CIT Rework, Modification, and Repair of Electronic Assemblies 2 — Questions and Answers
Question 1: When reworking a BGA component, what is the primary purpose of the soak phase in the reflow profile?
- To rapidly melt solder and reflow joints
- To activate flux and equilibrate board temperature before reflow (Correct answer)
- To cool the assembly quickly after soldering
- To pre-clean flux residues from previous rework
Correct answer: To activate flux and equilibrate board temperature before reflow
The soak phase allows flux to activate and the entire assembly to reach thermal equilibrium, reducing thermal shock before the reflow peak.
Question 2: According to IPC-7711/7721, which condition requires a Class 3 assembly to be scrapped rather than repaired?
- A lifted pad that can be re-bonded with conductive epoxy
- Conductor damage exceeding allowable width reduction limits after repair (Correct answer)
- A misaligned component within acceptable tolerance
- A single cold solder joint on a through-hole lead
Correct answer: Conductor damage exceeding allowable width reduction limits after repair
IPC-7711/7721 defines allowable repair limits; conductor damage that still exceeds width reduction limits after repair renders the assembly non-conforming for Class 3.
Question 3: What is the correct method to remove a through-hole component with multiple leads from a double-sided PCB without damaging plated-through holes?
- Apply heat to all leads simultaneously using a drag soldering iron
- Use a solder extractor on each lead sequentially while heating (Correct answer)
- Clip all leads flush and drill out remaining stubs
- Apply bulk heat from a hot plate and pull the component
Correct answer: Use a solder extractor on each lead sequentially while heating
Sequential heating and solder extraction on each lead prevents the thermal stress that could damage PTHs, ensuring barrel integrity.
Question 4: When conformally coating a reworked area, what must be done if the original coating was a silicone type?
- Apply urethane directly over the silicone residue
- Remove all silicone residue before applying any new coating (Correct answer)
- Thin the silicone and re-apply over existing coating
- Use an epoxy overcoat to seal the silicone residue
Correct answer: Remove all silicone residue before applying any new coating
Silicone residue prevents adhesion of most other coating types and must be fully removed before recoating to ensure conformal coating integrity.
Question 5: A technician notices a solder bridge between two fine-pitch QFP leads after rework. What is the IPC-recommended first corrective action?
- Reflow the entire component and re-inspect
- Use a flux-wetted solder wick to remove the bridge (Correct answer)
- Apply additional solder to balance the joint
- Scrape the bridge away with a sharp blade
Correct answer: Use a flux-wetted solder wick to remove the bridge
A flux-wetted solder wick is the IPC-recommended method for removing solder bridges on fine-pitch leads without damaging the pads.
Question 6: Under IPC-7711/7721, what is the maximum number of times a plated-through hole may be re-soldered before it is considered potentially compromised?
- One time
- Two times
- Three times (Correct answer)
- Five times
Correct answer: Three times
IPC-7711/7721 guidance generally limits PTH re-soldering to three cycles before barrel integrity may be compromised.
Question 7: When performing a modification to add a jumper wire on a Class 3 assembly, which wire type is preferred per IPC-7711/7721?
- Solid copper wire, uninsulated
- Solid nickel wire, bare
- Insulated, stranded copper wire (Correct answer)
- Bare tinned copper, solid
Correct answer: Insulated, stranded copper wire
IPC-7711/7721 requires insulated, stranded copper wire for jumper modifications on Class 3 assemblies to ensure flexibility and prevent shorts.
CIT Rework, Modification, and Repair of Electronic Assemblies 2
When reworking a BGA component, what is the primary purpose of the soak phase in the reflow profile?