PCB Quality Control & Assurance 5 — Questions and Answers
Question 1: What is the purpose of thermal stress testing (solder float test) on a bare PCB?
- To measure the melting point of the solder mask material
- To evaluate laminate and plated through-hole integrity after exposure to soldering temperatures (Correct answer)
- To verify the board's impedance characteristics at elevated temperature
- To test component attachment strength after reflow
Correct answer: To evaluate laminate and plated through-hole integrity after exposure to soldering temperatures
The solder float test exposes the PCB to molten solder at 288°C for 10 seconds to evaluate if the laminate, via barrels, and plating can withstand soldering process temperatures without delamination or cracking.
Question 2: What is 'outgassing' in the context of PCB quality, and when is it most likely to occur?
- Flux vapors released during wave soldering that cause bridging defects
- Volatile gases released from laminate materials during high-temperature processing (Correct answer)
- Copper oxidation gases that degrade solder wettability
- Air expelled from vias during aqueous cleaning
Correct answer: Volatile gases released from laminate materials during high-temperature processing
Outgassing occurs when volatile compounds trapped in the laminate material are released during high-temperature processes like soldering, potentially causing blow holes in solder joints or delamination.
Question 3: In a PCB quality audit, what does DPMO stand for and how is it used?
- Defective Parts per Manufacturing Order — counts total rejected boards
- Defects Per Million Opportunities — a Six Sigma metric for measuring process quality (Correct answer)
- Design Process Margin Output — evaluates design margin against specifications
- Drilling Precision Measurement Operation — tracks drill accuracy statistics
Correct answer: Defects Per Million Opportunities — a Six Sigma metric for measuring process quality
DPMO (Defects Per Million Opportunities) normalizes defect counts against the number of defect opportunities, allowing fair comparison of process quality across different product complexities.
Question 4: Which condition represents 'delamination' versus 'blistering' in PCB laminate defects?
- Delamination is a subsurface separation while blistering is a visible surface bubble (Correct answer)
- Blistering occurs between copper and laminate while delamination occurs between fiber and resin
- They are identical defects with different terminology
- Delamination always spans the full board while blistering is localized
Correct answer: Delamination is a subsurface separation while blistering is a visible surface bubble
Delamination is a separation between laminate layers or between laminate and copper that may not be externally visible, while blistering is a localized swelling visible as a raised bubble on the board surface.
Question 5: What is the role of a quality management system (QMS) certification like ISO 9001 for a PCB manufacturer?
- It guarantees that all manufactured PCBs will be defect-free
- It certifies that the manufacturer's processes and documentation meet standardized quality management requirements (Correct answer)
- It specifically validates electrical performance of the PCBs produced
- It replaces the need for IPC-A-610 acceptance inspections
Correct answer: It certifies that the manufacturer's processes and documentation meet standardized quality management requirements
ISO 9001 certifies that a manufacturer has implemented a documented quality management system with consistent processes, not that specific products are defect-free or meet any particular technical standard.
Question 6: A PCB shows 'haloing' around drilled holes, appearing as a light ring in the laminate. What is the most likely cause?
- Excessive copper plating thickness in the via barrel
- Mechanical delamination of the laminate caused by drill stress or improper parameters (Correct answer)
- Incorrect solder mask application near the hole
- Electrochemical migration between via and ground plane
Correct answer: Mechanical delamination of the laminate caused by drill stress or improper parameters
Haloing is a light-colored ring around drilled holes caused by separation of glass fibers from resin due to mechanical stress from drilling, indicating damage to the laminate structure.
Question 7: During incoming inspection of copper-clad laminate, which property is most critical to verify for controlled impedance PCB designs?
- Surface roughness of the copper foil
- Dielectric constant (Dk) and loss tangent of the laminate material (Correct answer)
- Peel strength of the copper foil from the substrate
- Moisture absorption rate of the laminate
Correct answer: Dielectric constant (Dk) and loss tangent of the laminate material
For controlled impedance designs, the dielectric constant (Dk) directly determines trace impedance calculations; variation from specified Dk values will cause impedance deviations in the finished board.
PCB Quality Control & Assurance 5
What is the purpose of thermal stress testing (solder float test) on a bare PCB?