What Is a Reflow Oven?

The reflow soldering technology is no stranger in the field of electronics manufacturing. The components on the various boards used in our computers are soldered to the circuit board by this process. There is a heating circuit inside this device that air or nitrogen After heating to a high enough temperature, it is blown to the circuit board where the component has been pasted, so that the solder on both sides of the component is melted and bonded to the motherboard. The advantage of this process is that the temperature is easy to control, oxidation can be avoided during the welding process, and the manufacturing cost is easier to control.

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According to the continuous improvement of the product's heat transfer efficiency and soldering reliability, reflow soldering can be roughly divided into five development stages.

The reflow soldering process is a surface-mounted board, and the process is more complicated, which can be divided into two types: single-sided mounting and double-sided mounting.

The temperature curve refers to the curve of the temperature of a point on the SMA as time passes when it passes through the furnace. The temperature curve provides an intuitive method to analyze the temperature change of a component throughout the reflow process. This is very useful for obtaining the best solderability, avoiding damage to components due to over-temperature, and ensuring the quality of soldering.

There are three main reasons for the uneven heating of components in the SMT reflow process: the difference in the thermal capacity or absorbed heat of the reflow component, the influence of the conveyor or heater edges, and the load of the reflow product.

1. Generally, PLCC and QFP have larger heat capacity than a discrete chip component, and it is more difficult to weld large area components than small components.

2. In the reflow furnace, the conveyor belt is used to reflow the product. It also becomes a heat dissipation system. In addition, the edge of the heating part is different from the central heat dissipation condition. The edge temperature is generally low, except the temperature in the furnace. In addition to different requirements, the temperature of the same load surface is also different.

3. Different effects of product loading. The adjustment of the temperature curve of reflow soldering should consider the good repeatability under no-load, load and different load factors. The load factor is defined as: LF = L / (L + S); where L = length of the assembled substrate and S = interval of the assembled substrate.

To achieve reproducible results in a reflow process, the larger the load factor, the more difficult it is. The maximum load factor of a reflow oven is usually 0.5 ~ 0.9. This depends on the product situation (component soldering density, different substrates) and different models of reflow furnaces. To get good welding results and repeatability, practical experience is important ^[1]

With the development of many electronic products in the direction of small size, light weight, and high density, especially the large-scale use of handheld devices, the original SMT technology has been severely challenged in terms of component material technology, which has led to the rapid development of SM. opportunity. The lC pin pitch has developed to 0.5mm, 0.4mm, 0.3mm, BGA has been widely used, CSP has also emerged, and has shown a rapid upward trend. No-clean, low-residue solder paste on materials is widely used. All these put forward new requirements for the reflow soldering process. A general trend is to require reflow soldering to adopt more advanced heat transfer methods to achieve energy saving and uniform temperature, which are suitable for the welding requirements of dual-panel PCBs and new device packaging methods. Gradually realize the comprehensive replacement of wave soldering. Generally speaking, reflow ovens are developing in the direction of high efficiency, multi-function and intelligence, and there are mainly the following development paths. Reflow soldering in these development areas has led the development direction of future electronic products.

Nitrogen reflow

The use of inert gas protection in reflow soldering has been used for some time and has been widely used. Due to price considerations, nitrogen protection is generally selected. Nitrogen reflow has the following advantages.

(1) Prevent reduction of oxidation.

(2) Improve the welding wetting force and speed up the wetting speed.

(3) Reduce the generation of solder balls, avoid bridging, and obtain good soldering quality.

Reflow soldering on both sides

Double-sided PCB has become quite popular and is becoming more and more complicated. It has become so popular, mainly because it provides designers with extremely good flexibility space to design more compact and compact low-cost products. Double-sided boards generally have the upper side (component side) soldered by reflow, and then the lower side (lead side) by wave soldering. There is a tendency to double-sided reflow, but there are still some problems with this process. The bottom component of the large board may fall during the second reflow process, or the bottom solder joint is partially melted, causing solder joint reliability problems.

Through-hole plug-in components

Through-hole reflow is sometimes referred to as reflow soldering of classified components. It is gradually emerging. It can remove the wave soldering link and become a process link in PCB mixing technology. One of the biggest benefits of this technology is that it can play a role in the surface. At the same time as the advantages of the mounting manufacturing process, the through-hole plug-in is used to obtain better mechanical connection strength. For the flatness of the large-sized PCB board, the pins of all surface-mounted components cannot be in contact with the pads. Even if both the pin and the pad can be in contact, the mechanical strength it provides is often not large enough, and it is easy to fall off and become a failure point in the use of the product.

