1. Green paint construction
The ball-planting pad on the underside of the BGA is created using the “green paint set limit” method. When the green paint exceeds a thickness of 1 mil and the backing surface is too small, it can lead to a “crater effect,” making it difficult for wave soldering to penetrate. Furthermore, during the ball planting process, the combination of excess flux and high heat can force solder to seep beneath the edge of the green paint, potentially causing it to lift. This scenario differs significantly from solder paste soldering on PCB pads. Typically, the SMD copper pad on such carrier boards is slightly larger (often featuring nickel and gold), allowing the green paint to extend up to a peripheral width of 4 mil. However, because tin cannot flow to the outer straight edge of the copper pad, the stress is concentrated. Consequently, the strength of these joints is inferior to that of NSMD solder joints formed by full copper pads. Additionally, the stress in SMD solder joints dissipates poorly, leading to a “fatigue life” that is generally only 70% of that of NSMD. In fact, designers and manufacturers of standard packaging substrates often lack insight into this logic, which poses increasing risks to the strength of various BGAs on mobile phone PCBs, especially in the context of lead-free soldering.
(1) Green paint plug hole
1. The primary function of the green paint plug hole is to facilitate the removal of empty space, enabling quicker fixes to the PCB during circuit testing. Additionally, it protects the circuit or solder pad near the through hole on the first side from being compromised by the second surface wave soldering process, which can be disrupted by Yong Tin. However, if the plug is not secure and breaks, it can lead to persistent issues due to the high pressure from soldering or wave processes pushing in tin dross. While the original table listed four plug hole methods, none are truly practical for mass production.
2. Reapplying wave soldering after fusion welding is common when certain PCB components require additional soldering once both sides are complete. This can lead to heat transfer through adjacent through holes to the first side, risking reflow of the soldered ball, potentially resulting in accidental cold welds or open circuits. In such instances, external heat shields, such as temporary Heat Shields and Wave Shields, can be employed to insulate the BGA area.
3. The construction methods for plugging holes include dry film covering and printing flooded holes, which involve inserting the hole into the printing plate surface. Professional plug holes are filled and cured with special resin before green paint is applied to both sides. Regardless of the method, achieving perfection in construction is challenging. Consequently, OSP boards with green paint often experience failures, particularly when liquid materials remain in slits post-plugging, damaging the perforated copper. Moreover, baking the back plug can harm the OSP film, presenting a significant dilemma.
4. Regarding BGA placement, the printing of solder paste is best executed using a narrow and wide trapezoidal opening in the steel plate to prevent disturbance during the lifting process. Typically, metal constitutes about 90% of solder paste, and solder particle sizes should not exceed 24% of the opening size to avoid edge blurring. The most frequently used BGA assembly printing paste features a particle size of 53μm, while CSP commonly uses 38μm particles.
5. For large BGAs with a pitch of 1.0-1.5mm, the printed steel plate thickness should be 0.15-0.18mm; for fine pitch BGAs under 0.8mm, the thickness should be reduced to 0.1-0.15mm. The opening’s aspect ratio should be maintained around 1.5 to facilitate paste application. The corners of square pads for close-spaced designs should be rounded to minimize tin particle adhesion. For small-pitch round pads, the width-to-depth ratio of the steel plate must be below 66%, ensuring that printed paste exceeds the pad surface by 2-3 mils for improved temporary adhesion before welding.
6. Hot air fusion welding has become the predominant method of Reflow since the 1990s. A greater number of heating sections in the production line allows for easier adjustments to the “temperature-time curve” and accelerates production speed. Current lead-free solderers typically feature over 10 heating segments (up to 14). However, prolonged exposure to high temperatures beyond the Tg of the plate can cause the PCB to soften and lead to Z expansion, resulting in board failure due to inner circuit or PTH fractures. The flux in solder paste must reach above 130°C to activate, maintaining activity for 90-120 seconds, while the average heat resistance limit for components is 220°C, which must not be exceeded for more than 60 seconds.
