Introduction to the Component
This tutorial centers on Chip Quik’s EXB-SN62PB36AG2 Solder Bar, a 1-pound bar of Sn62/Pb36/Ag2 alloy. This is a eutectic solder with a melting point of approximately 179°C, which is lower than standard Sn63/Pb37. The addition of 2% silver improves joint strength and reduces silver leaching from component terminations, making it ideal for surface-mount technology (SMT) prototypes and rework. Its bar form is perfect for hand-soldering or for use in a small solder pot, offering excellent wetting and a bright finish. This alloy is chosen for this tutorial because it provides a robust, reliable connection for constructing a precision voltage reference circuit, where thermal and mechanical stability are critical.
Design Requirements and Specifications
We will design a precision +5.000V reference powered from a 9V battery, suitable for calibrating multimeters or ADCs. Key requirements: output voltage accuracy within ±0.1% at 25°C, temperature drift under 50 ppm/°C, output current capability of 10 mA, and low output noise (< 50 µVrms). The circuit must be built on a single-sided FR-4 PCB using through-hole and SMT components, with all joints made using the EXB-SN62PB36AG2 solder bar.
Step-by-Step Design Process with Calculations
Begin by selecting the voltage reference IC: the LT1021-5 from Analog Devices. This offers 0.05% initial accuracy and 20 ppm/°C drift. The circuit requires a supply voltage of 9V; the LT1021-5 has a dropout voltage of 2V, so 9V is sufficient. Calculate the power dissipation: at 10 mA output, the total current drawn is about 12 mA (including quiescent). Power = (9V - 5V) * 0.012A = 0.048W, which is safe. For filtering, add a 10 µF tantalum capacitor at the input and a 10 µF output capacitor. The output impedance of the reference is low, but a 0.1 µF ceramic bypass at the IC pins reduces high-frequency noise. Use a 1kΩ potentiometer in series with a 1kΩ fixed resistor as a trim network from the output to the adjust pin (if the reference has an adjust pin; the LT1021-5 does not, so we rely on its fixed accuracy). Alternatively, for a fixed design, use a precision voltage divider with 0.1% resistors to create a 5.000V output if the reference is 5.000V nominal. For this tutorial, we assume the LT1021-5 provides exactly 5.000V ±0.05% without trimming.
Component Selection Rationale for the Complete BOM
BOM: LT1021-5 (U1); 10 µF 16V tantalum capacitors (C1, C2); 0.1 µF ceramic capacitor (C3); 1kΩ 0.1% resistor (R1) for output loading test; 9V battery snap connector; single-sided FR-4 PCB; and Chip Quik EXB-SN62PB36AG2 solder bar. The solder bar is chosen over standard Sn63/Pb37 because the 2% silver content prevents leaching of silver from the LT1021-5’s leadframe, ensuring a reliable metallurgical bond. The bar form allows us to melt a small amount into a solder pot for tinning wires or for use with a soldering iron tip that can be dipped. For hand-soldering, we will cut a small piece of the bar and melt it onto the iron tip, avoiding flux-core issues—this forces us to apply external flux (e.g., rosin flux pen) for optimal wetting.
Simulation Tips and What to Look For
Simulate the circuit in LTspice using the LT1021-5 model. Key checks: apply a 9V supply, monitor output voltage with no load and with a 500Ω load (10 mA). Look for output voltage ripple: it should be less than 1 mV peak-to-peak. Add a 100 µV, 60 Hz sine wave to the input to simulate ripple rejection—the output should show less than 10 µV of ripple (PSRR > 80 dB). Temperature sweep from 0°C to 70°C; output drift should be less than 0.1 mV (20 ppm/°C 50°C 5V = 5 mV, so 0.1 mV is excellent). If the simulation shows oscillation, add a 1Ω resistor in series with the output capacitor to dampen. Avoid simulating the solder joint itself—the alloy’s resistivity is negligible at these currents.
Prototype Build and Testing Methodology
Fabricate a small PCB with copper pads. Clean the board with isopropyl alcohol. Apply flux to all pads. Using a soldering iron set to 320°C, cut a 1 cm piece of the EXB-SN62PB36AG2 bar and melt it onto the iron tip. Solder the LT1021-5 first, ensuring all pins are wetted. The solder should flow smoothly and form a concave fillet. Add capacitors and the resistor. For the battery connector, tin the wire ends by dipping them into a small solder pot (a metal cup heated on a hotplate) filled with the same solder bar. Inspect joints with a magnifier—they should be shiny and smooth. Power the circuit with a 9V battery and measure output voltage with a 6.5-digit multimeter. Expect 5.000V ± 0.005V. Test load regulation by switching a 500Ω resistor on and off; the output should change less than 0.1 mV.
Performance Verification and Optimization
Measure temperature drift by placing the PCB in a temperature chamber from 0°C to 70°C, logging output every 10°C. The drift should be under 0.25 mV. If drift is higher, check for mechanical stress on the IC—the solder joints must be stress-free. For noise testing, use a low-noise preamplifier and an oscilloscope in AC mode; noise should be below 50 µVrms. Optimization: if the output is slightly off, you can add a precision voltage divider (two 0.01% resistors) to create a trim. However, the EXB-SN62PB36AG2 solder provides a reliable, low-stress connection that minimizes thermal EMFs at junctions, which is crucial for precision. For best results, reflow the board in a toaster oven at 200°C to homogenize the solder joints and relieve any residual stress. This tutorial demonstrates that the choice of solder alloy directly impacts circuit performance, especially in precision analog designs.
