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What is an SMT Reflow Carrier? Why is it sometimes necessary in SMT production? And when do we need an SMT Full Process Carrier? Are there situations where neither is required?
The so-called “SMT reflow carriers or reflow templates” are essentially trays or carriers used to hold PCB (Printed Circuit Board) during the reflow soldering process. These carriers often feature positioning pillars to secure the PCB, preventing misalignment or deformation. Some advanced carriers even come with a lid, typically designed for Flexible Printed Circuit (FPC) use to avoid it from floating or lifting, equipped with magnets on both the upper and lower carriers acting as suction cups to firmly hold the board. This ensures the prevention of board deformation. In the case of rigid PCBs, a cover plate may also be used to secure specific components during the reflow process, preventing sliding.
In SMT assembly, the use of these carriers depends on various factors, including the type of PCB, the components being assembled, and specific production requirements. Sometimes, advanced carriers like the Full Process Carrier are employed for more comprehensive support and protection during the entire assembly process.
The purpose of using ”SMT Reflow Carrier” or “SMT-Reflow-Template” typically includes:
Minimizing PCB Deformation: The use of Reflow Carriers aims to “reduce” PCB deformation rather than completely preventing it. Despite well-designed Reflow Carriers, some degree of deformation may still occur as the PCB undergoes high temperatures during reflow. The carriers feature positioning pillars to secure the PCB and ribs to provide support, thereby mitigating deformation caused by exceeding the PCB’s Tg value and the influence of gravity.
Preventing Dropping of Heavy Components: Reflow Carriers can be designed with support points to hold certain components that are prone to dropping during the second reflow after the first-side assembly. This is particularly relevant for heavier components such as network cable connectors and sockets.
Both of these points are closely related to the high-temperature zone of the SMT reflow oven. Considering that most products now adopt lead-free processes, the melting point of SAC305 solder paste is 217°C, and SAC0307 solder paste melts approximately between 217°C to 225°C. The recommended maximum reflow temperature typically falls between 240°C to 250°C. However, for cost considerations, the commonly used FR4 board materials often have a Tg (glass transition temperature) of either 150 or 170. When a PCB enters the high-temperature zone of the reflow oven, it surpasses its Tg and becomes rubbery. The rubbery state of the PCB reflects its material characteristics without permanent deformation.
Moreover, there is a trend toward thinner PCBs, reducing from the standard 1.6mm thickness to as low as 0.8mm or even 0.4mm. Thinner PCBs are more prone to deformation under the high temperatures of the reflow process.
The ”SMT-Reflow-Carrier” is designed to address PCB deformation and component dropping issues. It typically uses positioning pillars to secure the PCB in place through holes, effectively maintaining the shape of the PCB and reducing deformation caused by high temperatures. Additionally, ribs or support points are strategically placed to prevent gravity-induced bending and sinking issues.
Furthermore, the Reflow Carrier takes advantage of its resistance to deformation at high temperatures. Designing ribs or support points underneath heavier components ensures that the components do not fall. However, careful design is necessary to avoid issues such as excessive lifting of components due to support points, which could impact the accuracy of solder paste printing on the second side.
In summary, a well-designed “SMT-Full-Process-Carrier” should possess the following characteristic requirements:
The softening deformation temperature should be above 300°C, allowing for repeated use without deformation. This is the primary requirement.
It should have minimal thermal expansion. Excessive expansion could potentially damage the PCB.
The material should be processable.
Ideally, the material should be lightweight, as heavy carriers are not suitable for general use in electronic manufacturing where operators need to handle them.
The material should resist heat absorption and facilitate rapid heat dissipation. Efficient heat dissipation is crucial for the quick cooldown of the carrier after reflow, allowing for prompt handling and reducing the need for additional carriers, thus minimizing costs.
The material should be cost-effective and suitable for mass production.
Commonly used materials for reflow carriers include aluminum alloy. High carbon steel and magnesium alloy are also used to produce reflow trays. While aluminum alloy is lighter than conventional iron-based metals, it can still be somewhat heavy for operators on the production line. Additionally, aluminum alloy tends to absorb heat, requiring operators to wear heat-resistant gloves or wait for a cooling period before handling. Another material, the , is more economically viable but has a shorter lifespan. However, some reports suggest that it may cause allergic reactions in certain cases.
What is SMT Full Process Carrier?
Typically, SMT reflow carriers are only used when the PCB is going through the reflow oven, meaning processes before the reflow oven (such as solder paste application, component placement, etc.) do not require the use of carriers. This helps minimize the overall quantity of carriers needed.
However, with the trend towards thinner and high denser PCBs, coupled with increasing precision demands in solder paste printing, the potential for board deformation during solder paste printing becomes a concern. When PCB deformation occurs during solder paste printing, it can lead to misalignment in solder paste application, affecting the positioning and thickness of the solder paste. This is particularly problematic for fine-pitch components and those smaller chip than size of 0402.
When faced with the aforementioned issues, the best approach is to address them through design modifications. If design adjustments prove insufficient, it becomes necessary to consider the use of “SMT Full Process Carriers.” These carriers are essentially similar to “SMT reflow carriers,” with the key difference being the consideration of the solder paste printing process. Therefore, the PCB, when placed in the carrier, must protrude above the carrier’s surface, at least flush with the PCB surface. Even positioning pillars must adhere to this requirement; otherwise, issues may arise during solder paste printing.
It’s anticipated that the quantity of “SMT Full Process Carriers” will significantly increase compared to “SMT reflow carriers,” depending on the length of the SMT production line. While the cost may be relatively small for large-scale production, for products with low quantities and high diversity, the cumulative cost of these carriers could be comparable to that of a small car.
Postscript:
If you’re an RD or project manager, it’s crucial to thoroughly evaluate the production cost and quality risks associated with the design. Using SMT carriers requires consideration not only of carrier costs but also the addition of at least one person-hour for loading/unloading and transporting carriers. During these operations, there’s also the potential for quality issues arising from accidental contact with components. Keep in mind the concept of overall cost.
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