The Reflow Soldering temperature profile is not a mere pile-up of parameters, but a dynamic control over the entire process of Solder Paste activation, solvent volatilization, and intermetallic compound formation. If the heating rate in the preheating zone exceeds 3℃/s, the low-boiling-point components in the Flux will not have time to vaporize smoothly, and will instead burst out suddenly at the edge of the PCB pad, directly pushing the molten solder away from the joint to form solder beads. This phenomenon is particularly prominent in densely populated areas of 0201 and 0402 resistors and capacitors.

Key Control Points for the Preheating Zone

We typically set the preheating zone to a range of 120℃～160℃, with the heating slope strictly controlled between 1.0 and 2.5℃/s. This range ensures that the rosin-based carriers in the solder paste are fully softened, while preventing local collapse of the solder paste particles due to excessive temperature difference between the inside and outside. On a JiajinYuan production line, an abnormal fan in the reflow zone once caused the actual heating slope to reach 3.8℃/s, and the proportion of boards with excessive solder beads surged from 2% to 11% in a single day. Normal operation was restored immediately after the machine was shut down to calibrate the wind speed.

The Key Role of the Soaking Zone

The soaking zone temperature is maintained at 170℃～190℃ for a duration of 60 to 90 seconds. Its core task is to allow residual moisture and ethanol in the flux to escape completely, while ensuring that the oxide film on the surface of the solder powder is fully reduced. If the soaking time is less than 60 seconds or the temperature is below 170℃, micro-air pockets will already exist in the BGA bottom solder joints before entering the reflow zone. If these cannot be expelled later, they will solidify into voids. The problem of excessive void rate in QFN devices for a certain JiajinYuan customer was ultimately traced back to fluctuations in nitrogen flow, which caused the actual temperature in the soaking zone to drift by ±5℃.

Coordination of Reflow Peak Temperature and Time

The peak temperature must be at least 20℃ above the solder liquidus line and maintained for no less than 60 seconds, but it must not exceed 245℃ to avoid accelerating copper layer diffusion or damaging the chip package body. We found that when the reflow time was compressed to 45 seconds, the uneven thickness of the IMC in SAC305 solder joints was significantly aggravated, accompanied by concentrated voids appearing in the central area of the joints. This indicates that the intermetallic reaction had cooled and solidified before reaching an equilibrium state.

The Cooling Zone Should Not Be Overlooked

The cooling rate should be controlled between -1℃/s and -4℃/s. Too slow a rate will lead to coarse grains and reduce the fatigue resistance of the solder joint, while too fast a rate is likely to generate micro-cracks inside the joint and induce thermal stress voids. When JiajinYuan was debugging the reflow oven for a certain automotive controller, it was found that an excessively high fan speed in the cooling zone resulted in an average cooling rate of -5.2℃/s. X-ray inspection showed that the proportion of voids at the root of QFP pins increased by 7 percentage points.