QFN Chips Bad Soldering Analysis and Solution

PCBA Project:

Vehicle Oil Control Tracking System

Usage:

Vehicle fuel usage tracking and information prompts

Failure Produce:

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 displays a prompt indicating that it cannot find 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:

Allow the solder to fully solder with the chip pins, then perform a copy verification again. If the program can be written and runs normally, it can be determined that the PCBA product failure was caused by bad welding of the QFN chip.

QFN bad welding factors:

1. The chip factor Chip Soldering:

Upon observing the components under the microscope, one can find that the pin parts of the chip have varying degrees of oxidation, as illustrated in the figure below. There is blackened foreign matter on the surface of the pin, which results in poor soldering.

Solutions:

* 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 one determines the integrity of the incoming device, they must vacuum-pack the chip to seal it.At the same time, we should put the sealed chip in the dehumidification cabinet. Before production, you can open and use them. One must also store any extra chips in a dehumidification cabinet for preservation.

One must strictly control the temperature of the IC chip dehumidification cabinet and set it to 26 ± 2 ℃.

2. Substrate factors:

After soldering, it find that the soldering fluidity is not uniform at the other solder joints, some pads have no soldering. A preliminary judgment can be made that the surface of the PCB has poor tinning and the substrate is oxidized.

Solutions:

* 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.

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.

Solutions:

1. One must store the solder paste in a refrigerator within 30 minutes of its arrival at 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.To restore the solder paste to ambient temperature, one should keep it at room temperature for two hours.

4. Before using the solder paste, one must stir it with a blender for 2-5 minutes.One can use the solder paste in a state of natural dripping.

IPC standard

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. To ensure the final product quality, one must strictly operate each process in accordance with the regulations.

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