2.4 Components and Packages

A schematic symbol is a theoretical instruction; a component package is a physical constraint. In manufacturing, the "Package" refers to the dimensions, lead style, and material casing of the electronic part. The machine does not care if the chip is a microcontroller or a timer; it only cares about the geometry of the plastic body and how the metal leads contact the solder paste. If you select a package that exceeds the capability of the assembly line, the design fails regardless of the electrical logic.

Passives: The Dust and the Bricks

Resistors and capacitors (Passives) make up 80% to 90% of the placement count on a typical board. They are defined by standard imperial size codes (e.g., 0402, 0603).

The Size-Risk Trade-off

• 0201 or smaller: These are effectively microscopic dust.
  • Risk: "Tombstoning" (standing up vertically) due to uneven surface tension during reflow. Requires high-precision machines.
• 0402 / 0603: The industry standard.
  • Risk: Low. Cheap and easy to place.
• 1206 or larger: "Bricks."
  • Risk: Cracking. Large ceramic bodies are brittle. If the PCB flexes during depanelization, the capacitor cracks and shorts.

Integrated Circuits (ICs): Leads vs. No Leads

Chips come in two primary mechanical flavors: those with visible legs (Leaded) and those with pads underneath (Leadless).

Leaded Packages (SOIC, QFP, TSSOP)

These have metal legs protruding from the side.

• Benefit: The solder joint is visible. An inspector can verify it with a simple magnifying glass.
• Risk: Bent leads. If a leg is bent by 0.1 mm in the tray, it will not touch the pad.

Leadless Packages (QFN, BGA, LGA)

These have connections underneath the body.

• Benefit: High density. You can fit more silicon in less space.
• Risk: Invisible joints. You cannot inspect these visually.
• If you use BGA/QFN → Then you must budget for X-Ray inspection.

Connectors: The Stress Points

Connectors are the only interface where the user physically touches the PCBA (plugging in cables).

• Surface Mount (SMT) Connectors: Held on only by solder.
  • Risk: Ripping off the board. If the user pulls the cable hard, the pads peel off the FR-4.
• Through-Hole Connectors: Legs go through the board.
  • Risk: Manual assembly cost, but mechanically superior.
  • If the connector will be plugged/unplugged frequently (e.g., USB charging port) → Then use Through-Hole or SMT with additional mechanical anchor tabs.

Handling Risks: Physics and Chemistry

Components are not inert. They are sensitive to moisture, electrostatic discharge (ESD), and orientation.

Polarity (Directionality)

Some components work only one way.

• Parts: Diodes, Electrolytic Capacitors, ICs, LEDs.
• The Trap: Silkscreen markers (dots/lines) must match the physical indentation or stripe on the component.
• If a Tantalum capacitor is placed backwards → Then it will likely explode or catch fire upon power-up.

Moisture Sensitivity Level (MSL)

Plastic absorbs water from the air.4 When a moisture-laden chip enters the 260˚C reflow oven, that water turns to steam instantly. The pressure cracks the plastic body from the inside out. This is called "Popcorning."

• MSL 1: Immune to moisture.
• MSL 3: Must be mounted within 168 hours of opening the vacuum bag.
• If the MSL floor life is exceeded → Then the parts must be "Baked" (heated slowly) to dry them out before assembly.⁶

Packaging Formats: Tape vs. Tray vs. Tube

Machines feed components from specific carriers.

• Tape and Reel: The standard for 99% of parts. Fast, continuous feeding.
• Tray: Used for large, flat chips (QFP, BGA).
• Tube: Legacy format. Avoid if possible; requires special vibrating feeders.

Pro-Tip: Never send "Cut Tape" (loose strips of tape) for mass production. Machines need a "Leader" (empty tape) to thread the feeder. If you send a strip of 10 resistors, the machine cannot pick them up.

Final Checklist

Category	Parameter	Risk Factor	Critical Control
Passives	Size Code	Tombstoning / Cracking	Avoid 0201 unless necessary; use 0603+ for ruggedness.
ICs	Lead Style	Hidden Solder Joints	Require X-Ray for all QFN/BGA parts.
Mechanical	Stress	Connector Strength	Use through-hole anchors for I/O ports.
Handling	MSL	Popcorning	Track "Time Open" for MSL 3+ parts; bake if expired.
Handling	Polarity	Explosion / Short	Explicitly mark Pin 1 on Silkscreen and Assembly Drawing.
Sourcing	Format	Pick Failure	Order Tape & Reel; avoid Cut Tape/Loose parts.