With the rapid advancement of mobile communication technology, EMS (Electronic Manufacturing Services) providers worldwide are facing severe labor shortages. Furthermore, the Industry 4.0 trend has heightened the demand for automation in EMS facilities. As a result, many components that were previously not suited for Surface Mount Technology (SMT) are now required to meet the Paste-In-Hole (PIH) process. These components include Type A USB connectors, RJ45 Ethernet connectors, power sockets, transformers, and other relatively bulky parts. In the past, these components were typically soldered after SMT using wave soldering equipment or manual touch-up.
Due to the scarcity of manual labor and the need to save on subsequent manufacturing costs, along with quality considerations, many system manufacturers and EMS companies are gradually transitioning these non-SMT components to at least meet the PIH process requirements. This enables all electronic components to be soldered to the printed circuit board through a reflow oven.
However, converting components to SMD or PIH processes involves specific requirements. Please refer to the article “What Process Impact of Substituting SMDs with Paste-In-Hole components?” to understand the material and design requirements for these components.
Additionally, when transitioning all electronic components to SMT processes, a new challenge arises. During the second pass of the PCB through the reflow oven, previously soldered components on the first side, especially heavier components, face the risk of falling into the reflow oven. Although most companies define in their Design for Manufacturability (DFx) specifications that heavier components must be designed on the same side of the circuit board, technological advancements and the aforementioned requirements are making it increasingly challenging for Research and Development (RD) teams to adhere to this rule.
If you are unfamiliar with the concepts of the first and second sides of a circuit board, please refer to the articles “Introduction to PCBA Double-Sided Reflow Process (SMT) and Considerations” and “Q&A for Beginners: Understanding SMT Stencils, Component Placement Sequence, DFM, and DFX.“
SMT factories face the challenge of preventing heavier components from falling during the second reflow. Here are some methods to address this issue:
Method 1: Dispensing “SMT Red Glue”
In the early days of SMT production lines, glue dispensers were essential because components treated with glue could undergo wave soldering. However, most SMT lines no longer use this equipment since only few boards go wave soldering now. Applying glue manually should be avoided because it consumes more labor and time, and it’s challenging to control quality as it might lead to accidents when working close to other components.
The purpose of applying red glue is to adhere components to the PCB surface. Thus, the red glue must be applied on the PCB surface and be in contact with the component to hold it in place. The high temperature in the reflow oven will cure the red glue, making it an irreversible adhesive that cannot be softened with heat.
If you need to apply glue directly beneath the component, it must be done immediately after solder paste printing on the PCB, just before placing the component. Be cautious as applying glue under the component might slightly raise the risk of component misalignment. Typically, this method is used with larger and heavier components, and glue quantity must be controlled carefully.
Another approach is to apply glue on the side of the component. This should be performed after solder paste printing and positioning of the component on the PCB. Be aware that there’s a risk of accidentally displacing the component. This method is often used with PIH components as their leads pass through the PCB, reducing the risk of dislodgment.
If using a machine to apply glue on the component’s side, precise control of glue quantity and application location is required. Apply the red glue to the PCB where the component will be placed, and then gently press the component into position using the pick-and-place machine’s nozzle. This ensures that the component won’t become misaligned or dislodged.
Advancements in robotics have offered solutions to many issues previously associated with difficult-to-control glue dispensing. Some companies have employed simple robotic arms to set up glue dispensing on SMT production lines. While this solution is cost-effective and suitable for lines with less glue dispensing, its speed is slower. It’s best suited for situations where high precision isn’t critical, as excessive robotic handling can introduce the components to oxidation, which might be problematic for later soldering operations.
Method 2: Using Reflow Carriers or Trays
Reflow carriers or templates can be designed with ribs or supports precisely positioned to hold the heavier components in place. This prevents heavier components from falling during the second reflow process. However, reflow carriers come at a cost. The number of carriers must exceed the length of the reflow oven, accounting for the simultaneous processing of multiple PCBs and allowing buffer and spare carriers. This can become a significant expense.
Reflow carriers are typically made of metal or special high-temperature-resistant plastics to withstand multiple passes through the high-temperature reflow oven. Using carriers might require additional labor because placing PCBs on carriers needs to be done manually. Additionally, using carriers may impact the reflow process by affecting temperature uniformity due to the heat-absorbing properties of metallic carriers. Therefore, when configuring reflow temperatures, it’s essential to monitor temperature consistently with carriers. Carrier designs should aim to minimize heat absorption by eliminating unnecessary material, ensuring that the carriers support the components without deformation.
Method 3: Adjusting Upper and Lower Reflow Oven Temperatures
Most reflow ovens allow separate control of upper and lower oven temperatures. In the early days of SMT production, when the size of electronic components were larger (e.g., 1206-size components), it was common to set the lower oven temperature 5-10°C lower than the upper oven temperature. This was done to prevent components from falling due to reflowing solder when passing through the second time reflow. However, most SMT engineers now don’t use this approach, as components have become much smaller, and there’s a minimal risk of components falling.
However, with the challenge of transitioning large components to SMT, the risk of components falling when large components on the first side reflow during the second reflow is almost certain. Even with adjustments to upper and lower oven temperatures, it’s impossible to prevent the components from falling if there are large, heavy connectors on the bottom side.
Adjusting upper and lower oven temperature differences is only effective for smaller components. If all components are at risk of falling, this method doesn’t work. It’s essential to be aware that a significant temperature difference between the upper and lower ovens can lead to PCB warping.
Method 4: Mixing High and Low Temperature Solder Paste
I don’t particularly recommend this solution because solder paste with a low melting point often has weaker solder joints. Prior to usage, it should undergo rigorous reliability evaluation. Also, having different solder pastes with varying melting points within a factory can risk misapplication.
The idea is to use high-temperature solder paste for the first side reflow and low-temperature solder paste for the second side reflow. This prevents the reflowing of solder on the first side components during the second reflow. For example, you might use SAC305 solder paste (melting point 217°C) for the first side reflow, and Sn42Bi58 low-temperature solder paste (melting point 138°C) for the second side reflow. The reflow profile for the second side should not exceed 220°C (preferably below 200°C). This way, the solder joints on the first side with SAC305 solder paste will not remelt during the second reflow.
Method 5: Consider Touch-Up Soldering (Machine or Manual)
Calculate the cost of touch-up soldering and compare it with the cost of dealing with components falling during the reflow process. Sometimes, pursuing full automation without considering cost-effectiveness might not be the best approach. It’s essential to weigh the costs and assess whether touch-up soldering is more cost-effective.
Touch-up soldering can be done manually or using robotic soldering. The quality of manual soldering might be questionable, so automated soldering is preferred.