“With the rapid development of the panel industry in recent years, after domestic manufacturers have expanded their production madly, the relationship between product supply and demand has been basically eased, and manufacturers have begun to focus on the improvement of product “quality”. The Display panel industry will develop towards the trend of high-resolution images, curved surfaces, ultra-thin planes, thinness, flexibility, high dynamic HDR, high contrast ratio and wide color gamut, thus Mini-LED emerges as the times require.
With the rapid development of the panel industry in recent years, after domestic manufacturers have expanded their production madly, the relationship between product supply and demand has been basically eased, and manufacturers have begun to focus on the improvement of product “quality”. The display panel industry will develop towards the trend of high-resolution images, curved surfaces, ultra-thin planes, thinness, flexibility, high dynamic HDR, high contrast ratio and wide color gamut, thus Mini-LED emerges as the times require.
According to the forecast, Mini-LED will officially break out in 2019 and enter the commercialization stage. At present, the global mainstream manufacturers have basically completed the research and development process of Mini-LED backlight, and entered the stage of small batch samples or large batch supply. Major domestic enterprises are also intensively carrying out process research to speed up the process of putting them on the market.
Mini-LED uses LED chips with a size of micrometers. Each Mini-LED circuit board usually has thousands of chips and tens of thousands of solder joints to connect RGB three-color chips. Such a huge number of solder joints brings great difficulty to the packaging and soldering of the chip. Today, let’s take a look at the welding process.
Compared with the traditional reflow soldering process, Mini-LED’s requirements for the process have reached the extreme. According to statistics, more than 40% of the poor soldering is caused by the printing process, and 40% is caused by the soldering. The other 20% has a lot to do with solder paste and substrate material. For the reliable soldering of Mini-LED, higher requirements are put forward for equipment reflow soldering, soldering process, and material (solder paste), all of which are indispensable.
Equipment – Vacuum Reflow Soldering:
Mini-LED has smaller pads, less solder paste, and smaller chips, which puts forward higher requirements for process parameters such as welding equipment requirements and temperature uniformity. Current welding problems include:
1. Chip displacement: There is movement after the chip is welded, and it is necessary to reduce the movement of the bare chip after welding.
2. Chip rotation: Because the spacing between the Mini-LED chips itself is only 0.8mm, 0.6mm, 0.4mm or even smaller, then during the welding process, the chip itself is easy to rotate in the atmosphere, which has a bad influence.
3. High void rate: At present, after nitrogen reflow soldering, solder paste with low void rate is used, and the void rate after soldering is controlled to about 10%. For ordinary solder paste, the void rate after soldering may reach more than 15%. The void rate is too high, and long-term use may lead to defective products due to thermal conductivity or reliability problems.
4. False soldering: When choosing reflow soldering, the oxygen content is a very important indicator. If the oxygen content cannot be controlled within 100ppm or even lower, it may lead to false soldering of the product. At the same time, the temperature uniformity is not up to standard is also a very important factor. If the temperature uniformity in the entire furnace cavity is less than 2 degrees, it will lead to poor soldering of some chips.
The above problems are also very important parameters for users to choose a vacuum welding furnace. Be sure to test more and strictly control the technical indicators. After all, there are more than 9,000 chips on a circuit board, and it is a big thing that there are several defects that lead to the defects of the final product.
The author has tested many reflow and vacuum reflow ovens in the industry, and has failed many times. It took more than a year to test successfully. The detour ahead is too long.
We finally tested and purchased the V3 vacuum welding furnace from Zhongke Comrade Technology Company. The effect fully meets the requirements. The products of P0.9 and P0.6 were successfully welded in small batches. The void rate has been controlled to within 2%. This vacuum furnace has several advantages:
1. The temperature uniformity is very high. It really reached within 2 degrees. Some companies previously claimed that the temperature uniformity was within 2 degrees. After our actual test, it did not meet the standard at all. False propaganda is too powerful, which is also the main reason for our detours.
2. The temperature uniformity in the process of heating slope, we didn’t pay much attention to this before, but after introduction by the manufacturer’s technicians, we found that this index is also very important.
3. The temperature uniformity during the cooling process, we only paid attention to the cooling slope before. Didn’t pay much attention to temperature uniformity in this process. In the actual welding process in the later stage, it was found that this indicator is also a very core indicator. Come to think of it, the solder has melted. When it starts to cool, some places cool quickly, and some places cool slowly. We tested a vacuum furnace product before, and the temperature uniformity was actually 70 degrees worse when cooling. Think about how much the defect will be caused by this factor. And this factor is rarely paid attention to. We tested poorly before, and the manufacturer’s technician said that it may be a problem with our materials, and their process is no problem. We repeatedly tossed and spent nearly half a year, and later on this V3 vacuum furnace, these problems did not occur.
All in all, because there are too many chips for Mini-LED products, the process is relatively complicated. If you find a knowledgeable supplier, the product progress can be shortened by half or even more than half.