Quick video of how I assemble Moteinos. This includes using a home made metal stencil to spread solder paste on the pads and a manual pick and place apparatus for speedy placement of SMD components. In a future post I will show details about this tool.
UPDATE: See this post for a video on how the pick-and-place apparatus was built for less than $20.
In previous posts (here and here) I described my efforts and research to develop a DIY method to make good quality SMD metal stencils at home. I have since experimented some more and I believe I found the best method (so far) to do this with very cheap materials, yielding very good results.
Since transferring the toner to the soda can aluminum alloy was the most challenging part, I kept experimenting with different transfer mediums and other ways of possibly facilitating and perfecting the transfer.
Finally the solution is here: consistent, repeatable, “perfect” toner transfer, every time.
I posted this video of the latest SMD stencil I made for my Moteino project. I ended up shrinking the pads by 8mil on this one, the reason being that the previous stencils I made I shrunk by 5mil and the stencil would dump too much solder paste, causing some solder bridging. See the stencil step through post for details how to make this.
I also discovered another very good method to transfer toner: silicone coated paper. I wiped a very thin layer of clear silicone on a blank sheet of paper (on half of it). After it dried I printed the mask directly to it. The iron method will give very good results, the toner will either transfer 100% or not at all. The spots where it doesn’t transfer are fixable with a thin Sharpie pen. The tool you use to spread the silicone has to be very flat and even, any unevenness will result in less perfect toner transfer.
Once I assemble Moteinos, I will need a way to load the bootloader and test programming from the Arduino IDE. Since I plan to sell them without soldered headers (unless people want that option), it’s impossible to easily program the ATMega328P chips after the boards are assembled. Unless … I use a programming jig, a perfect DIY project to help in the Moteino manufacturing process.
Steps for making the jig and programming a Moteino:
I ordered some spring loaded test probes (aka pogo pins) on ebay
I used 2 Moteino PCBs to align and hold the pogo pins in place
The bottom PCB is soldered to the pogo pins, the PCB in the middle is used to keep the pins straight and aligned. This can also be soldered.
I only soldered enough pogo pins to be able to program and test the on-board LED and the transceiver. If those connections work, the other Moteino pins are almost 100% guaranteed to work.
Connect the AVR programmer and the FTDI adapter/cable. The FTDI adapter provides power, the AVR programmer expects the board to be powered separately.
Set the fuses and burn the bootloader with the AVR programmer
This is a step through to make your own metal stencils for PCB solder paste application. In the last post I described my trial and error details of picking the materials. I am using beverage can aluminum for the stencil material and transparency film for toner transfer. In the meantime I discovered that many other materials can be used, especially for the toner transfer medium, with various degrees of success. Read the notes at the end for details. This etching method will not produce the highest quality you can get. Laser cut stencils are higher quality but a lot more $$$. For my purposes such a home-made stencil serves me very well, and saves me a TON of time (I can now apply all the solder paste in one wipe instead of using a syringe to hand apply it on all the pads). Solder paste is very forgiving and even the most imperfect stencils I produced this way give good results after reflow. The whole point is to try to apply an approximately even amount of solder paste to the pads, such that during reflow, all your pins will look the same and won’t form bridges. Overall I think the cost vs outcome/quality ratio is very good. Once you get the hang of this and develop your own routine and choice of materials, in less than 30 minutes, you can produce a high quality home-made metal stencil (almost comparable to a stainless steel laser cut). Continue reading →
Stencils for SMD solder pasting are becoming less expensive. Companies like ohararp.com makes your custom Kapton-polymide film stencils for $25 per 8×11″ sheet. Not bad, but I’m not sure if they allow you to fill that sheet with more than one design. It’s plastic and it will wear out fast, fold easily, etc. My DIY adventures involve mistakes sometimes and I appreciate mistakes because they teach me a lot, but not at a rate of $25+shipping and a week of waiting per mistake. I want better.Continue reading →
For some time I had this old ATX power supply gathering dust and getting moved from one place to another. I kept wanting to transform it into a bench supply. ATX supplies are switched power supplies that can provide a lot of current. They have a set fixed of voltages (5V, 3.3V, 12V, -12V), nonetheless very useful for most electronics and low voltage high-power projects. Once nice thing is that you can combine outputs to get less conventional voltages, for instance you can use 3.3V as GND and 5V as VDD to get an effective 1.7V potential difference which could come in handy in some projects; you could also use -12v as GND and 12V as VDD to get a 24V range for your project. Be warned if you try that though – if your project has other GND sources that are connected to mains ground/earth you could potentially cause some sparks/smoke/flame/fireworks, since all the grounds in this supply are directly connected to mains earth – as revealed by a simple connectivity test between the supply input and output ground terminals.
In my first post I’ll describe how I etch PCBs at home with a less conventional but cleaner method, using vinegar as etchant. Yes, vinegar! Vinegar is somewhat slow compared to strong acids like ferric chloride or muriatic (hydrochloric) acid, but its much safer and less hazardous, and there’s an easy way to speed it up. When I first tried it I instantly liked it and never looked back.