Expert in Comprehensive PCBA Full-Process Services
After soldering, there are frequent instances of cold or false solder joints, resulting in unstable signals or intermittent signal loss upon power-up. How should this be addressed?
First, select automotive-grade lead-free solder (such as SAC305) and a low-residue high-temperature flux; avoid chlorine-containing formulations to prevent corrosion. Next, adjust the reflow profile—specifically the temperature and dwell time at each stage—based on the thermal tolerance of the components (e.g., ceramic capacitors and power chips) to ensure complete melting and wetting of the solder without damaging the parts. Prior to reflow, use plasma cleaning to remove oxide layers from pads and leads, and then perform X-ray inspection of the solder joints: rework any voids or cold joints, and promptly replace any contaminated components.
Pin bridging and solder balls—especially in high-density chips—can easily cause short circuits. What should be done?
Use high-precision laser cutting to fabricate the steel stencil; when the pin pitch is ≤0.5 mm, reduce the stencil aperture by 10%–15% and round the edges to minimize solder volume. Calibrate the pick-and-place nozzle to control mounting pressure and accuracy, ensuring that component placement deviation does not exceed ±0.1 mm. During reflow soldering, slow down the heating rate in the soak zone to reduce solder turbulence. After soldering, use an anti-static desoldering tape to remove any solder balls; do not use sharp tools to scrape the solder pads, as this could damage the circuit traces.
After surface-mount component placement, misalignment and tombstoning occur, affecting subsequent assembly. What are the causes, and what are the solutions?
This is mainly caused by asymmetric solder pads and improper placement parameters. First, design the solder pads at both ends of the component to be symmetrical in size and shape. Select a nozzle of the appropriate size for the pick-and-place machine and carefully control the pressure and speed; for small components such as 0402 and below, enable the vision alignment function to improve accuracy. During reflow soldering, use a gentle temperature profile to minimize differences in solder pull forces. If misalignment or tombstoning occurs after soldering, correct the position using a hot-air gun in combination with tweezers, then perform a rework solder joint.
Irregular PCBA design—such as small component spacing and overlapping of pins with silkscreen—leads to manufacturing difficulties and a higher defect rate. How can this be optimized?
Design according to SMT process requirements: standard component spacing ≥ 0.3 mm, power components ≥ 0.5 mm; avoid overlapping of leads with silkscreen and solder mask; ensure pad sizes match lead dimensions; and provide sufficient inspection and rework space in high-density areas. Minor design deviations can be corrected by adjusting stencil apertures or optimizing placement parameters; for significant deviations, the PCB layout must be revised. Prior to mass production, conduct first-article testing to identify design–process mismatches early on.
For high-density chips such as BGAs, internal solder joint defects cannot be detected by visual inspection—how can they be identified?
For mass production, X-ray inspection is used to examine internal solder joints and identify porosity, cold solder joints, and voids (automotive-grade requirements stipulate a void ratio of ≤5%). Visual defects are detected using AOI equipment, with parameters carefully calibrated to match automotive-grade components. Cross-sectional or ultrasonic testing may be performed on first articles and random samples. Defects are precisely repaired using a BGA rework station, with temperature and time meticulously controlled to prevent damage to adjacent components; the repaired assemblies are then re-inspected by X-ray.
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