Joy Technology Co.,Limited

The key process of SMT reflow welding rework technology 

Reflow before the appropriate preheating PCB board; re-flow after the rapid cooling of the solder joints. Two of the most critical processes for the successful repair of SMT are also the two most likely to overlook the problem:

It is true that PCB processing at high temperatures (315-426 ° C) for a long time poses a lot of potential problems. Heat damage, such as pad and lead warping, substrate delamination, raw white spots or blistering, discoloration. Board Alice and Burning are usually caused by the inspector's attention. However, it is precisely because it will not "burn bad plate" does not mean that "plate is not damaged." High temperature on the PCB "invisible" damage even more serious than the above list of problems. For decades, numerous tests have repeatedly demonstrated that PCBs and their components can "pass" rework after the test and test, the decay rate higher than the normal PCB board. This kind of substrate warpage and its circuit element attenuation and other "stealth" problem comes from different materials of different expansion coefficient. Obviously, these problems will not be self-exposed, even in the beginning of the circuit test was not found, but still lurking in the PCB components.

Although the "repair" looks good, but as people often say a word: "surgery is successful, but the patient unfortunately died."Due to the large thermal stress, the PCB assembly at room temperature (21 ° C) has a temperature difference of about 349 ° C for the circuit board and its components when it is exposed to a hot iron with a heat source of about 370 ° C, a welding tool or a hot head. Variety,Produce "popcorn" phenomenon."Popcorn" phenomenon refers to the existence of an integrated circuit or SMD in the device inside the moisture in the repair process of rapid heat, so that moisture expansion,the phenomenon of micro-burst or rupture.

Therefore, the semiconductor industry and circuit board manufacturing industry requires production staff in the reflow before, as far as possible to shorten the warm-up time,quickly rose to reflow temperature. In fact, the preheating stage of the PCB assembly reflow process has been included. Regardless of PCB assembly plant is the use of wave soldering, infrared vapor phase or convection reflow, each method are generally preheated or heat treatment, the temperature is generally 140-160 ℃. In the implementation of reflow before the use of a simple short-term preheating PCB can solve many problems when the repair. This has been in the reflow process has been a few years of successful history.the benefits of preheating the PCB assembly before reflowing are multifaceted.Since the preheating of the plate reduces the reflow temperature, both wave soldering, IR / vapor welding and convection reflow can be performed at about 260 ° C.The benefits of preheating are multifaceted and integrated. First, preheating or "heat-treating" components prior to commencing reflow helps to activate the flux, remove the oxides and surface films of the surface to be welded, and the volatiles of the flux itself. Accordingly, this cleaning of the flux prior to reflow will enhance the wetting effect. Preheating is to heat the entire assembly below the melting point of the solder and the temperature of the reflow. This can greatly reduce the risk of thermal shock to the substrate and its components. Otherwise rapid heating will increase the temperature gradient within the assembly and produce thermal shock. The large temperature gradient generated inside the assembly will form thermomechanical stresses that cause the material with low thermal expansion to be brittle, causing rupture and damage. SMT chip resistors and capacitors are particularly susceptible to thermal shock.

In addition, if the entire assembly is preheated, the reflow temperature and the reflow time can be reduced. If there is no preheating, the only way to only further increase the reflow temperature, or extend the reflow time, no matter which way is not appropriate, should be avoided.Reduced rework makes the board more reliable.As a benchmark for welding temperature, the use of different welding methods, welding temperature is not the same, for example: Most of the wave soldering temperature of about 240-260 ℃, steam welding temperature of about 215 ℃, reflow temperature of about 230 ℃. To be fair, the rework temperature is not higher than the reflow temperature. Although the temperature is close, it is never possible to achieve the same temperature. This is because all the rework processes require only one local component to be heated, and the reflow requires the entire PCB assembly to be heated, both for wave soldering and vapor phase reflow.

