PCB Case Studies & Practical Application 4

Question 1

A PCB assembly line reports excessive tombstoning of 0402 resistors during reflow. Which design or process change most directly addresses this defect?

Accepted Answer

Ensure both pads are the same thermal mass and reflow temperature, and verify pad sizes are symmetric

Answer

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?

Accepted Answer

Moisture or flux residue trapped under the conformal coating compromised the dielectric properties

Answer

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?

Accepted Answer

Large switching loop area radiates EMI and increases parasitic inductance, causing voltage spikes

Answer

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?

Accepted Answer

Net connectivity issues that ICT cannot detect, such as opens masked by ICT probing or timing-dependent faults

Answer

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?

Accepted Answer

ENIG, because it provides a flat, co-planar surface critical for fine-pitch BGA solder joint formation

Answer

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.

PCB - Design Practice Test

PCB - Design Practice Test

PCB Case Studies & Practical Application 4