A 15cm PCB ruler designed for SMD (Surface Mount Device) engineering typically serves several purposes in electronics design and troubleshooting. Here are some common details and features you might find on such a ruler:

1. Measurement Scales: It includes both metric (centimeters and millimeters) and possibly imperial (inches) measurement scales. These scales are crucial for precise measurements of components, traces, and dimensions on printed circuit boards (PCBs).

2. PCB Trace Widths: The ruler might have markings indicating standard PCB trace widths, which are important for designing and troubleshooting circuit boards.

3. Component Sizing: It often includes markings for standard SMD component sizes, such as 0402, 0603, 0805, 1206, etc. This helps in identifying the physical dimensions of surface mount components.

4. Pin Pitch Measurements: There could be a scale for measuring the pitch between pins on various packages like QFP, SOP, and others, aiding in layout and verification.

5. Tool Integration: Some rulers may include additional tools like a protractor for measuring angles or a notch for determining the polarity of diodes.

6. Reference Information: It might feature reference tables or charts for resistor color codes, capacitor values, or other quick reference information relevant to PCB design and assembly.

7. Material: Usually made from durable materials like stainless steel or plastic, ensuring longevity and resistance to wear and tear in a workshop environment.

8. Color Coding: Often, different colors are used to differentiate between measurement scales or to highlight important markings.

9. Units of Measurement: Clear indications of which units (metric, imperial) are being used on each part of the ruler.

10. Additional Features: Depending on the specific model, additional features such as a magnifying strip for reading small print or a straight edge for cutting or scoring PCBs may be included.

These rulers are indispensable tools for electronics engineers, technicians, and hobbyists working with PCBs, providing both measurement accuracy and convenience in a compact form factor.