Today, I would like to share how to check whether the PCB assembly services in China design drawings are appropriate. We have sorted them out!
After the PCB wiring is completed, the PCB design rules should be checked to ensure that the PCB design scheme meets the requirements and all networks are connected correctly. The common new inspection items in PCB assembly services in China are as follows:
The chemical deposition is used to deposit gold, and a chemical oxidation-reduction reaction forms a coating. Generally speaking, the thickness is relatively thick. It is one of the electroless nickel gold deposition methods, which can obtain a thicker gold layer. Gold plating is based on the principle of electrolysis, also known as electroplating. The main way of metal surface treatment is electroplating.
Gold is used in the surface treatment of PCB assembly services in China. People cannot tell the difference between them accurately. Some people even think that there is no difference between them, which is a very wrong view. In the product application, 90% of the gold plate is the sinking gold’s circuit board. The most fundamental difference between the gold-plated circuit board and the gold-plated circuit board is that the gold plating is hard gold (wear-resistant), and the sinking gold is soft (not wear-resistant) because the poor solderability of the gold-plated plates is his fatal defect, which is also the direct reason many companies give up the gold plating process!
Ordinary people choose gold plating, so what is gold plating? We call the whole plate gold plating, which generally refers to “electroplated gold,” “electroplated nickel gold plate,” “electrolytic gold,” “electroplated gold,” and “electroplated nickel gold plate.” Soft gold is different from hard gold (gold finger uses ordinary hard gold). Its principle is to dissolve nickel and gold (commonly known as gold salt) in a chemical solution, immerse circuit board into electricity, form nickel gold coating on the circuit board’s copper foil surface in plating cylinder, and connect with the current. Nickel gold coating has been widely used in electronic products because of its high hardness, wear-resistance, and nonoxidation.
Gold is used in the surface treatment of circuit boards because of its strong conductivity, good oxidation resistance, and long service life. It is generally used for keyboard, gold fingerboard, etc. the fundamental difference between the gold-plated circuit board and the gold-plated circuit board is that gold plating is hard gold (wear-resistant) while sinking gold is soft gold (not wear-resistant).
Successful repair technology of SMT reflow soldering
Preheat PCB properly before reflow and cool solder joint quickly after reflow.
The two most critical processes that contribute to the successful repair of SMT are also the two most easily overlooked problems.
Because repair technicians often ignore these two basic processes, sometimes the repair condition is worse than that before repair.
Although some “repair” defects can sometimes be found by the next process inspector, in most cases, they are always not seen, but they will be exposed immediately in the future circuit test.
Preheating — the premise of successful repair
Indeed, PCB processing at high temperatures (315-426 ℃) for a long time will bring many potential problems. Thermal damage, such as warpage of pad and lead, substrate delamination, white spot or blister, discoloration.
Warping and burning usually cause the inspector’s attention. However, it is precisely because it will not “burn out the board” does not mean that “the board is not damaged.” The “invisible” damage of high temperature to PCB is even more serious than the problems listed above.
Over the past decades, countless tests have repeatedly proved that PCB and its components can “pass” the inspection and test after the rework, and its attenuation speed is higher than that of normal PCB. The “invisible” problems, such as the substrate’s internal warpage and the attenuation of its circuit components, come from the different expansion coefficients of different materials. Obviously, these problems will not expose themselves, even at the beginning of the circuit test, but still, lurking in the PCB assembly.
Although “repair” looks good, but as people often say: “the operation was successful, but the patient died.”.
The reason for the huge thermal stress is that when the PCB assembly at room temperature (21 ℃) suddenly contacts the soldering iron, solder removal tool, or hot airhead with a heat source of about 370 ℃ for local heating, there is a temperature difference of about 349 ℃ for the circuit board and its components,
The “popcorn” phenomenon occurs.
