Introduction to the Development of 9 Reflow Soldering Processes

Due to the continuous miniaturization of electronic products, sheet metal components have emerged, and traditional welding methods can no longer meet the needs. Firstly, reflow soldering technology is used in the assembly of hybrid integrated circuits, with most of the components assembled and soldered being chip capacitors, inductors, mount on transistors, and diodes. With the continuous improvement of SMT technology and the emergence of various surface mount components (SMC) and surface mount devices (SMD), reflow soldering technology and equipment, as part of SMD technology, have also undergone corresponding development. Its application is becoming increasingly widespread, and it has been applied in almost all electronic product fields. Reflow soldering technology has also gone through the following stages of development around equipment improvement.

1. Hot plate and push plate conductive reflow soldering

This type of reflow soldering furnace relies on a heat source under the conveyor belt or push plate to heat up the components on the substrate through thermal conduction. It is used for single-sided assembly of thick film circuits using ceramic (Al2O3) substrates. Only by sticking the ceramic substrate on the conveyor belt can sufficient heat be obtained. Its structure is simple and the price is cheap. Some thick film circuit factories in our country introduced such equipment in the early 1980s.

2. Infrared radiation reflow soldering

This type of reflow soldering furnace is also mostly conveyor belt type, but the conveyor belt only serves as a support and conveying substrate. Its heating method mainly relies on infrared heat sources to heat in a radiation manner. The temperature in the furnace is more uniform than the previous method, with larger mesh holes, suitable for reflow soldering heating of double-sided assembled substrates. This type of reflow soldering furnace can be said to be the basic type of reflow soldering furnace. It is widely used in our country and the price is relatively cheap.

3. Infrared heating air reflow soldering. This type of reflow soldering furnace is based on the IR furnace and adds hot air to make the temperature inside the furnace more uniform. When using infrared radiation heating alone, it is found that different materials and colors absorb heat differently in the same heating environment, that is, the Q value in equation (1) is different, resulting in a temperature rise Δ T is also different, for example, the packaging of SMDs such as ICs is black phenolic or epoxy, while the lead wire is white metal. When heated alone, the temperature of the lead wire is lower than its black SMD body. Adding hot air can make the temperature more uniform, while overcoming differences in heat absorption and poor shading, IR+Hot air reflow soldering furnaces have been widely used internationally.

4. Nitrogen reflow welding

With the increase of assembly density and the emergence of Fine pitch assembly technology, nitrogen filled reflow soldering technology and equipment have emerged, improving the quality and yield of reflow soldering, and have become the development direction of reflow soldering. Nitrogen reflow soldering has the following advantages:

(1) Prevent and reduce oxidation

(2) Improve welding wetting force and accelerate wetting speed

(3) Reduce the generation of solder balls, avoid bridging, and achieve the listed welding quality

It is particularly important to obtain the listed welding quality by using solder paste with lower activity flux, which can also improve the performance of the solder joint and reduce the discoloration of the substrate. However, its disadvantage is that the cost significantly increases, and this increased cost increases with the amount of nitrogen used. When you need to achieve 1000ppm oxygen content and 50ppm oxygen content in the furnace, the demand for nitrogen is different. Nowadays, solder paste manufacturers are committed to developing wash free solder paste that can perform well in environments with high oxygen content, which can reduce nitrogen consumption.

For the introduction of nitrogen in reflow soldering, a cost-benefit analysis must be conducted, which includes product yield, quality improvement, reduction of rework or maintenance costs, etc. A complete and accurate analysis often reveals that nitrogen introduction does not increase the final cost, but on the contrary, we can benefit from it.

Most of the furnaces currently used are forced hot air circulation type, and controlling nitrogen consumption in such furnaces is not an easy task. There are several methods to reduce the consumption of nitrogen and the opening area of the furnace inlet and outlet. One important point is to use partitions, rolling shutters, or similar devices to block the unused space of the inlet and outlet. Another method is to use the principle that the hot nitrogen layer is lighter than air and not easy to mix. When designing the furnace, the heating chamber is made higher than the inlet and outlet, forming a natural nitrogen layer in the heating chamber, Reduced the compensation amount of nitrogen and maintained it at the required purity.

5. Double sided reflow soldering

Double sided PCBs have become quite popular and are gradually becoming popular. The main reason why they have become so popular is that they provide designers with extremely good elastic space, thereby designing smaller, compact, and low-cost products. So far, double-sided boards are generally soldered on the top (component surface) by reflow soldering, and then on the bottom (pin surface) by wave soldering. The current trend is towards double-sided reflow soldering, but there are still some issues with this process. The bottom components of the large board may fall off during the second reflow soldering process, or partial melting of the bottom solder joints may cause reliability issues with the solder joints.

Several methods have been found to achieve double-sided reflow soldering: one is to use adhesive to stick the first side component, so that when it is flipped over and enters the reflow soldering for the second time, the component will be fixed in position and will not fall off. This method is very common, but requires additional equipment and operation steps, which increases costs. The second method is to use soldering alloys with different melting points. When making the first surface, a higher melting point alloy is used, while when making the second surface, a lower melting point alloy is used. The problem with this method is the selection of low melting point alloys