Die Attach

// Die-attach thermal resistance meter and power cycler.
// Board: ESP32 DevKit v1.
//
// Measures the thermal resistance of a die-attach joint, then power-cycles it and
// re-measures - because the interesting result is not the initial number but how it
// DRIFTS as the expansion mismatch works the joint.
//
// Silicon expands 2.6 ppm/K, copper 17. Every power cycle is one repetition of that
// argument, and a joint that will delaminate does so progressively: the thermal
// resistance rises long before anything visibly fails. Watching that rise is the
// whole point, and it is the same diagnostic shape as flash erase time and as solder
// joint resistance elsewhere in this batch.
//
// WIRING
//   GPIO 25   heater MOSFET gate - drives the resistor bonded to the paddle
//   GPIO 4    1-Wire bus: TWO DS18B20s, one on the die side, one on the paddle
//   GPIO 34   ADC1 - voltage across a 0.1 ohm shunt, to get real power
//
// Rth = (T_die - T_paddle) / P     in kelvin per watt.
// Measure the POWER, do not assume it: the resistor's value changes as it heats.

#include <OneWire.h>
#include <DallasTemperature.h>

#define HEATER_PIN   25
#define ONE_WIRE_PIN  4
#define SHUNT_PIN    34
#define SHUNT_OHMS   0.1f
#define SUPPLY_V     12.0f
#define VREF         3.30f
#define ADC_MAX      4095.0f

#define SETTLE_MS    240000UL   // 4 min - thermal settling is SLOW, do not rush it
#define ON_MS         120000UL
#define OFF_MS        120000UL
#define CYCLES_PER_MEASURE  50

OneWire oneWire(ONE_WIRE_PIN);
DallasTemperature sensors(&oneWire);

float readPower() {
  uint32_t acc = 0;
  for (int i = 0; i < 64; i++) { acc += analogRead(SHUNT_PIN); delay(1); }
  float v_shunt = (acc / 64.0f) * (VREF / ADC_MAX);
  float i_amp = v_shunt / SHUNT_OHMS;
  return i_amp * (SUPPLY_V - v_shunt);
}

// index 0 = die side, index 1 = paddle. Check which is which before trusting it.
float measureRth() {
  digitalWrite(HEATER_PIN, HIGH);
  unsigned long t0 = millis();
  float lastDelta = -999;
  while (millis() - t0 < SETTLE_MS) {
    delay(15000);
    sensors.requestTemperatures();
    float d = sensors.getTempCByIndex(0) - sensors.getTempCByIndex(1);
    // stop early once it has genuinely settled
    if (lastDelta > -900 && fabs(d - lastDelta) < 0.05f) break;
    lastDelta = d;
  }
  sensors.requestTemperatures();
  float tDie = sensors.getTempCByIndex(0);
  float tPad = sensors.getTempCByIndex(1);
  float p = readPower();
  digitalWrite(HEATER_PIN, LOW);
  if (p < 0.05f) { Serial.println("# power too low to measure - check the shunt"); return -1; }
  return (tDie - tPad) / p;
}

void setup() {
  Serial.begin(115200);
  pinMode(HEATER_PIN, OUTPUT); digitalWrite(HEATER_PIN, LOW);
  analogSetPinAttenuation(SHUNT_PIN, ADC_11db);
  sensors.begin();
  if (sensors.getDeviceCount() < 2) {
    Serial.println("# need TWO DS18B20 sensors - die side and paddle");
    while (1) delay(5000);
  }
  delay(3000);

  Serial.println("# die attach thermal resistance, with power cycling");
  Serial.println("cycles,rth_k_per_w,t_die_c,t_paddle_c,power_w");

  float r0 = measureRth();
  Serial.printf("# baseline Rth = %.3f K/W\n", r0);

  for (int block = 0; ; block++) {
    for (int c = 0; c < CYCLES_PER_MEASURE; c++) {
      digitalWrite(HEATER_PIN, HIGH); delay(ON_MS);
      digitalWrite(HEATER_PIN, LOW);  delay(OFF_MS);
    }
    float r = measureRth();
    sensors.requestTemperatures();
    Serial.printf("%d,%.3f,%.2f,%.2f,%.2f\n",
                  (block + 1) * CYCLES_PER_MEASURE, r,
                  sensors.getTempCByIndex(0), sensors.getTempCByIndex(1), readPower());
    if (r0 > 0 && r > r0 * 1.30f) {
      Serial.println("# Rth up 30% from baseline - the joint is delaminating");
      Serial.println("# Shear it off now and photograph the fracture face while it is");
      Serial.println("# partially failed. That is the most informative moment.");
    }
  }
}

void loop() { }