The demand for electronic products of portable communication systems and the high performance requirements of electronic machines have greatly promoted the development of electronic products in the direction of miniaturization, integration, multi-function, high frequency and high reliability. Electronic packaging technology directly affects the high-speed transmission, power consumption, complexity, reliability and cost of electronic devices and integrated circuits, so it has become a key technology in the electronic field.
Low Temperature Co-Fired Ceramics, LTCC package can assemble and integrate different types of chips and other components into the same package to realize certain functions of the system. It is an important means to realize system miniaturization, integration, multi-function and high reliability.
What is LTCC(Low Temperature Co-Fired Ceramics)?
Low Temperature Co-Fired Ceramics (LTCC) uses glass/ceramic materials as the dielectric layer of the circuit, and uses high-conductivity metals such as Au, Ag, Pd/Ag as inner and outer electrodes and wiring, and is printed in parallel. It is a kind of ceramic made by printing multi-layer circuit in the way of lamination and co-sintering in a sintering furnace below 950 ℃.
Advantages of LTCC(Low Temperature Co-Fired Ceramics)
The LTCC substrate has the advantages of small wiring conductor square resistance, many wiring layers, high wiring density, low sintering temperature, low dielectric loss, excellent high-frequency performance, thermal expansion coefficient matching with various chips, etc., thus becoming an ideal leading substrate for high-density integration.
LTCC can embed passive components such as resistors, capacitors, inductors, antennas, filters, baluns, couplers, duplexers, etc. It is easy to form cavities with various structures, which can be combined with thin-film precision wiring technology or realized by laser processing Hybrid multi-layer substrate MCM-C/D with higher routing accuracy and better performance.
Ceramic packaging of LTCC substrates can improve the packaging requirements of components (modules) for high frequency, low loss, high-speed transmission, and miniaturization.
What is the LTCC package used for?
LTCC package products have been widely used in aerospace, aviation, communications, radar and other fields, and LTCC products have also been widely used in the 5G field that requires higher data transmission rates, bandwidth and lower latency. The frequency of LTCC package products has exceeded 100 GHz , has broad development prospects and application markets.
LTCC package material characteristics
LTCC package materials refer to the base materials used to carry electronic components and their interconnections, and play the roles of mechanical support, sealing environment protection, signal transmission, heat dissipation and shielding. Including LTCC substrate, wiring, housing, frame, heat sink, cover plate, solder and other materials, there are generally three categories: LTCC substrate material, packaging metal material and soldering material.
LTCC substrate material
LTCC substrate materials include LTCC green tape and materials such as conductors and resistors matched with green tape. The conductor materials used in LTCC wiring and through-hole connections use noble metals such as Au, Ag, Pd, Pt or their alloys (binary or ternary alloys PdAg, PtAg, PtAu, PtPdAu, etc.) as the conductive phase, and its performance is stable. The technology is mature and can be sintered in air atmosphere.
Cu is also a material with high electrical conductivity, high thermal conductivity, excellent welding performance, and suitable for low-temperature sintering.
However, since Cu is easily oxidized after being heated in the air, it is different from noble metal materials such as Au and Ag, and a neutral atmosphere (usually nitrogen) is required as a protective gas during sintering.
LTCC package metal material
The metal material for LTCC package is mainly selected according to the characteristics of the metal packaging material. It is necessary to comprehensively consider the thermal conductivity, thermal expansion coefficient, density, solderability, and process maturity of the metal material.
The Fe-Ni-Co alloy containing 29% nickel and 18% cobalt is called Kovar alloy. Its thermal expansion coefficient is small, which matches the thermal expansion coefficient of commonly used LTCC substrates. With better processability and lower cost, it is a more commonly used metal shell material; but its thermal conductivity is not high, which also limits its application range as a metal shell package.
CuW and CuMo alloys combine many excellent properties of W, Mo and Cu, so they have good thermal conductivity, arc corrosion resistance, welding resistance, high-temperature resistance, oxidation resistance, etc., and the thermal expansion coefficient can be in a certain range. It is mainly used in large-scale integrated circuits and high-power microwave devices, such as thermal control boards, heat dissipation elements (heat sink materials) and lead frames.
However, due to the high density of CuW and CuMo, the scope of use is limited, and it is not suitable for application in portable electronic products and aerospace equipment. It is less used in LTCC package that requires lightweight electronic equipment.
