Electronic enthusiast network report (text/Liang Haobin) Some time ago at Murata's media exchange meeting, we learned about a "burial" plan launched by Murata. In the past, capacitors needed to be attached to motherboards or chip substrates, but Murata has introduced an innovative solution that integrates capacitors into the PCB without taking up the same space on the PCB surface as traditional packaging, which Murata calls the Integrated Package Solution.
The built-in embedded capacitor adopts a lamination process, which is actually a product of a combination of different layers and different materials, just like the chip capacitor. One of the features is the through-hole, which can directly connect the top and bottom electrodes of the buried capacitor, and there are many through-hole structures inside the buried capacitor, which can realize vertical power supply, shorten the power supply distance in the package, and thus reduce the loss.
In fact, in addition to capacitors, power devices also have packages that are embedded in the PCB.
What are the benefits of embedding power devices into a PCB
Taking the power module as an example, the current power module on the main drive inverter of electric vehicles is basically injection molded or frame-type packaging. Since power chips generate a lot of heat when working, most of them use substrates with high thermal conductivity and electrical insulation, and solder the power chips on the substrate, such as copper-clad ceramic substrates, to achieve good chip heat dissipation.
This kind of power module based on ceramic substrate, in which the chip can only be single-layer wiring through the copper coating on the ceramic surface, and the circuit connection is realized by means of overhead bonding wire, etc., which makes the electrical performance and heat dissipation are greatly limited, especially in reducing the miscellaneous inductance of the commutation loop and gate control loop and the thermal coupling between the chips.
Vitesco Technologies previously said in a seminar that PCBs have natural advantages in terms of electrical performance, such as the ability to perform multi-layer wiring, which can reduce the impact of EMC by controlling line spacing and layer spacing; The insulating materials used in the PCB can meet the requirements of 400V to 1000V high voltage insulation; The electronic devices embedded in the PCB can achieve excellent heat dissipation performance through high heat dissipation materials and reasonable heat dissipation layer design. Therefore, the technology of PCB embedding power chips for power module packaging has great performance potential.
According to Vitesco's technical evaluation data, firstly, in terms of the ability to pass current, the power module in the traditional package is about 101A per 29 square millimeter chip, while the power module in the PCB embedded power module is 142A per 29 square millimeter chip, which increases the unit current capacity by about 40%, which also means that the power chip usage can be reduced by one-third with the same current output. With the same power output requirements, the bill of materials cost of the power module can be reduced by up to 20%.
Specific to the application of inverters, taking 800V inverters and SiC power chips as an example, after the inverter uses an embedded package SiC module, the WLTC cycle loss of the inverter is reduced by 60% compared with the SiC module in frame packaging, and the inverter size can be reduced.
For traction inverters, reliability and service life are important, and Vitesco Technologies has already developed samples of SiC modules based on 400V and 800V systems in PCB embedded packages, and the AQG324 critical reliability verification of the samples shows that the design life of PCB embedded packages can be several times longer than that of conventional packages. Among the 800V SiC module samples, each power switch uses 8 power chips with an area of 20 square millimeters, the half-bridge PCB size is 70mm×40mm, the stray inductance of the module is less than 1nH, the slew rate is more than 25kV per microsecond, and the peak effective value of single-phase output current reaches 850A.
Implementation of PCB embedded packaging
In order to embed power devices into the PCB, higher requirements are also put forward for the materials and manufacturing processes of the PCB, such as the need for PCB materials to have good thermal conductivity, be able to withstand high voltage and high current, and have the characteristics of low resistance and low parasitic inductance to reduce power loss; The PCB material must also have good insulation properties to ensure electrical isolation between devices, prevent short circuits and breakdowns, etc. Therefore, it also requires close cooperation from PCB manufacturers to achieve large-scale mass production of PCB embedded packaging power devices.
Schematic diagram of Shennan circuit packaging patent Source: Patent Star CN 118173455 A
In February this year, Shennan Circuit applied for a patent called "a power chip embedded packaging substrate and packaging method", which was disclosed in June. As shown in the figure above, 1 is a bare chip, 2 is a rigid substrate, 3 is the first core board, 4 is an insulating layer, 5 is the first external metal layer, 6 is a blind hole on the surface of the metal layer, 7 is a connecting post, and 8 is the first inner metal layer.
In this patent, the steps of such an encapsulation are described: first, the first core board is obtained, and a through groove along the thickness direction is made in the first core board; Fix the bare die on a rigid substrate; The bare die and the rigid substrate are embedded in the slot of the first core board as a whole; The insulating layer is pressed on the first core board, and the insulating layer covers the bare die in the slot; forming the first external metal layer on the insulating layer; processing a plurality of blind holes from the surface of the first outer connected metal layer, and the blind holes realize the three-dimensional vertical interconnection between the bare chip and the rigid substrate and the first outer metal layer, so that the bare die directly fans out the signal through the blind vias, reduces the signal transmission path, and reduces the transmission loss. The bare die is encapsulated in the first core board, which frees up space on the surface of the first core board, which can place more electronic components and reduce the footprint of the package.
Source: Fan Jiajie, Qian Yichen. A high-reliability embedded SiC power device package design method
In addition to embedding PCBs, in 2022, Hou Fengze, associate researcher of the Institute of Microelectronics, Chinese Academy of Sciences, and Fan Jiajie, a young researcher from Fudan University, also proposed a new SiC power module packaging and reliability optimization design method based on substrate embedding technology: using a new photosensitive molding medium (PID) to prepare the interconnection blind vias on the electrodes of SiC MOSFET power devices through photolithography; Based on board-level physical vapor deposition (PLPVD) technology, the metal on the electrodes of power devices such as SiC MOSFETs is changed in batches; Replace the traditional bonding technology of power devices with a double-sided rewiring layer (DSRDL) process.
As you can see from Figure b above, the substrate embedded package volume is much smaller than that of a discrete device in the TO-247 package.
brief summary
After the power device adopts the PCB embedded package, the circuit layout design is more flexible than the traditional package, which can greatly improve the development efficiency. At the same time, it can also greatly reduce the overall system volume, such as in the application of inverters, under the same output power demand, reduce the amount of power chips, and bring overall cost-effectiveness to the system. Another benefit is that some advanced circuit topologies, such as three-level, IGBT/SiC MOSFET hybridization, due to the complex structure and the large number of components used, traditional packaging is still less used in the field of electric vehicle inverters, but in the future, through more flexible PCB embedded packaging, these topologies may be promoted in the field of traction inverters.