Advice: Soldering boards directly onto another board without headers and other things

[Grimmdus]

Soo I am after taking time away from my lightsaber project I am back at it but redesigning it from the ground up including trying to make it much more compact. Starting with using the teensy 3.6, the teensy prop shied, a 4ch pico buck based on the Sparkfun 3ch one that I found on OSHpark (https://oshpark.com/shared_projects/cMroyCWV) and hopefully i'd like to add Adafruit Bluefruit LE UART Friend -BLE so i can change settings via my phone and what ever other tiny parts that might be required like a clash sensor, an adjustable regulator etc.. With all that said I am still quite the noob at this stuff, haven't studied up on it as much as i should have over the last year. The crux of my issue is the space requirements of my design don't give me to much room to work. I have a 4 cell 2S/2P 18650 battery pack and the above eletronics to fit inside a 1.45 to 1.50in ID tube and about 6 inches long space. I might be able to fit a pcb/header/teensy stacked configuration and spread out over about 6 inchs in length with a couple of the smaller boards on the opposite side as well. The height and width is really tight and i can't expand the size of the tube and still hold it.

Long intro aside i am wondering if I can seat/directly solder like a SMD the teensy, prop shield, and other parts and connectors to a PCB without using headers that I would design to make all the connections between boards that I need and just plain convenience to limit how many wires I have connecting between devices. I assume it would be a more secure connection and be more reliable and i could design it more to my needs. I obviously don't have the abilty to do anything super complicated at this point and i am not wanting create any new dev board but just adapt the ones available to me. Instead of stacking them they would be laid end to end one boards one after another. I would still need to make some modifications anyways since the connections on the prop board dont quite match the 3.6 anyways all in all if its possible that would make things a lot cleaner. I'm not sure if there is a term from what i am describing and that limits my reseach options on how to do this. Also is there a eagle file of the back side traces of the teensy 3.6 and prop shield that i could use to line things up in the design software? Any advice, thoughts, etc?

Things also to think about is if it's possible to jack into the micro USB connector on the teensy and move the connector to the end of the board else I would need to get some super slim extension cable since it or the micro sd card would be trapped up against the prop shield.

Also, I noticed on the teensy 3.6 it says the pins aren't 5v tolerant yet the Vin is labeled as 3.6v to 6v. Can i assume that the vin is regulated and its just talking about the other pins that can't have higher voltage's attached to them? (i.e. over 3.3v 250ma) It is a little confusing since i can't see a reason why i would feed electricity into the board except via the main Vin. Wouldn't any power crossing those pins be coming from the board itself going out to power other devices like the prop shield for example, or accent led's etc. My led driver for the main rgbw star would be taking power from the battery pack not through the teensy, only using the pwm's to adjust colors.

I know enough to be dangerous about this stuff but in most things its a process of figurung things out one bit at a time.

Grimmdus

 

[MichaelMeissner]

I would think if space is an issue, you might think about the Teensy 3.2 instead of 3.6. Particularly since the prop shield was designed for the 3.2. But perhaps you need things that the 3.6 brings. If you need access to more pins, FrankB designed a board that is meant to be soldered directly to the Teensy 3.2 and brings out the pins that are on the solder pads below the Teensy (https://oshpark.com/shared_projects/0T6ZdhhG).

If you are mating the boards directly, then you need to make sure each board doesn't have any solder pads that could cause unintentional shorts, etc.

The VIN pin is attached to the voltage regulator (and the USB pin unless you cut the traces underneath the Teensy). You can feed power to the Teensy via this pin, or if you are using USB, you can draw power from the pin. All other pins are 3.3v only.

You can solder wires to the pads underneath the USB socket to move the USB elsewhere.

In terms of attaching the 3.6 to the prop shield, FrankB designed this board: https://pcbs.io/share/8gXO3.

One note about soldering to pads, etc. I would worry about wires coming off in a light saber, particularly as you tend to move them around.