Aluminum-silicon alloy material has the advantages of light weight, low thermal expansion coefficient, good thermal conductivity, high strength and rigidity, and can be plated with gold, silver, and nickel, and has good wetting between silicon and aluminum. It has excellent properties such as easy precision machining, non-toxicity, and low cost, and has become one of the electronic packaging materials with broad application prospects.
Al/SiC has the characteristics of high thermal conductivity, low expansion coefficient, high strength, low density, good electrical conductivity, etc., and is being paid more and more attention by more and more people. Al/SiC has been applied in batches in the packaging field as a substrate or heat sink material.
LTCC package solder material
LTCC package soldering materials are mainly used as connecting materials for welding LTCC substrates and metal base plates, metal enclosures, and pins, assembly of components on substrates, solder ball connections, and vertical interconnection of substrates. The melting point of welding materials for LTCC package is generally lower than 450 °C, which belongs to solder.
During the production process of LTCC package, processes such as metal and ceramic welding, component assembly, solder ball array production, and vertical interconnection are required. These assembly and packaging processes are often completed through multi-step welding. In order to ensure that the subsequent process does not affect the soldering results of the previous process (component remelting and displacement), the melting point of the solder used in different processes often has a certain temperature difference to form a temperature gradient.
Research status of LTCC package technology
LTCC metal case package
LTCC metal case packaging is similar to traditional thick-film multi-layer alumina substrate metal case packaging. It is a method of welding or bonding the LTCC substrate on the inner bottom surface of the metal case, and realizing the electrical connection inside and outside the case through insulators or connectors embedded in the metal case. It is usually used in high-reliability electronic products or customized military or aerospace products with special performance requirements.
The metal shell can be welded to the LTCC substrate with one-sided cavity opening, and can also be welded to the LTCC substrate with double-sided cavity opening. After the components are assembled on the LTCC substrate, the metal casing can be sealed by parallel seam welding or laser welding. The advantages of LTCC metal shell packaging are good airtightness, strong versatility, and relatively mature technology. It is one of the most widely used LTCC packaging forms.
LTCC pin grid array package
The LTCC Pin Grid Array(PGA) package is a package in which a metal frame is welded on the surface of the LTCC substrate as the package frame, and a metal PGA is welded on the bottom as the I/O terminal. The LTCC circuit substrate is used as the packaging carrier, and the I/O terminals of the package are directly drawn out on the substrate, so that the substrate, frame and cover are integrated into a package.
LTCC ball grid array package
The LTCC Ball Grid Array (BGA) package is a package in which the surface of the LTCC substrate is soldered with a metal frame as the package frame, and the bottom surface is soldered with solder balls as the I/O terminal. LTCC BGA hermetic package also belongs to LTCC integrated package.
The BGA package is a more efficient package with a higher I/O pin count than a perimeter-exited metal case package. PGA package pin pitch is difficult to be smaller than 1.27mm, while BGA ball pitch can be smaller, and I/O pin density will be higher than PGA package. LTCC BGA packages have very short pins and short current paths for vertical connections, and BGA packages have much lower lead inductance than other lead connections . The BGA solder balls are distributed in an area array, which is beneficial to the heat dissipation of the substrate.
LTCC QFP package
LTCC Quad Flat Package(QFP Package) is a package in which the metal frame welded on the surface of the LTCC substrate is used as the package frame, and the leads welded on the bottom edge of the substrate are used as the I/O terminal.
LTCC LCC package
LTCC Leadless Chip Carrier(LCC) package is a packaging form in which the surface of the LTCC substrate is welded with a metal frame as the package frame, and the I/O terminal is a conductor film layer drawn from the inside of the LTCC substrate to the bottom of the substrate.
LTCC 3D-MCM package
LTCC Three-Dimensional Multichip Module (3D-MCM) package is a packaging of multi-piece (not less than 2) two-dimensional board-level LTCC module (2D-MCM) stacked vertically and realized electrical and mechanical connections to form the components.
Development trend of LTCC package technology
With the advent of the era of digitalization, informationization and networking, electronic packaging has put forward higher requirements for miniaturization, integration, multi-function, high speed and high frequency, high performance, high reliability and low cost. LTCC packaging products have obvious characteristics in terms of miniaturization, integration, high speed and high frequency, and high performance, and will continue to develop in the future to maintain technological advantages.