The ball-planting pad on the underside of the BGA is created using the “green paint set limit” method. When the green paint exceeds a thickness of 1 mil and the backing surface is too small, it can lead to a “crater effect,” making it difficult for wave soldering to penetrate. Furthermore, during the ball planting process, the combination of excess flux and high heat can force solder to seep beneath the edge of the green paint, potentially causing it to lift. This scenario differs significantly from solder paste soldering on PCB pads. Typically, the SMD copper pad on such carrier boards is slightly larger (often featuring nickel and gold), allowing the green paint to extend up to a peripheral width of 4 mil. However, because tin cannot flow to the outer straight edge of the copper pad, the stress is concentrated. Consequently, the strength of these joints is inferior to that of NSMD solder joints formed by full copper pads. Additionally, the stress in SMD solder joints dissipates poorly, leading to a “fatigue life” that is generally only 70% of that of NSMD. In fact, designers and manufacturers of standard packaging substrates often lack insight into this logic, which poses increasing risks to the strength of various BGAs on mobile phone PCBs, especially in the context of lead-free soldering.
(1) Green paint plug hole
1. The primary function of the green paint plug hole is to facilitate the removal of empty space, enabling quicker fixes to the PCB during circuit testing. Additionally, it protects the circuit or solder pad near the through hole on the first side from being compromised by the second surface wave soldering process, which can be disrupted by Yong Tin. However, if the plug is not secure and breaks, it can lead to persistent issues due to the high pressure from soldering or wave processes pushing in tin dross. While the original table listed four plug hole methods, none are truly practical for mass production.
2. Reapplying wave soldering after fusion welding is common when certain PCB components require additional soldering once both sides are complete. This can lead to heat transfer through adjacent through holes to the first side, risking reflow of the soldered ball, potentially resulting in accidental cold welds or open circuits. In such instances, external heat shields, such as temporary Heat Shields and Wave Shields, can be employed to insulate the BGA area.
3. The construction methods for plugging holes include dry film covering and printing flooded holes, which involve inserting the hole into the printing plate surface. Professional plug holes are filled and cured with special resin before green paint is applied to both sides. Regardless of the method, achieving perfection in construction is challenging. Consequently, OSP boards with green paint often experience failures, particularly when liquid materials remain in slits post-plugging, damaging the perforated copper. Moreover, baking the back plug can harm the OSP film, presenting a significant dilemma.
4. Regarding BGA placement, the printing of solder paste is best executed using a narrow and wide trapezoidal opening in the steel plate to prevent disturbance during the lifting process. Typically, metal constitutes about 90% of solder paste, and solder particle sizes should not exceed 24% of the opening size to avoid edge blurring. The most frequently used BGA assembly printing paste features a particle size of 53μm, while CSP commonly uses 38μm particles.
5. For large BGAs with a pitch of 1.0-1.5mm, the printed steel plate thickness should be 0.15-0.18mm; for fine pitch BGAs under 0.8mm, the thickness should be reduced to 0.1-0.15mm. The opening’s aspect ratio should be maintained around 1.5 to facilitate paste application. The corners of square pads for close-spaced designs should be rounded to minimize tin particle adhesion. For small-pitch round pads, the width-to-depth ratio of the steel plate must be below 66%, ensuring that printed paste exceeds the pad surface by 2-3 mils for improved temporary adhesion before welding.
6. Hot air fusion welding has become the predominant method of Reflow since the 1990s. A greater number of heating sections in the production line allows for easier adjustments to the “temperature-time curve” and accelerates production speed. Current lead-free solderers typically feature over 10 heating segments (up to 14). However, prolonged exposure to high temperatures beyond the Tg of the plate can cause the PCB to soften and lead to Z expansion, resulting in board failure due to inner circuit or PTH fractures. The flux in solder paste must reach above 130°C to activate, maintaining activity for 90-120 seconds, while the average heat resistance limit for components is 220°C, which must not be exceeded for more than 60 seconds.
