1. Green Paint Construction
The ball pad at the bottom of the BGA of FR4 PCB is soldered using the “green paint limit” method. If the green paint layer is too thick (more than 1mil) and the pad surface is too small, it can result in a “crater effect” that makes it difficult for wave soldering to penetrate. Additionally, when exposed to a large amount of flux and high heat during the ball planting process on the circuit board, the solder can penetrate into the bottom edge of the green paint, causing it to peel off. This is unlike the solder paste method used for PCB processing pads.
Most of the time, if the SMD copper pad on such a board is slightly larger (sometimes including nickel and gold), the green paint can extend up to 4mil along the circumference. However, since the tin cannot flow to the outer straight wall of the copper pad, the strength of the solder joint is not as strong as the NSMD solder joint formed by all copper pads under stress. The stress on the SMD solder joint is not easily dissipated, resulting in a “fatigue life” that is generally only 70% of NSMD.
In reality, the designers and manufacturers of general packaging carrier boards are often unaware of this logic, leading to the strength of various BGA bearing pads on mobile phone circuit boards becoming increasingly compromised during lead-free soldering.
1) Green paint hole plugging
The green paint plug hole on the fr4 PCB serves two main purposes. Firstly, it facilitates vacuum pumping and quick fixation of the board surface during circuit board testing. Secondly, it protects the lines or solder pads near the through-hole on the first side from being affected by tin surges during wave soldering on the second side. However, if the filling is not firm and breaks, it can lead to severe aftereffects caused by tin spraying or wave soldering.
2) Re-wave soldering after reflow soldering
After reflow soldering is completed on both sides of some components, it is often necessary to insert and solder additional components. This can transfer the wave soldering heat to the first side and potentially cause the previously reflow-soldered ball pads to melt again, resulting in unexpected cold soldering or open circuits. Temporary heat shields and wave shields can be used to insulate the upper and lower sides of the BGA area in order to prevent this.
3) Hole plugging construction
Construction methods for green paint hole plugging on fr4 PCBs include dry film hole cover and print flooding hole methods. Both methods involve plugging holes on both sides of the printed circuit board, but may result in air bursting out at high temperatures. Professional plugging involves intentionally plugging and curing the holes with special resin before printing green paint on both sides. Regardless of the method used, achieving perfect hole plugging is difficult. For OSP boards, it is not feasible to plug the front or back with green paint, and there have been many cases of tragic failure. OSP after front plugging can leave liquid medicine in the slit and damage the copper pores during baking, creating a dilemma.
2. BGA installation
1) Solder paste printing
The steel plate used for printing should ideally have trapezoidal openings with a narrow top and wide bottom to facilitate stepping on the foot and lifting the steel plate after printing without disturbing the solder paste. The metal part of the commonly used solder paste accounts for about 90%, and the size of the tin particles should not exceed 24% of the opening to avoid blurring of the printed paste edge. Particle size for BGA assembly paste is commonly 53 μm, while for CSP, the common particle size is 38 μm. The thickness of the printed steel plate for large BGAs with a foot distance of 1.0-1.5mm should be 0.15-0.18mm, and reduced to 0.1-0.15mm for less than 0.8mm. The “width to depth ratio” of the opening should be about 1.5 to facilitate paste removal. In the case of closely spaced rounded pad openings, an arc must be present at the corner to reduce tin particle sticking. For small-piece close-spaced round pad openings with a width-depth ratio of less than 66%, the applied printing paste must be 2-3 mil larger than the pad surface for better temporary adhesion before reflow soldering.
2) Hot air reflow soldering
Forced convection hot air has become the mainstream of reflow soldering since the 1990s. The more heating sections in its production line, the easier it is to adjust the “temperature time curve”, and the production rate will be accelerated. Lead-free solders must have more than 10 sections on average for heating, with some requiring up to 14 sections. Exceeding the Tg of the plate and prolonged exposure to high temperatures can cause the circuit board to become soft and expand, leading to potential disasters such as inner line or PTH fractures. The flux in the solder paste must be above 130 ℃ to show activity, and its activity time should be 90-120 seconds. The heat resistance limit of various components in the fr4 PCB averages at 220 ℃ and should not exceed 60 seconds.