However, conventional LTCC packaging products still have deficiencies in thermal matching, heat dissipation, cost, etc., which affect the development of LTCC package products and their application in a wider range of fields. Solving the key problems of LTCC package products in certain application requirements has become a technical problem that needs further research.
High thermal expansion coefficient LTCC package
LTCC packages have ways of increasing integration density, such as high-density wiring and multi-chip assembly. The use of LTCC substrates with high thermal expansion coefficients, selection of appropriate interconnect materials and appropriate processes for packaging are important means to improve the reliability of LTCC package modules applied to PCB masters.
In addition, after using the LTCC substrate with a high thermal expansion coefficient, the metal enclosure can use materials such as AlSi with lower density and higher thermal conductivity, which is conducive to the selection of metal materials and module heat dissipation. The high thermal expansion coefficient LTCC package plays an important role in promoting the application of LTCC in the field of high-speed, ultra-large-scale circuits and matching with PCB mother boards.
High thermal conductivity LTCC package
The development of electronic equipment in terms of miniaturization, multi-function, and high power will further increase the assembly density and power density of the modules in the equipment. Therefore, the effective heat dissipation of the packaged modules is an important factor to ensure the reliability of the equipment. If the LTCC substrate material with higher thermal conductivity can be developed, it will be the best solution to solve the problem of high thermal conductivity LTCC packaging, but there is no commercial LTCC substrate material with high thermal conductivity at present.
Therefore, whether it is to improve the heat dissipation capability of LTCC packaging through substrate materials, thermal conductive materials, micro-channels and other processes, the realization of high thermal conductivity LTCC package will enable LTCC modules to play a greater role in more fields.
Low cost LTCC package
At present, LTCC package products have been used in aviation, aerospace, communication, radar and other fields, but at this stage, high-end LTCC products are still mainly imported LTCC materials, The relevant supporting slurry system is mainly a precious metal material system based on composite materials such as Au, Ag, Pt, and Pd, and the cost is relatively high. Obviously, this is inconsistent with the low-cost development trend of electronic information products, which affects the popularization and application of LTCC packaging products.
Therefore, it is necessary to develop LTCC green ceramic tape and low-cost supporting conductor paste. Cu conductors are not only cheap, but also have excellent electrical, thermal, and soldering properties. By developing highly reliable, low-cost LTCC materials that can be wired with Cu conductors, the cost of LTCC packaging can be effectively reduced.
System-in-LTCC package
System-in-Package (SiP) refers to the integration of multiple chips and components in one package to realize a system or subsystem with complete basic functions. System-in-Package strives for higher assembly density and functional density, and can shorten lead times. At present, the LTCC package is usually assembled in the system as a module to realize some functions of the system.
With the successful development of new materials for LTCC substrates (such as materials with high strength, high thermal conductivity, and low cost) and the maturity of advanced packaging and assembly processes, LTCC package will integrate more and more complex components, giving full play to the advantages of LTCC miniaturization, integration, high speed and high frequency, etc., and realize system-level LTCC packaging.
Conclusion
LTCC substrates can be packaged in different forms. The LTCC metal shell package can be realized by choosing a metal shell with a thermal expansion coefficient similar to that of the LTCC substrate and having an appropriate density to be welded to the LTCC substrate.
In the future, The development of LTCC package technology will develop in the direction of high thermal expansion coefficient LTCC packaging, high thermal conductivity LTCC packaging, low-cost LTCC packaging and system-level LTCC packaging.
Low Temperature Co-Fired Ceramics (LTCC) uses glass/ceramic materials as the dielectric layer of the circuit, and uses high-conductivity metals such as Au, Ag, Pd/Ag as inner and outer electrodes and wiring, and is printed in parallel. It is a kind of ceramic made by printing multi-layer circuit in the way of lamination and co-sintering in a sintering furnace below 950 ℃.
LTCC package products have been widely used in aerospace, aviation, communications, radar and other fields, and LTCC products have also been widely used in the 5G field that requires higher data transmission rates, bandwidth and lower latency.
- LTCC metal case package
- LTCC pin grid array package
- LTCC ball grid array package
- LTCC QFP package
- LTCC LCC package
- LTCC 3D-MCM package
