A brief discussion on the protection scheme of Bluetooth headsets

As people's living standards continue to improve, Bluetooth headsets have gradually entered the lives of ordinary people. In particular, the emergence of TWS true wireless Bluetooth headsets represented by Apple Airpods has made the use of Bluetooth headsets more convenient and the application scenarios more diverse.

Since Bluetooth headsets are free from the constraints of wires, they are more popular in the sports field.

It is inevitable for people to sweat during exercise, so sports headphones are more or less waterproof. However, the compactness of the headphone structure limits the size of the sealing ring, and the usual bumps, temperature changes, sweat corrosion, etc. can cause the sealing structure to fail, thus causing abnormal functions of the headphone motherboard.

When the headphone manufacturers in the industry conduct product reliability tests, they usually use the relatively strict method of soaking the whole machine in sweat. However, this test cannot fully reflect the actual usage, so it often happens that even if the reliability of the preliminary evaluation is passed, there will be a high defect rate after the product is put on the market. The main reasons are as follows:

1. Liquid sweat (test environment) VS gaseous sweat (use environment)

The artificial sweat used in the immersion test environment is in liquid form, while sweat from the human body or during the drying process of sweat will exist in gaseous form. Most of the sweat is water. There is a strong surface tension between liquid water molecules. The molecules pull each other to form a very long chain structure, so it is difficult to penetrate into the headset. However, in the real use environment, the surrounding temperature is high when people sweat, and most of the sweat exists in the form of steam (sweat evaporates in the natural environment and is also gaseous). Since there is no surface tension between gaseous molecules, they all exist in the form of single molecules. The diameter of water molecules is very small, and it is difficult to completely isolate them with general structural protection. This is also the reason why Bluetooth headset motherboards often fail due to corrosion.

2. Tests are all conducted in a single environment, while the actual use process is often the superposition of multiple environmental factors at the same time. Even if a product is subjected to multiple reliability tests (such as salt spray test, high temperature and high humidity test, and water (sweat) test), it cannot reflect the actual use environment. Therefore, the actual use environment is generally more stringent than the test environment. If the test standards are too loose, it may cause a high defect rate after the finished product is shipped.

3. During use, Bluetooth headsets may fail to seal due to external pulling (mostly for noodle-type headsets), bumping, or squeezing. This is not always visible to the naked eye, so the risk of corrosion is greater.

In response to the above problems, many brand manufacturers with higher quality requirements have made considerable attempts, including gluing, nano-hydrophobic, Parylene nano-waterproofing technology, etc. Below we analyze the protection of Apple Airpods by disassembling them.

• After heating and opening it with professional tools, you will find that the earphone joints and the charging port at the bottom are sealed with a thick layer of glue. The red part in the picture shows that the glue was stretched when it was disassembled.

• The earphones use a combination of hard and soft boards. From the red frame of the soft board, you can clearly see that there is a layer of white film wrapped around the entire board.

After analysis, it was found that the film layer is a nano-film layer of Parylene film. This film layer has excellent acid, alkali and sweat resistance. It is used on the headset motherboard to prevent sweat from entering during wearing and causing circuit board corrosion.

Due to the small size of Bluetooth headset boards, high component density and relatively large production capacity, the application of Parylene coating technology in this industry has a very high technical threshold.

From the details of the circuit board in the picture, it can be seen that the masking area is very regular, the breaks in the film layer are also very regular, and there are no obvious signs of pulling. The masking and film removal should be carried out in an automated manner.

Since Bluetooth headset panels have very high requirements for shielding accuracy, achieving process automation requires comprehensive consideration of everything from materials to customized equipment. Currently, most Parylene coating companies do not have mature solutions.

Xiayu Nano Technology (Shenzhen) Co., Ltd. is a company that focuses on the overall solution of Parylene nano protection. After several years of hard work, the company's technical team has developed a complete set of automated solutions, focusing on high-level protection of Bluetooth headsets. The bare board of Bluetooth headsets with nano protection can pass the 24-hour artificial sweat immersion test in working condition.

Summarize

Since the anti-sweat effect of traditional three-proof technology is relatively poor, and its thickness is several hundred microns, which will affect product assembly, coupled with the increasingly stringent environmental protection requirements, its application in the industry is subject to certain restrictions. However, Parylene nano-membrane coating has excellent anti-sweat corrosion performance, the film thickness is very thin, and it can provide 360-degree protection in an environment with dense components, so it is recognized and widely used by brand customers. As the trend of miniaturization and integration of headphones becomes higher and higher, customers' requirements for protection are becoming more and more stringent, and the application of Parylene nano-protection technology will become more and more extensive.