In PCBA manufacturing, preventing large components from falling off during the second reflow soldering process of double-sided PCB assembly is a persistent challenge for engineers. Why do some components stay in place while others drop off? What are the underlying technical principles, and how can we improve reflow soldering stability? This article explores the science behind component retention and offers practical solutions for more reliable PCBA production.
Understanding the Double-Sided PCB Assembly Process
A double-sided PCB features electronic components on both sides, including surface-mount devices (SMDs) such as resistors, capacitors, and LEDs, as well as through-hole components like connectors. The typical assembly process follows these steps:
- First Side Soldering:
The lighter and smaller SMDs are soldered first to minimize the risk of detachment during the second reflow.
- Second Side Soldering:
Where larger or heavier components, such as connectors, are soldered.
If heavy components are soldered first, they may face downward during the second reflow cycle. The high temperatures could re-melt the solder joints, and gravity may cause the components to detach, increasing the defect rate.
The Science Behind Component Retention in Reflow Soldering
The shift of Solder Melting Point
Lead-free solder pastes, such as SAC305, have an initial melting point of 217°C. However, after the first reflow, the solder alloy composition slightly changes, often increasing the melting point to 220°C or higher. This means that even if the oven temperature remains the same during the second reflow, the first-side solder joints become more resistant to re-melting, improving component stability.
Surface Tension of Molten Solder
Surface tension forms when solder is molten. Small or lightweight components benefit from this force, which helps keep them in place. In contrast, larger or heavier components have a downward force due to gravity, making them more prone to detachment, even if their solder joints do not completely melt.
Oxide Film and Flux Effects
Solder joints form a thin oxide layer after reflow, enhancing structural stability. Additionally, flux evaporation during the first reflow reduces the likelihood of re-melting in the second reflow process.
How to Prevent Component Detachment from Double-Sided PCB Assembly?
Optimize Assembly Order, Process, and Equipment
- Follow the principle of “light components first, heavy components last” to ensure the first-side components remain stable during the second reflow.
- Use advanced SMT equipment to ensure evenly heating and minimize the risk of component movement.
- Select high-performance solder paste, which alloy material with a suitable melting point should be chosen based on components, to improve joint reliability.
Temperature Profile & Pad Design Optimization
- Precisely control the reflow temperature profile to prevent excessive heat from re-melting first-side solder joints.
- Optimize pad size and solder volume to enhance mechanical strength of solder joints.
Key Factors Affecting Stability
Component Weight vs. Solder Joint Support
- The risk of detachment increases with component weight and decreases with larger solder pad areas.
- Engineers can mitigate this risk by adjusting pad dimensions or choosing alternative, lighter components when possible. For example, lightweight chip capacitors and resistors remain stable during the second reflow due to strong surface tension and less gravitational pull.
Synergy Between Flux and Solder Performance
- Since solder melting points increase by 5–10°C after the first reflow, small components typically stay in place during the second heating cycle.
- However, if the oven temperature is set too high, first-side solder joints may re-melt, leading to component detachment.
Case Study: RK3566 Linux Development Board
An example is the RK3566 Linux Development Board | Available on LCSC. This PCB features large surface-mounted connectors, including USB 2.0 ports, HDMI interfaces, and SMD pin headers—all of which have relative heavy weight.
To prevent detachment, heavy components are placed on the second soldering side. This ensures that the previously soldered components are not exposed to excessive gravitational force and the risk of re-melting.
This strategic placement improves manufacturing efficiency and yield rates, ensuring the assembly quality of components.
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