Due to the need of miniaturization of electronic products, chip components have appeared, and traditional welding methods can no longer meet the needs. Firstly, reflow soldering process is adopted in the assembly of hybrid integrated circuit board. Most components assembled and soldered are chip capacitors, chip inductors, mounted transistors and diodes. With the development of SMT technology, various SMT components (SMC) and SMD components (SMD) appear. As a part of SMT technology, reflow soldering technology and equipment have also developed accordingly, and its application has been widely used in almost all electronic product fields. Reflow welding technology has also experienced the following development stages around the improvement of equipment.
(The picture shows Rio Tinto Mcr series reflow soldering) Rio Tinto Chuangneng Electronic Equipment Co., Ltd.
1. Conductive reflow welding of hot plate and push plate;
This reflow oven relies on the heat source under the conveyor belt or push plate to heat the components on the substrate through heat conduction, and is used for single-sided assembly of thick film circuits on ceramic (Al2O3) substrates. Only when the ceramic substrate is attached to the conveyor belt can enough heat be obtained, and the structure is simple and the price is low.
2. Infrared radiation reflow welding;
Most of these reflow ovens are conveyor belts, but the conveyor belts only play the role of supporting and conveying substrates, and their heating methods are mainly infrared heat sources. The temperature in the furnace is more uniform than before, and the mesh is larger, which is suitable for reflow soldering and heating the substrate assembled on both sides. This kind of reflow oven can be said to be the basic type of reflow oven.
The early reflow soldering design was mainly based on infrared radiation, which was sensitive to the color difference of its devices, and there were unstable factors in temperature control. Products with high welding requirements are not recommended.
3. Infrared hot air reflow welding;
This reflow oven is based on the infrared oven, and hot air is added to make the temperature in the oven more uniform. When heated by infrared radiation alone, it is found that under the same heating environment, different materials and colors absorb different heat, that is, different Q values in the formula (1) lead to different temperature rise δ T.. For example, the package of SMD such as IC is black phenolic or epoxy, and the lead is white metal. When heated alone, lead is white. Adding hot air can make the temperature more uniform and overcome the problems of poor heat absorption and poor shadow. IR+ hot air reflow furnace is widely used in the world, and IR+ hot air is widely used in Rio Tinto M series reflow welding.
4. Full hot air reflow welding:
M-series reflow soldering IR+ hot air has been widely used, but it is difficult for IR+ hot air to meet higher welding requirements, such as motherboard, various control boards, BGA, and products with more IC, such as Rio Tinto's MCR series and BTW series, all-hot air welding is adopted to meet the uniformity of IC heating during reflow soldering. In the total hot air model, there are two circulation methods. Small circulation, independent air outlet nozzles and centralized return air make the furnace temperature heated more evenly. The improved recovery air duct based on small circulation shows that the temperature uniformity is better in practical use.
5. Nitrogen (N2) hot air reflow welding:
With the increase of assembly density and the appearance of fine-pitch assembly technology, nitrogen-filled reflow soldering technology and equipment have emerged, which has improved the quality and yield of reflow soldering and become the development direction of reflow soldering. Nitrogen reflow welding has the following advantages:
Preventing and reducing 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 welding quality.
It is particularly important to obtain the listed welding quality. Solder paste with low active flux can be used, which can improve the performance of solder joints and reduce the discoloration of substrate. But its disadvantage is that the cost is obviously increased, which increases with the increase of nitrogen content. When you need to reach 1000ppm oxygen content and 50ppm oxygen content in the furnace, the demand for nitrogen is very different. At present, all solder paste manufacturers are committed to developing clean-free solder paste that can weld well in an atmosphere with high oxygen content, thus reducing the consumption of nitrogen.
For introducing nitrogen into reflow soldering, cost-benefit analysis must be carried out, and its benefits include product yield, quality improvement, rework or maintenance cost reduction, etc. A complete and correct analysis often reveals that the introduction of nitrogen does not increase the final cost, on the contrary, we can still benefit from it.
At present, most of the furnaces used are forced hot air circulation, which is not easy to control the consumption of nitrogen. There are several ways to reduce the consumption of nitrogen and the opening area of the inlet and outlet of the furnace. It is very important to block the idle space at the entrance and exit with partitions, roller blinds or similar devices. Another method is to use the principle that the hot nitrogen layer is lighter than air and difficult to mix, so that the heating chamber is higher than the inlet and outlet when designing the furnace, thus forming a natural nitrogen layer in the heating chamber, reducing the compensation amount of nitrogen and maintaining the required purity. This technology is in Rio Tinto MCR-N2Roh. .....