CIT Rework, Modification, and Repair of Electronic Assemblies 3 — Questions and Answers
Question 1: What is the primary reason for pre-baking PCB assemblies before BGA rework?
- To activate solder paste on the stencil
- To remove absorbed moisture and prevent popcorning (Correct answer)
- To cure conformal coating before component removal
- To pre-tin pads for better wetting
Correct answer: To remove absorbed moisture and prevent popcorning
Pre-baking drives out absorbed moisture from the PCB and component packages, preventing steam-induced delamination (popcorning) during reflow.
Question 2: When repairing a lifted SMT pad, the technician must ensure the epoxy adhesive cure extends under the replacement pad. Why is this critical?
- To improve thermal conductivity of the pad
- To provide mechanical anchoring so the pad does not lift again under thermal cycling (Correct answer)
- To increase the electrical conductivity of the repair
- To prevent the pad from oxidizing during reflow
Correct answer: To provide mechanical anchoring so the pad does not lift again under thermal cycling
Epoxy under the replacement pad anchors it mechanically so thermal expansion and contraction during cycling do not cause re-lifting.
Question 3: According to IPC-A-610, a solder joint on a Class 2 through-hole assembly is acceptable if solder fill in the barrel is a minimum of:
- 25%
- 50% (Correct answer)
- 75%
- 100%
Correct answer: 50%
IPC-A-610 requires a minimum of 50% barrel fill for Class 2 through-hole solder joints to be considered acceptable.
Question 4: A technician must repair a cracked solder joint on a BGA ball that is not accessible for direct inspection. What technique is used to verify joint quality after repair?
- Visual inspection with 10x magnification
- X-ray inspection (2D or 3D) (Correct answer)
- Electrical continuity test only
- Pull testing of the BGA package
Correct answer: X-ray inspection (2D or 3D)
X-ray inspection (2D or 3D CT) is used to verify BGA solder joint quality, including voids and opens, because the joints are hidden beneath the package.
Question 5: During chip removal on an SMT assembly, which tool setup is preferred to avoid pad damage on a 0201 component?
- Standard soldering iron with a conical tip at full power
- Tweezers with hot-air rework station at controlled temperature (Correct answer)
- Infrared reflow oven set to full profile
- Desoldering braid with high-wattage iron
Correct answer: Tweezers with hot-air rework station at controlled temperature
Tweezers combined with a controlled hot-air station allows simultaneous reflow of both terminations, preventing pad damage from mechanical force.
Question 6: What does the term 'orange peel' describe in the context of conformal coating rework?
- A flux residue pattern caused by poor pre-cleaning
- A surface texture defect indicating improper curing or application of conformal coat (Correct answer)
- A solder mask discoloration from excessive heat
- An oxidation pattern on copper conductors
Correct answer: A surface texture defect indicating improper curing or application of conformal coat
Orange peel describes a rough, dimpled surface texture in conformal coating resulting from improper application viscosity or curing conditions.
Question 7: When selecting a replacement component for repair, IPC-7711/7721 requires that the replacement part must be:
- A different but functionally equivalent part number from any supplier
- Electrically equivalent and meet or exceed the original component specifications (Correct answer)
- The lowest-cost available component with the same footprint
- Approved only if the original part number is no longer available
Correct answer: Electrically equivalent and meet or exceed the original component specifications
IPC-7711/7721 requires that replacement components be electrically equivalent and meet or exceed original specifications to maintain assembly reliability.
CIT Rework, Modification, and Repair of Electronic Assemblies 3
What is the primary reason for pre-baking PCB assemblies before BGA rework?