PCB Case Studies & Practical Application 4 — Questions and Answers
Question 1: A PCB assembly line reports excessive tombstoning of 0402 resistors during reflow. Which design or process change most directly addresses this defect?
- Increase solder paste volume on both pads equally
- Ensure both pads are the same thermal mass and reflow temperature, and verify pad sizes are symmetric (Correct answer)
- Switch to 0603 components
- Reduce reflow oven conveyor speed
Correct answer: Ensure both pads are the same thermal mass and reflow temperature, and verify pad sizes are symmetric
Tombstoning occurs when one pad melts before the other, creating uneven surface tension; ensuring symmetric pad design and equal thermal exposure prevents one end from lifting.
Question 2: A high-reliability aerospace PCB requires conformal coating. After coating, the board fails dielectric withstanding voltage (DWV) testing. What is the most likely cause?
- Conformal coating increased trace impedance
- Moisture or flux residue trapped under the conformal coating compromised the dielectric properties (Correct answer)
- The coating is too thick, causing capacitance between traces
- Wrong coating type was used for aerospace applications
Correct answer: Moisture or flux residue trapped under the conformal coating compromised the dielectric properties
Flux residues left from soldering are ionically conductive; when encapsulated by conformal coating, they trap moisture and cause leakage paths that fail DWV testing.
Question 3: A designer is laying out a switching power supply. The catch diode's current loop (diode, inductor, output capacitor) is very large. What is the consequence?
- Increased voltage drop across the diode
- Large switching loop area radiates EMI and increases parasitic inductance, causing voltage spikes (Correct answer)
- Reduced efficiency due to longer trace resistance
- The diode may not turn on fast enough
Correct answer: Large switching loop area radiates EMI and increases parasitic inductance, causing voltage spikes
The high-frequency switching current loop in a SMPS is the primary EMI radiator; a large loop area acts as a loop antenna and increases parasitic inductance causing damaging voltage spikes.
Question 4: A test engineer discovers that a board passes ICT (In-Circuit Test) but fails functional test. Components are correct and soldering is good. What should the designer investigate?
- ICT fixture probe contact resistance
- Net connectivity issues that ICT cannot detect, such as opens masked by ICT probing or timing-dependent faults (Correct answer)
- Incorrect test program logic in ICT
- Component orientation that ICT cannot verify
Correct answer: Net connectivity issues that ICT cannot detect, such as opens masked by ICT probing or timing-dependent faults
ICT verifies component values and basic connectivity but cannot detect timing-dependent failures, marginally open connections that close under probe pressure, or functional logic errors.
Question 5: A PCB designer is selecting between ENIG (Electroless Nickel Immersion Gold) and HASL surface finish for a board with fine-pitch 0.4mm BGA components. Which is more appropriate and why?
- HASL, because it provides a thicker solder layer for better joint strength
- ENIG, because it provides a flat, co-planar surface critical for fine-pitch BGA solder joint formation (Correct answer)
- HASL, because it is RoHS compliant
- ENIG is only needed for gold wire bonding applications
Correct answer: ENIG, because it provides a flat, co-planar surface critical for fine-pitch BGA solder joint formation
HASL creates uneven, domed pad surfaces that cause coplanarity issues with fine-pitch BGAs; ENIG provides a flat, oxidation-resistant surface ideal for 0.4mm pitch BGA assembly.
Question 6: An engineer is reviewing a design with a single-point ground (star ground) topology. The high-current motor return is connected at the same star point as the analog sensor ground. What problem arises?
- The star topology increases ground inductance
- Motor return currents flowing through shared impedance to the star point create voltage offsets on the analog ground reference (Correct answer)
- Single-point ground violates IPC design rules
- The motor and sensor share the same current, overloading the ground trace
Correct answer: Motor return currents flowing through shared impedance to the star point create voltage offsets on the analog ground reference
Even in star grounding, if the motor's high return current shares any impedance (trace resistance) with the analog reference point, it creates millivolt-level ground offsets that corrupt sensor readings.
Question 7: A PCB for outdoor use must meet IP67 rating. The design uses a standard through-hole connector on the board edge. What modification is required?
- Apply conformal coating over the connector
- Replace with an IP67-rated sealed connector or add a gasketed housing enclosure sealing around the connector (Correct answer)
- Use SMD instead of through-hole connector
- Add a moisture sensor IC to the board
Correct answer: Replace with an IP67-rated sealed connector or add a gasketed housing enclosure sealing around the connector
IP67 (dust-tight, 30-minute immersion to 1m) requires environmental sealing at all board openings; a standard connector must be replaced with an IP67-rated sealed version or properly gasketed in the enclosure.
PCB Case Studies & Practical Application 4
A PCB assembly line reports excessive tombstoning of 0402 resistors during reflow.
Which design or process change most directly addresses this defect?