Reduces rework the circuit board more reliable
As a benchmark for welding temperature, using different welding methods, welding temperature are not the same, such as: the majority of wave soldering temperature is about 240-260 ℃, vapor phase soldering temperature is about 215 ℃, the reflow temperature is about 230 ℃. Properly speaking, rework reflow temperatures not higher than the temperature. Although the temperature close to, but never could reach the same temperature. This is because: that all rework process components only need to take a local heating, and then flow required for heating the entire PCB assembly, wave soldering both IR and vapor phase reflow are true.
The same restrictions reduce rework reflow temperature, another factor is the industry-standard requirements, that is, to rework point temperature of the surrounding components which must not exceed 170 ℃. Therefore, the rework of the PCB reflow temperature should again flow component itself and the size of the component size to adapt, because the PCB is essentially a local repair, why limit the repair process temperature maintenance of PCB board. Repair of localized heating in the range of temperature than the production process higher to offset the entire circuit board assembly endothermic.
So to say, still no good reason to explain the entire board rework temperature not higher than the production process of reflow temperature, and thus close to the semiconductor manufacturer recommended target temperature.
Repair or rework of PCB components before preheating of three methods:
Today, the warm-u China PCB assembly methods into three categories: oven, hot plate and hot-air tank. In rework and reflow oven used to remove components before preheating the substrate, to be effective. Also, preheat oven baking away in some IC internal moisture and prevent popcorn phenomenon, the use of baking is a favorable method. The so-called popcorn phenomenon is the rework of SMD components on the humidity higher than normal humidity devices sudden rapid warming will occur when the micro-crack. PCB in the preheated oven baking time is longer, usually up to 8 hours.
Preheat oven to a defect is different from the hot plate and hot-air tank, preheating time and preheating by a technician and also repair is not feasible. Moreover, to achieve rapid cooling of the oven in terms of solder joints is impossible.
Preheat the hot plate is the most ineffective way to the PCB manufacturer. Because of failure to repair is single-sided PCB assembly, mixed technology world of today, all flat or flat side of the PCB assembly is indeed rare. PCB substrate are generally required to install the components on both sides. The uneven surface is not possible to preheat the hot plate.
The second hot-plate defects in solder reflow is once achieved, the hot plate will continue the release of heat to the PCB components. This is because, even unplug the power, the hot plate will have to transfer the stored residual heat to the PCB solder joints impede the cooling rate. This will cause unnecessary obstruction of the cooling pad lead to form lead precipitation liquid pool, the solder joint strength and reduce variation.
Advantages of using hot air preheat tank is: hot air tank with no regard to PCB component shape (and the bottom of the structure), hot air directly into the PCB and components quickly and cracks in all the corners. So that uniform heating throughout the entire PCB assembly and shorten the heating time.
PCB components in the secondary cooling pad
As mentioned earlier, SMT on the PCBA (printed circuit board assembly) the challenge is to rework rework process should mimic the production process. Proved: First, preheat the PCB before reflow component is necessary for successful production of PCBA; second, then immediately after the rapid cooling flow component is also very important. This simple process has been two people ignored. However, in the through-hole technology and micro-welding-sensitive components, preheating and secondary cooling is more important.
Common equipment such as chain reflow oven, PCB components and then flow through the area immediately after entering the cooling zone. With the PCB assembly into the cooling zone, in order to achieve rapid cooling, ventilation components to the PCB is very important, general repair and production equipment itself became one.
PCB components and then after a slow cooling flow in the liquid solder will not produce the lead-rich liquid pool will reduce the solder joint strength. However, the use of rapid cooling to prevent precipitation of lead, the grain structure tighter, stronger joints.
In addition, the faster the cooling will reduce the PCB solder reflow assembly when the accidental movement or vibration due to produce a series of quality problems. For the production and repair, small SMD possible to reduce the dislocation and the tombstone is a secondary phenomenon, another advantage of the cooling PCB components.