In PCBA processing, there are several key points in the whole process of reflow and reflow soldering.
① The reflow soldering curve should be close to the original soldering curve. The hot air reflow soldering curve can be divided into four areas: preheating zone, immersion temperature zone, reflow zone and cooling zone. The temperature and time parameters of the four areas can be set separately, and these programs can be stored and recalled at any time by connecting with a computer.
Wave soldering furnace temperature curve
②In the process of reflow welding, the heating temperature and time of each area should be selected correctly, and the heating speed should be paid attention to. Generally, before 100℃, the maximum heating rate does not exceed 6℃/s; after 100℃, the maximum heating rate does not exceed 3℃/s; in the cooling zone, the maximum cooling rate does not exceed ℃/s. Because too high heating rate and cooling rate may damage the PCB and reworked components, this damage is sometimes invisible to the naked eye. For different components and different solder pastes, different heating temperatures and times should be selected. For example, the reflow temperature of CBGA chips should be higher than the reflow temperature of PBGA, 90% Pb, 10% Sn should be higher than 37% Pb, 63% Sn solder paste. For no-clean solder paste, its activity is lower than that of non-no-clean solder paste, so the soldering temperature should not be too high and the soldering time should not be too long to prevent the oxidation of solder particles.
③When reflow soldering with hot air, the bottom of the PCB must be able to heat up. Heating has two purposes: one is to avoid warping and deformation due to heat on one side of the PCB; the other is to shorten the melting time of the solder paste. For large-size board rework BGA, bottom heating is particularly important. Generally, there are two heating methods for the bottom of rework equipment: hot air heating and infrared heating. The advantage of hot air heating is uniform heating. This heating method is generally recommended for rework processes; the disadvantage of infrared heating is that the PCB is not evenly heated.
④Choose a good hot air return nozzle. The hot air reflow nozzle is a non-contact heating, which relies on the high-temperature air flow to melt the solder at the solder joints on the BGA chip at the same time.
The cleaning after rework is generally partial cleaning, and there are two methods: one is to directly use a solvent matching the soldering material and flux to clean, this method may still have unclear marks after cleaning; the other is to use a cleaning fluid Washing with water, this process is often followed by drying treatment due to the presence of water components, but the cleanliness is good and can meet the requirements of relevant process standards.
Regardless of the rework method and the rework tool used, due to the influence of the use of the device and the skill of the operator, although the printed circuit assembly can meet the quality level requirements, there are more or less uncontrollable factors in the process.
Objectively speaking, the quality of hand-soldering repair depends largely on the skill level and comprehension ability of the operator. It is impossible to achieve a very consistent work effect in a short period of time. Therefore, there is a certain degree of repair for some printed circuit components. risk.
Although the current rework workstation system has been greatly improved in terms of functions and capabilities, and its accuracy and repeatability are comparable to automated placement equipment, it is still operated by humans, so the training of operators is very important. important. Secondly, in terms of the structure of the welding device, due to its limited functions and functions, it cannot be compared with modern automatic reflow welding equipment with seven temperature zones and ten temperature zones. The thermal atmosphere environment in a very small area has limited controllable parameters, and it is difficult to set and adjust the soldering temperature curve. The solder joints formed by the soldering of large-scale packaged devices will be very different, especially for BGA, CSP and other components. The shape, gloss and smoothness of the local solder joints are worse than those of the reflow furnace.
