During PCB assembly ,mostly happen PCB Rework . Before reflow soldering properly preheat the PCB; cool the solder joints quickly after reflow.The two most critical processes that helped to successfully SMT rework which also are two of the most overlooked issues:

Since these two fundamental processes are often overlooked by rework technicians, in fact, sometimes after rework is worse than before rework.

Although some “rework” flaws can sometimes be detected by later process inspectors, they are not always seen in most cases but are exposed in circuit testing later on.

Admittedly, long term PCB processing at high temperatures (315-426 ° C) poses a number of potential problems. Thermal damage, such as pad and lead warpage, substrate delamination, with white spots or blisters, discoloration.

Board warpage and burned will usually cause the inspector attention. However, precisely because it does not “burn the board” does not mean that “the board is undamaged.” High temperature on the PCB “invisible” damage even more serious than the problems listed above.

For decades, countless tests have repeatedly proved that PCBs and their components “pass” the rework check and tests, which decay faster than normal PCBs. This “invisible” problem of internal warpage of the substrate and attenuation of its circuit components results from different expansion coefficients of different materials. Obviously, these problems are not self-exposed and are not even detected at the beginning of the circuit test, but are still lurking in the PCB assembly.

Although “after repair” looks good, but as people often say the sentence: “The operation was successful, the patient died of unfortunate.”

Large thermal stress causes, at room temperature (21 ℃) PCB components sudden contact with a heat source of about 370 ℃ soldering iron, de-welding tools or hot air head for local heating, the circuit board and its components have a temperature difference of about 349 ℃ Variety.

“Popcorn” phenomenon refers to the existence of an integrated circuit or SMD in the moisture inside the device in the repair process quickly heated, so that moisture expansion,Micro-burst or rupture appears. Therefore, the semiconductor industry and circuit board manufacturing industry require production staff to reflow, try to shorten the warm-up time,Rapid rise to reflow temperature. In fact, PCB assembly reflow process already includes reflow preheating phase. Regardless of PCB assembly plant is the use of wave soldering, infrared vapor or convection reflow, each method generally have to preheat or heat treatment, the temperature is generally 140-160 ℃. Before implementing reflow soldering, using a simple short-term preheating PCB can solve many problems when reworking. This has been the successful history of reflow soldering for several years.

Therefore, the benefits of preheating PCB components before reflow are manifold.

Wave soldering, IR / vapor phase and convection reflow soldering are all solderable at about 260 ° C because preheating of the plates reduces the reflow temperature.

First of all, preheating or “heat treatment” before starting reflow helps to activate the flux, removing oxides and surface films on the metal surface to be soldered, as well as the volatiles of the flux itself. Accordingly, this cleaning of the flux before the reflow will enhance the wetting effect. Preheating is to heat the entire assembly below the melting point of the solder and the reflow temperature. This greatly reduces the risk of thermal shock to the substrate and its components. Otherwise rapid heating will increase the temperature gradient within the module and produce thermal shock. Large temperature gradients created inside the module will create thermo-mechanical stresses, causing these materials with low thermal expansion to embrittle, rupture and damage. SMT chip resistors and capacitors are particularly vulnerable to thermal shock damage.

In addition, if the entire assembly is preheated, the reflow temperature can be reduced and the reflow time can be shortened. If there is no warm-up, the only way to further increase the reflow temperature or to extend the reflow time should be avoided, whichever is less appropriate.

As a benchmark for welding temperature, using different welding methods, welding temperature is not the same, for example:

Most wave soldering temperature is about 240-260 ℃, vapor phase temperature is about 215 ℃, reflow temperature is about 230 ℃. Correctly speaking, rework temperature is not higher than reflow temperature. Although the temperature is close, but it is impossible to achieve the same temperature. This is due to the fact that all rework processes require only one partial component to be warmed up and the re-flow requires the entire PCB assembly to be warmed up, both in wave soldering IR and vapor phase reflow.

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Another factor that also limits the rework temperature during rework is the industry-standard requirement that the components surrounding the point to be repaired should never exceed 170 ° C. Therefore, the rework temperature should be reworked with PCB components and components to be reflow the size of the size to adapt, due to the PCB board is essentially the partical rework, so rework process limits the PCB board repair temperature. Partical rework has a higher heating range than the production process to offset the heat absorption of the entire board assembly.

In this case, there is no good reason why the rework temperature of the entire board can not be higher than the reflow temperature in the production process, so that it is close to the target temperature recommended by the semiconductor manufacturer.

Nowadays, there are three types of preheating PCB assembly methods: ovens, hotplates and hot blast tanks. It is effective to use an oven to preheat the substrate before reworking and reflow soldering disassembly components. Furthermore, baking is an advantageous method of preheating the oven to bake the internal moisture in some integrated circuits and prevent popcorn. The so-called popcorn phenomenon refers to the repair of SMD devices in humidity higher than the humidity of normal devices will suddenly occur when the rapid temperature rise micro-cracking. PCB baking in the preheating oven longer, usually up to 8 hours.

1).One of the drawbacks of preheating the oven is that unlike a hot plate and hot blast tank, it is not feasible for a technician to preheat and concurrently rework when preheating. Moreover, it is impossible for the oven to cool the solder joints rapidly.

2).Hot plate is the most invalid way to preheat the PCB. Because the PCB components to be repaired are not all single-sided,

Today is the world of mixed technologies, and it is indeed rare for PCB components that are all flat or planar. PCB on both sides of the substrate generally have to install components. It is not possible to heat these uneven surfaces with a hot plate.

The second drawback of hotplates is that the hotplates continue to release heat to the PCB assembly once the solder reflow is achieved. This is because, even after unplugging the power supply, the residual heat stored in the hot plate continues to be conducted to the PCB and hinders the cooling rate of the solder joint. This obstruction of solder joint cooling will lead to the formation of unnecessary lead precipitation lead pool, cause the strength and deterioration of solder joints.

3).Advantages of using a hot-air tank to warm up are: the hot-air tank no need to consider the outline (and the bottom structure) of the PCB assembly at all, and the hot air can directly and quickly enter all corners and cracks in the PCB assembly. So that the entire PCB assembly heating evenly,and shortens the heating time.

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Facts have proved:

First, preheating the PCB assembly before reflow is necessary for the successful production of PCBA. Second, it is also important to quickly cool the assembly immediately after reflow. These two simple processes have always been neglected by people. However, pre-heating and secondary cooling are even more important in through-hole technology and micro-soldering of sensitive components.

With PCB components into the cooling zone, in order to achieve rapid cooling,for PCB component ventilation is very important, the general repair and production equipment itself is one.

Decreasing the cool-down of the PCB assembly after reflow will cause the unwanted Pb-rich liquid pool in the liquid solder to reduce the solder joint strength. However, the use of rapid cooling can prevent the precipitation of lead, the grain structure tighter, more solid for solder joints.

In addition, cooling the solder joints faster reduces the PCB assembly’s subsequent series of quality problems due to accidental movement or vibration. For production and rework, reducing the potential for misplacement and tombstoning phenomenon of small SMDs is another benefit of secondary cooling of the PCB assembly.

There are many benefits to secondary cooling PCB assemblies when properly preheating and reflow, and these two simple procedures need to be incorporated into the rework of technicians. In fact, when preheating the PCB, the technician can do other preparations at the same time, such as applying solder paste and flux to the PCB.

Of course, there is a need to solve the process issues of newly reworked PCB assemblies because it has not yet passed the circuit test and this is a real time-saver. Obviously, there is no need to scrap the PCB during rework to save the cost and prevent a one-point, stop-gap treatment.