Another factor limiting the reduction in reflow temperatures in rework is the industry standard, which means that the temperature of the components around the repair point must never exceed 170 ° C. Therefore, the reflow temperature in the reflow should be with the PCB component itself and to reflow the size of the components to adapt to the size of the PCB board is essentially due to the local rework, so the repair process limits the PCB board maintenance temperature. The temperature range of the localized rework is higher than the temperature in the production process to counteract the endotherm of the entire board assembly.

In this case, there is no good reason to say that the repair temperature of the entire board can not be higher than the reflow temperature in the production process, thus approaching the target temperature recommended by the semiconductor manufacturer.Three methods of preheating PCB components before repair or repair:Today, preheating PCB components are divided into three categories: oven, hot plate and hot air tank. It is effective to use an oven to preheat the substrate before reworking and reflow the components. Moreover, the use of baking is a favorable way to preheat the oven in baking out some of the internal circuits in the internal moisture and prevent the popcorn phenomenon. The so-called popcorn phenomenon refers to the repair of the SMD device in the humidity above the normal device humidity suddenly in the rapid warming will occur when the micro-crack. PCB in the preheating oven baking time is longer,Generally up to 8 hours or so.

One of the drawbacks of the preheating oven is that it is different from the hot plate and the hot air tank, and it is not possible to preheat by a technician and to rework it at the same time. Moreover, it is impossible to quickly cool the solder joint in the oven.Hot plate is the most ineffective way to preheat PCB board. Because the PCB components to repair is not all single-sided,Today is the world of hybrid technology, one side is all flat or flat PCB components is indeed rare. PCB on both sides of the substrate are generally installed components. It is not possible for these uneven surfaces to be preheated with hot plates.The second drawback of the hot plate is that once the solder reflows, the hot plate will continue to release heat to the PCB assembly. This is because, even after unplugging the power supply, there will still be stored residual heat in the hot plate to continue to conduct the PCB to prevent the solder joint cooling rate. This obstruction of solder joints can cause unnecessary lead precipitation to form a lead bath, reducing the solder joint strength and deteriorating.

The advantage of using a hot air tank to preheat is that the hot air tank does not take into account the shape (and bottom structure) of the PCB assembly, and the hot air can enter the corners of the PCB assembly directly and quickly. So that the entire PCB assembly heating evenly,And shorten the heating time.Secondary cooling of solder joint in PCB assembly

As mentioned earlier, the challenge of SMT to PCBA (PCB components) is that the repair process should mimic the production process. Facts have proved:
First, preheating the PCB assembly before reflow is necessary for the successful production of the PCBA; secondly, it is also important to quickly cool the components immediately after reflow. And these two simple craft has been ignored by people. However, in the through-hole technology and micro-welding of sensitive components, preheating and secondary cooling is even more important.

Common reflow equipment such as chain furnace, PCB components through the re-flow zone immediately after entering the cooling zone. With the PCB assembly into the cooling zone, in order to achieve rapid cooling,ventilation of PCB components is very important, the general repair and production equipment itself is integrated.The slow cooling of the PCB assembly after recirculation causes the unwanted lead-filled liquid pool in the liquid solder to reduce the solder joint strength. However, the use of rapid cooling can prevent the precipitation of lead, so that the grain structure is more tight, more solid solder joints.In addition, the faster cooling of the solder joint reduces the quality of the PCB assembly due to accidental movement or vibration during reflow. For production and rework, it is another advantage of the secondary cooling of the PCB assembly by reducing the possible misalignment and tombstone of small SMD.

Summary:
There are many benefits of secondary cooling PCB components when properly warmed up and reflowed, and these two simple procedures need to be included in the technician's rework. In fact, when preheating the PCB, the technician can do other preparatory work at the same time, such as solder paste and flux on the PCB.Of course, the need to solve the problem of new refurbished PCB component process, because it has not passed the circuit test, which is a real time to save. Obviously, there is no need to repair the PCB in the repair and save the cost.