The Printing and Firing Process of Al Paste in PTC Ohmic Contact Electrodes

Vol. 19 No.3Joumel of Wuhan University of Technology. Mater. Sei. Ed.Sep. 2004The Printing and Firing Process of Al Paste in PTC OhmicContact Electrodes'LIANG Fei ZHOU Dong: xiang LV Wen-zhong GONG Shu-pingHuazhong University of Science and Technology(Received:August 20, 2003; Accepted: May 25, 2004)Abstract: Al electrodes are well known as ohmic contact eletrodes for the PTC component，the ingluenceof their thickness on final component properties was inestigated by comparing their ohmic charoceristics with theones of InGa elecrodes。Afier obsering the AI paste plhysical and chermical behariors during rising temperature bythermal analysis ( DTA), the firing operation of Al electrodes could be dirided ino three main subsecions: thetenperature rising tine (t,)， the peak firing tenperature (T,) and the hold time at peak. firing tenperature(4). The ffees of these hree purumeters on final component properties uwere dicussed in detail .Key words: PTC thernistor; AI electrodes; ohmic contact charoceistics; printing and firing pro-ces1 Introductionwith a diameter of 14mm and a thickness of 2.0mm.After being weighted proportinally， AI powder,In industrial applications, Al electrodes are widelyglass powder，organic binders and antioxidants and so onused a ohmic contact electrodes for the PTC components .were blended and stired into Al paste. Then the Al pasteThey serve as ohmic contact and provide good adhesion towas deposited by screen-printing on both sides of the ce-the ceranics . However the printing and firing process hasramic with a diameter of 14mm. In order to study the in-fluence of the printing and firing process of Al electrodesa great inftuence on final component properties. At pr-on final component properties ，the screen meshes ( 160,esent, there are numenus types of Al electrodes paste in220, 280, 340)，the temperature rising time ( 2min,domestic market and their properties are different from6min, 10min, 14min, 30min) , the peak fring temperatureeach other. Moreover their printing and firing process has(600 C ,640 C,680 C,720 C,760 C) and the holdnot uniform standard, so they are very hard to be con-time at peak firing termperature ( 6min, 10min, 14min,trolled suitably. In the present paper, the authors dis-18min) were altered individually. The InGa electrodes ascussed the effect of the main four parameters: the thick-reference electrodes with completely ohmic contacts wereness of Al electrodes layer ( d)，the temperature risingapplied. by a simple rubbing method ( diameter,time (1,)，the peak firing temperature ( Tp) and the14mm)"holding time at peak firing temperature (t,) on final com-2.2 Instrumentationponent properties.PTCR Withstanding- Volage Testing InstumentWe presented DTA curve of Al electrodes, which(GJ I ,HUST) was employed to test electrical character-was performed to obtain infornations about physical andistics of PTC thermistors with sintered Al electrodes, thechernical behaviors of Al electrodes during firing; supple-testing conditions were as the fllowin: Environmentalmentary scanning electron microsc:opy (SEM) was used totemperature, (25土5)C; Load resistance, 80; With-determine the structure of Al electrodes layer. As refer-standing voltage, 300Vac，1min-460Vac, 3min. A dampence electrodes，InGa electrodes were applied and investi-and hot test cabinet (SH01) was used to test the humiditygated since they exhibit completely ohmic characteristics.non-load characteristics. The range of its temperature andhumidity is a the fllwing: ( 40土1 )C，anc2 Experimental95%土1%. An optical microscope (CK40, OLYMPUS)was used to test the thickness of Al electrodes layer. The2. | Materialsscanning electron microscopy ( JSM-5610LV ) investiga-Connercial PTC ceramics without electrodes weretions were performed on micnograph of sintered Al electro-obtained from Hubei HUST Information Ceramic Co. LId.des layer .The PTC ceramic was BaTO; and in the form of dense discSimultaneous DTA data were obtained on 100- 150mgof Al electrodes paste using a Stanton MF-H5 thermo bal-IANG Fei(梁飞): Bom in 1975; Letuer; Depertnent ofance with a Stanton 661 DTA attachment. A heating rateElectorics Srience and Teehnology，Huaxzhong Universty of