The core difficulty of rework soldering precision motherboards has never been removing defective components, but ensuring heat only acts on the target solder joint without affecting surrounding 0201 resistors, 0402 capacitors, and RF chips under the shielding cover. We used JJY JRY-850B paired with a dual-channel infrared temperature gun for actual measurement and found that when the hot air head is 28mm from the board surface, the air speed is set to 4 levels, and the temperature is set to 260°C, the actual reflow peak temperature of the BGA center solder ball reaches 237°C, but the temperature of the adjacent 0201 resistor body has soared to 142°C, far exceeding the tolerance threshold of its ceramic dielectric. At this time, the bottom heating table must be activated synchronously to maintain the back of the PCB at 95°C, so as to reduce the temperature difference between the upper and lower surfaces and reduce thermal gradient impact.

The preheating stage must not be skipped, otherwise direct heating of the cold board will cause micro-voids between the BGA bottom solder balls and the pads. We usually linearly increase the bottom table temperature from room temperature to 85°C over 90 seconds, while the hot air head sweeps the entire area at a low air speed of 200°C for 30 seconds, so that the temperature rise rates of the solder balls, PCB pads, and IC solder balls are as consistent as possible. If this process is compressed to within 40 seconds, abnormal solder joint wetting angles and some solder balls showing dumbbell-shaped necking will definitely be visible under the microscope, indicating that the Flux is not fully activated and the intermetallic compound growth is uneven.

The removal timing depends on the actual measurement data of the infrared gun rather than empirical judgment. When the infrared readings of the four-corner solder balls of the BGA are stable at 233～236°C and last for more than 3.2 seconds, the vacuum nozzle is allowed to start. The suction is set at 42kPa and the lifting speed is controlled at 0.8mm/s. Too fast will break the residual solder balls and lead to open circuits in the inner PCB traces, while too slow will cause surrounding components to continue bearing heat. There was once a batch where the lifting was delayed by 0.5 seconds, resulting in edge microcracks in two adjacent 100nF X7R capacitors during AOI re-inspection, and the crack extension depth reached 83μm in X-ray sampling inspection.

Before ball mounting, the pads must be wiped back and forth three times with JJY special cleaning paste, and then the residual paste must be wiped clean with a dust-free cloth dipped in 99.8% ethanol. Otherwise, residual organic matter will carbonize and turn black during reflow, causing missed detection in subsequent ICT testing. We once encountered a medical motherboard that failed functionally after three consecutive reworks, and final slicing revealed a 32μm thick carbon film above the second-layer pad, which was caused by incomplete removal of the cleaning paste.

The first reflow after new BGA placement must use the same furnace temperature curve as mass production, and local hot air repair welding cannot be used for speed. Because the IMC thickness between the BGA bottom solder balls and the PCB pads needs to form a 2.1～3.4μm Cu6Sn5 stable phase in complete reflow, local hot air can only achieve surface melting, and insufficient internal IMC growth will lead to a 37% decrease in solder joint shear force after high-temperature aging, which is particularly fatal in automotive-grade motherboards.