CIS Soldering Techniques & Standards 3 — Questions and Answers
Question 1: What soldering defect is characterized by solder that bridges two adjacent pads or leads that should not be connected?
- Solder bridge (short) (Correct answer)
- Icicle
- Cold solder joint
- Tombstoning
Correct answer: Solder bridge (short)
A solder bridge is an unintended solder connection between two conductors that should be electrically isolated, creating a short circuit.
Question 2: Tombstoning of an 0402 chip resistor during reflow is most commonly caused by:
- Uneven heating causing one pad to wet before the other (Correct answer)
- Excessive solder paste volume on both pads
- Too slow a reflow profile ramp rate
- Using nitrogen atmosphere in the reflow oven
Correct answer: Uneven heating causing one pad to wet before the other
Tombstoning occurs when one pad melts and wets before the other, creating an unbalanced surface tension force that lifts one end of the component.
Question 3: A 'cold solder joint' is primarily identified by which visual characteristic per IPC-A-610?
- Dull, grainy, or rough surface texture with poor wetting (Correct answer)
- Bright, shiny appearance with excess solder
- Dark brown discoloration around the joint
- Absence of solder on the lead
Correct answer: Dull, grainy, or rough surface texture with poor wetting
Cold solder joints exhibit a dull, grainy, or crystalline surface caused by insufficient heat or movement during solidification, indicating poor wetting.
Question 4: What is 'solder balling' and what is its primary cause in SMT reflow processes?
- Small solder spheres adjacent to joints, caused by solder paste spattering from moisture or rapid outgassing (Correct answer)
- Large solder balls formed from too much paste volume
- Solder balls inside BGA packages from incorrect ball attach
- Solder droplets on the PCB surface from wave soldering splatter
Correct answer: Small solder spheres adjacent to joints, caused by solder paste spattering from moisture or rapid outgassing
Solder balling creates tiny solder spheres near joints when flux solvents or moisture cause explosive outgassing during the preheat phase, scattering solder particles.
Question 5: Per IPC-A-610 Class 2, what is the maximum allowable amount of lifted land (pad lifting) on a surface mount component?
- Any lifted land that reduces electrical performance is a defect; minor lifting without separation may be a process indicator (Correct answer)
- Up to 25% of pad area may be lifted
- Lifted lands are always acceptable in Class 2
- No lifted lands are permitted under any classification
Correct answer: Any lifted land that reduces electrical performance is a defect; minor lifting without separation may be a process indicator
IPC-A-610 classifies lifted lands based on whether the electrical function is compromised; any lift that separates the conductor is a defect across all classes.
Question 6: Which defect type describes excess solder that forms a sharp spike or point protruding from a solder joint?
- Solder icicle or solder peak (Correct answer)
- Solder void
- Blow hole
- Skew
Correct answer: Solder icicle or solder peak
A solder icicle (or solder peak) is a pointed protrusion of solder that can violate minimum electrical clearance requirements.
Question 7: What is 'dewetting' in soldering, and how does it differ from 'non-wetting'?
- Dewetting is when molten solder initially wets then recedes, leaving a rough irregular surface; non-wetting means solder never adhered (Correct answer)
- Dewetting means the solder never melted properly; non-wetting means it melted but didn't flow
- Dewetting and non-wetting are interchangeable terms in IPC standards
- Dewetting applies only to through-hole joints; non-wetting applies to SMT only
Correct answer: Dewetting is when molten solder initially wets then recedes, leaving a rough irregular surface; non-wetting means solder never adhered
Dewetting indicates the base metal surface was initially solderable but became contaminated or oxidized, while non-wetting means adhesion never occurred.
CIS Soldering Techniques & Standards 3
What soldering defect is characterized by solder that bridges two adjacent pads or leads that should not be connected?