Reflow soldering green lead-free

Due to environmental considerations, lead will be severely restricted in the 21st century. Although the amount of lead used in the electronics industry is very small, less than 1% of the total, it is also banned and will be gradually phased out in the development trend. Eliminated, many places are developing reliable and economical lead-free solder, and the various alternatives developed generally have melting points about 40 tuns higher than tin-lead alloys, which means that reflow soldering must be at higher temperatures In the next step, nitrogen protection can partially eliminate the increased oxidation and damage to the PCB itself due to temperature increase.

Continuous reflow soldering

Special furnaces have been developed to handle continuous flexible boards with SMT components attached. The biggest difference from ordinary reflow ovens is that they require special tracks to transfer flexible boards. Of course, this kind of furnace also needs to be able to deal with the problem of continuous boards. For separated PCB boards, the flow in the furnace has no dependence on the status of the previous stations. However, for continuous flexible boards in rolls, the flexible boards are in The entire line is continuous. Any special problem on the line, the pause means that the entire line must be paused. This creates a special problem. The part that is paused in the furnace will be damaged due to overheating. Therefore, such a furnace must have strain. The ability to stop randomly, continue to process this section of flexible board, and return to normal working status when the full line resumes continuous operation.

Reflow soldering vertical oven

The market's demand for shrinking volume has made CSP, MPM, and even POP more applications, so components have a smaller footprint and a higher signal transmission rate after placement. Filling or filling is used to strengthen the solder joint structure so that it can withstand the stress caused by the inconsistent thermal expansion coefficient between the silicon wafer and the PCB material. Generally, the dripping or filling method is often used to seal the wafer with glue. .

Simulation and optimization of reflow soldering curve

The method of using computer technology to simulate the reflow soldering process has received widespread attention. This method can greatly shorten the process preparation time, reduce the experimental cost, improve the welding quality, and reduce the welding defects.

By using PCBCAD data to build the product model structure, the reflow soldering process simulation model can replace the traditional online parameter setting process, and can even be used to ensure the compatibility of the PCB design and reflow soldering process before production, and guide the manufacturability design ( DFM), this simulation model can also eliminate the defects that cannot cover all product areas when using thermocouple testing. The PCB component model solver and the reflow furnace model built can accurately predict the PCB components for specific process settings. Reflow temperature profile. Using this method to optimize the process of new products at the PCB design stage can easily ensure the compatibility of product design and process equipment.

Reflow soldering replaceable assembly

Alternative Assembly and Reflow Technology (AART) has aroused interest in the PCB assembly industry. The AART process can perform reflow soldering of through-hole components and surface-mount components at the same time, eliminating wave soldering and manual soldering. Compared with the traditional AART process, it has fewer costs, cycles and defect rates. Through the AART process, complex PCB assembly processes can be established. AART must consider materials, design, and process factors that affect it. A Decision Support System (DSS) can help engineers implement AART processes.

Reflow process control

The intelligent reflow furnace has a built-in computer control system, which can easily input various data under the Windows operating environment, and can quickly remove or replace the reflow soldering process curve from the memory, saving adjustment time and improving production efficiency.

The purpose of process control is to achieve both the required quality and the lowest possible cost. In the past, process control mainly focused on the detection of shortages to improve quality; after development, the most fundamental connotation of control was to continuously monitor various processes and find out deviations that did not meet the requirements. Process control is the ability to obtain relevant data in a particular operation that affects the final result. Once a potential problem arises, it can receive relevant information in real time, take corrective actions, and immediately adjust the process to the best conditions. Monitoring actual process data is considered true process control. This means that in the reflow process control, it means monitoring the thermal curve of each board manufactured.

An automatic management system capable of continuously monitoring the reflow furnace, which can indicate whether the process is out of control before the process shift actually occurs. This is the Automatic Reflow Management (ARM) system. This system converts continuous SPC histograms , Line balance network, documentation and product tracking form a complete software package, and can automatically measure process data in real time and make judgments to affect product cost and quality. The basic function of the automatic reflow management system is to accurately automatically detect and Collect product data through the furnace, it provides the following functions: no need to verify the process curve; automatically collect reflow soldering process data; provide real-time feedback and alarms for zero defect production; provide automatic SPC charts and correction process capability index , Cpk) variable alarm ^[1] .