Common Solder Defects and Root Cause Analysis

General guidance

it is well known that 60-70% of all solder defects can be traced back to the printing defects such as solder bridging, cold slump and Uneven Solder Paste Deposit while fine tuning the reflow profile do benefit and avoid some soldering defects

Tombstoning

Tombstoning defects can be attributed to unbalanced wetting forces, generally found when assembling passive components (see Figure 1).

To reduce tombstoning, minimizing the thermal gradient between pads prior to reflow is crucial, this can be accomplished by slowing the ramp rate from ambient to peak, allowing the PWB assembly to gradually/evenly rise in temperature.

Fig.1

Solder balling/beading Solder beading

Solder beading is big solder ball, it occurs when isolated solder paste aggregates are formed under low stand-off components. At reflow, the aggregate melts and emerges from the underside of the component, separate from the solder mass/joint.

Fig.2

Solder balling occurs if the solder paste spatters (small explosive) or the solder paste slump and the flux spreads. 1- 1.5°C/second ramping rate is recommended to minimize those defects.

Voiding

Voiding occurs in components such as BGA, SGA, QFN, and LGA and increase with a higher reflow temperatures and greater surface tension with lead free alloy. Voiding is affected by the ability of the flux chemistry to effectively (how quickly and how completely) remove surface oxides. As the solvent volatizes, the viscosity of the flux remnant increases, making the residue less mobile or difficult to be excluded from the interior of the solder.