Properly preheat the PCB before reflow; cool the solder joints quickly after reflow. The two most critical processes that help to successfully repair SMT are also the two issues that are most likely to be overlooked:

Because these two fundamental processes are often overlooked by rework technicians, in fact, sometimes the condition is worse after rework than before rework. Although some “rework” defects can sometimes be found by the back-end process inspectors, they are not always visible in most cases, but will be exposed immediately in subsequent circuit tests.

Warm-up ,the prerequisite for successful rework

It is true that long-term processing of PCBs at high temperatures (315-426 ° C) will bring many potential problems. Thermal damage, such as warped pads and leads, delamination of the substrate, white spots or blistering, and discoloration. Warping and burning are often noticed by inspectors. However, just because the board does not “burn out” does not mean that the board is not damaged. The “invisible” damage to PCBs from high temperatures is even more serious than the problems listed above. For decades, countless tests have repeatedly demonstrated that PCBs and their components can “pass” inspections and tests after rework, and that their decay rates are higher than normal PCB boards. Such “invisible” problems such as internal warpage of the substrate and attenuation of its circuit components arise from different expansion coefficients of different materials. Obviously, these problems will not be self-exposed, not even discovered when starting the circuit test, but still lurking in the PCB assembly.

Although it looks good after “rework”, it is like a saying often said: “The operation was successful, but the patient died unfortunately.” The cause of the huge thermal stress. When the PCB assembly at normal temperature (21 ° C) suddenly contacts a soldering iron with a heat source of about 370 ° C, a desoldering tool or a hot air head for local heating, the circuit board and its components have a temperature difference of about 349 ° C Change, producing a “popcorn” phenomenon.

“Popcorn” phenomenon refers to the phenomenon that the moisture existing in an integrated circuit or SMD inside the device is rapidly heated during the repair process, which causes the moisture to expand and micro-burst or rupture. Therefore, the semiconductor industry and the circuit board manufacturing industry require production personnel to shorten the preheating time as quickly as possible and quickly rise to the reflow temperature before reflow. In fact, the PCB component reflow process already includes a preheating stage before reflow. Regardless of whether the PCB assembly plant uses wave soldering, infrared vapor phase or convection reflow soldering, each method generally requires preheating or thermal insulation treatment, and the temperature is generally 140-160 ° C. Prior to reflow soldering, a simple short-term preheating of the PCB can solve many problems during rework. This has been successful for several years in the reflow soldering process. Therefore, the benefits of preheating PCB components before reflow are manifold.
Because the preheating of the board will reduce the reflow temperature, wave soldering, IR / vapor phase welding and convection reflow welding can all be performed at about 260 ° C.

The benefits of preheating are multifaceted and comprehensive

First, pre-heating or “insulating” components before starting reflow helps to activate the flux, remove oxides and surface films from the surface of the metal to be soldered, and the volatiles of the flux itself. Accordingly, this cleaning of the activated flux just before reflow enhances the wetting effect. Preheating is the heating of the entire assembly to a temperature below the melting point of the solder and the temperature of the reflow soldering. This can greatly reduce the risk of thermal shock to the substrate and its components. Otherwise rapid heating will increase the temperature gradient inside the module and cause thermal shock. The large temperature gradients generated inside the module will cause thermomechanical stress, causing these low thermal expansion materials to embrittle, resulting in cracking and damage. SMT chip resistors and capacitors are particularly vulnerable to thermal shock.

In addition, if the entire component is preheated, the reflow temperature and reflow time can be reduced. If there is no preheating, the only way is to further increase the reflow temperature or extend the reflow time. Either method is not suitable and should be avoided.

Reduced rework makes circuit boards more reliable

As a benchmark for welding temperature, different welding methods are used, and the welding temperature is different. For example: most wave welding temperature is about 240-260 ℃, vapor phase welding temperature is about 215 ℃, and reflow welding temperature is about 230 ℃. Correctly speaking, the rework temperature is not higher than the reflow temperature. Although the temperature is close, it is never possible to reach the same temperature. This is because all the rework processes only need to heat one local component, and reflow needs to heat the entire PCB assembly, whether it is wave soldering IR and vapor phase reflow soldering.
Another factor that limits the reduction of reflow temperature during rework is the requirement of industry standards, that is, the temperature of the components around the rework point must not exceed 170 ° C. Therefore, the reflow temperature during rework should be compatible with the size of the PCB assembly itself and the size of the component to be reflowed. Since it is essentially a local rework of the PCB board, the rework process limits the repair temperature of the PCB board. The localized rework heating range is higher than the temperature in the production process to offset the heat absorption of the entire circuit board assembly.
In this case, there is still no good reason to say that the rework temperature of the entire board cannot be higher than the reflow soldering temperature in the production process, so as to approach the target temperature recommended by the semiconductor manufacturer.

