CID Design for Testability & Reliability 2 — Questions and Answers
Question 1: What is the primary purpose of a boundary scan (JTAG) architecture in PCB design?
- To increase signal routing density
- To enable in-circuit testing of board interconnects without physical test probes (Correct answer)
- To reduce power consumption during manufacturing
- To improve impedance matching on high-speed lines
Correct answer: To enable in-circuit testing of board interconnects without physical test probes
Boundary scan (IEEE 1149.1/JTAG) shifts test data through cells at component pins, allowing interconnect faults to be detected without bed-of-nails probes.
Question 2: Which failure mode describes an intermittent open caused by thermal cycling stress on a solder joint?
- Electromigration
- Tin whisker growth
- Solder fatigue cracking (Correct answer)
- Conductive anodic filament (CAF)
Correct answer: Solder fatigue cracking
Repeated thermal expansion and contraction cycles induce mechanical fatigue in solder joints, eventually producing cracks that cause intermittent opens.
Question 3: In reliability testing, what does MTBF stand for and what does it quantify?
- Mean Time Before Failure — the average uptime before any degradation begins
- Mean Time Between Failures — the average operational time between successive failures in repairable systems (Correct answer)
- Maximum Thermal Boundary Factor — the thermal limit before component failure
- Minimum Tested Burn-in Factor — the burn-in duration required before shipment
Correct answer: Mean Time Between Failures — the average operational time between successive failures in repairable systems
MTBF (Mean Time Between Failures) is the average elapsed time between inherent failures of a repairable system under normal operating conditions.
Question 4: A designer wants to verify via-to-via clearance on an inner layer without physical cross-section. Which DFT technique best supports this?
- Flying probe test
- JTAG boundary scan
- Automated X-ray inspection (AXI) (Correct answer)
- Functional end-of-line test
Correct answer: Automated X-ray inspection (AXI)
AXI uses X-ray imaging to inspect internal PCB features such as buried vias, inner-layer clearances, and hidden solder joints non-destructively.
Question 5: Which design practice improves the ability to isolate faults during in-circuit test (ICT) by preventing back-driving issues?
- Adding test point vias directly on component pads
- Inserting series resistors (typically 10–100 Ω) on signal lines (Correct answer)
- Removing all decoupling capacitors from the board
- Increasing via aspect ratio beyond 10:1
Correct answer: Inserting series resistors (typically 10–100 Ω) on signal lines
Series resistors isolate driving sources from ICT fixture drivers, preventing back-drive currents from damaging active devices during test.
Question 6: What is 'test coverage' in the context of PCB manufacturing test?
- The percentage of board area covered by solder mask
- The fraction of potential fault sites detected by a given test method (Correct answer)
- The number of test points per square inch of board
- The ratio of tested boards to total production volume
Correct answer: The fraction of potential fault sites detected by a given test method
Test coverage is the percentage of possible fault sites (opens, shorts, wrong components) that a given test strategy can detect.
Question 7: Which reliability screening method accelerates failure mechanisms by exposing assembled boards to elevated temperature and voltage stress before shipment?
- Environmental Stress Screening (ESS) / Burn-in (Correct answer)
- Highly Accelerated Life Test (HALT)
- Automated Optical Inspection (AOI)
- Design Rule Check (DRC)
Correct answer: Environmental Stress Screening (ESS) / Burn-in
ESS/burn-in applies thermal and electrical stress to precipitate early-life (infant mortality) failures before products reach customers.
CID Design for Testability & Reliability 2
What is the primary purpose of a boundary scan (JTAG) architecture in PCB design?