Causes of Solder Beads and On-Site Intervention Logic

Solder beads mostly appear on both sides of the solder terminals of CHIP components. Essentially, the Flux in the Solder Paste volatilizes violently during the preheating stage, pushing the liquid solder to splash outward of the pad. If the stencil thickness is selected as 0.12mm but the squeegee pressure is not adjusted to 4.5kg synchronously, or the reflow preheating slope exceeds 2.5℃/s, the incidence of solder beads will rise sharply from the normal 0.3% to 2.1%. The rework statistics of a certain batch of 0402 resistors from Jia Jinyuan show that 76% of solder beads are accompanied by blackened pad oxidation, indicating that when the nitrogen purity is lower than 99.99%, the flux residue aggravates metal splashing.

Excessive Void Rate and Key Reflow Window

Voids at the bottom of QFN exceeding 25% trigger reliability risks, which is not solely determined by the reflow peak temperature, but caused by insufficient residence time in the soaking zone (150–170℃) leading to insufficient flux escape, while excessively fast cooling rate traps gas inside the solder joint. Jia Jinyuan's actual measurement found that when the chain speed is increased from 70cm/min to 85cm/min, the standard deviation of BGA void rate in the same furnace expands to ±9%, indicating that conveyor vibration has interfered with the uniformity of solder paste collapse.

Tombstoning Phenomenon and Hard Constraints on Component Coplanarity

A sudden increase in the tombstoning rate of chip components of 0603 size and above often exposes inconsistent downward pressure depth of the Z-axis nozzle of the placement machine. If the same nozzle uses a compression amount of 0.15mm for both 0805 and 1206 components, excessive force on small-sized components will cause solder paste offset at the solder terminals, while insufficient pressure on large-sized components will lead to a placement height deviation exceeding 0.08mm. Jia Jinyuan confirmed through actual measurement with a laser confocal microscope that when the coplanarity error of component solder terminals is >0.05mm, the tombstoning probability increases by 4.3 times.

Bridging Defects and Stencil Aperture Process Coupling

Bridging of 0.4mm pitch QFP is often misjudged as excessive solder paste amount, but in reality, it is caused by the mismatch of the aspect ratio of the stencil aperture leading to solder paste adhesion during demolding. When the aperture width is set to 0.18mm but the length is not compressed to below 0.22mm, the edge burrs of the solder paste after printing are squeezed and extended during placement. Jia Jinyuan confirmed with 3D SPI scanning that 92% of bridged solder joints have the double abnormality of solder paste volume exceeding the design value by 18% and center offset >0.03mm.

Rapid Production Line Attribution Path

When encountering sudden SMT defects, first retrieve the SPI data of the most recent three furnaces to compare the changes in the solder paste volume CV value, then check whether the stencil tension has fallen below 35N/cm, and finally verify whether the deviation between the measured temperature and the set value of the third temperature zone of the reflow oven is >±1.2℃. Jia Jinyuan has solidified these three steps as mandatory inspection items for the first piece after line change, reducing the average fault location time from 47 minutes to 11 minutes.