1. **PCBA Double-Sided Reflow Soldering Process (SMT) and Precautions**
Currently, the leading circuit board assembly technology in the PCB industry is “full-board reflow soldering (Reflow).” While other soldering methods exist, full-board reflow can be categorized into single-sided and double-sided reflow soldering. Single-sided boards are now rarely utilized, as double-sided reflow soldering optimizes space on the circuit board, allowing for smaller product designs. Consequently, most boards available in the market employ the double-sided reflow soldering process.
2. **Note on Cost Efficiency**
If space constraints are not an issue, the single-sided process can actually reduce one SMT step. When weighing material costs against SMT labor costs, single-sided may prove to be more cost-effective.
3. **Process Considerations**
The “double-sided reflow soldering process” involves two reflow stages, leading to certain process limitations. A common issue arises when the board enters the second reflow oven; components on the first side may fall due to gravity, particularly as the board passes through the high-temperature zone. This article will outline the precautions necessary for component placement during the double-sided reflow soldering process.
(Another point to consider is, why do most small components that have been tinned on the first side not re-melt and fall off when the second side is passed through the reflow oven? Why do only the heavier components tend to fall?)
Which SMD components should be processed on the first side in the reflow oven?
Generally, it is advisable to place smaller components on the first side during reflow soldering. This is because the PCB experiences less deformation when the first side is processed, ensuring higher precision in solder paste printing, making it more suitable for small components. Additionally, smaller parts are less likely to fall off during the second pass through the oven. Since these components rest directly on the bottom of the PCB during the second side’s reflow, their lighter weight makes them less prone to detachment under high temperatures. Moreover, components on the first side must withstand the temperatures of two reflow processes, so they should be temperature-resistant. Typical resistors and capacitors can endure reflow soldering at least three times to accommodate boards that may require re-processing.
Which SMD components should be placed on the second side during reflow? This is crucial.
▪ Larger or heavier components should be positioned on the second side to minimize the risk of falling during the reflow process.
▪ LGA and BGA components should ideally be placed on the second side to avoid unnecessary re-melting, reducing the chances of cold solder joints. If small BGAs with fine pitches are present, they may still be considered for placement on the first side.
The debate over whether to place BGA components on the first or second side remains ongoing. While second-side placement avoids re-melting risks, it can lead to greater PCB deformation, potentially impacting solder quality. Therefore, some experts suggest placing fine-pitch BGAs on the first side. However, significant PCB warping poses a serious risk for delicate components on the second side, as it can lead to inaccuracies in solder paste application and volume. Hence, the focus should be on minimizing PCB distortion instead of automatically opting for first-side placement of BGAs.
▪ Components that cannot tolerate multiple high-temperature exposures should be placed on the second side of the reflow oven to prevent damage.
▪ PIH/PIP components should also go on the second side unless their lead lengths do not exceed the board thickness. Longer leads protruding from the PCB may interfere with the stencil on the second side, causing solder paste application issues.
▪ Some components, such as network cable connectors with LEDs, may require soldering internally. Ensure these parts can withstand passing through the reflow oven twice; otherwise, they should be positioned on the second side.
When components are placed on the second side of the reflow oven, the circuit board has already been subjected to high temperatures, which can lead to warping. This deformation makes it challenging to control solder paste volume and positioning, increasing the risk of cold solder joints or short circuits. Thus, it is advised not to place 0201 or fine-pitch components on the second side, and BGA components should preferably use larger solder balls.
Referring to the images of the front and back sides of the SD card board at the beginning of the article, you should be able to clearly identify which components should be arranged for processing on the first side and which on the second side.
Additionally, in mass production, there are various methods for soldering and assembling electronic components on circuit boards. Each process is generally determined during the initial design phase, as component placement directly influences the assembly sequence and quality, while the layout indirectly affects assembly efficiency.
The current PCB soldering processes can be broadly categorized into full-board soldering and partial soldering. Full-board soldering includes reflow and wave soldering, while partial methods encompass carrier wave soldering, selective soldering, and non-contact laser soldering.
