Optimization of three key process issues for LTCC substrates

LTCC substrate production process,application.

LTCC technology is a multilayer circuit technology born in the 1980s, which first uses raw porcelain powder to form a raw porcelain strip by flow-delaying, then perforates each layer of raw porcelain strip, through-hole metal paste filling, circuit graphics printing, resistance printing, and finally, each layer of raw porcelain sheet is stacked in alignment, pressed together and sintered into one at 850°C to 900°C to form a multilayer ceramic circuit. LTCC technology is widely used in RF circuit systems with its unique technical characteristics.

The LTCC multilayer circuit substrate manufacturing process is long and complex, and the substrate shrinkage, warpage, and interlayer alignment accuracy are all important factors affecting the product performance. This paper addresses the above technical difficulties and describes how to optimize the process parameters to solve these problems. Through extensive process experiments and data testing, the results show that the new process scheme can effectively solve these process problems. Here we will briefly describe optimization of three key process issues for LTCC substrates.

Analysis of the three Process Issues LTCC Substrates

1、Deviation of shrinkage rate

The shrinkage rate of LTCC is directly related to its density, the higher the density the smaller the shrinkage rate, and the lower the density the larger the shrinkage rate, which is reflected in the process parameters, the density corresponds to the laminating pressure, so the shrinkage rate of LTCC products can be changed by regulating the laminating pressure to meet the design requirements, as shown in Figure 1 below is the relationship curve of “laminating pressure – density – shrinkage rate” provided by the LTCC material manufacturer for this batch of material.

2、Substrate warpage

(1) sintering reasons: sintering process and substrate warpage, shrinkage has a direct relationship, the sintering process of LTCC substrate is actually an exothermic heat absorption reaction process. Discharge stage (room temperature to 500 ℃ or so) in the substrate organic matter decomposition volatilization, mass reduction; sintering stage (700 ℃ ~ 850 ℃ or so) substrate crystallization and precrystallization reaction, along with the reaction, the substrate shrinkage. Therefore, the sintering time at low temperature and high temperature, the heating rate and the degree of substrate shrinkage and warpage are closely related, so it is necessary to optimize the sintering curve, and adjust the heating rate and time at the gluing stage, the heating rate and time at the sintering stage, the air flow rate at each stage and other important process parameters through experiments.

(2) Substrate structure and metal distribution: The structure of LTCC substrate is also a key factor to determine the sintering warpage of LTCC substrate. When there are multiple sizes of cavity structure on LTCC substrate, it is difficult to balance the symmetry of the structure, and because LTCC substrate contains a large number of through-holes and dense metal wires, these are difficult to be evenly distributed, which will easily lead to its warpage.

(3) Paste selection problem: Different silver, gold, and raw porcelain paste with different thermal expansion coefficient match, so the large area printing layer selection of different pastes on the warpage of the substrate is particularly obvious.

3、Alignment deviation between layers of through holes

The LTCC substrate interlayer alignment deviation is related to many factors such as punching accuracy, shrinkage of raw ceramic tiles themselves, printing conductors of each layer, and stacking alignment accuracy, etc. It is a difficult control point, so the whole process needs to be monitored to find out the main influencing factors and optimize the control. LTCC manufacturing is carried out according to the process flow with film, and the whole process is monitored for misalignment.

From the above test, it can be seen that the actual LTCC substrate due to the existence of metal through-hole, printed metal wire, the shrinkage rate of these metals is different from the shrinkage rate of white porcelain, so the shrinkage rate of LTCC products under the same lamination pressure is somewhat different from that of white porcelain, and the shrinkage rate deviation can be solved by accumulating process data and making corresponding adjustments to the lamination pressure. For LTCC substrate sintering shrinkage, it is not only related to the sintering process, but also closely related to the substrate symmetry design, circuit layout, and paste selection; the process with film is more difficult than the process without film in the through-hole interlayer alignment accuracy control, and the stress release when tearing the film is the main reason for the deviation of interlayer through-hole alignment, and the process without film is more suitable for the application with higher interlayer alignment accuracy requirements.