Basic Knowledge of SMT Reflow Oven Fixtures
2024-08-26 17:25
Solder paste printing, SMD placement, and reflow soldering are all core processes of Surface Mount Technology (SMT), which require the use of SMT tools to maximize SMT production efficiency. Common SMT tools include stencils, test fixtures, and reflow oven fixtures. Today, we will introduce the reflow oven fixture—also known as SMT fixture, PCB tray, or SMT tray—which plays a role in supporting and transporting Printed Circuit Boards (PCBs) throughout these SMT processes.
In rigid PCB assembly, the use of reflow oven fixtures may or may not be necessary. However, if you are working with flexible PCB (FPC) assembly, such fixtures are absolutely essential for your SMT process.Reflow oven fixtures are often a critical component in achieving consistent, top-quality results during New Product Introduction (NPI) or high-volume PCB assembly runs. This tool is no stranger to the electronics manufacturing industry—it is a well-established part of designing robust PCB assembly processes. When deciding to use a reflow oven fixture, mechanical support is usually the only factor considered, but the fixture actually offers numerous uses and advantages.
Purposes and Advantages of SMT Fixtures
Whenever you consider and/or evaluate a reflow fixture, take time to remind yourself of the benefits it provides and how you intend to design it. There are several factors to consider when reviewing the advantages of reflow fixtures and key design elements.
Providing mechanical support throughout the entire SMT production line (for both top-side and bottom-side assembly) reduces warping of rigid PCBs, rigid-flex PCBs, and flexible circuits—thereby significantly improving yield rates.SMT fixtures are truly indispensable when handling thinner substrates, but they also prove beneficial for substrates considered "thick." The specific design features selected for a given SMT tray are crucial to minimizing warping. During the reflow process, threaded inserts can be used to secure the PCB to the fixture. Tapered tooling pins, positioned in the tooling holes (plated or non-plated) of the circuit board, create a wedge effect to fix the PCB in the fixture’s PCB pocket during solder paste screen printing, pick-and-place, and reflow processes. Additionally, by using flush-mounted "pushers" with different spring K-factors (depending on PCB thickness and stacking characteristics), tension or pressure can be applied against these tapered pins—helping to maintain the flatness of the PCB throughout the entire process.Stencil alignment after PCB depaneling is also improved: In applications involving fine-pitch components, printing smaller patterns after depaneling operations can boost yield rates. PCBs are typically received in a depaneled state, and reflow fixtures are the only solution for transporting and handling these SMT assemblies.Exercise caution when determining the number of patterns to include in a single fixture. While a higher number of patterns will obviously increase productivity, the accuracy of stencil-to-PCB alignment decreases exponentially with each additional pattern.
Vision and Alignment
Reflow fixtures can provide the necessary or additional fiducials required to achieve optimal stencil registration and SMD placement precision. It is recommended to ask your fixture supplier to explain how they design and manufacture these fixture fiducials—ensuring the fiducials are durable and easily detectable by your process equipment.
Conveyance
Many "irregularly shaped" PCB layouts are difficult to convey through SMT process equipment. However, fixtures feature sturdy edge step designs, which make smooth conveyance trouble-free. When designing a fixture, pay attention to the thickness of these edge steps.
Edge Clearance
If components are too close to the edge of the PCB, they may get damaged during the assembly process of the first side or the second side without the use of an SMT fixture. SMT fixtures can be designed to leave clearances around these edge-proximal components (on both sides of the PCB) while still providing firm support for the rest of the circuit board.
Process Standardization
If all assemblies in a product portfolio use standard fixture designs (in terms of width, thickness, and material), changeovers for machines and/or production lines can be reduced. For single-sided processing, reflow fixtures can often be used in other "downstream" processes, such as Automated Optical Inspection (AOI) or electrical testing.SMT fixtures also eliminate the need for high-cost vacuum systems or custom support plates in printing or placement equipment. Additionally, fixtures offer the extra benefit of requiring no additional operator training, engineer support, or equipment maintenance support.
Protection/Thermal Shielding
Heat-sensitive components can usually be shielded through the secondary reflow process. Additionally, a robust fixture design helps distribute temperature evenly across the PCB, thereby minimizing thermal impact.
Critical Alignment of Components
Cover alignment plates or integrated fixture features can help achieve critical alignment or spacing for components such as edge connectors during the reflow process.
Creating Custom Reflow Fixtures
It is crucial for success to create a custom SMT fixture for each unique circuit board assembly. A thorough understanding of thermodynamics and the impact of composite fixtures in the reflow environment is a key factor in determining whether any fixture will succeed or fail.
When visiting a PCB assembly factory, you may notice dark gray SMT fixtures on every machine and conveyor belt in the SMT production line. Many people are concerned about the question: What materials are these fixtures made of?
At MADPCB, the primary material for SMT fixtures is always nanocomposite stone—also known as Durostone—though aluminum is sometimes used. Durostone is the most commonly used material for manufacturing reflow fixtures. It is a heavy-duty glass fiber-reinforced plastic that exhibits extremely high strength at high temperatures, along with excellent mechanical and thermal properties. It maintains dimensional stability and keeps the fixture flat through repeated reflow cycles in the SMT process.
Durostone is manufactured using polyester (PET), vinyl ester, epoxy, or modified epoxy resins, combined with carbon, glass, or basalt fiber reinforcements. SMT fixtures made of Durostone can be used continuously at high temperatures for more than 20,000 cycles.
SMT fixtures made from Durostone offer numerous benefits to all PCB and assembly manufacturers. The cost of SMT fixtures falls under Non-Recurring Engineering (NRE) expenses, and they can be regarded as a valuable tool option with a short return on investment (ROI) period.
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