Preface

As electronic products evolve toward miniaturization, portability, networking, and higher performance, increasingly stringent demands are placed on circuit assembly technologies and the number of I/O pins. Chip sizes continue to shrink, while the number of pins grows, posing significant challenges for manufacturing and rework.

It turns out that the quad flat pack (QFP), widely used in surface-mount technology (SMT), has a minimum lead pitch of only 0.3 mm. At this pitch, leads are prone to bending, deformation, or breakage, placing stringent demands on SMT assembly processes, equipment precision, and soldering materials. Moreover, QFPs with narrow leads and tight pitches can exhibit defect rates as high as 6,000 ppm, thereby limiting their widespread adoption. In contrast, ball grid array (BGA) packages distribute the device’s pins across the bottom surface of the package, replacing the traditional four-sided lead-out configuration with an array of lead‑tin solder bumps. This design accommodates a significantly higher number of I/O pins while employing larger lead pitches—such as 1.5 mm or 1.27 mm—compared to the 0.4 mm or 0.3 mm pitches of QFPs. As a result, BGA packages facilitate straightforward soldering and interconnection with SMT‑compatible PCB traces. Not only do they enable greater pin density within the same footprint as QFPs, but their wider lead pitches also substantially improve SMT assembly yield, reducing defect rates to as low as 0.35 ppm. This ease of manufacturing and repairability has led to the widespread adoption of BGAs in electronic product manufacturing.

To enhance the quality and reliability of BGA solder joints, this study investigates defect manifestations and reliability issues associated with BGA solder joints.

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BGA Soldering Quality and Inspection BGA solder joints are located beneath the die, making it difficult to assess solder quality with the naked eye after assembly. In the absence of dedicated inspection equipment, one can first visually check whether the collapse around the chip’s perimeter is uniform, then align the die under a light source; if light passes through every row and column, this provides a preliminary indication that no bridging has occurred. However, this method cannot detect internal defects in the solder joints or voids on the joint surfaces. To more accurately evaluate solder‑joint quality, X‑ray inspection equipment is essential.

Commonly used X-ray inspection instruments include two-dimensional direct‑view X-ray imagers and X-ray circuit‑board inspectors. Traditional two-dimensional direct‑view X-ray systems are relatively inexpensive; however, their drawback is that all solder joints on both sides of a PCB are simultaneously imaged in a single radiograph. When components are mounted on both sides at the same location, the shadows cast by the solder joints overlap, making it impossible to distinguish which side the component belongs to. Moreover, if defects are present, it becomes difficult to determine whether they originate from one layer or another, thus failing to meet the requirements for accurately identifying soldering defects.

The X‑ray circuit board inspector is an X‑ray computed tomography system specifically designed to examine solder joints. It can inspect not only BGA packages but also the solder joints of all components mounted on a PCB. The device employs X‑ray computed tomography, enabling it to visualize solder balls in cross‑sectional layers and generate tomographic images. These X‑ray tomograms can be compared against the original CAD design data and user‑defined parameters, allowing for timely determination of whether the solder joints meet quality standards. Its main drawback is its high cost.

2.1

Acceptance Criteria for BGA Solder Joints Regardless of the inspection equipment used, a clear standard is essential to determine whether BGA solder joints meet quality requirements. IPC‑A‑610C, section 12.2.12, defines the acceptance criteria for合格BGA solder joints as follows: the joints should be smooth and rounded with well‑defined edges and no voids; all joints must exhibit uniform diameter, volume, grayscale, and contrast; alignment must be precise, with no offset or rotation; and no solder balls are permitted. After soldering, the preferred approach is to verify compliance with these criteria; however, in practical inspections, the standards may be slightly relaxed. For example, regarding alignment, an offset of up to 25% relative to the pad is acceptable. Similarly, the presence of solder balls is not strictly prohibited, provided their size does not exceed 25% of the spacing between the two nearest adjacent solder balls.

