PCB Design 3 — Questions and Answers
Question 1: What is the primary reason to avoid 90-degree (right-angle) trace bends at high frequencies?
- They increase manufacturing cost
- The outer corner of a 90° bend can cause acid traps and slightly increases effective trace width, affecting impedance (Correct answer)
- They are impossible to manufacture
- They increase board thickness
Correct answer: The outer corner of a 90° bend can cause acid traps and slightly increases effective trace width, affecting impedance
At high frequencies, 90° bends slightly increase the effective trace width at the corner, changing characteristic impedance and potentially causing reflections; 45° chamfers or arcs are preferred.
Question 2: What does 'layer stackup' refer to in multilayer PCB design?
- The order in which components are placed on the board
- The arrangement and thickness of copper and dielectric layers that make up the PCB (Correct answer)
- The sequence of manufacturing steps
- The number of vias connecting layers
Correct answer: The arrangement and thickness of copper and dielectric layers that make up the PCB
Layer stackup defines how copper signal, ground, and power layers are arranged, along with the dielectric materials and thicknesses between them, directly impacting impedance and EMI.
Question 3: What is 'crosstalk' in PCB design?
- Interference from external electromagnetic sources
- Unwanted coupling of a signal from one trace onto an adjacent trace (Correct answer)
- Noise caused by power supply ripple
- Signal reflection from an impedance mismatch
Correct answer: Unwanted coupling of a signal from one trace onto an adjacent trace
Crosstalk is electromagnetic coupling between adjacent traces where a changing signal on an aggressor trace induces a voltage or current on a nearby victim trace.
Question 4: What is the purpose of a 'via-in-pad' technique?
- To reduce the number of layers needed
- To place a via directly within a component pad, saving space and improving electrical performance for fine-pitch components (Correct answer)
- To connect the silkscreen to copper
- To add thermal relief to SMD pads
Correct answer: To place a via directly within a component pad, saving space and improving electrical performance for fine-pitch components
Via-in-pad places the via directly under an SMD component pad, reducing inductance and saving routing space, commonly used with BGAs and high-frequency components; the via must be filled and capped.
Question 5: What is 'net tie' used for in PCB design?
- Connecting power rails to the same voltage potential
- Intentionally connecting two different net names at a single point to allow separate simulation or analysis (Correct answer)
- Tying unused pins to ground
- Connecting multiple vias together
Correct answer: Intentionally connecting two different net names at a single point to allow separate simulation or analysis
A net tie allows two separately named nets (such as AGND and DGND) to be connected at a single star point on the board while keeping them electrically distinct in simulation.
Question 6: What does 'tombstoning' refer to in SMT assembly?
- Placing a component upside down
- A defect where one end of a two-terminal component lifts off its pad during reflow soldering (Correct answer)
- A via drill defect
- Excessive solder paste application
Correct answer: A defect where one end of a two-terminal component lifts off its pad during reflow soldering
Tombstoning (also called Manhattan effect) occurs when surface tension from molten solder on one pad lifts one end of a passive component, leaving it standing vertically on the other pad.
Question 7: In PCB design, what is the function of a 'test point'?
- A mechanical mounting hole for the board
- A dedicated pad or via used to probe or test electrical signals during manufacturing and debugging (Correct answer)
- A via that connects all ground planes
- A pad reserved for future component population
Correct answer: A dedicated pad or via used to probe or test electrical signals during manufacturing and debugging
Test points are dedicated accessible locations on a PCB where automated test equipment (ICT) or manual probes can contact a net to verify electrical functionality.
PCB Design 3
What is the primary reason to avoid 90-degree (right-angle) trace bends at high frequencies?