PCB assembly through SMT double-sided reflow soldering has become the mainstream process in contemporary electronic manufacturing. However, occasionally, many individuals inquire, “Is there a method to proactively determine, or is there any calculation formula to predict the risk or rate of components falling off during the second reflow on the first side?”
Workingbear have actually written several posts regarding considerations in SMT double-sided reflow soldering design and processes in this blog. For those interested in this topic, I recommend taking a look at these articles:
Methods to Prevent Heavier Components from Falling Off on the First Side during Second Reflow
Introduction to PCBA Double-Sided Reflow Process (SMT) and Component Placement Considerations
5 Key Aspects of PCB Layout Design to Be Considered Prior to Assembly and Soldering Production
The process and procedure for double-sided reflow soldering in PCB assembly are as follows:
- Application of solder paste and placement of SMD components on the first side
of the PCB.
- First pass through the reflow oven (1st reflow).
- Flip the PCB.
- Application of solder paste and placement of SMD components on the second side
of the PCB.
- Second pass through the reflow oven (2nd reflow).
Therefore, during the second-side component soldering, the electronic devices already soldered on the first side of the PCB will undergo reflow on their bottom side. When the PCB enters into reflow oven, the high temperature will re-melt the solder that has already solidified on the first side, potentially causing heavier electronic components to fall off.
So, what factors could influence the occurrence of component falling during the second reflow on the first side?
If you’ve closely observed devices that are prone to falling during reflow, you’ll notice they share some common characteristics, such as fewer solder joints, heavier or taller component bodies. Examples include SMD models of LAN transformers, coil transformers, coil filters, larger electrolytic capacitors, coaxial connectors, LGAs, MCMs, and similar components.
Here are some factors that could potentially affect the likelihood of components falling off during the second reflow:
Due to the influence of gravity, heavier components are more susceptible to falling off the circuit board.
Solder Paste Alloy Composition:
The industry mainly uses two types of solder paste alloys: lead-free (e.g., SAC305) and tin-lead (e.g., Sn61.9%/Pb38.1%). Lead-free solder paste typically has a higher melting point (around 217°C for SAC305), while tin-lead solder paste melts at a lower temperature (about 183°C for SnPb). The viscosity of the molten solder may also affect the likelihood of components falling off, but information on this aspect is less readily available.
Reflow Oven Temperature:
Specifically, the actual temperature of the PCB during reflow, especially the temperature of the PCB’s bottom side, can influence the extent of solder liquefaction. Higher temperatures improve the wetting properties of solder, causing it to flow more evenly onto the pads and the component pins. Some people lower the reflow oven’s bottom temperature by approximately 5-10°C to address component falling issues.
PCB Surface Finished:
Different PCB surface finished result in different Intermetallic Compounds (IMCs) and wetting characteristics. However, this factor is generally considered to have a minor impact on the likelihood of components falling off during the second reflow.
Component Pin Solderable Area:
Besides component weight, the area available for soldering on component pins is a significant factor affecting solder’s ability to hold components in place. Components with larger pin solderable areas are generally better at preventing bottom-side components from falling off.
Solder Paste Printing Volume:
This factor should be considered in conjunction with the area of component pins. More solder paste does not necessarily equate to better results; the goal is to ensure that the solder paste adequately wets the bottom and side surfaces of component pins. Excess solder paste can sometimes lead to undesirable outcomes.
In summary, we can identify two primary factors affecting the likelihood of components falling off during the second reflow: “component weight” and “solderable area of component pins.” We can further categorize these factors using “solder paste composition.” As a result, some individuals have developed a simple formula, based on these three factors, to calculate and assess the risk of components falling off during the second reflow.
Please don’t ask me how this formula was derived because I don’t have that information. It seems to be a set of calculations that has surfaced from mainland China… so let’s just take it as a reference!
Formula for Evaluating the Risk of Components Falling Off During the Second SMT Reflow (For Reference Only)
For double-sided reflow soldering board designs, components on the first side with solderable areas that meet the following requirements for mass (Cg) and pad area (Pa), generally do not pose a risk of falling off.
In the formula below, Cg represents the component’s mass, while Pa is the total pad area (though personal opinion suggests that component pin area would be more appropriate):
(1) For lead-containing processes, the standard to avoid falling off: Cg/Pa ≤ 0.048g/mm²
(2) For lead-free processes, the standard to avoid falling off: Cg/Pa ≤ 0.038g/mm²
Under normal circumstances, small chip resistors, capacitors, inductors, thin flexible board connectors, thin BGAs, QFPs, QFNs, and similar components placed on the first side of the circuit board during the second reflow as the bottom side should not experience component falling issues. However, BGAs require additional consideration regarding solder reflow during the second reflow (please note that this formula is an empirical value and should be used for reference purposes only).
(Again, Don’t ask me where this formula comes from; asking would spoil the mystery. It’s a kind of empirical knowledge passed down, and it’s only meant as a reference!)
What are some methods to prevent components from falling off during SMT reflow?
If the PCB layout engineer can’t place components on the second side for reflow to avoid the risk of falling off on the bottom side during the second reflow, the process can be more challenging. However, with some extra effort and cost, it’s still possible to overcome component falling issues:
Method 1: Apply adhesive underneath or beside the components to secure them.
Method 2: Use reflow carriers or trays to support heavier components.
Method 3: Adjust the temperature difference between upper and lower reflow zones. Lowering the lower zone temperature slightly below the upper zone can help, but this method is effective only for components in the borderline weight category.
Method 4: Employ a combination of high and low-temperature solder pastes. Use a high-temperature paste for the first side and a low-temperature paste for the second side reflow.
Method 5: Consider re-soldering after the fact. This could involve machine or manual soldering.
For more in-depth information, it’s recommended to read: “How to Prevent Components from Falling Off on the First Side During the Second Reflow.”