Why Does SMT Require Reflow Oven Trays with Full Carriers?
2024-08-27 17:05
In Surface Mount Technology (SMT), the reflow oven is an indispensable soldering equipment, essentially a combination of baking ovens. Its primary function is to melt the solder in the solder paste at high temperatures inside the reflow oven, thereby soldering Surface Mount Device (SMD) components to the circuit board. Without SMT reflow soldering equipment, the SMT process cannot achieve the soldering of electronic components to circuit boards. The SMT reflow oven tray, meanwhile, is the most critical tool when products go through the reflow oven. At this point, you may have questions: What is an SMT reflow oven tray? What is the purpose of using an SMT reflow oven tray or carrier? Let’s first understand what an SMT reflow oven tray is.
1. What Is an SMT Reflow Oven Tray?
An SMT reflow oven tray (also known as an SMT reflow carrier) is essentially a tray or carrier used to hold PCBs and transport them into the reflow oven. Typically, the tray/carrier is equipped with positioning pins to secure the PCB, preventing it from shifting or warping. Some advanced trays/carriers are also fitted with a cover—commonly used for Flexible PCBs (FPCs)—and have magnets installed. During the loading process, suction cups are used to fasten the cover, allowing SMT assembly factories to avoid PCB warping.
2. Purpose of Using SMT Reflow Oven Trays or Carriers
The use of reflow oven trays in SMT production aims to reduce PCB warping and prevent heavy components from falling off. Both issues are closely related to the high-temperature zone of the SMT reflow oven. Currently, most products adopt lead-free processes: the melting temperature of lead-free SAC305 solder paste is 217°C, while that of SAC0307 solder paste ranges approximately from 217°C to 225°C. The maximum recommended reflow temperature is generally between 240°C and 250°C. However, for cost considerations, FR4 boards with a glass transition temperature (Tg) of 150°C or higher are usually selected. This means that when the PCB enters the high-temperature zone of the reflow oven, its temperature has already exceeded the glass transition temperature, causing the PCB to enter a rubbery state—and warping of the PCB in this state is exactly a manifestation of its material properties.
Additionally, PCB thickness has been reduced: from the standard 1.6mm to 0.8mm, or even 0.4mm. Such thin circuit boards are prone to warping when exposed to the high temperatures of the reflow oven.
SMT reflow oven trays or carriers are designed to address the problems of PCB warping and component falling. They typically use positioning pins to fix the PCB’s positioning holes, effectively maintaining the PCB’s shape to reduce warping when the board material is exposed to high temperatures. Additionally, auxiliary support rods are usually required to prevent the middle part of the PCB from bending and sagging due to gravity.
Furthermore, leveraging the low-warpage property of the carrier material, reinforcing ribs or support points can be designed under heavy components to ensure they do not fall off. However, the design of such carriers must be meticulous: excessive support points should be avoided, as they may lift components and cause inaccurate solder paste printing on the second side of the PCB.
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