The reflow oven is not a simple heating device but a thermodynamic execution terminal

The essence of a reflow oven is to implement millisecond-level response temperature field regulation on PCBs in a nitrogen or air environment. The heating rate in its preheating section must be controlled between 1.0℃/s and 2.0℃/s to avoid PCB warpage and MLCC microcracks, while the soaking section must maintain a temperature range of 150℃ to 180℃ for more than 120 seconds to ensure full activation of rosin-based Flux and discharge of residual moisture in the Solder Paste; otherwise, popcorning or solder balls will occur in the reflow zone.

Temperature curve settings must match solder characteristics and board-level structure

When using SAC305 Solder Paste, the peak temperature must reach 235℃±5℃ and the duration must be controlled between 45 seconds and 90 seconds. If it exceeds 120 seconds, the copper foil on PCB pads is prone to excessive erosion, while if it is less than 30 seconds, insufficient wetting of solder joints at the bottom of BGA will occur. The independent eight-zone design of the JJY reflow oven allows coordinated fine-tuning of wind speed, heating power, and chain speed for each zone. For example, for the working condition where a large-size heat sink is already attached to the back of a double-sided board, the heating slope of the rear four zones can be reduced by 0.3℃/s and the soaking bandwidth can be extended, so as to balance the thermal penetration difference between the upper and lower surfaces.

Daily inspection cannot only focus on temperature deviation values

We found that a temperature fluctuation ≤±1.5℃ at each measurement point displayed by the furnace temperature tester does not mean qualified soldering, because a slight offset in the thermocouple attachment position will cause reading distortion. It is necessary to simultaneously check whether the tension of the conveyor mesh belt is uniform, whether the fan speed in the cooling section has attenuated, and whether there is liquid accumulation blockage in the nitrogen flow meter. Any abnormality in any of these links will cause disordered air distribution in the actual furnace chamber, resulting in a difference of more than 30% in the degree of solder joint fillet pulling between adjacent IC chips on the same board.

Chain speed and zone power must be verified in a coupled manner

After replacing a new batch of PCBs, even if the old temperature curve is still used, solder bridging may still occur in batches. The root cause is that the change in the TG value of the board material leads to a change in heat capacity. At this time, the chain speed should first be reduced by 15%, and then the heating power of each zone should be increased by 5%. At the same time, an infrared thermal imager should be used to capture the board surface temperature cloud map, with a focus on observing whether local overheating occurs in the 0.4mm pitch QFP pin area in the front section of the reflow zone. If so, the heating proportion of that zone needs to be compressed and the turbulence intensity of the air knife needs to be enhanced.

The effect of nitrogen protection depends on the matching of inlet concentration and flow rate

The standard nitrogen interface of the JJY reflow oven requires an inlet purity of ≥99.99%, but actual measurement shows that when the flow rate is set to 25L/min, if the negative pressure in the furnace chamber is not maintained between -5Pa and -8Pa, nitrogen will escape in large quantities from the rail gaps, causing the oxygen content in the furnace to rise back to above 800ppm. At this time, the opening of the exhaust vent and the frequency of the blower must be adjusted simultaneously, so that nitrogen forms a top-down laminar flow coverage in the furnace chamber instead of turbulent mixing, which can ensure that OSP surface pads are not oxidized at peak temperature.