Effects of Co-Firing and Plating Processes on Magnetic Properties and Reliability for Ni-Cu-Zn Based Ferrite Multilayer Chip Inductors

Abstract

A huge residual stress is generated in sintering process and interacts with stresses generating in following processes of manufacture of multilayer chip inductors and this complicated stress system impacts the characteristics and limits the application of this type of inductor Besides the stress the current passes through coil and produces joule heat to increase the MLCIs’ (Multi-Layer Chip Inductors) temperature during working period as time goes by the inductance of the components within defect will lower and fail To achieve the future demands in higher power and smaller size the interaction mechanism the effectiveness and the origin of inductance-stress-heat of MLCIs should be researched systematically and completely In this dissertation we chose Ni-Cu-Zn ferrite as a material for MLCIs and divided whole experiment into 2 major parts which are co-firing (sintering) and plating process The first part has 4 chapters including crack thermal characteristic stress characteristic and eliminating stress and second part has 2 chapters including plating and reliability We attempted to find the root causes mechanism and the solutions for improving MLCIs’ performance further through the studies of these 6 chapters The chapter 4 is dealing with relation with stress which causes the crack of MLCIs In this study we found the stress is generated by mismatch of silver coil and ferrite core during cofiring furthermore due to the need of low DCR (Direct Current Resistance) of coil the thickness of coil is increased and it will cause mismatch and stress even larger By recognizing the root causes we can select finer silver particles as the raw material of coil to make the silver coil shrink at lower temperature which the ferrite is still in ductile status and therefore to lower the impact level to ferrite core and the crack can be inhibited The chapter 5 is the study of the temperature effect to the MLCIs We arranged different temperature conditions and crosschecked their results we found 3 parts co-contributing to change of inductance they are stress releasing magnetic domain change and exchange between stress-sensitive inductance and magnetic-sensitive inductance and the third part also revealed a phenomenon that the interaction between stress and temperature We also found that the influence of thermal treatment to the inductance relating magnetic field of MLCIs can be reduced by a pre-treatment undergoing sub-zero temperature condition In this study we utilized temperature cycling (TC) to release the residual stress for evaluating its level inside the MLCIs We studied the influence and origin of stresses which are caused by manufacture processes in chapter 6 The materials of circular type cores and multilayer chip inductors (silver coil and ferrite core co-exit inside the components) and the processes of termination making (dipping and firing process) and electroplating were all studied in sequence to indentify each contribution and their interaction in these materials and processes We also proposed a characteristic plot of inductance-stress to elaborate the trend of inductance change following with the processes The plot also revealed that if the stress can not be avoided inside the MLCIs the compressive stress is preferable in the perspective of ratio of induction and stress By utilizing different construction of multilayer chip inductors and temperature cycling handling we can analyze the local stress status in different location and the location where the stress released The result showed that the stresses are located in the interfaces of silver coil-ferrite core end termination-body and Sn-Ni-Ag plating layers The levels of these stresses are not equal and they can be eliminated by undermining the connection of interfaces locally and limitedly We proposed solutions to avoid the stress completely in chapter 7 As the analysis in chapter 4 the residual stress after sintering is caused by shrinkage mismatch of silver coil and ferrite core and therefore we can utilize a concept which is “shrinking separately” by making silver shrink at lower temperature Furthermore by extending the concept of shrinking separately we developed an alternative approach which applied carbon layer under and below the silver layer to prevent the direct contact from silver coil and ferrite core to achieve the shrinking separately and the residual stress-free However some side effects of process are remained to be overcame The chapter 8 is an analysis of inductance change after electroplating of MLCIs Comparing and crosschecking the inductances of pure ferrite coil winding-ferrite core and MLCIs which underwent soaking plating solution and alternatively electroplating conditions we found that there were 3 factors contributing the change of inductance they are (1) the plating layer covered the body and generated an excess stress and interacted with residual stress inside the body the inductance will increase when the stresses counteracts and the inductance will decrease when the stress accumulates; (2) plating solution corrodes and undermines the interface between end termination and body to release the stress and then increase the inductance moreover when the surface status of chip inductor are changed by different sintering profiles the wetting ability will be changed and as well as the corrosion; (3) the current electroplating generates a magnetic field and interacted with remanence of chip inductors and caused the inductance fell In the last chapter (chapter 9) of the dissertation the reliability failure was analyzed What we found was the Cu precipitating on the end termination and we deduced that the existence of ions/ matter exchange during electroplating By matter exchanging the ions of plating solution penetrates into the surface of chip’s body and then migrates into the chip body deeper by the electrical field in working or reliability test and the irreversible change inside the chip body happens and causes function failure of MLCIs To improve this failure both changing electroplating conditions and changing sintering temperature can be taken

Date of Award	2015 Aug 3
Original language	English
Supervisor	Wen-Shi Lee (Supervisor)