2.2

Common BGA soldering defects include bridging, open circuits, missing solder balls, voids, large solder balls, and blurred solder joint edges. Voids are not unique to BGAs; they can typically be observed visually in surface-mount and through-hole component joints without the need for X-ray inspection. However, in BGA assemblies, because the solder joints are concealed beneath the package, X-ray inspection is required to detect voids. Some even argue that voids may be beneficial to reliability. The IPC‑7095 committee acknowledges that very small voids that cannot be entirely eliminated might contribute to reliability, but a clear threshold for what constitutes an acceptable size remains to be defined.

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Void formation mechanisms: BGA solder balls comprise the component layer (adjacent to the BGA package on the substrate), the pad layer (adjacent to the PCB substrate), and the intermediate layer. Depending on the specific circumstances, voids may occur in any of these three layers. BGA solder balls can already contain voids prior to soldering, resulting in voids after the reflow process—this may stem from voids introduced during ball fabrication or from issues with the solder paste applied to the PCB surface.

In addition, PCB design is another major cause of void formation. For example, if vias are placed beneath solder pads, ambient air can enter the molten solder balls through these vias during soldering; upon cooling, voids remain within the solder joints. Voids that appear in the pad layer may result from the flux contained in the solder paste printed on the pads volatilizing during reflow soldering—gases escaping from the molten solder and leaving behind voids as the joint cools.

Poor gold plating on the pads or contaminants on the pad surface can both lead to void formation. Voids are most commonly observed in the component layer, specifically in the region between the center of the solder ball and the BGA substrate. This is likely because, during reflow soldering, air bubbles and volatilized flux gases trapped beneath the BGA pads on the PCB remain; as the eutectic solder balls fuse with the applied solder paste in the reflow zone, these trapped gases and bubbles give rise to voids. Furthermore, if the reflow profile does not maintain sufficient dwell time in the reflow zone, the air bubbles and flux gases do not have enough time to escape before the molten solder enters the cooling phase and solidifies, resulting in voids. Therefore, the settings of the reflow temperature profile are a critical factor in void formation.

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Process Improvement Recommendations for Enhancing BGA Solder Joint Reliability

1) Preheat circuit boards and chips to remove moisture; for BGA packages in tray form, bake at 120°C for 4–6 hours prior to soldering.

2) Clean the solder pads to remove any flux or solder paste remaining on the PCB surface.

3) When applying solder paste or flux, always use fresh materials and ensure even coverage. The solder paste must be thoroughly mixed, and its viscosity as well as the amount applied should be appropriate to prevent bridging during the soldering process.

4) During component placement, each solder ball on the BGA chip must be precisely aligned with its corresponding pad on the PCB.

5) During reflow soldering, it is essential to select appropriate heating temperatures and dwell times for each zone, while also paying close attention to the rate of temperature rise. Generally, before reaching 100°C, the maximum heating rate should not exceed 6°C/s; after 100°C, it should not exceed 3°C/s. In the cooling zone, the maximum cooling rate should be no more than 6°C/s. Excessively rapid heating or cooling can damage both the PCB and the components, and such damage may sometimes be invisible to the naked eye. Furthermore, different components and solder pastes require tailored temperature and time settings. For no‑clean solder paste, which has lower activity than conventional solder paste, the soldering temperature should be kept moderate and the soldering time shortened to prevent oxidation of the solder particles.

6) During PCB design, all BGA pads on the board should be designed to the same size. If certain vias must be routed beneath the pads, it is essential to work with a qualified PCB manufacturer to ensure that all pads are uniform in size, with consistent solder volume and uniform height. 5. Conclusion: As the mainstream trend in electronic products moves toward miniaturization, the pitch of BGA package leads becomes increasingly tight, making soldering progressively more challenging. Consequently, ensuring the soldering reliability of BGAs remains an ongoing area of research.

References

[1] Mechanism of BGA Void Formation and Its Impact on Solder Joint Reliability

[2] IPC – International Electronics Industry Association. IPC‑A‑610D Acceptance Criteria for Electronic Assemblies