 

[Edward]

If space is at a premium you can save a few cubic CM by soldering one board onto another directly. It's not uncommon to open very cheep products and find a manufacturer has made a basic two layer board, with a small multi-layer board soldered ontop. Here's a quick example of what I mean


They do this to save on manufacturing costs.

Using teensy I've designed a board to sit directly below the it. I've lined up the holes I needed, then taking the individual pins out of a header, soldered the connection and cut of the excess pin. Sadly I can't share it as the design technically belongs to my emplyer at the time, but I hope I've explained the concept well enough.
I've also designed boards to sit in the footprint of parts where I need to change the part, essentailly to mate one footprint to another. It's not cleaver but it can be done.

You might well permanently marry the two boards together for life. The first method I've mentioned was beyond permanent, particularly as I plated the holes. Using the mouse-bte style pictured you can with a little love get the two to part company if you've room to wiggle the board. If you're 100% on your design or dont mind sacrificing a teensy at the alter of trial and error go nuts! If you're feeling paticularly brash you could shave the outside mm from a teensy, to get mouse bites and them solder it SM from the side like the board pictured. Maybe check the layout file first and make sure you'd not going to shave of internal copper traces.

Michael makes a valid point about fouling the board by connecting using the exposed pads under the teensy. You should apply your preffered insulator! Perhaps a bit of mylar.

You are correct not 5V tolerant means the pins operate at 3v3 and applying 5v to those pins will damage the teensy. The distinction is made becuase some versions of the teensy, while only outputing at 3v3 will accept 5V logic on some of thier inputs.

 

[Grimmdus]

Thanks for the replies guys
@MichaelMeissner I actually have a 3.2 I started out with that chip and had wires soldered to the underside to get enough i/o. I was previously using a cut down audio board and breakouts for the amp, imu, etc. I hadn't seen the prop shield or it wasn't out when I originally started the project. When I came back to look and saw the new boards that is when I noticed it. It's a little inconvenient to have to pair up the 3.6 with it but in the end, it will be much easier than using sperate breakouts for each function. As far as space my issue is more of a height issue than a length one. That is why I'm trying to cut the headers out entirely and do board to board soldering. Also, the 3.6 being faster and more i/o means a lot since I was hard pressed to get everything done with the amount of i/o in the 3.2 it was doable but felt messy the way I was doing it. My main concern is running event tripped audio off the sd card and the accelerometer, and led effects all at the same time would be maybe a little taxing on the 3.2. I don't mind having an obscenely long setup by going with the 3.6 and laying the boards out if it means it will be easier for me to program, less worry about lag, more i/o, and less messy/hackish. It's probably also my desire to have the latest and greatest but that's the PC enthusiast in me too I guess. Good advice on the rest I appreciate it.

@Edwards that's exactly what I was thinking about just hadn't seen examples of it yet to know it it was done or possible.

The underside I think is the real issue. I was wondering if attaching the board to another like I would an SMD using solder paste and an oven would work or if the possibility to having the solder flow between pads and connect pads together that I do want to be might be an issue. I guess in my mind I was thinking that if I designed it so the leads were the only thing on the surface of the bottom board and just bury the traces inside on the middle layer or on the opposite side depending on 2layer or 4 layer board that I would be reasonably safe from those kind of issues. Is my logic right on that? Having different examples like the ones you two have linked definitely help and give me examples in the direction I need to go. And i'm gonna use the eagle files for the different boards to get the spacing as best i can.
As far as permanent joining I don't mind if they are so, hopefully that makes the setup sturdier in the end since i am hopeing it to be dualable not just a show piece. Gonna try and be as careful and meticulous as possible, get other peoples eyes on it so I can minimize the chances of wasting my money. I'd prefer not to kill any boards. X fingers. If at first you don't succeed make sure the second time works because it's freaking expensive.

I am trying to essentially replace this http://www.plecterlabs.com/shop/product_info.php?cPath=1&products_id=53 mimic the functionality as well as add my own personal preferences to the setup. If it turns out cheaper in the end (minues hte equipment costs since i get to keep those for other projects down the road) then that will be all the better.

Grimmdus

p.s. wouldn't the mylar melt?