1. After the first Reflow Soldering, the solder joints have formed a double-layer intermetallic compound of Cu6Sn5 and Cu3Sn. If a second reflow soldering is performed, the thickness of the Cu3Sn layer will increase by more than 40% and an obvious columnar crystal structure will appear. This structure is extremely prone to cracking along the interface under thermal cycle loads. Therefore, when the product involves BGA chip rework, the peak temperature of the secondary reflow must be controlled below 235°C, the time above the liquidus line must be compressed to within 60 seconds, and a low-silver lead-free Solder Paste must be used simultaneously to inhibit excessive IMC growth.
2. When a mixed-mount double-sided SMT board passes through the reflow oven, the already soldered Bottom-side solder joints will experience a second thermal shock. At this time, if a high-activity solder paste is used on the Top side or the heating rate exceeds 3°C/s, the Sn-Cu-Ni ternary IMC at the interface of the Bottom-side solder joints will accelerate precipitation and induce micropore aggregation. Therefore, it is recommended to preferentially use Ni/Pd/Au immersion gold pads on the Bottom side, select a no-clean medium-activity solder paste on the Top side, and extend the preheating zone of the entire board to 90 seconds to balance the thermal stress distribution.
3. During the secondary reflow of stacked board modules (such as camera module + main control board stacking), the solder joints on the underlying PCB not only undergo remelting of their own solder, but also superimpose the additional heat capacity load conducted by the upper board body, resulting in a local peak temperature deviation of up to 15°C. Therefore, a 0.2mm thick ceramic heat insulation gasket must be inserted into the stacking gap, and on the basis of reducing the reflow oven chain speed by 15%, ±2°C closed-loop temperature control compensation must be implemented for Zone4 to Zone6.
4. X-ray inspection shows that the void rate of QFN solder joints after three reflows generally rises to 28% to 35%, far exceeding the 25% upper limit allowed by IPC-A-610G. The main reason is that the Flux residues carbonize into a film after multiple pyrolysis, hindering gas escape. Therefore, a 75°C/30-minute vacuum baking process under nitrogen protection should be added after the first reflow, and then a water-washable solder paste should be used to achieve a total flux residue of less than 0.05mg/cm2.
5. OSP boards with a pad copper thickness of less than 12μm are prone to copper dissolution and migration during secondary reflow, resulting in an increase in the solder joint wetting angle by more than 12° and accompanied by edge solder shrinkage. At this time, the wind speed in the cooling zone of the reflow oven must be increased to 1.8m/s to shorten the high-temperature residence time, and a solder paste model containing a copper ion stabilizer must be used. At the same time, the PCB supplier is required to uniformly plate the copper thickness before OSP to 18±2μm.
6. For 0201 and smaller passive devices, secondary reflow will cause the solder joint volume shrinkage rate to reach 9.3%, causing the tombstoning rate to increase by more than 3 times. This is because the small pad has low heat capacity and insufficient solder paste slump height. Therefore, it is necessary to make the stencil opening direction form a 15° angle with the long axis of the device, reduce the printing pressure to 4.2kgf, and add a 30-second 60°C infrared pre-bake before reflow to enhance the adhesion of the solder paste.
7. Measured data shows that the SAC305 solder joints after three standard reflow soldering have their failure cycles shortened to 57% of the original life in the -40°C to 125°C temperature cycle test, and the main failure mode is the peeling of the IMC layer at the root of the solder joint. Therefore, any design involving three or more thermal cycles must switch to the SAC-Q series of high-reliability solder paste, and strictly control the post-reflow cooling rate within the range of -3.5°C/s to -4.0°C/s to avoid excessive residual tensile stress.
