Fabrication Of The Mechanical Key Calculator:

After designing the board, this is the process I followed to create two working prototypes. These were the first two devices I made from start to finish (design, fabrication and then testing). This process is better documented in my drone swarm project.

Steps:

1. Ordering the parts:

Here is the autogenerated BOM (bill of materials) for the calculator. I used it to create shopping carts of Digikey, Aliexpress and Ebay in order to source all of the components I needed. The circuit boards and stencils were ordered from JLCPCB.

2. DispenSING SOLDER PASTE

In order to assemble the board, a paste made of flux and microscopic beads of solder is first dispensed using a stencil to precisely cover the pads.

3. Placing Components

Using tweezers, components are carefully placed on the board. The paste is sticky enough to hold placed components down and prevent them from falling off. The precise alignment of components on their pads is not important, since the paste melts during the reflow baking process and surface tension automatically aligns them.

4. ReFLOWING

Once the board has been prepared with paste and components, it is placed into an oven and baked at high temperatures precisely controlled by a specific curve designated by the solder paste manufacturer. To accomplish this, I used my custom Controleo 3 reflow oven that I built, which is based off of a heavily modded toaster oven.

5. Visual Inspection

After reflowing, the board is carefully inspected using a magnifying glass to find any unwanted solder “bridges” which may cause short circuits.

6. Final ASSEMBLY

The reflow process only works for surface mounts parts. Through hole parts (which have pins that extend through the circuit board) such as pin headers and LEDs must be added by hand.