Passive Embedding for higher Performance and Reliability
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NTC thermistors on top and bottom surfaces of the chip
A key advantage of the NTC thermistors manufactured from wafers is the configuration of their electrical contacts. In this case, they are located on the top and bottom surfaces of the chip. This allows the lower terminal to be contacted directly and with complete surface contact onto the semiconductor substrate using conventional semiconductor processes. The upper terminal is contacted via conventional wire bonding, as is usual for IGBT modules. The contact surfaces are optionally available in gold or silver plating in order to achieve the best possible bonding results.
Among the other advantages of these chip NTC thermistors are their minimal electrical and thermal tolerances. This precision is achieved by means of a special process technology: Before separating the individual elements from the wafer, the total resistance of the wafer is measured with respect to a rated temperature of 100 °C. The size of the thermistors to be separated is then determined based on this. This ensures that the tolerances of the separate components is much smaller than those of standard NTC thermistors rated at 25 °C, as is shown in Figure 4.
Because EPCOS chip NTC thermistors have a narrow tolerance of only ±1.5 K at 100 °C, IGBT modules can then be operated without premature derating at temperatures very close to their maximum permissible values and thus be utilized more efficiently. This solution is also suitable for new power semiconductor generations such as those based on SiC.
3D Integration with LTCC and SESUB
As smartphones and other portable electronic devices are designed to support more bands and offer greater functionality, a maximum level of integration that goes beyond the miniaturization of the single components is required in order to keep these devices compact. LTCC technology (low temperature co-fired ceramic) is an established technology that enables the functions of passive components such as inductors, capacitors and resistors to be embedded within the thin ceramic layers. Depending on the level of integration, LTTC technology, which is used mainly to manufacture RF modules for smartphones, can save up 80 percent space compared with discrete solutions.
However, because the LTTC sintering process takes place at temperatures higher than 500 °C, heat-sensitive components such as semiconductors must be mounted in piggyback mode on the upper side of the modules after sintering. By actually embedding the ICs in the substrate, TDK’s SESUB technology (semiconductor embedded in substrate) represents a new approach to integration. Even including the embedded ICs, the overall thickness of the SESUB substrate is only 300 µm, (Figure 5).
Discrete passive components on the surface of the substrate
The discrete passive components required can be placed on the surface of the substrate. In order to increase the integration density even further, thin passive components will also be embedded in the substrate in a next step. Because SESUB modules make use of the third dimension, their area is 50 to 60 percent smaller than conventional discrete solutions, depending on the design.
The shorter line connections within the substrate layers of the modules lead to improved parasitics and thus support better system performance. EMC performance is also improved due to the shielding effect of the metal layers inside the SESUB substrate. In addition, SESUB delivers excellent thermal attributes due to the fact that the IC is completely embedded. All surfaces of the chip are in full contact with the laminate, which optimizes the heat transfer from the semiconductor into the substrate layers. These layers themselves contain the copper micro-interconnection grids, which provide for a very homogenous and efficient heat dissipation. In particular, their superior thermal performance is important for applications in the area of power management, transceivers, processors, and the power amplifier – or all the main components of a smartphone. In addition to miniaturization, a key criteria for the use of both LTCC and SESUB technologies are their high reliability and significantly reduced logistics outlay.