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BGA welding technology for OEM factory technicians
In today’s electronics manufacturing, the quality control of Surface Mount Technology (SMT) is well-established. However, BGA (Ball Grid Array) soldering remains a critical challenge. The process capability of an OEM factory largely depends on its BGA soldering expertise. With the rapid development of the electronics industry, components are continuously upgraded in functionality, while their performance improves and their size shrinks. Some components now use more advanced manufacturing processes, which constantly pose new challenges for SMT production.
One such challenging component is the "flower ball pad bow chip." As electronic packages move toward higher density and thinner designs, these chips become smaller, lighter, and more powerful. For example, CPUs used in mobile phones and ultrabooks exhibit these characteristics clearly: they require high performance integration while occupying minimal space. These chips often have thousands of solder joints, with pads designed to be very small—typically less than 0.35mm in diameter—and arranged irregularly. Additionally, the chip body tends to warp significantly, making it difficult to achieve good coplanarity during reflow. This type of chip is referred to as the "flower ball pad bow chip," as shown in Figure 1.
Figure 1: Material body warping
During the SMT phase, several key control points must be carefully managed. First, the reflow profile must be precisely set to ensure that the chip and PCB fit perfectly after soldering, avoiding issues like warpage. Second, the stencil opening design must be optimized to prevent problems such as bridging, the pillow effect (HoP), and non-wetting open (NWO). Finally, selecting the right solder paste is essential for improving the overall yield of the production line.
2.1 Reflow Temperature Control
For conventional lead-free BGA components, SAC305 or SAC405 alloys are commonly used, with peak temperatures typically between 228°C and 250°C. However, for flower ball pad bow chips, the thin substrate makes them highly sensitive to temperature. If the peak temperature is too high, the chip may deform at the corners, leading to HoP or NWO. Conversely, if the temperature is too low, the chip may not fully flatten, resulting in bridging or virtual soldering. Therefore, the peak temperature should be moderate, and a multi-zone reflow oven with a gentle temperature curve is recommended to reduce thermal shock and deformation.
2.2 Stencil Opening Design
Due to the thinness of the package and increased warpage, the solder balls and PCB pads can experience significant relative movement during reflow. To compensate, the stencil openings must be adjusted. In areas where the package warps severely—such as the edges and center of the chip—the stencil openings should be enlarged to increase the volume of solder paste, ensuring proper coverage and preventing defects. In contrast, when the package is compressed, reducing the number of openings helps avoid bridging. For other areas, a 1:1 aperture ratio is usually sufficient.
2.3 Solder Paste Selection
BGA components are prone to thermal deformation during reflow. If the thermal expansion coefficient (CTE) of the carrier plate differs significantly from that of the chip or encapsulating material, the chip may lift at the corners. For flower ball pad bow chips, the solder ball diameter is very small (around 0.35mm), and the amount of solder paste on the pad is limited. Due to the large surface-to-volume ratio, the flux in the solder paste evaporates quickly, reducing its activity. This increases the risk of separation between the solder paste and the ball, leading to poor solder joints. To address this, a high-adhesion solder paste with strong flux activity is recommended. It ensures better adhesion and maintains flux activity even under deformation, helping to prevent HoP and NWO.
3. Verification Process
For new or specially designed devices, suppliers often provide recommendations for temperature profiles and stencil openings. However, even with strict adherence to these guidelines, achieving satisfactory results can be challenging. Differences in product design, equipment, materials, and quality requirements mean that field engineers must independently adjust and verify process parameters based on real-world conditions. Through repeated trials and adjustments, accurate process data can be obtained.
The following steps were taken to optimize the SMT process for the flower ball pad bow chip:
3.1 Chip Data
The DPS product uses a CORE I7-6600U chip with a BGA1356 package (42 × 24mm). The BGA ball diameter is 0.35mm, but the pad design is varied, as shown in Table 1.
3.2 Formulation and Implementation Plan
Based on the three key SMT control points, the following plan was implemented:
- Adjust the reflow temperature curve to 240°C with 12 temperature zones and a slope of 2–3°C/sec.
- Increase the solder paste volume by adjusting the stencil openings, especially around the four corners and center of the chip.
- Use a high-adhesion solder paste with No. 4 powder particles and nano film technology, ensuring the stencil time is kept within 2 hours.
After implementation, the BGA and PCB achieved good coplanarity, with no bridging, stretching, or voids observed in X-RAY inspections (Figures 4 and 5).
3.3 Post-Verification
This process was also applied to similar chips like the CORE I7-4650U and Intel C612. The results showed stable performance with no soldering defects.
Conclusion
In typical BGA soldering, the selection of solder paste and the setting of furnace temperature are crucial. For flower ball pad bow chips, additional attention must be given to the following:
1. Maintain a reflow temperature of around 240°C to ensure BGA leveling and PCB flatness.
2. Expand stencil openings to increase solder paste volume while avoiding bridging.
3. Use stepped stencils and pay special attention to various components during temperature measurement.
4. Control the stencil time to maintain flux activity, using high-adhesion solder paste.
5. Ensure proper PCB support to prevent deformation during the entire SMT process.
This analysis aims to contribute to the industry and encourage technical exchanges to improve production quality. Due to limitations in detection equipment and personal experience, any shortcomings in this article are welcome for feedback and correction.