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After SMT Assembling, the PCB board substrate works abnormally in the copy program process.
Bad phenomenon: The communication serial port can not connect after the PCBA power on, the serial port lights up red. The PC host prompts that can not found the connected device and cannot be programmed.
Product defects Analysis:
1.Power supply: The power supply of this product substrate is 12V, so it is necessary to start from the power supply terminal, analyze whether the substrate is powered on normally, whether there is a short circuit, etc., and whether the relevant voltage drop chip is normally used to supply voltage voltage to each part. If the confirmation is correct, proceed to the next step of research and analysis.
2.Communication section: Since the product is connected to the test point of the circuit board substrate through the probe, and the bad phenomenon is poor communication, the test point responsible for communication can be used as a breakthrough for research. Use a multimeter to perform point-by-point test confirmation. See the figure below for the test point diagram of the substrate connection:
By measuring the operating voltage of the test point, it is found that the 5V supply voltage provided by the serial port is normal, but the working voltage of the two pins responsible for signal transmission is abnormal and is not within the normal working range.
Going up the test point checking, it is found that these test points are directly connected to the QFN chip ATMEGA168PA-MU. By looking up the model specification of the product, the working pins of the entire chip can be fully understood. As shown below:
After testing, the chip did not work properly. Troubleshoot the peripheral circuits and find that they are all normal. Finally, find this chip location causes problem.
By using microscope, find that the soldering state of the chip is abnormal, and a good state of tin was not formed on the side of the chip, as shown in the following figure:
Solder deposits on the outside of the pad, forming different degrees of spherical shape. The chip is re- tinned by a manual soldering iron, as shown below:
Let the solder fully soldering with the chip pins, then copy verification again, it is found that the program can be written and runs normally. It is thus determined that the QFN chip bad welding cause the PCBA products failure.
QFN bad welding factors:
1. The chip factor:
Observing the components under the microscope, it is found that the pin parts of the chip have different degrees of oxidation. See the figure below: There is blackened foreign matter on the surface of the pin, which leads to poor soldering.
* Strengthen the inspection of the pin for the incoming chip, and conduct random sampling according to the ratio. If this problem occurs again, feedback to the supplier to replace the complaint
* After determining the integrity of the incoming device, the chip needs to be vacuum-packed to seal the chip. At the same time, we should put the sealed chip in the dehumidification cabinet. Before production, you can open and use them. The extra chips also need to be placed in a dehumidification cabinet for storage.
The temperature of the IC chip dehumidification cabinet is set to 26 ± 2 ℃, strictly in accordance with its control.
2. Substrate factors:
After soldering, it find that the soldering fluidity is not uniform at the other solder joints, some pads have no soldering. It is preliminarily judged that the surface of the PCB is poorly tinned and the substrate is oxidized.
* Check the stock substrate, take photos of the bad position and feedback. At the same time, increase the sampling ratio of the substrate. Effective control of the temperature and humidity of the warehouse. The substrate can not be exposed to the air for a long time, avoiding direct sunlight.
Warehouse storage humidity: 30-70%, temperature 15-30 ° C, shelf life should be less than 6 months.
* Choose a quality supplier to provide good plates.
3. Solder paste factor:
The reasonable use and control of solder paste directly affects the subsequent quality. Therefore, we must give full play to the active characteristics of solder paste.
1. The solder past must be placed in the refrigerator for storage within 30 minutes after it arriving the warehouse. At the same time, we should paste the identification card on the bottle and do record management work carefully.
2. Refrigeration Temperature: 0-10 degrees, storage time is 6 months.
3. Solder paste return ambient temperature : Two hours at room temperature, if do not open the lid and not exceeding 12 hours, it can be still used. More than 12 hours need to be re-inserted in the refrigerator for storage.
4. Before using the solder paste, it needs to be stirred with a blender for 2-5 minutes. The solder paste can be used in a state of natural dripping.
Side pad length H = 0.34mm
Stencil thickness is 0.12mm
After first go through wave soldering, if the solder thickness (F) is greater than 0.12mm, F/H is greater than 35%, meet the secondary standard.
QFN bad soldering is often happening in the PCB SMT assemble industry, and can cause very serious impact. In order to better control the badness, we need to start from many aspects, such as PCB pad design, PCB storage, chip control and management, solder paste management and use, furnace temperature control, etc. Each process needs to be operated in strict accordance with the regulations to ensure the final product quality.