Build Instructions

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• 1

  STENCIL PROCESS

  We started the circuit assembly process by applying solder paste to the PCB, but there was a slight problem. The number of components used in this project is quite large, which also means a very high number of pads. Because of this, using a solder paste dispensing syringe would be impractical. Instead, we used a custom stencil made specifically for this circuit.

  A stencil is essentially a thin metal sheet with laser-cut slots and holes that match the component pads on the PCB. Using a stencil allows solder paste to be applied evenly across all pads. In industrial settings, where circuits are mass-produced, stencils are commonly used for this purpose.

  1. We first set up a makeshift PCB and stencil holder using extra PCBs. This setup helped keep the main PCB stationary and ensured it did not move during the process.
  2. After aligning the stencil openings with the PCB component pads, we applied solder paste over the stencil and used a scraper to spread it evenly across the slots. This process deposited a thin, uniform layer of solder paste onto each component pad, preparing the board for component placement.
• 2

  SMD COMPONENTS PLACEMENT PROCESS

  Now comes another tedious process: SMD component placement. It is tedious because of the sheer number of LEDs used in this project. In total, we used 280 LEDs along with several other SMD components, bringing the overall component count to over 300. All of these components had to be placed manually.

  1. We started by placing the 100 nF decoupling capacitors in their designated locations. For the pick-and-place process, we used ESD-safe tweezers.
  2. Next, we placed all 280 SMD RGB LEDs onto their pads, making sure each LED was oriented correctly, as incorrect orientation would prevent the LEDs from working.
  3. After the LED placement, we added the remaining SMD components, which included the SMD push buttons and all the parts related to the power management circuit.
  4. Once all components were placed, the entire board was carefully lifted and placed onto an SMD reflow hotplate. The hotplate heats the PCB from below up to the solder paste melting temperature, which is around 200°C. As a result, all the components are soldered securely to their pads.
• 3

  TESTING SMD LEDs

  Before we move ahead with the placement of the RP2040 board on the backside of the PCB, we tested whether the LEDs were working or not. If the LEDs were not working and the RP2040 was already mounted on the back side of the PCB, we would not be able to use a hot plate to correct the LEDs.

  We added jumper wires to the 5V, GND, and GPIO0 terminals of the RP2040. These jumper wires were then connected to a XIAO SAMD21 M0 development board.

  We uploaded the sketch below to the XIAO, which turns on each LED in the chain one after another in green. This sketch was taken from the example sketches of the Adafruit NeoPixel library.

  #ifdef __AVR__
  #endif
  #define PIN 0
  #define NUMPIXELS 280
  Adafruit_NeoPixel pixels(NUMPIXELS, PIN, NEO_GRB + NEO_KHZ800);
  #define DELAYVAL 50
  void setup() {
  #if defined(__AVR_ATtiny85__) && (F_CPU == 16000000)
  clock_prescale_set(clock_div_1);
  #endif
  pixels.begin();
  }
  void loop() {
  pixels.clear();
  for(int i=0; i<NUMPIXELS; i++) { // For each pixel...
  pixels.setPixelColor(i, pixels.Color(0, 150, 0));
  pixels.show();
  delay(DELAYVAL);
  }
  }

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