Component packages explained: through-hole and SMD
Electronic component packages explained: through-hole (axial, radial, DIP, TO-220) and surface-mount (0402 to 2512, SOT-23, SOIC, TSSOP, QFN, QFP, BGA), with metric and imperial codes and hand-solderability.
Through-hole (THT)
Leads go through holes in the board. Axial (resistors, diodes: leads at both ends), radial (electrolytic capacitors, LEDs), DIP (two rows of pins at 2.54 mm pitch, fits breadboards), TO-92 (small transistors), TO-220 (regulators and power transistors with a tab for a heatsink). Easy to handle and robust, but large and expensive to assemble by machine. Today used mainly for connectors and power parts.
Two-terminal SMD: the size codes
Passive SMD parts are named by size. The common code is imperial (inches), but the metric code uses the same digits for different sizes, which causes real ordering mistakes.
| Imperial | Metric | Size (mm) | By hand? |
|---|---|---|---|
| 0201 | 0603M | 0.6 × 0.3 | No |
| 0402 | 1005M | 1.0 × 0.5 | Difficult |
| 0603 | 1608M | 1.6 × 0.8 | Yes, with practice |
| 0805 | 2012M | 2.0 × 1.25 | Yes |
| 1206 | 3216M | 3.2 × 1.6 | Easy |
| 2512 | 6332M | 6.3 × 3.2 | Easy |
Integrated circuits
| Package | Leads | Typical pitch | By hand? |
|---|---|---|---|
| SOT-23 | 3 to 6 gull-wing | 0.95 mm | Yes |
| SOIC | Gull-wing, two sides | 1.27 mm | Yes |
| TSSOP / MSOP | Gull-wing, two sides | 0.65 / 0.5 mm | With flux and practice |
| QFP | Gull-wing, four sides | 0.5 to 0.8 mm | With drag soldering |
| QFN / DFN | Pads under the edge, thermal pad | 0.4 to 0.65 mm | Hot air or reflow |
| BGA | Balls under the body | 0.4 to 1.27 mm | No: reflow and X-ray inspection |
Why the package matters
The same chip often comes in several packages. The choice affects size, heat dissipation, assembly cost, whether a prototype can be reworked by hand, and whether the assembler can inspect the joints. It is a design decision, not a detail.
Package choice interacts with thermal design, assembly yield and cost at volume. We choose packages in the schematic phase with the assembly process in mind, and avoid parts that make prototypes impossible to rework.
Parts and tools to practise
| Item | What to look for | Buy |
|---|---|---|
| SMD resistor and capacitor sample book | 0603 or 0805 values in a book. Handy for rework once you solder SMD. | View on Amazon ↗ |
| ESD-safe precision tweezers | Fine straight and curved tips to hold small parts. | View on Amazon ↗ |
| Magnifier or USB microscope | To inspect joints. Most bad joints look fine to the naked eye. | View on Amazon ↗ |
| Solderless breadboard kit with jumper wires | Full-size breadboard, pre-cut jumpers and flexible Dupont wires. | View on Amazon ↗ |
Links go to Amazon. As an Amazon Associate we earn from qualifying purchases, at no extra cost to you. We only list tools of the kind we use on our own bench.
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