Manufacturability and Reliability of Trimmed Buried Resistors in LTCC

摘要

Embedding resistors in the substrate is a smart solution to achieve a higher component density/area. In general, Low Temperature Cofired Ceramic (LTCC) technology offers this feature. Though this opportunity exists for years, the usage is still very low. Embedding "foreign" materials in the LTCC body might be a challenge. One has to find a system, which shows no or little interactions with both the LTCC-glass and the termination while offering a matched shrinkage to the tape during free sintering. Secondly, the usual methods to finetune target resistor values by mixing of pastes and adjusting the sintering profile with subsequent firings/measurements do not apply due to the long sintering cycle. Further application related post/ire steps are influencing the final resistance as well. All these parameters must already be available during the design process. Thirdly, the typical printed resistor tolerance of +-25 percent is not adequate to the majority of circuit designs. The latter can be solved by high voltage pulse trimming. The use of very short pulses avoids great Joule heating in certain regions, which have a higher current concentration (hot spots). The efficiency of the high voltage pulses depends on the setting of various parameters (pulse amplitude, pulse frequency, burst duration, number of bursts and kind of resistor system). The energy supplied and the resistor length is proportional. The microparticles within the microstructures of the fired resistors will be affected during the supply of the energy of high voltage pulses. This effect is based on the chemical composition of the resistors pastes, the physical construction of the networks of microstructures in the resistor system and the above mentioned pulse parameters. Optimization of pulse parameters allowed to reduce the pulse amplitude to a few hundreds volts. Therefore, trimming of buried resistors in complex hybrid circuits can be accomplished without any damage of adjacent components. By means of this method it is possible to achieve a trimming rate of +-25 percent. The paper focusses on the selection of different pastes (10 Ω/sq ― 10 kΩ/sq) based on manufacturability, LTCC compatibility and refiring behavior. The selected candidates will be evaluated in terms of their trim-sensitivity and stability after trimming. Typical processing conditions for assembly will be applied (reflow, underfill/glop top curing cycles etc.) and a qualification (1000h temperature storage, thermal cycling etc.) will be performed.