Fabrication Of DRONE SWARM (vErsion 2):
This is a log of the work I did to fabricate the control board I designed in version 2. I was planning on fabricating an entire drone with the radio board as well, but the results of experiment 1 indicated a major flaw in my design that forced me to redesign the control board instead of moving on to programming.
I fabricate my pcbs using a reflow soldering process. The process involves spreading a solder paste onto specific points on the board, placing the components and baking the board along a precisely controlled temperature profile in order to melt the solder and affix the components to the board.
Paste application
The paste is stored in a syringe with a wide opening since it is viscous. It is spread onto a stencil made of a laser cut stainless steel sheet. When properly aligned, the stencil ensures that exactly the right amount of paste is dispensed in exactly the right location. Dispensing the paste can be done by hand, but the stencil greatly increases the margin of error and lowers the risk of applying too much paste, which can form unwanted connections. The paste is dispensed slightly above the holes in the stencil, because the spreading motion consists of a single downward pass with a spare piece of stainless steel sheet acting as a squeegee. Extra passes across the stencil with the squeegee often do more harm than good since extra paste can by pushed underneath the stencil. Once the paste is applied, the stencil is carefully lifted, leaving tiny squares of paste as high as the thickness of the stencil.
Component Placement
A variety of technologies exists for picking up the extremely light and small components and dropping them accurately on the board, including dedicated industrial machinery and tiny vacuum powered suction cups. I use a set of antistatic tweezers. Most components are too small to pick up with bare hands, and if they are, it often results in bent pins. Some are small enough that they can easily be launched off of the table with a careless exhale, or even inhaled. Reorienting them often involves tossing them up slightly and hoping they flip over on impact. Luckily, the paste is sticky enough that it will often hold a component on the board even if it is turned upside down. The placement of a components also does not need to be as precise as one might expect, since the paste melts during the reflow process and surface tension is often able to pull them into alignment.
REflowing
The solder paste consists of microscopic balls of metal suspended in a flux gel. When heat is applied, the acidic flux gel liquefies first, removing any corrosion or oxidization on the surfaces where the solder adheres. As the temperature increases, it boils off, and the suspended metal balls melt, affixing the component to the board. There is a delicate balance between raising the temperature high enough and maintaining it long enough for this process to occur, and making sure none of the plastic on the components melts or anything is damaged by the heat. All of this takes place inside my controleo3 based modified toaster oven.
Parts:
Here is the autogenerated BOM (bill of materials) for the control board. I used this along with a similar one for the radio board to create a list of all of the parts I needed to order to assemble a drone. I also created dedicated shopping carts on amazon, eBay and Banggood with the parts unique to those websites.