Currently, the leading circuit board assembly technology in the PCB industry is “full-board reflow soldering (Reflow).” While other soldering methods exist, full-board reflow can be categorized into single-sided and double-sided reflow soldering. Single-sided boards are now rarely utilized, as double-sided reflow soldering optimizes space on the circuit board, allowing for smaller product designs. Consequently, most boards available in the market employ the double-sided reflow soldering process.
2. **Note on Cost Efficiency**
If space constraints are not an issue, the single-sided process can actually reduce one SMT step. When weighing material costs against SMT labor costs, single-sided may prove to be more cost-effective.
3. **Process Considerations**
The “double-sided reflow soldering process” involves two reflow stages, leading to certain process limitations. A common issue arises when the board enters the second reflow oven; components on the first side may fall due to gravity, particularly as the board passes through the high-temperature zone. This article will outline the precautions necessary for component placement during the double-sided reflow soldering process.
(Another point to consider is, why do most small components that have been tinned on the first side not re-melt and fall off when the second side is passed through the reflow oven? Why do only the heavier components tend to fall?)
Which SMD components should be processed on the first side in the reflow oven?
Generally, it is advisable to place smaller components on the first side during reflow soldering. This is because the PCB experiences less deformation when the first side is processed, ensuring higher precision in solder paste printing, making it more suitable for small components. Additionally, smaller parts are less likely to fall off during the second pass through the oven. Since these components rest directly on the bottom of the PCB during the second side’s reflow, their lighter weight makes them less prone to detachment under high temperatures. Moreover, components on the first side must withstand the temperatures of two reflow processes, so they should be temperature-resistant. Typical resistors and capacitors can endure reflow soldering at least three times to accommodate boards that may require re-processing.
Which SMD components should be placed on the second side during reflow? This is crucial.
▪ Larger or heavier components should be positioned on the second side to minimize the risk of falling during the reflow process.
▪ LGA and BGA components should ideally be placed on the second side to avoid unnecessary re-melting, reducing the chances of cold solder joints. If small BGAs with fine pitches are present, they may still be considered for placement on the first side.
The debate over whether to place BGA components on the first or second side remains ongoing. While second-side placement avoids re-melting risks, it can lead to greater PCB deformation, potentially impacting solder quality. Therefore, some experts suggest placing fine-pitch BGAs on the first side. However, significant PCB warping poses a serious risk for delicate components on the second side, as it can lead to inaccuracies in solder paste application and volume. Hence, the focus should be on minimizing PCB distortion instead of automatically opting for first-side placement of BGAs.
▪ Components that cannot tolerate multiple high-temperature exposures should be placed on the second side of the reflow oven to prevent damage.
▪ PIH/PIP components should also go on the second side unless their lead lengths do not exceed the board thickness. Longer leads protruding from the PCB may interfere with the stencil on the second side, causing solder paste application issues.
▪ Some components, such as network cable connectors with LEDs, may require soldering internally. Ensure these parts can withstand passing through the reflow oven twice; otherwise, they should be positioned on the second side.
When components are placed on the second side of the reflow oven, the circuit board has already been subjected to high temperatures, which can lead to warping. This deformation makes it challenging to control solder paste volume and positioning, increasing the risk of cold solder joints or short circuits. Thus, it is advised not to place 0201 or fine-pitch components on the second side, and BGA components should preferably use larger solder balls.
Referring to the images of the front and back sides of the SD card board at the beginning of the article, you should be able to clearly identify which components should be arranged for processing on the first side and which on the second side.
Additionally, in mass production, there are various methods for soldering and assembling electronic components on circuit boards. Each process is generally determined during the initial design phase, as component placement directly influences the assembly sequence and quality, while the layout indirectly affects assembly efficiency.
The current PCB soldering processes can be broadly categorized into full-board soldering and partial soldering. Full-board soldering includes reflow and wave soldering, while partial methods encompass carrier wave soldering, selective soldering, and non-contact laser soldering.
