Why Does SMT Require Reflow Carriers and Full-Process Carriers?
2024-08-26 17:22
What is an SMT Reflow Carrier? Why is an SMT Reflow Carrier sometimes used in SMT production? And why is a Full-Process Carrier (or Full-Line Carrier) sometimes needed?
The so-called "SMT Reflow Tray or Reflow Carrier" is essentially a tray or carrier used to hold PCBs (Printed Circuit Boards) and transport them through a Reflow Oven. Typically, such trays or carriers are equipped with positioning pins to secure the PCBs, preventing them from shifting or deforming. Some more advanced trays or carriers even come with an additional cover, which is usually designed for FPCs (Flexible Printed Circuits). Magnets are installed on both the upper and lower parts of the carrier to act as suction fasteners, ensuring a tighter hold and more effectively preventing the boards from deforming.
The purposes of using "SMT Reflow Trays or Reflow Carriers" usually include the following:
1.Reducing PCB deformation.
Here, the term "reducing" is used instead of "preventing" because no matter how well a reflow carrier is designed, PCBs will still experience some degree of deformation after being exposed to the high temperatures of the reflow oven. As long as the deformation stays within the range that does not affect quality, it is acceptable.
2.Preventing heavy components from falling off.
In fact, the two points mentioned above are both related to the high-temperature zone of the SMT Reflow Oven. For the lead-free process adopted by most products nowadays, the melting temperature of the lead-free SAC305 solder paste is 217°C, while the melting temperature of SAC0307 solder paste generally ranges from 217°C to 225°C. The maximum recommended reflow soldering temperature is usually between 240°C and 250°C. However, due to cost considerations, the FR4 substrate we usually choose only has a Tg (Glass Transition Temperature) of above 150°C (for those who are not clear about what Tg is, please refer to the article "What is Glass Transition Temperature (Tg)"). This means that when the PCB enters the high-temperature zone of the reflow oven, it has already exceeded its glass transition temperature and turned into a rubbery state. The deformation of the PCB in the rubbery state is just a natural manifestation of its material properties.
Additionally, PCBs have become thinner — their thickness has decreased from the usual 1.6mm to 0.8mm, and Honglijie (Shenzhen) has even seen PCBs as thin as 0.4mm. Such thin circuit boards are more prone to deformation when subjected to the high temperatures of the SMT Reflow Oven.
As for the heavy components mounted on the first side of the PCB, the reason why they tend to fall off during the reflow soldering of the second side probably requires no further explanation. Instead, many people have questions like "Why don't the components on the first side fall off during the second SMT reflow soldering? Will the solder melting temperature increase during the reflow soldering process?" For answers to these questions, please refer to the relevant articles on "Solutions to Prevent Components on the First Side from Falling Off During the Second Reflow Soldering".
The "SMT Reflow Tray or Reflow Carrier" is designed to address the issues of PCB deformation and component falling off. It generally uses positioning pins to fix the positioning holes of the PCB, effectively maintaining the shape of the PCB and reducing substrate deformation when the substrate is exposed to high temperatures and tends to deform. Of course, additional ribs are also necessary to prevent the middle part of the substrate from bending and sagging due to the influence of gravity.
Additionally, the characteristic of reflow carriers being resistant to deformation can also be utilized to design ribs or support points beneath "heavy components" to ensure no components fall off. However, the design of this carrier must be extremely careful to prevent the support points from pushing up the components excessively, which would cause inaccuracies in the solder paste printing on the second side.
To summarize, a high-quality SMT Reflow Tray or Reflow Carrier must possess the following characteristic requirements:
- Its softening and deformation temperature should be above 300°C. It should be reusable without deformation — this is the primary requirement.
- It should have low thermal expansion. Thermal expansion and contraction are common properties of most materials; if the carrier exhibits excessive thermal expansion, it will instead damage the PCB.
- The material should be machinable.
- The material should preferably be lightweight. Since operators need to pick up and place the carrier, an overly heavy carrier is unsuitable for operations in general electronics factories.
- The material should preferably be either slow to absorb heat or fast to dissipate heat. As the carrier must be reused in cycles, if it cannot cool down quickly to a temperature manageable by hand after passing through the reflow oven, more backup carriers will need to be prepared — which will increase costs.
- The material should be as low-cost as possible and suitable for mass production.
The commonly used materials for reflow carriers include aluminum alloy; in addition, high-carbon steel and magnesium alloy are also used to make reflow trays. Although aluminum alloy is lighter than ordinary ferrous metals, it is still somewhat heavy for the young female workers on the production line to handle. Moreover, aluminum alloy tends to absorb heat easily — after passing through the reflow oven, workers must either wear heat-insulating gloves or wait for a certain cooling period before they can pick it up, making the operation quite inconvenient.
Additionally, there is another type of synthetic stone reflow carrier, which is a relatively economical and cost-effective material. However, it has a shorter service life, and in recent times, there have been frequent reports of allergic reactions associated with it.
Full Process Carrier?
A typical SMT Reflow Carrier is only used when PCBs pass through the reflow oven. That is to say, the carrier does not need to be installed during the processes prior to the reflow oven (such as solder paste printing, component mounting, etc.). This approach helps reduce the number of carriers required.
However, as PCBs become increasingly thinner and denser, the precision requirements for solder paste printing are also growing higher. If a circuit board is already deformed during solder paste printing, the printing position of the solder paste will shift, and the thickness of the solder paste will also change. All these issues are highly detrimental to fine-pitch components and components smaller than 0402 (in size code).
When the aforementioned problems occur, the best solution is to address them through design changes. If all design adjustments fail to help, it is necessary to start considering the use of SMT Full Process Carriers. In essence, this type of carrier is similar to a Reflow Carrier; the only difference is that the solder paste printing process must be taken into account. Therefore, after a PCB is placed into the carrier, it must protrude above the carrier’s surface — even the positioning pins must comply with this requirement. Otherwise, problems will arise during solder paste printing.
It is foreseeable that the number of SMT Full Process Carriers used will increase exponentially compared to Reflow Carriers, depending on the length of the SMT production line. For high-volume production products, the overall cost may be negligible when spread out. However, for low-volume, high-mix products, the total cost of these carriers can be nearly enough to buy a small car.
If you are an RD (Research and Development) professional or a project manager, please carefully evaluate the production costs and quality risks caused by the design. When using SMT carriers, you not only need to calculate the cost of the carriers themselves but also account for the additional labor hours required for at least one worker to transfer the carriers. Moreover, quality issues may occur during operation if components are accidentally touched. Please always keep the concept of total cost in mind.
Previous Page
Next Page
Previous Page
Next Page
More news
What are the factors that affect SMT through reflow soldering?
2024/08/27
2024/08/27
Why does SMT require reflow soldering of trays and the entire process carrier
2024/08/27
Let’s Solve Your Test Challenge Together
Whether you need a custom socket, fixture, or complete solution, our engineers will review your files and design hardware that meets your exact requirements.
Copyright© 2025 Shenzhen LD Electronic Technology Co., Ltd.
