DO-35 and SOT-23 are two widely used semiconductor packages that represent fundamentally different eras of electronics manufacturing: through-hole (THT) versus surface-mount (SMT). While both house small-signal devices like diodes, transistors, or DIACs, they differ significantly in structure, performance, and application suitability.
1. Physical Structure & Dimensions
DO-35: An axial-leaded glass package—cylindrical (1.8–2.2 mm diameter, 4–5 mm long) with wires extending from both ends. Commonly used for Zener diodes, switching diodes, or DIACs (e.g., DB3).
SOT-23: A flat, plastic surface-mount package (typically 2.9×1.3×1.1 mm) with three gull-wing leads spaced ～1.9 mm apart. Ubiquitous for MOSFETs, BJTs, and small analog ICs.
2. Mounting & PCB Assembly
DO-35 requires through-hole insertion and wave soldering—suited for manual assembly or low-density boards. SOT-23 is placed directly on the PCB surface and reflow-soldered, enabling high-speed automated production, higher component density, and double-sided assembly. In smartphones and wearables, SOT-23 has largely replaced DO-35.
3. Electrical & High-Frequency Performance
The long leads of DO-35 introduce significant parasitic inductance (several nH), causing ringing or signal delay above 10 MHz—making it unsuitable for high-speed circuits. SOT-23’s short leads and optimized layout minimize parasitics, excelling in RF front-ends, fast switching, and precision analog applications.
4. Thermal & Power Handling
Both are low-power packages, but SOT-23 dissipates heat more effectively via its exposed leads and optional thermal pad connected to copper pours. Typical power ratings: SOT-23 = 300–500 mW; DO-35 = 200–300 mW, with heat primarily conducted through leads—less efficient.
5. Reliability & Environmental Suitability
DO-35’s glass body offers excellent insulation and high voltage tolerance (often >600 V), ideal for AC mains applications like light dimmers. However, it’s fragile under mechanical stress. SOT-23’s epoxy molding provides superior shock/vibration resistance but usually limits voltage ratings to<200 V, restricting it to low-voltage DC systems.
6. Cost & Industry Trends
DO-35 is extremely cheap per unit and remains in cost-sensitive appliances. Yet, its incompatibility with SMT lines is driving it out of modern manufacturing. SOT-23 has slightly higher component cost but lower total assembly, test, and rework expenses—making it a de facto industry standard.
Selection Guidance:
Choose DO-35 for high-voltage, low-frequency, through-hole designs (e.g., TRIAC trigger circuits);
Choose SOT-23 for low-voltage, high-density, SMT-based products (e.g., sensor interfaces, PMIC peripherals).
In Summary:
DO-35 embodies legacy reliability; SOT-23 represents modern efficiency. They serve distinct design philosophies—not one is universally “better.” Understanding their trade-offs ensures optimal component selection in real-world engineering.
