PCB Quality Control & Assurance 4 — Questions and Answers
Question 1: What is the function of an X-ray inspection system in PCB quality assurance?
- To measure surface roughness of copper traces
- To inspect hidden solder joints such as BGA balls and buried vias (Correct answer)
- To verify dielectric constant of the laminate material
- To detect electromagnetic interference patterns
Correct answer: To inspect hidden solder joints such as BGA balls and buried vias
X-ray inspection reveals solder joint quality under BGA packages, hidden voids, and internal via integrity that cannot be seen with optical inspection.
Question 2: Which IPC standard defines the requirements for the design of rigid printed boards?
- IPC-A-610
- IPC-6012
- IPC-2221 (Correct answer)
- IPC-7711
Correct answer: IPC-2221
IPC-2221 'Generic Standard on Printed Board Design' establishes the generic requirements for the design and manufacture of rigid PCBs.
Question 3: A PCB shows 'pink ring' around drilled holes. What does this indicate?
- Correct copper plating thickness
- Delamination of the glass-resin bond in the hole annular ring area (Correct answer)
- Successful desmear treatment
- Oxidation of the outer copper layer
Correct answer: Delamination of the glass-resin bond in the hole annular ring area
Pink ring appears when the glass-resin interface separates around a drilled hole, exposing the glass fiber bundles which appear pink/white, indicating a lamination or drilling quality issue.
Question 4: In PCB testing, what is the difference between in-circuit test (ICT) and functional test (FT)?
- ICT tests individual components while FT tests the board operating as a complete system (Correct answer)
- ICT uses flying probes while FT uses a bed-of-nails fixture
- ICT is performed before soldering while FT is performed after soldering
- ICT tests analog circuits while FT tests digital circuits only
Correct answer: ICT tests individual components while FT tests the board operating as a complete system
ICT tests individual component values and connections using bed-of-nails or flying probes, while functional test powers up the assembly and verifies it performs its intended function.
Question 5: What is 'copper voids' in plated through-hole barrels, and why are they a quality concern?
- Excess copper that creates shorts between layers
- Gaps in the copper plating within the hole barrel that reduce current-carrying capacity and reliability (Correct answer)
- Copper deposition on the non-conductor areas of the board
- Overplating of copper that reduces hole diameter
Correct answer: Gaps in the copper plating within the hole barrel that reduce current-carrying capacity and reliability
Copper voids are plating discontinuities within PTH barrels that reduce current-carrying capability, create weak points susceptible to thermal cycling failures, and can cause open circuits.
Question 6: What is the purpose of a microsection (cross-section) in PCB quality verification?
- To measure the board's dielectric constant electrically
- To physically cut and polish the PCB to examine internal layer registration, plating thickness, and hole quality under a microscope (Correct answer)
- To test the mechanical flexural strength of the laminate
- To verify correct net connectivity using continuity testing
Correct answer: To physically cut and polish the PCB to examine internal layer registration, plating thickness, and hole quality under a microscope
Microsectioning involves cutting, mounting, polishing, and etching a PCB sample to allow microscopic examination of internal features like layer registration, copper plating thickness, and via quality.
Question 7: According to IPC-A-610, which condition is classified as a 'defect' rather than a 'process indicator'?
- Minor solder bridging that can be reworked without affecting function
- A lifted pad on a surface mount component (Correct answer)
- Slightly dull solder joint surface appearance
- Minor flux residue on the board surface
Correct answer: A lifted pad on a surface mount component
A lifted pad is an IPC Class 1/2/3 defect because it represents a condition that may result in failure of the assembly in its intended environment and cannot be acceptably reworked.
PCB Quality Control & Assurance 4
What is the function of an X-ray inspection system in PCB quality assurance?