The ball pad at the bottom of the BGA of FR4 PCB is soldered using the “green paint limit” method. If the green paint layer is too thick (more than 1mil) and the pad surface is too small, it can result in a “crater effect” that makes it difficult for wave soldering to penetrate. Additionally, when exposed to a large amount of flux and high heat during the ball planting process on the circuit board, the solder can penetrate into the bottom edge of the green paint, causing it to peel off. This is unlike the solder paste method used for PCB processing pads.
Most of the time, if the SMD copper pad on such a board is slightly larger (sometimes including nickel and gold), the green paint can extend up to 4mil along the circumference. However, since the tin cannot flow to the outer straight wall of the copper pad, the strength of the solder joint is not as strong as the NSMD solder joint formed by all copper pads under stress. The stress on the SMD solder joint is not easily dissipated, resulting in a “fatigue life” that is generally only 70% of NSMD.
In reality, the designers and manufacturers of general packaging carrier boards are often unaware of this logic, leading to the strength of various BGA bearing pads on mobile phone circuit boards becoming increasingly compromised during lead-free soldering.
1) Green paint hole plugging
The green paint plug hole on the fr4 PCB serves two main purposes. Firstly, it facilitates vacuum pumping and quick fixation of the board surface during circuit board testing. Secondly, it protects the lines or solder pads near the through-hole on the first side from being affected by tin surges during wave soldering on the second side. However, if the filling is not firm and breaks, it can lead to severe aftereffects caused by tin spraying or wave soldering.
2) Re-wave soldering after reflow soldering
After reflow soldering is completed on both sides of some components, it is often necessary to insert and solder additional components. This can transfer the wave soldering heat to the first side and potentially cause the previously reflow-soldered ball pads to melt again, resulting in unexpected cold soldering or open circuits. Temporary heat shields and wave shields can be used to insulate the upper and lower sides of the BGA area in order to prevent this.
3) Hole plugging construction
Construction methods for green paint hole plugging on fr4 PCBs include dry film hole cover and print flooding hole methods. Both methods involve plugging holes on both sides of the printed circuit board, but may result in air bursting out at high temperatures. Professional plugging involves intentionally plugging and curing the holes with special resin before printing green paint on both sides. Regardless of the method used, achieving perfect hole plugging is difficult. For OSP boards, it is not feasible to plug the front or back with green paint, and there have been many cases of tragic failure. OSP after front plugging can leave liquid medicine in the slit and damage the copper pores during baking, creating a dilemma.
2. BGA installation
1) Solder paste printing
The steel plate used for printing should ideally have trapezoidal openings with a narrow top and wide bottom to facilitate stepping on the foot and lifting the steel plate after printing without disturbing the solder paste. The metal part of the commonly used solder paste accounts for about 90%, and the size of the tin particles should not exceed 24% of the opening to avoid blurring of the printed paste edge. Particle size for BGA assembly paste is commonly 53 μm, while for CSP, the common particle size is 38 μm. The thickness of the printed steel plate for large BGAs with a foot distance of 1.0-1.5mm should be 0.15-0.18mm, and reduced to 0.1-0.15mm for less than 0.8mm. The “width to depth ratio” of the opening should be about 1.5 to facilitate paste removal. In the case of closely spaced rounded pad openings, an arc must be present at the corner to reduce tin particle sticking. For small-piece close-spaced round pad openings with a width-depth ratio of less than 66%, the applied printing paste must be 2-3 mil larger than the pad surface for better temporary adhesion before reflow soldering.
2) Hot air reflow soldering
Forced convection hot air has become the mainstream of reflow soldering since the 1990s. The more heating sections in its production line, the easier it is to adjust the “temperature time curve”, and the production rate will be accelerated. Lead-free solders must have more than 10 sections on average for heating, with some requiring up to 14 sections. Exceeding the Tg of the plate and prolonged exposure to high temperatures can cause the circuit board to become soft and expand, leading to potential disasters such as inner line or PTH fractures. The flux in the solder paste must be above 130 ℃ to show activity, and its activity time should be 90-120 seconds. The heat resistance limit of various components in the fr4 PCB averages at 220 ℃ and should not exceed 60 seconds.
