The ball mounting process is not simply stacking solder balls, but requires the Solder Paste printing thickness, reflow profile, and solder ball size to be tightly matched. We found in a certain automotive MCU rework that when the solder paste printing thickness deviation exceeds 12μm, even with JGY-708A solder paste from JiajinYuan, 32% solder ball tombstoning and 18% collapse will occur. This indicates that the starting point of the ball mounting process must be the joint control of stencil aperture accuracy and squeegee pressure.

Solder Paste Selection Determines Ball Mounting Success or Failure

JiajinYuan JGY-708A solder paste has a thixotropic index of 4.8, melting point of 179°C, and Tg value of 62°C. Its flux residue volatilization rate is >92% at a peak temperature of 260°C, which directly determines whether additional cleaning is required after BGA ball mounting. JGY-708A has an X-ray detected void rate of <15% under no-clean conditions, provided that the nitrogen concentration during reflow is maintained above 99.95% and the oxygen content is ≤50ppm. Otherwise, the void rate will jump above 28%, accompanied by abnormal wetting angles >90°.

Solder Balls and Solder Paste Must Be Used as a Set

We have tested the results of mixing solder balls from different manufacturers with JiajinYuan solder paste. When using non-matching solder balls, even if the particle size tolerance is controlled within ±2.5μm, 11% of ball rolling offset still occurs. This is because the rosin system in the solder paste does not match the activation energy of the oxide layer on the solder ball surface. Only when paired with original JiajinYuan solder balls can synchronous wetting be achieved at the initial stage of reflow, and ball self-alignment be completed within the liquidus temperature range (179°C~217°C). At this time, the solder joint thrust is stable between 5.2N~5.8N, which is much higher than the 4.0N lower limit required by IPC-7095B.

Key Control Points for Ball Mounting Rework

Before ball mounting, the BGA pads must be treated with a plasma cleaner for 30 seconds to remove organic contaminants. Otherwise, the spread diameter of JiajinYuan solder paste on the pads will shrink by 0.08mm, causing the solder balls to fail to fully cover the pad edges, and increasing the risk of solder joint necking by 4 times after reflow. Meanwhile, the clamping force of the ball mounting fixture must be set to 8.5N±0.3N. Too low pressure will cause the solder balls to float due to insufficient sinking depth into the solder paste, while too high pressure will cause the solder paste to be squeezed out of the pads and form bridges. Both situations result in a single-ball missing false detection rate >6.7% in AOI inspection.

Reflow Parameters Must Be Dynamically Adapted to Solder Paste Characteristics

The activity window of JiajinYuan JGY-708A solder paste is concentrated between 195°C~212°C. Therefore, the heating slope in the reflow zone must be controlled at 0.8°C/s±0.1°C/s, with a peak temperature of 224°C±2°C and holding time of 65 seconds±5 seconds. If the holding time is shortened to 55 seconds, the thickness of the solder joint IMC layer is only 2.3μm, which is lower than the 3.1μm threshold required for reliable soldering, and interface cracking occurs at the 327th cycle in the thermal shock test. Extending it to 75 seconds, on the other hand, will cause excessive growth of Cu6Sn5 into Cu3Sn, increasing the brittleness of the solder joint by 22%.