When a large number of solder beads appear on the PCB and are concentrated on both sides of the CHIP component pads, the root cause is that the heating rate in the preheating zone is too high, causing the Flux to boil violently and the Solder Paste particles to be splattered and scattered. The practical solution is to reduce the heating rate in the 120℃～160℃ section from 2.8℃/s to 1.6℃/s, synchronously extend the time of this interval to 65 seconds, and use a K-type thermocouple attached to the board to actually measure the actual heating curve of the board. The preventive point is that the first piece of each shift must use a furnace temperature tester to collect data at more than 3 points, and the ambient humidity must be controlled between 45% and 60%RH, otherwise uneven solvent volatilization will amplify the risk of solder explosion.

If it is continuously found that the large-area void rate at the bottom of the BGA is >25% and the X-ray image shows that the voids are regularly circularly distributed, it indicates that the reflow peak temperature has not truly penetrated to the bottom solder joints. The reason is that the temperature zoning response in the furnace lags, or the thermocouple measurement point position deviates from the BGA center area. The practical solution is to increase the peak temperature from 245℃ to 248℃, increase the holding time from 6 seconds to 9 seconds, and attach a thermocouple to the back of the PCB directly below the BGA for closed-loop verification. The preventive point is to calibrate the furnace cavity infrared sensor with a black body at least once a month, and at the same time ensure that the nitrogen flow rate is stable at 500L/min and the O? residue is <100ppm.

When the tombstoning rate of 0201 resistors suddenly rises to more than 3.7%, it is usually because the heat capacity difference between the pads on both sides is amplified by the reflow curve. Specifically, the transition zone between the end temperature of the holding zone (175℃) and the reflow starting temperature (205℃) is too narrow, resulting in uneven heating of small components. The practical solution is to slow down the heating rate in the 175℃～205℃ interval from 1.2℃/s to 0.7℃/s, and insert a 185℃±2℃ steady-state platform for 22 seconds. The preventive point is that the stencil opening must adopt a trapezoidal design instead of a rectangular one, and the placement and reflow must be completed within 30 minutes after printing, to avoid the surface of the flux forming a film causing wetting imbalance.

If QFP device pin bridging and cold soldering coexist in the same furnace, the essence is that the reflow peak temperature fluctuates more than ±2.5℃ in the 235℃～245℃ interval. The direct inducement is that the aging of the heating module leads to PID response delay, or the carbon deposition of the conveyor belt in the furnace cavity causes uneven reflected heat radiation. The practical solution is to stop the machine to clean the reflectors of the heating chambers in Zone 3 and Zone 4, replace the SSR solid-state relay in Zone 2, and fine-tune the chain speed from 75cm/min to 72cm/min to extend the residence time. The preventive point is to use an infrared thermal imager to scan the temperature uniformity of the inner wall of the furnace cavity every two weeks, and if the temperature difference exceeds 8℃, the power output of each temperature zone must be recalibrated.

When solder joints appear in batches that are gray and have no metallic luster and the thrust test is 15% lower than the standard value, it indicates that the cooling rate in the cooling zone is too slow, resulting in excessive IMC growth. The typical manifestation is that it takes >120 seconds to drop from 217℃ to 150℃. The practical solution is to turn on the forced air cooling mode in Zone 6, increase the air speed at the furnace outlet from 1.8m/s to 2.4m/s, and install a -10℃ circulating water cooling plate at the end of the conveyor belt. The preventive point is that the inlet temperature of the cooling zone must not be higher than 180℃, and the cooling curve must pass through the 150℃～100℃ phase transition interval within 60 seconds, otherwise the grain coarsening of the SnAg3.0Cu0.5 alloy is irreversible.