When the ramp-up slope of Reflow Soldering exceeds 3°C/sec, the Flux solvent in the Solder Paste vaporizes violently in a short period, causing solder paste particles to be ejected and form solder balls or splash onto non-pad areas. Meanwhile, rapid heating prevents the oxide film on the solder powder surface from being reduced in time, directly weakening the wetting force and inducing cold solder joints. For components with different heat capacities (such as 0201 resistors and QFPs), the difference in heating rate is exacerbated, resulting in uneven wetting thrust at the solder ends and further causing tombstoning.

We have observed a large number of isolated solder balls after solder paste printing and reflow, mostly concentrated on both sides of CHIP component pads. In this case, the production line should be immediately suspended, and thermocouples should be used to measure the actual furnace temperature profile to confirm whether the ramp-up slope at the end of the preheating zone exceeds 2.5°C/sec. If it exceeds the limit, the belt speed of the first preheating zone should be reduced by 5%, and the belt speed of the second preheating zone should be synchronously reduced by 3%, then the temperature should be re-measured and verified. If it still exceeds the limit after adjustment, the aging status of the heating tubes and the uniformity of the furnace wind speed must be checked. Never simply increase the set temperature to cover up the actual out-of-control ramp-up problem.

Tombstoning defects are particularly prominent on small-sized chip components such as 0402 and 0201, manifesting as one end standing upright while the other end adheres to the pad. The main cause is that the melting time difference between the solder joints at both ends is greater than 0.3 seconds, and an excessively large ramp-up slope will amplify this difference. In practice, X-ray scanning should first be used to confirm whether there is a solder paste volume deviation at the bottom solder joint of the tombstoned component, then compare the aperture size tolerance of the solder paste stencil for components of the same package on the same PCB. If the aperture consistency is worse than ±5μm, the machine must be stopped to calibrate the stencil tension and redo the SPI first article inspection. At the same time, the total time of the reflow oven preheating zone should be extended by 8 seconds to compress the slope fluctuation window.

Poor wetting is often accompanied by dull solder joints and a climb height of less than 0.3 times the pin thickness. This is not simply due to insufficient temperature, but because the ramp-up is too fast, causing the flux to completely decompose and fail before the solder paste melts. In this case, the peak temperature cannot be simply increased; instead, the soaking zone temperature should be lowered by 10°C and extended by 2 seconds, allowing the residual organic acid sufficient time to activate the metal surface. Simultaneously, check whether the solder paste batch is approaching the end of its shelf life. Solder paste that has been opened for more than 72 hours without nitrogen protection must be scrapped and must not be mixed with new paste for continued use.

Solder paste splashing is often accompanied by increased visible smoke in the furnace and accelerated carbon accumulation on the track, indicating that rosin-based flux has undergone pyrolysis rather than controlled volatilization. In this case, an infrared thermometer should be used to spot-check the actual temperature distribution in each temperature zone of the furnace, with a focus on verifying whether there is a local hot spot at the entrance of the third temperature zone. If present, clean the dust accumulated in the heating module of this section and replace the corresponding fan filter. Afterwards, add a 15-minute empty burn run before the next furnace loading to ensure that residual organic matter is completely removed; otherwise, subsequent continuous production will still repeatedly contaminate the solder paste.

All adjustment actions must be bound to the furnace temperature test board number and the solder paste LOT number of the day, and recorded in the SMT process change log. Verbal handover or skipping the actual measurement step based on experience is prohibited. Because for the same model of reflow oven, when the ambient humidity is >65%RH, the measured ramp-up slope will be 0.8°C/sec higher than that in a dry environment. This variable must be included in the calibration benchmark before each line change.