SAW Devices are one of the key parts for realizing the miniaturization and functionality of mobile communication terminals, and are used mainly as a Band-pass filters of RF and IF stages. SAW Devices are indispensable for wireless sections or mobile TV tuners of mobile information terminals, such as cell phones or smart phones and its market has expanded along with the expansion of the mobile communication market.
In recent years, the modularization of wireless sections of mobile information terminals has progressed. Resin-mold type modules are especially sought after. They possess excellent shielding properties for protecting chips and bonding wires from the outer stress or moisture, while realizing miniaturization, excellent flatness and easy of mounting.
The miniaturization, especially low-profile structure has been greatly desired for each part in order to advance the modularization. However, SAW Devices are structured in a way that the chips stored in the package vibrate physically, so a certain space is required on the chips. Therefore, it was difficult to produce a product of small and low-profile type. Previously, it has been common practice to mount them separately from the module due to height constraints.
However, the SAW Devices are now desired to be integrated into a module.
Realizing small and low profile structure by WLP
SAW Devices, due to their nature, must have the space internally in order for chips to vibrate. Therefore, in a general structure the chips are mounted on the Ceramic Substrate and are then housed. Due to this structure, the minimum height is limited to 0.5mm.
In order to realize the low-profile product, the factor that increases the height has to be eliminated as much as possible. Looking at it from a totally different angle, we examined the structure providing the internal space without using the Ceramic Substrate. Our solution was making use of the Piezoelectric Substrate of the chip itself for its packaging. This structure is called WLP (Wafer Level Package). We succeeded in realizing the miniaturization and lowered the profile of the SAW Device from 1.4×1.1×0.5mm as in the previous package to 0.8×0.6×0.35mm now. Furthermore, the basic performance of the SAW Devices has been maintained.
We made the mounting simple by adopting the BGA (Ball grid array) structure with the terminal electrode formed hemi-spherically with solder.
Realizing the integration into modules by our own sealing technology of hollow parts.
It is very meaningful to have realized the height of 0.35mm. That is, the height required for the module of Resin-mold type to be 0.35mm. Furthermore, when the Resin-molding is performed on the whole module, large pressure is applied to each part. As stated above, SAW Devices must have a structural space for the chips to vibrate internally, but having such space means securing a certain height. It also has a shortcoming of being weak against the pressure from the outside.
As the WLP structure uses the chip itself as a package, we developed technology to strengthen the package. Also, due to its structure, it is easy for mold resin to enter while sealing parts. By using our ingenuity, we established a new sealing technology for hollow parts. This realized heat resistance of 50 atmospheric air pressure (about 5MPa) minimum at 150deg.C.
As a result of the realization of a low profile structure and heat resistance, SAW Devices can now be integrated into the module of Resin-mold type, which previously were attached externally, as a common practice.