“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, causing the moisture to expand,
Microburst or fracture occurs. Therefore, the semiconductor industry and the circuit board manufacturing industry require the production personnel to shorten the preheating time as much as possible before reflow,
Rapidly rising to reflow temperature. In fact, the preheating stage before reflow has been included in the PCB reflow process. Whether wave soldering, infrared vapor phase, or convection reflow soldering are adopted in the PCB assembly plant, preheating or heat preservation treatment is generally required for each method. The temperature is generally 140-160 ℃. Before reflow soldering, using a simple short-term preheating PCB can solve many problems during the repair. This has been successful in reflow soldering for several years.
There are three ways to preheat PCB before or during a repair.
Nowadays, there are three ways to preheat PCB components: oven, hot plate, and hot air trough. It is effective to use the oven to preheat the substrate before repairing and reflow soldering. Also, preheating the oven is a good way to remove moisture and prevent popcorn in some integrated circuits. The so-called popcorn phenomenon is that the humidity of repaired SMD device is higher than that of a normal device, and it will break when it is suddenly heated rapidly. The baking time of PCB in preheating the oven is longer,
It usually lasts about 8 hours.
One of the preheating oven defects is that it is different from the hot plate and hot air trough. It is not feasible to preheat by one technician and repair at the same time. Moreover, the oven cannot cool the solder joint quickly.
The hot plate is the most ineffective way to preheat PCB. Because the PCB components to be repaired are not all single-sided,
In today’s world of hybrid technology, PCB components with a flat or flat surface on one side are indeed rare. PCB on both sides of the substrate are generally installed components. It is impossible to preheat these uneven surfaces with a hot plate.
The hot plate’s second defect is that once solder reflow is realized, the hot plate will continue to release heat to PCB assembly. Even after the power is removed, there will still be residual heat stored in the hot plate to continue to transfer to the PCB, which hinders the cooling speed of solder joints. This kind of resistance to soldering joint cooling will cause unnecessary lead precipitation and lead pool formation, which will reduce and deteriorate the strength of the solder joint.
The advantage of using hot air trough preheating is that the hot air trough completely does not consider the shape (and bottom structure) of PCB components. Hot air can directly and quickly enter all corners and cracks of PCB components. The whole PCB assembly is heated evenly,
The heating time is shortened.
The second cooling of solder joints in PCB assembly is as mentioned above. The challenge of SMT to PCBA (printed board assembly) repair is that the repair process should imitate the production process. Facts have proved that:
Firstly, preheating PCB before reflow is necessary for successful PCBA production; secondly, cooling PCB immediately after reflow is also essential. Moreover, these two simple processes have been ignored. However, in the through-hole technology and micro-welding of sensitive components, preheating and secondary cooling are more important.
Common reflow equipment, such as chain furnace, PCB components, enter the cooling zone immediately after passing through the reflow zone. As PCB components enter the cooling zone, in order to achieve rapid cooling,
Ventilation is essential for PCB components. Generally, repair and production equipment itself is integrated.
Slow down cooling after reflow of PCB assembly will cause unnecessary lead rich liquid pool in liquid solder and reduce solder joint strength. However, rapid cooling can prevent the precipitation of lead, making the grain structure tighter and the solder joint firmer.
Also, faster cooling of solder joints will reduce a series of quality problems caused by unexpected movement or vibration of PCB assembly during reflow. For production and repair, reducing the possible dislocation and tombstone phenomenon of small SMD is another advantage of secondary cooling PCB assembly.
There are many advantages in the correct preheating and reflow of the secondary cooling of PCB components, so it is necessary to bring these two simple procedures into technicians’ repair work. In fact, when preheating the PCB, the technician can do other preparatory work simultaneously, such as coating solder paste and flux on the PCB.
Of course, we need to solve the new repaired PCB assembly process problem because it has not passed the circuit test, which is also a real time-saving. Obviously, there is no need to scrap PCB during repair, which saves the cost. One point of prevention is worth 12 points of treatment.