PCB Quality Control & Assurance 3 — Questions and Answers
Question 1: In PCB quality control, what is a 'lifted land' defect?
- A via that has separated from its barrel
- A copper pad that has partially or fully detached from the board surface (Correct answer)
- A solder mask opening that exposes too much copper
- A trace that rises above the PCB plane due to warpage
Correct answer: A copper pad that has partially or fully detached from the board surface
A lifted land occurs when a copper pad separates from the substrate, often caused by excessive heat during soldering or mechanical stress.
Question 2: What is the primary purpose of ionic contamination testing (such as ROSE testing) on a PCB?
- To measure copper trace resistivity
- To detect residual flux and ionic contaminants that could cause corrosion or leakage (Correct answer)
- To verify solder joint integrity under vibration
- To check dielectric constant of the laminate
Correct answer: To detect residual flux and ionic contaminants that could cause corrosion or leakage
ROSE (Resistivity of Solvent Extract) testing measures ionic contamination levels on PCBs, which if excessive can cause electrochemical migration, corrosion, and field failures.
Question 3: What does the term 'tombstoning' refer to in SMT assembly quality?
- A component that sinks into the solder during reflow
- A component that stands up on one end due to uneven solder wetting forces (Correct answer)
- Solder bridging between two adjacent pads
- A BGA ball that collapses completely during reflow
Correct answer: A component that stands up on one end due to uneven solder wetting forces
Tombstoning (also called drawbridging) occurs when one end of a small SMD component lifts off its pad, typically caused by unequal wetting forces or thermal imbalance.
Question 4: Which test method applies electrical current between opposing copper features to detect short circuits or leakage paths?
- Time Domain Reflectometry (TDR)
- Flying probe test
- Insulation resistance test (Correct answer)
- Boundary scan test
Correct answer: Insulation resistance test
Insulation resistance testing applies voltage between conductors to measure the resistance of dielectric material and detect leakage paths or shorts.
Question 5: A PCB manufacturer reports a 'bow and twist' measurement of 0.75% on a standard FR4 board. According to IPC-6012, is this acceptable?
- Yes, IPC-6012 allows up to 1.5% bow and twist
- No, the maximum allowed is 0.5% for SMT boards (Correct answer)
- Yes, any value below 1% is always acceptable
- No, bow and twist must be exactly 0% for all boards
Correct answer: No, the maximum allowed is 0.5% for SMT boards
IPC-6012 limits bow and twist to 0.75% maximum for boards with SMT components and 1.5% for through-hole only boards; 0.75% on an SMT board is at the boundary and typically rejected.
Question 6: What is 'negative etch back' (also called resin smear) in plated through-holes?
- Copper plating that extends beyond the hole wall
- Resin from the laminate that covers the glass fibers and copper foil in the hole wall (Correct answer)
- Excessive removal of dielectric material during desmear
- Underetching that leaves copper residue between traces
Correct answer: Resin from the laminate that covers the glass fibers and copper foil in the hole wall
Resin smear occurs during drilling when heat melts laminate resin onto the hole wall, covering conductor layers and potentially causing open circuits in the via barrel.
Question 7: In statistical process control (SPC) for PCB manufacturing, what does a point outside the 3-sigma control limit indicate?
- Normal process variation that should be ignored
- A special cause variation requiring immediate investigation (Correct answer)
- That the process is operating within acceptable limits
- That the specification limits need to be widened
Correct answer: A special cause variation requiring immediate investigation
A point beyond 3-sigma control limits signals a special cause variation — an unusual event outside normal process behavior — requiring root cause investigation and corrective action.
PCB Quality Control & Assurance 3
In PCB quality control, what is a 'lifted land' defect?