fits on a standard panel. I totally get it, that's what is common and well known, but we know there is a different option and we think there is some merit to trying it. To give you a cost reference, current commercially available products that do this start at $3k, and often quickly fail. If we could build one for $5k that worked that would be a huge win.

               

              I have no contacts in China and no experience in that market, so I don't have clear path into the space. But it's worth it to try and figure out.

               

              V/r

              James Whitlock

            • Re: Flexible PCB Strip Design
              ninjatrent

              Hello James,

               

              What do you mean "Digital Thermometer Chain?

                • Re: Flexible PCB Strip Design
                  Keitarusm

                  Hey Trent,

                   

                  That's almost an industry term. But if you think about it like a LED strip, and replace the LEDs with digital thermometers like the Maxim DS1721 chip. A whole bunch of digital thermometers in a chain. You can read from the DTC and get the temperature at each point, say 2cm apart. Yeah that's probably not useful for your bedroom but really useful if you want to understand the thermodynamics at play in something like sea-ice.

                   

                  V/r

                  James Whitlock

                    • Re: Flexible PCB Strip Design
                      shabaz

                      Hi James,

                       

                      Interesting challenge. I do believe the PCB manufacturer can help you, I suspect if Newbury Electronics do not know of a way then they should be able to point you towards some other manufacturer who can, or some other suggestions. I wish I knew more about such an environment that you wish to subject it to, but I don't have experience/knowledge in this area : (

                       

                      By the way, you probably already know, but just in case, even with addressing, it could get very difficult to have so many of these devices in a chain, all on a single I2C bus, because of the capacitance on the SDA/SCL lines. In practice you may need to break it up into separate domains (e.g. using I2C repeater type devices) say every few dozen devices. So, the strip will need a small circuit after every few dozen temperature sensors. If you can, it could be worth prototyping the whole length of it up (even if it is all on conventional rigid PCB pieces) to test out the electronics/software, just to be sure of your final design for the flexible PCB.

                      1 of 1 people found this helpful
                      • Re: Flexible PCB Strip Design
                        mcb1

                        get the temperature at each point, say 2cm apart

                        What is the total distance.

                         

                        Does the device end up in the water or locked into the sea ice.?

                         

                        Do you have a link or photo of these commercial units?.

                        It may help someone think outside the box.

                         

                        Cheers

                        Mark

                          • Re: Flexible PCB Strip Design
                            Keitarusm

                            Hi Mark,

                             

                            The total distance is about 4 meters and the device spends some of it's life in sea water, and some frozen into the ice cover. I don't have a photo handy, but probably the biggest commercial player in these types of instruments is http://beadedstream.com/

                             

                            Their products are great and very reliable, I'm sure they'd last for years. The only issues are density, they have a minimum spacing that is quite large, and also they're quite expensive. There is nothing wrong with their price, but for my needs, it's hard to justify such a robust instrument when I know for a fact that it won't last longer than 2 years at most. The units I deploy are not recovered, because the cost of doing so is high.

                             

                            Someone recently recommended a DIY approach to using copper tape and a toner transfer method PCB etch to make a custom strip. That might be what I go for in the near term. It's entirely possible that I'm just wrong and what I'm looking for doesn't exist. Which is a totally fine answer if it's true. I guess I'm still hoping for someone to come along who can say "Hey, I make LED strips for living. Here's how we do it." I really haven't been able to find much info at all.I'm looking into attending a trade show...so if you know of any good ones let me know, I figure that might be the most sure fire way to meet someone with the inside track.

                             

                            V/r

                            James Whitlock

                              • Re: Flexible PCB Strip Design
                                mcb1

                                My sideways thought for this was a series of Dallas One wire sensors.

                                If you used the sealed version, they have three wires exiting, which can be extended to a common point and joined.

                                https://cdn-shop.adafruit.com/1200x900/381-00.jpg

                                https://www.adafruit.com/product/381

                                 

                                 

                                The business end can be fixed to a flexible strip and in theory you could make them in 1m sections with 50 per section. (at 20mm spacing)

                                 

                                The specs don't indicate how many can be in a single bus, and I'm aware there was some trial and error in one of the Design Challenges which eventually got sorted.

                                However it's an option that is affordable and could work.

                                 

                                 

                                I'm not sure that the pcb is the issue, it's the ability to talk to each device that makes this one a bit more challenging.

                                At 20mm spacing and 4m long there are 200 individual items providing data in some organised manner.

                                 

                                You have one other minor issue of the temperature. Most IC's are happy with zero or -10C but go below and you start to operate on the edge, so while the actual measuring device might work, the interface unit buried in the ice may have problems.

                                 

                                Cheers

                                Mark

                                  • Re: Flexible PCB Strip Design
                                    Keitarusm

                                    Hi Mark,

                                     

                                    You're right on the money with the Maxim one wire devices. I actually use their DS28EA00 chips. One wire is a great technology in my opinion. Perhaps limited use cases, but it's excellent for doing these long chains of devices. I've done 200 plus sensors with no issues. The devices are slow, and all individually addressed. I just pair the whole thing with a 1-wire bus master from maxim and my interface to an MCU is just i2c.

                                    The PCB is definitely the hard part of this task. The design has been done for nearly 2 years, but finding just the right flex laminate and someone who can handle it is the big issue. Another thing I notice when asking for help is that people tend to apply their own operating principles onto their responses, which is normally exactly the kind of advice someone might needs as a beginner. But in my case, I'm sort of searching for what people know is possible, not really what you might do in practice.

                                     

                                    As it turns out, after a solid week of searching, I have found someone who can do exactly what I mentioned in my first comment, even after convincing myself it was impossible. All Flex in MN does this type of work exactly. They can produce flex circuits up to 40' in length(One continuous board). It's highly specialized work and you certainly pay the premium, but this is exactly what I was looking for. I plan to contract with them to produce these boards. I should maybe have prefaced all these comments with a statement that this problem wasn't a hobbyist one, this is work for academic research. Normally I wouldn't ask time of the community for such things, but I had become desperate while trying to find anything out about the technology.

                                     

                                    V/r

                                    James Whitlock