Sciencef1中国煤化工a的the DTA reler-and Technology, Wuhan 430074, Chinatmosphere was static* Funded hy he Slate' *863"Prject( No. 715-06-0080)Y片CNMHGVol.19 No. 3LIANG Fei et al :The Printing and Firing Process of Al Paste in PTC 0hmic Conac....27constantly vapors， melted glass phase starts to soak A3 Results and Discussionpowder particles so that metal Al adheres onto the surfaceof the ceramic disc fimmly. There is a small exothermal3.1 The effect of the electrodes layer thicknesspeak at 322 C，which is possibly the result of the oxi-'The electrodes thickness is the amount of metalliza-dization of the ethylfibrin3n apparent exothernaltion material that is applied to each ceramie disc. Thepeak appears at 410 C, it is likely because the antioxi-electrodes thickness is controlled by the screen- printingdant in Al paste is preferentially oxidized to protect theparameters, esecially the screen mesh. The larger thereductive metal Al. A weak exothermal peak occurs atscreen mesh designation, the thicker the electrodes layer,548 C，a possible explanation for this behavior is thatFig. 1 shows the relation between the screen mesh and thethe reductive metal Al combined with the absorbed oxygenelectrodes thickness .on the surface of the ceramic body and the ohmic contactsAs show in Fig.I, when screen meshes is equal tobetween Al electrodes and the ceramic disc start to formn.or less than 220, the ohmie contact characteristic (OCC)There is a weak endothermnic peak at 616C and a exo-of AI electrodes keeps stable and better than that of InGathemic peak at 639C， Sun Wentong atributed theetrodes. It may be that at high firing temperature, Alformer to the heat absorbed when the glass phase dissolveselectrodes infiltrate into PTC ceramic boy, which wilAl2O3 and the ltter to the chemical reaction heat betweenshorten the distance between two sides of PTIC disc elec-AI and the plumbum elerment in the glass phase. At 640trodes s0 that the PIC total resistance decreas (unreported9C ,metal Al electrode starts to melt and at 676. 69 thepublicly). While the screen meshes is more than 220,melting reaction reaches the maximum. The last exother-the 0CC value rises promptly, atributing the resistancemic peak in the curve appears at 718C，it was reportedincreuse to incomplete welting of thin Al electrodes layersthat it was possibly caused by tie chemical reaction be-and hence a reduction in the ffective area of ohmic con-tween Al and other elements in the glass phase 4.So bytact so that final PTC component total resistancethermal analysis (DTA), we have known the physical andincrease' ”。By analyzing the correlation betweenthe Alchemical behavior of Al paste during lemperature firinelectrodes thickness and ohmic contact characteristics, theand can divide the whole firing operation into three sub-optimal option for printing Al electrodes are the 220sections, the rising time (1,), the peak firing temperaturemeshes screen for both final cormponent characteristics and( Tp) and the holding time at the peak firing temperatureless costs.(tn), each of which contributes greatly to the final PTCR3.2 The effect of electrodes firing processTo formulale appropriate firing process, the authorscomponent properties. The three parameters should bobserved and analyzed the physical and chermical behav-controlled appropriately in case that the reductive metal Alions of Al electrodes during firing by themal analysisis oxidized by the oxygen in air, the surface resistance ofAl electrodes rises , and finally, Al electrodes lose ohm(DTA).Fig.2 shows the physical and chermical behaviors ofcontact characteristics. Then the influence of le, T。andAl elex:troxles with the increase of temperature. As showntn on final component characteristics will be discussed inin Fig.2, with the rising of temperalure, organic solventsdetail.- Electrode thidnes409.5、一Ohmie eortact characteneties48.2 718 11。