Matching the reflow profile for different Alloy Solders is by no means a problem that can be solved by simply applying a template. Our production line once directly used the SAC305 profile to solder Sn63/Pb37 Solder Paste, which resulted in a large number of cold solder joints on QFP pins. The root cause is that the eutectic melting point of Sn63/Pb37 is only 183°C, while the actual melting range of SAC305 is 217~220°C, with a liquidus temperature difference of 34°C between the two. If the temperature zones are not re-divided, it will inevitably cause premature collapse of low-melting-point solder paste or insufficient melting of high-melting-point solder paste.

When actually adjusting the line, you must first confirm the TSD curve graph provided by the Solder Paste Manufacturer, focusing on its recommended time above liquidus (TAL), peak temperature tolerance band, and cooling rate. For example, some lead-free solder pastes require the cooling slope to be controlled between -1.5°C/s and -3.0°C/s, otherwise it is easy to cause brittle cracking of solder joints. For bismuth-containing alloy solder pastes, since the melting point is as low as 138°C, the heating rate in the preheating zone must be suppressed to 0.5~1.2°C/s to avoid premature boiling and splashing of the Flux.

We commonly use an infrared furnace temperature tester to measure the multi-point temperature on the PCB surface, especially focusing on the temperature difference at the bottom of BGA, thick copper layer areas, and around fine-pitch chips. It is found that the temperature difference at different positions in the same reflow oven can reach 8~12°C. Therefore, you cannot only look at the data of thermocouples attached to the test board, but also combine the solder joint wetting angle and bridging rate returned by AOI to reverse-infer the real temperature field distribution. For example, when 0201 components continuously exhibit tombstoning, it is most likely caused by asymmetric heating on both sides of the PCB, resulting in a melting time difference of more than 0.8 seconds for the solder paste at both ends.

Fluctuations in the metal powder oxidation rate between solder paste batches will also affect the reflow performance. For the same batch of SAC305, if the oxygen content rises from 0.15% to 0.22%, the measured time above liquidus must be extended by 8~12 seconds to ensure that the solder joint void rate is below 15%. Sn63/Pb37 is more sensitive to oxygen content; once it exceeds 0.18%, the soaking zone temperature must be lowered by 5°C and extended by 15 seconds to compensate for the loss of surface tension.

The on-site rapid verification method is to take three standard test boards and run three parameter combinations respectively: the current profile, peak temperature reduced by 3°C, and liquidus time increased by 10 seconds. Then use X-ray to spot-check the solder joint voids and IMC layer thickness. As long as one of the boards meets the IPC-A-610 Class 2 Acceptance standard, immediately lock this set of parameters and put it into mass production. It is strictly forbidden to modify the reflow profile based on experience without the support of measured data.