In electronic manufacturing and maintenance, the problem of false soldering of packaged devices such as BGA, QFP, and SOP is regarded as the "number one public enemy". It is highly concealed, sometimes good and sometimes bad, and extremely difficult to locate, which poses a huge challenge to product quality and reliability.

This article will systematically sort out the prevention, detection and troubleshooting of false soldering, providing you with a complete solution from design to maintenance。

I Preventing problems before they occur - Systematic Prevention of False Soldering

Eliminate false soldering. Prevention is far better than cure. It is necessary to start from the source and strictly control every link.

1. Design is the foundation: PCB and pads

· Pad design: Strictly follow the recommended dimensions in the component datasheet. For BGA, the shape of the solder balls is determined by the opening of the solder pads and the steel mesh, which is of Paramount importance.

· Via hole treatment: It is strictly prohibited to drill holes on BGA pads. The adjacent vias must be filled, prevent solder loss.

· Layout and thermal balance: Large components should be kept away from the edge of the board and stress areas. The layout needs to take thermal balance into account to avoid excessive local temperature differences during reflow soldering。

2. Materials are the guarantee: PCB and components

· PCB quality: Ensure that surface treatments (such as gold plating, ENIG, OSP) are uniform and free of oxidation.

· Component status: Ensure good coplanarity of pins/solder balls and no oxidation. For humidity-sensitive devices, they must be baked in a standardized manner according to their grades to prevent the "popcorn" effect.

· Solder paste management: Select solder paste with the appropriate alloy and particle size, and strictly follow the refrigeration, tempering and usage norms. Do not use expired solder paste.

3. The process is at the core: printing, mounting and reflow soldering

· Solder paste printing

  · Steel mesh design: This is the key! Laser cutting and electro-polishing are the basics, and fine-pitch devices often use stepped steel mesh

  · Printing quality: Precisely control the parameters of the scraper blade and use SPI to conduct 100% inspection of the volume, area and height of the solder paste to ensure perfect printing formation.

· Component mounting: High-precision surface mount technology (SMT) machines and regular calibration ensure accurate component placement without deviation。

· Reflow soldering：

Reflow soldering curve: This is the soul! The furnace temperature tester must be used to measure and optimize the curve in real time. The settings of the preheating zone temperature, constant temperature zone, reflow zone (the time above the peak temperature and the liquidus line), and cooling zone are of vital importance, directly determining the quality of the solder points.

II  Sharp eyes - Precise detection of False soldering.

When the PCB steps out of the reflow oven, we need multiple "eyes" to detect hidden defects.

1. Online detection

· AOI：BGA For visible solder joints such as QFP and SOP, AOI can efficiently detect defects like bridging, solder deficiency, and drift. But it's impossible to see through BGA。

· X-Ray： The top-best method for detecting BGA false soldering. It can clearly present the bubbles, bridging, cracks and alignment inside the solder joints, and judge the quality by analyzing the cavity rate.

2. Functional and electrical testing

· ICT： The circuit connectivity is checked through test points, but it is not sensitive to false soldering that is not completely open.

· Functional test: By simulating real working scenarios, it can detect some unstable hidden false soldering related to temperature and vibration.

3. Boundary scanning: Specialized in addressing "invisible" open paths

Boundary Scan (JTAG) is a powerful non-physical contact testing technique, especially adept at detecting open interconnections in BGA/QFP.

· Principle: Through the built-in boundary scanning unit of the chip, the output pins are forcibly driven and the values of the input pins are captured, thereby determining whether the connection between the two is intact.

· Advantages: No probe required, capable of testing the bottom solder joints of BGA, fast testing speed, and able to precisely locate faulty pins.

· Limitations：

· The main detection is complete open circuit. Intermittent false soldering with increased resistance and fluctuating performance is prone to be missed.

  · It is impossible to test non-digital pins such as power and simulation

  · The device needs to support JTAG and have corresponding connections when designed.

4. Destructive analysis (for process validation)

· Dye detection test: Penetrate the dye into the bottom of the BGA, then split the chip. The areas with dye are cracks or false solder joints. The results are apparent and reliable.

· Cross section analysis: Cutting and polishing the solder joints and observing their microstructure under a microscope is the most accurate analytical method, but it is costly and can damage the sample.

III  Fault tracking - On-site investigation of false soldering issues

When a finished board card shows a suspected false soldering fault, how can the real cause be identified step by step?

Step1： Information collection

· Record the fault phenomenon: Is it completely unpowered or is it dysfunctional? Is it related to temperature and vibration? This is a typical feature of false soldering.

Step 2： Visual and microscopic examination

· Carefully inspect the pins of all QFP and SOP devices with a magnifying glass or stereomicroscope to see if there is any lifting, misalignment, or cracks in the solder joints.

Step 3： Physical probing method

· Pressing method: When powered on, gently press the suspected component with an insulating rod. If the fault appears or disappears with pressing, there is a high suspicion that the device is poorly soldered.

· Hot air/cooling method

· Slightly heat the device with a hot air gun. Thermal expansion may cause the false solder joint to temporarily come into contact, and the fault will disappear.

  · Local cooling with cooling spray may cause the fault to stably occur due to cold contraction.

  · Note: This method requires caution to avoid damaging the surrounding components.

Step 4： Multimeter and oscilloscope

· Multimeter: Measure the resistance, voltage or diode voltage drop of the suspected pin, compare it with the good board, and look for abnormalities.

· Oscilloscope: It measures the waveforms of data lines and clock lines. Poor soldering may cause waveform distortion, abnormal amplitude or burrs.

Step 5： X-Ray and Thermal imaging

· X-Ray: Directly taking an "X-ray" of the suspected BGA is the most intuitive evidence for confirming false soldering.

· Thermal imaging: When powered on and in operation, scan. Due to the high contact resistance, the false solder joints may overheat locally, forming "hot spots".

Step 6： Repair verification

· For QFP/SOP: Use a soldering iron or hot air gun for patch soldering.

· For BGA: Professional desoldering, ball placement and resoldering must be carried out using a BGA rework station to ensure uniform and reliable soldering. After repair, it must be verified through X-Ray and functional tests.

IV  Summary: Build an impregnable wall for preventing and controlling false welding

To deal with false soldering, a single method is doomed to fail. A systematic defense line must be established:

· Prevention: Relying on excellent design + precise printing + perfect curves

· Inspection: Relying on a combination of AOI, X-Ray, and boundary scanning

· Troubleshooting: Through a process-oriented operation of phenomenon analysis → physical probing → electrical measurement → image confirmation.

Only by organically integrating pre-event prevention, in-event detection and post-event investigation can we fundamentally overcome the stubborn problem of false soldering and create highly reliable electronic products.