The rise of full-screen mobile phones in the past two years has driven the under-screen fingerprint recognition technology to sweep the entire smartphone industry at a lightning speed. According to statistics from IHS Markit, in 2019, the global shipments of smartphones equipped with under-screen fingerprint recognition technology will exceed 100 million units. It is expected that this technology will maintain rapid growth in the market in the next three years.
Figure 1: Under-screen optical fingerprint shipment trend
The mainstream under-screen optical fingerprint recognition solution on the market is the COB (chip on board) solution - the chip and peripheral components are directly placed on the FPC soft board. The under-screen optical fingerprint solution uses this solution to be widely used in mobile terminals, but its disadvantages are also obvious - the COB package module size is large, squeezing the battery space.
On the one hand, because the COB module is based on Wire Bonding technology, the inner pins and gold wire leads on the flexible board require a certain amount of space; at the same time, the peripheral components welded on the flexible board occupy a certain amount of space. This results in the XY dimensions of the COB module being relatively large, and the battery size is forced to be sacrificed.
On the other hand, because the module factory must use COB assembly lines to assemble the current under-screen optical fingerprint recognition COB modules, and the COB assembly line production capacity is relatively tight (based on the current rapid growth in the number of terminal cameras, more COB lines are needed to assemble the cameras on mobile phones), further limiting the production capacity of smartphones.
Figure 2: Comparison between CSM and COB
With the arrival of 5G mobile phones, 5G mobile phones consume more power and have a stronger demand for large-capacity batteries. Sacrificing battery size to accommodate the fingerprint module has become a loss-making issue. In order to solve this problem, Silead, an industry-leading human-computer interaction and biometric solution provider, recently launched the CSM (chip-scale module) solution GSL7001F for under-screen optical fingerprint recognition, which is suitable for OLED screens and has great advantages in module size (XY direction). Compared with the traditional COB (chip on board) solution, Silead's new solution can reduce the module XY area by 50%, freeing up more space for large-capacity batteries, and making the internal stacking of the entire machine more operational.
Figure 3: Schematic diagram of the package
Silead's CSM solution uses the Flip Chip process to solder the chip on the PCB, then use glass to protect the chip surface, and then use solder balls to lead out the chip circuit; at the same time, because peripheral components can also be soldered on the PCB board, this makes the entire design change from 2D to 3D. This design forms a chip package that is about the same size as the chip, and does not occupy additional space in the XY direction. Based on this chip package, developers only need to use SMT to solder the chip package on the FPC soft board at the module stage, and then directly install the Holder + Lens on the chip package. The module assembly becomes simple, and the module XY direction size is comparable to the chip size, saving the internal space of the mobile phone.
According to Silead, the CSM solution they launched has the following advantages:
1. Save space:
Compared with COB modules, the CSM solution reduces the space in the XY direction by 50%, leaving more space for the battery; this is especially important for 5G mobile phones;
2. Reduce costs:
You can use the mature SMT production line of the module factory, without COB, which saves costs. The COB solution requires Die Bond/Wire Bond machines and workshops with corresponding purification levels (generally requiring Class 1000 workshops or above);
3. Improve efficiency:
Because the CSM package is completed in the packaging factory, it is more efficient and has higher quality assurance than COB, which makes the new solution more advantageous in mass production than COB.
Figure 4: CSM physical comparison
The CSM solution is the result of Silead's intensive research and development for more than a year. All quality and reliability tests have been completed, and many customers have entered the mass production and debugging stage. Various technical difficulties were also overcome during the testing phase. For example, because the chip package is a confined space, it is easy to have a "popcorn" phenomenon during SMT, which leaves room for water vapor to survive, affecting the reliability or performance of the product. In order to solve this problem, Silead experimented with various processes, using curved pipe control, vacuuming, etc., and finally met the mass production requirements.
Regarding the outlook for future technology, Silead said that the future under-screen optical fingerprint recognition will inevitably develop towards being safer, faster (higher recognition accuracy, lower recognition error rate, faster unlocking), smarter (changing with the fingerprint of a person, different ages, different seasons), and smaller products. Smaller products are not limited to the XY direction, but also require thinner products in the Z direction.
*Disclaimer: This article is originally written by the author. The content of the article is the author's personal opinion. Semiconductor Industry Observer reprints it only to convey a different point of view. It does not mean that Semiconductor Industry Observer agrees or supports this point of view. If you have any objections, please contact Semiconductor Industry Observer.
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