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Co fired ceramic board components and assemblies can be divided into two types: high-temperature co fired ceramics (HTCC) and low-temperature co fired ceramics (LTCC). HTCC refers to a co fired ceramic plate with electrical interconnection characteristics sintered together with metals with higher melting points above 1450 ℃. With the development of communication towards high frequency and high speed, LTCC has emerged in order to achieve low loss, high speed, and high-density packaging, with a sintering temperature of around 900 ℃. However, human exploration of science has never stopped. With the advancement of technology and the continuous improvement of social demand, people have discovered the new concept of ultra-low temperature co fired ceramics. Ultra Low Temperature Co fired Ceramics (ULTRACC) is a new type of dielectric material developed from low-temperature co fired ceramics.
Ultra low temperature co fired ceramics are a new type of multi-layer ceramic with numerous advantages
It can be sintered at extremely low temperatures ranging from 400 ℃ to 700 ℃. The ultra-low sintering temperature allows the dielectric to co sinter with aluminum electrodes and various electronic devices to achieve integration and multifunctionality of electronic devices. At the same time, it can also reduce costs and save energy, making it suitable for the integration of electronic components. And the low sintering temperature allows for a wider range of conductor data for functionalization, making it possible to mix technology (transistor technology, polymer based microcircuit manufacturing).
One of the most critical processes in low-temperature co firing ceramic technology - sintering
Low temperature co fired ceramic substrate is a three-dimensional interconnected circuit substrate formed by parallel drilling, filling, printing and other processes on different layers of ceramic strips, then stacking the different layers of ceramic strips together and finally sintering them together. Sintering is one of the most critical processes in LTCC technology, which directly affects the microstructure of ceramics and subsequently affects various performance indicators of ceramics. The sintering process involves complex physical and chemical changes, and the heating rate, peak temperature, and holding time are three important parameters in the sintering process. Improper selection of heating rate can easily cause substrate warping or even cracking. LTCC data can be divided into three categories based on composition and structure, with the first category being glass ceramic systems. The second type is the traditional glass ceramic composite system. The third type is glass bonded ceramic system. The first type of ceramics is currently widely used.
Ceramic substrate
The variation of dielectric constant and dielectric loss with heating rate is related to the microstructure changes of ceramics
The heating rate increased from 4 ℃/min to 16 ℃/min, and the internal density of the ceramic gradually decreased, while the porosity gradually increased. The samples sintered at heating rates of 4 ℃/min and 8 ℃/min are denser. At a heating rate of 12 ℃/minute, obvious pores appear inside. When the heating rate is 16 ℃/minute, the pores on the sample cross-section further increase. This is because after the completion of the rubber extrusion, due to the slow heating rate, the glass ceramic material can grow in an orderly manner with the increase of temperature. With the addition of crystal phases and the growth of grains, the internal pores can be slowly discharged, achieving the densification of the glass ceramic material. When the heating rate is too fast, the internal crystal phase fails to fully crystallize and the internal pores cannot be discharged in a timely manner, resulting in an increase in internal pores. According to the mixing law of dielectric constants in composite materials, the introduction of low dielectric constant substances will reduce the dielectric constant of the composite material. Due to the dielectric constant of air being 1, which is lower than the dielectric constant of crystal phases such as CaSiO3 and CaB2O4, as the heating rate increases, the dielectric constant decreases and the dielectric loss increases.
Different heating rates will affect substrate warpage
When the heating rate is between 4 ℃/min and 8 ℃/min, the substrate warpage is about 0.21%. As the heating rate increases to 12 ℃/min to 16 ℃/min, the substrate warpage also gradually increases. At 16 ℃/min, the warpage is about 0.82%.
From this, it can be seen that the flatness is better when the heating rate is 8 ℃/min, and the middle protrusion is obvious when the heating rate is 16 ℃/min. This is mainly because the glass ceramic material and silver electronic paste are heated and sintered together. When the heating rate is between 4 ° C/min and 8 ° C/min, the sintering shrinkage rate of silver electronic paste is close to that of glass ceramic. However, when the heating rate increases to 12 ° C/min to 16 ° C/min, the sintering shrinkage rate of silver electronic paste is much higher than that of glass ceramic material, resulting in severe sintering mismatch and substrate arching.
Different heating rates will affect the adhesion of the film layer
With the increase of heating rate, the adhesion of the solder pad film layer shows a decreasing trend. This is because when the heating rate is between 4 ℃/min and 8 ℃/min, the ceramic sintering produces more liquid phase, which can form good adhesion with the metal film layer. At the same time, the ceramic body is denser, with fewer pores, and fewer pores between the metal film layer and the ceramic, resulting in a higher bonding force. However, when the heating rate increases to 12 ℃/min to 16 ℃/min, the liquid phase content produced by ceramic sintering decreases, and there are more pores between the metal film layer and the ceramic, which reduces the bonding force between the metal film layer and the ceramic. As the heating rate increases, the co firing compatibility between the metal slurry and the ceramic may deteriorate, which may also lead to a decrease in the bonding force between the metal film layer and the ceramic.
LTCC liquid-phase sintering
LTCC data is generally composed of glass ceramics or glass composite ceramic powders, which have a high glass content. Therefore, LTCC sintering belongs to liquid-phase sintering. When LTCC data is in the high temperature range (≥ 500 ℃), the glass phase softens into a viscous liquid, which brings ceramic powder particles closer and closer together, and activates the powder particles. Under the promotion of concentration difference and interfacial tension, it promotes the growth of pores and glass flow in the substrate, achieving ceramic volume shrinkage and substrate densification. Single layer LTCC ceramic strips are formed by casting, while multi-layer ceramic strips are formed into dense bodies by isostatic pressing. After the LTCC substrate was subjected to a peak temperature of 450 ℃ for adhesive extrusion, the green body became relatively loose after extrusion foaming, and most of the particles were in a separated state with many gaps between them. As the sintering temperature increases and the time prolongs, especially after 650 ℃, the DSC curve of the ceramic powder shows that the ceramic powder begins to absorb heat and soften, with a glass transition temperature of 668 ℃. During this period, the ceramic particles continuously come into contact and rearrange, the large pores gradually disappear, and the mass transfer process between substances gradually begins. The contact state between particles gradually expands from point contact to surface contact, the solid solid contact area increases, and the solid gas surface area decreases accordingly.
conclusion
(1) The sintering heating rate significantly affects the microstructure of LTCC substrates. With the increase of sintering heating rate, the internal pores of the prepared ceramic substrate increase, resulting in a significant decrease in the dielectric constant of the substrate, an increase in dielectric loss, and a decrease in the adhesion and impact resistance of the film layer. When the sintering heating rate is 8 ℃/min, the prepared LTCC substrate not only has low porosity and high strength, but also has good dielectric and thermodynamic properties.
(2) The sintering heating rate significantly affects the matching of sintering shrinkage between silver electronic paste and glass ceramic. When the sintering heating rate increased from 4 ℃/min to 16 ℃/min, the warpage degree increased from 0.21% to 0.82%, resulting in ceramic substrate warpage and a mismatch between the sintering shrinkage rate of silver electronic paste and glass ceramic.
(3) The sintering of LTCC ceramic substrates requires an appropriate heating rate. The heating rate will affect the mass transfer, crystal growth, pore discharge, and densification process during the sintering process, thus affecting the mechanical and electrical properties.