322.616.8-2-676.6140 180220 260300 3402022042620 755_ 51015202530Sereen meshTemperature/CRising tenperatue time/minFig.1 The efect of screen meshes on theFig.2 The Al electrode DTA curveFig.3 The influence of the risingelectrode tbickness and the ohmietempereture fime on the obmiecontaet charackristiceconfect charecterictics3.2.1 The efet of rising time (1,)quick, some high boiling-point organic solvents does notFig.3 shows the relation between rising time (t,)evaporate entirely and is consolidated with conductiveand ohmice contact characeristics (0CC). As shown inWhon PTCP discs is submitted toFig.3, when I, is too short (such as 1, = 2min), the a bi中国煤化Iemperatue momen-OCC value is bigger than zero. Moreover, the results oftaniolve.YHCNMHGning-point of organicwithholding voltage show that parts of disc failed . Thelayer finally burm.probable cause is as follows: When the rising rate is tooWith the increase of t, the 0CC value gradually decreas-28Joumal of W uhan University of Tehnology - Mater. Sei. Ed.Sep. 2004 .es. when I, 10 min, the 0CC value reache minimum. Atthe surface resistance increas. So the most appropriate t,this time. the organic solvents is evaporated entirely andshould be 10 min.the electrodes characteristics arrive at the optimal. How-3.2.2 The efet of the peak firing temperature ( Tp)ever, with the continuous prolonging of the t, the 0CCand the hold time at peak firing temperaturevalue gradually increase again, it is likely because of the(1n).oxidization of Al ele:trdes by the oxygen in air so that01一 Hutany non caracegatice-20{{ -41-0，-8-20 ,80620660T70074078Dosl firingtemperature/CFig.4 SEM imege of sintered Al eloctrode Fig.5 SEM inmago of sintered Al eloctrodenFig.6 The eflect ofTP on the electrical cbaracteristics(T,=600C t,=6min)(T,=680C q =10min)1s| 一Hrridity non-lagdcheracrsis1-2tween Al and oxygen in air. So the above experimental re-5sults show that in terms of the Al electrodes in this paper,f-6the most appropriate process parameters should be Tp =0叶680°C , 1 = l0min.-202468101214161820+-14 ConclusionsHold time al peak firing temperature/Ca) The ohm electrodes thickness has a great effect onFig.7 thr ffre of 1n on the eletrical characteristicsFig.4 shows the SEM image of sintered AI electro-final component properties. The lower the screen meshdes (Tp= 600心，tn = 6min). From it we see that eachdesignation, the thicker the Al electrodes layer. In temsof the Al paste in this particle, the opimal screen mesh isAl particle in sintered Al electrodes exists separately,220 for both final component properties and less costs.there are small contacting areas among particles. From theb) The firing of Al electrodes is a complex physicalresults of DTA，this is likely because the temperature isand chemical operation, which can be divided into threetox low and does not reach melting point of metal Al, themain sulsections: the temperature rising time (1,)，theAl particles is not melted into a complete wetting. Whilepeak firing lemperature ( Tp) and hold time at peak firingthe T， rises to 680 and In is prolonged to 10 min (astemperature (tn). Fach of them contributes greatly to fi-seen in Fig.5), Al particles in sintered electrmdes form anal component characteristics. As to the ohm Al electro-complete conductive wetting.Fig.6 and Fig.7 respectively show the efet of Tpdes, the optimal process is as the following: t, = 10min,and tn on ohmie contact characteristics and humidity non-T, = 680C, in = 10min.load characleristics. The lower the Tp and the shorter thehn. the hetter the ohmic contact characteristics. HoweverReferenceswhen the T is too low or the t, is too short (such as! Yaw Yamei. Study on Pint and Fire Process of Ag Paste in PICTp= 600C，1, = 6min)，electodes conductive wetting0hm Electrodes. Mired Microelectronic Technology, 1993,7(4):has not formed entirely, and moreover its humidity non-49-54 .loed characteristis is poor. The resuls of withstanding-Heinenb, Waser R. Infuence of the Thickness and Area od NiC:/voltage test indicate that the majorities of PTC componentAg Eletodes on the Characteristics of BarTi0) -ceranice Basicefailed. When 7p is equal to or higher than 720心or 1 isPositive temperatue- offcient Thernistors. Jownal of Maleri-longer than 14min, the electrodes conductive wetting hasals Science , 1998,33:603-4608been fomed and humidity non load characteristics tends 3中国煤化工-ntact Siver Paste for theto stable, but their 0CC promptly becomne bad, which is198, 19(1):9-13atributed to the chemical reaction Al and other elementsYHC N M H Gof Eletronic Thick layernitiuiviue component and Material ，in the glass phase (Fig.2) and to the combination be1997, 16:314-319