In the realm of PCB assembly for vehicle oil control tracking systems, as well as addressing QFN soldering issues, ensuring flawless soldering is paramount for seamless functionality. This article delves into the analysis and resolution of soldering issues encountered during the assembly process, including those specific to QFN.
PCBA Project
Vehicle Oil Control Tracking System
Usage
Vehicle fuel usage tracking and information prompts
Identifying Anomalies
Upon completion of SMT assembling, abnormalities surfaced during the copy program process of the PCB substrate. Symptoms included failure to establish communication via the serial port upon powering on the PCBA, indicated by a red light on the serial port. The PC host displayed prompts indicating an inability to detect the connected device or execute programming tasks.
Root Cause Analysis
1. Power Supply Evaluation
The product’s substrate requires a 12V power supply, so initial scrutiny focused on power terminals. The analysis encompassed normal power-on behavior, potential short circuits, and the functionality of voltage drop chips supplying power to individual components.
2. Communication Circuit Examination
Given the product’s connectivity via a probe to the circuit board substrate and the communication-related malfunction, attention shifted to the test points responsible for communication. Utilizing a multimeter for point-to-point testing revealed normal 5V supply voltage but abnormal operating voltage on pins facilitating signal transmission. See the figure below for the test point diagram of the substrate connection:
Traceback analysis highlighted the direct connection of problematic test points to the QFN chip ATMEGA168PA-MU. Reference to the chip’s specifications elucidated its operational pin configuration. Peripheral circuit inspection yielded no irregularities, ultimately pinpointing the chip as the locus of malfunction.As shown below:
Microscopic examination unveiled abnormal soldering on the chip, with inadequate tin formation on its side, as shown in the following figure:
Solder deposits on the pad periphery exhibited varying degrees of spherical shape. Manual soldering iron intervention was employed to re-tin the chip, ensuring full soldering with its pins, as shown below:
Understanding QFN Soldering Issues
In scrutinizing components under magnification, it’s evident that the chip’s pin parts exhibit varying degrees of oxidation, manifesting as blackened foreign matter on the pin surface, leading to subpar soldering outcomes.
1. Addressing Chip Soldering Concerns
Enhanced Pin Inspection: Intensify scrutiny of incoming chip pins, implementing random sampling protocols. Any recurrence prompts feedback to the supplier for complaint resolution.
Optimal Chip Storage: Ensure the integrity of incoming devices by vacuum-sealing chips and storing them in a dehumidification cabinet. Strictly regulate the cabinet temperature to 26 ± 2 ℃ for optimal preservation.
2. Substrate Considerations
Post-soldering observations reveal non-uniform soldering fluidity at various joints, with some pads exhibiting inadequate soldering. This suggests poor tinning on the PCB surface and substrate oxidation.
3. Mitigating Substrate Issues
Thorough Stock Inspection: Document and report instances of substrate flaws, augmenting sampling rates for enhanced quality control. Maintain optimal temperature and humidity levels within the warehouse to preserve substrate quality.
Quality Supplier Selection: Partner with reputable suppliers to procure high-quality substrates, ensuring reliability in production processes.
4. Solder Paste Optimization
The judicious application and control of solder paste significantly influence subsequent quality outcomes, necessitating the maximization of its active characteristics.
5. Effective Solder Paste Management
Prompt Refrigeration: Refrigerate newly arrived solder paste within 30 minutes of receipt, affixing identification cards for meticulous record-keeping.
Optimal Refrigeration Conditions: Store solder paste at temperatures ranging from 0 to 10 degrees Celsius with a maximum storage duration of 6 months.
Temperature Adjustment: Allow solder paste to attain ambient temperature over two hours before use.
Thorough Mixing: Prior to application, stir solder paste for 2-5 minutes using a blender to ensure uniform consistency. Utilize solder paste in its naturally dripping state for optimal results.
IPC standard
Side pad length H = 0.34mm
The stencil thickness is 0.12mm
After first going through wave soldering, if the solder thickness (F) is greater than 0.12mm, F/H is greater than 35%, meet the secondary standard.
QFN soldering issues are prevalent in the SMT assembly industry and can significantly impact the overall product quality. To mitigate these issues effectively, a comprehensive approach is essential. This includes meticulous attention to PCB pad design, proper PCB storage practices, stringent chip control, and management, effective solder paste handling, and meticulous furnace temperature regulation.
By meticulously adhering to these guidelines and standards throughout each step of the assembly process, the quality of the final product can be ensured, ultimately enhancing customer satisfaction and brand reputation.
We invite you to explore our PCB Assembly Projects and contact us to discuss your specific project requirements.
