For Reflow Soldering of bottomless-lead devices such as BGA/QFN, the heating rate is not an option but a make-or-break threshold. This is because the narrow volatilization window of the Flux in the Solder Paste, the lagging melting response of the solder, and the uneven heat capacity distribution of the PCB are superimposed. A slight mistake will lead to batch occurrences of cold solder joints or tombstoning. Our production line once adjusted the chain speed by 0.3 m/min, causing the heating slope in the second zone of the reflow oven to jump from 2.1℃/s to 3.4℃/s. As a result, X-ray sampling inspection of two consecutive batches found that the void area of BGA corner solder joints exceeded 18%.

Excessive heating rate in the preheating section directly tears the ceramic substrate

The copper frame of the QFN package and the plastic package body have a large difference in thermal expansion coefficient. When the PCB surface temperature rises from 30℃ to 150℃ in less than 90 seconds, microcracks will occur at the stress concentration at the edge of the frame. Such cracks are not obvious in ICT testing, but after 72 hours of high temperature and high humidity aging, the open circuit rate soared to 11%. Jia Jinyuan verified that 2.3℃/s is the safety threshold for QFN-48 pin devices. After exceeding this threshold, the pad peeling force decreased by 35%.

Insufficient dwell time in the soaking section causes carbonized flux residue

If the soaking section cannot keep the board surface stable in the 145℃~155℃ range for more than 100 seconds, the rosin-based flux cannot be fully decomposed and escaped. The semi-cured residue remaining in the bottom cavity of the QFN will be carbonized again at the reflow peak. This not only blocks the heat conduction path and causes local overheating, but also hinders the spreading of the solder and forms a non-wetting ring. We used flying probe testing for comparison and found that a 20-second reduction in soaking time corresponds to a 19 percentage point drop in the solder joint thrust pass rate.

A sudden change in slope before the peak temperature induces solder ball offset

BGA devices enter the solder collapse critical zone near 183℃. At this time, if the heating rate sharply increases from 1.8℃/s to 2.9℃/s, the imbalance speed of the liquid solder surface tension far exceeds the self-calibration ability of the solder balls, resulting in 12% lateral displacement of solder balls for 0.4mm pitch BGA. X-ray images show that the offset direction is highly consistent with the furnace airflow direction. This phenomenon disappeared after adjusting the air volume balance valve, indicating that fluctuations in the heating rate are more fatal than the absolute value.

Uncontrolled temperature difference across the board amplifies the probability of local defects

If the temperature difference between the center and the edge of the BGA on the same PCB exceeds 3℃, the time difference at the reflow peak reaches 4.2 seconds. This means that the center solder joints have begun to cool and crystallize while the edges are still in a molten state. Coarse solder joint grains and uneven IMC layer thickness occur simultaneously. Jia Jinyuan used an infrared thermal imager to actually measure a 6-layer board. Under a heating rate of 2.6℃/s, the temperature difference reached 2.8℃, and the repair rate was 3.6 times higher than that under the 2.1℃/s condition.