How to solder a really tiny IC - Tue, Dec 10, 2024

This week was wildcard week which allowed us to explore many differnt facets of making. In the interest of time I opted to hoan a skill that I had been building all semster: PCB milling. The twist was that this week I needed to make a board using a compoent with a footprint not even a 1/64" endmill could manufacutre!

I decided to tango with the ATTINY84A. I had the choice between a QFN or BGA package. I was scared of the BGA package so I opted for the QFN package. Nonetheless this was still going to be a challening soldering job.

Image 1

I started by designing a board for it in kicad. My initial plan was to leverage the small size of this chip to make a really small PCB that could be woren like an earring. The IMU would ge at the middle controling a ring of 8 0805 LEDs around it. On the back I would put a CR2032 which would be the main limitation in how small I could make the chip. Image 1 However, when making the circular cutout for the board, I accidentally made it 25mm in radius rather than in diameter. I only noticed this after it was being milled out. Oh well, I then thought to myself, what are earrings but undersized ornaments (or maybe ornaments are oversized earings…)? Anyways, now I was expressing my holiday spirit and making an ornament! Image 1 It was now time for me to solder the board together. This is when the absolute tinyness of the IC really kicked in. Image 1 Nonetheless, I soldered it on with some solder paste and hot air under the microscope. I then used some solder wick to clean up the sides and hope for the best. This took all the SMD soldering experience that I had built up over the past year and a half at MIT through Solar Car and HTMAA, and I am happy that I can now solder the things that would have scared me when I was an incoming freshmen. The rest of the componets were 0805s that I had a lot of experience with.

Image 1

I now needed to program the board. This was the moment of truth. I soldered on the programming wires, Image 1 connected the board to an Atmel-ICE programmer, Image 1 and pressed upload using the programmer from the Arduino IDE. I held my breath as the sketch compiled and tried to flash. It compiled and hit me with an error.

Well Shit.

I knew it had been too smooth sailing up to this point. Debugging circuity this small could take as little as a couple of minuites to hours diagonising cold solder joints. I went back and reflowed some of the joints on the ATTINY84A and tried to flash again to no avail. I asked Anthony and he suggested to check the programming connections. Sure enough the random google image I used to refrence the ISP pinout was wrong and after chaning the connection from the programmer to my board I got a successful flash. Yay!

I first tested all the LEDs.

Next I flashed this program to it to display a secret message with the device.

const byte pins[] = {2, 1, 0, 10, 9, 8, 5, 3};
const char message[] = "********";


void setup() {
  // Set all pins as output
  for (byte i = 0; i < 8; i++) {
    pinMode(pins[i], OUTPUT);
  }
}


void loop() {
  // Iterate through each character in the message
  for (int i = 0; message[i] != '\0'; i++) {
    byte asciiValue = message[i]; // Get ASCII value of the character


    // Display the ASCII value on the LEDs
    for (byte j = 0; j < 8; j++) {
      // Set each LED to the corresponding bit of the ASCII value
      digitalWrite(pins[j], (asciiValue >> (7 - j)) & 1);
    }


    delay(500); // Wait for half a second to show the character


    // Turn off all LEDs for a brief pause between characters
    for (byte j = 0; j < 8; j++) {
      digitalWrite(pins[j], LOW);
    }
    delay(200); // Pause between characters
  }
  delay(1000);
}

I then put a hook on it to make it a true ornament.

Image 1

And put a battery connector on the back. The ornament does not have a switch to turn it on or off because I originally was trying to save as much space as possible for the earrings. Image 1

The ornament could now be viewed in its full glory.

I was happy when the ornament caught the eye of the local escape room guru and I got to waste her time trying to decode it! Image 1 Can you figure out what the secret message is?

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