Three methods for preheating PCB components before or during rework:

Today, there are three methods for preheating PCB components: ovens, hot plates, and hot air tanks. It is effective to use an oven to preheat the substrate before rework and reflow soldering to remove components. Moreover, preheating the oven is an advantageous method for baking out internal moisture in certain integrated circuits and preventing popcorn. The so-called popcorn phenomenon refers to the micro chipping of the SMD device being repaired when the humidity is higher than that of a normal device when it is suddenly heated up rapidly. PCB baking time in the preheating oven is longer, generally up to about 8 hours.
A disadvantage of the preheating oven is that unlike a hot plate and a hot air tank, it is not feasible for a technician to perform preheating and simultaneous repair during preheating. Moreover, it is not possible for the oven to cool the solder joints quickly.
Hot plate is the most ineffective way to preheat PCB board. Because the PCB components to be repaired are not all single-sided, in today’s mixed technology world, PCB components whose flat and flat surfaces are all rare are indeed rare. PCBs generally have components mounted on both sides of the substrate. These uneven surfaces cannot be preheated with a hot plate.
The second drawback of hot plates is that once solder reflow is achieved, the hot plate will continue to release heat to the PCB assembly. This is because even after the power is unplugged, the residual heat stored in the hot plate continues to be conducted to the PCB, which hinders the cooling rate of the solder joints. This obstruction to the cooling of the solder joint will cause the precipitation of unnecessary lead to form a lead liquid pool, which will reduce and deteriorate the solder joint strength.

The advantages of using hot air tank preheating are: The hot air tank does not consider the shape (and bottom structure) of the PCB component at all, and the hot air can directly and quickly enter all the corners and cracks of the PCB component. The entire PCB assembly is heated uniformly, and the heating time is shortened.

Secondary cooling of solder joints in PCB assemblies

As mentioned earlier, the challenge of SMT for PCBA (printed board assembly) rework is that the rework process should imitate the production process. Facts have proven: first, preheating PCB components before reflow is necessary for successful PCBA production; second, it is also important to cool the components immediately after reflow. These two simple processes have been ignored by people. However, in through-hole technology and micro-welding of sensitive components, preheating and secondary cooling are even more important.

Common reflow equipment such as a chain furnace, PCB components immediately enter the cooling zone after passing through the reflow zone. As the PCB components enter the cooling zone, in order to achieve rapid cooling, it is important to ventilate the PCB components. Generally, the rework is integrated with the production equipment itself.

Slow cooling after reflow of PCB components can cause unwanted lead-rich liquid pools in the liquid solder and reduce solder joint strength. However, the use of rapid cooling can prevent the precipitation of lead, making the grain structure tighter and the solder joints stronger.

In addition, faster solder joint cooling will reduce the PCB assembly’s series of quality problems due to accidental movement or vibration during reflow. For production and repair, reducing the possible misalignment and tombstone phenomenon of small SMD is another advantage of secondary cooling PCB components.

Summary

The benefits of secondary cooling PCB components when properly preheating and reflowing are many, and these two simple procedures need to be incorporated into the technician’s repair work. In fact, when preheating the PCB, the technician can do other preparations at the same time, such as applying solder paste and flux on the PCB.

Of course, it is necessary to solve the process problem of the newly returned PCB assembly because it has not passed the circuit test, which is also a real time saving. Obviously, it is not necessary to scrap PCBs during rework to save costs, and one point of prevention is worth twelve points of treatment.

Correspondingly, excessive waste can be eliminated due to substrate delamination, spotting or bubbles, warping, discoloration and premature vulcanization. Proper use of preheating and secondary cooling are two of the simplest and most necessary rework processes for PCB components.