Downward Trimming of Polymer Thick Film Resistors through High Voltage Pulses

Transcription

1 Downwar Trimming of Polymer Thick Film Resistors through High Voltage Pulses Y.Srinivasa Rao E.C.E. Department Shri Vishnu Engineering College For Women Vishnupur, Bhimavaram-540 W.G.Dt.,Ap, Inia Phone : FAX : E- Mail: yarravarapu@yahoo.co.in M.Satyam Devices Lab., E.C.E. Department Inian Institute Of Science Bangalore ( Inia ) Phone : FAX : or msece@ece.iisc.ernet.in Abstract The effect of high voltage pulses on the resistance of polymer thick film resistors is stuie. It is foun that the resistance ecreases with both the amplitue an uration of the high voltage pulses an with the number of pulses. The change in resistance is attribute to the ielectrophoretic motion of graphite granules in the PVC meium when high voltage pulses are applie to polymer thick film resistors. A moel has been propose through which the percentage change in resistance can be estimate in terms of the number of pulses, the uration of pulses, an the length of the pulses. Base on these investigations, we suggest a ownwar trimming metho for polymer thick film resistors through the application of high voltage pulses. This metho of trimming is a clean process an trimme resistors are free from hot spots. It is foun that there is consierable improvement in TCR an Current Noise of the trimme resistors. This improvement is attribute to increase in the conuctive paths in the trimme resistors. The International Journal of Microcircuits an Electronic Packaging, Volume 4, Number 4, Fourth Quarter, 001 (ISSN ) 88

2 Key wors Polymer Thick Film Resistors, Trimming of Thick Film Resistors, High Voltage Trimming. 1.Introuction Trimming of thick film resistors is resorte to obtain the require values from the processe ones. Generally thick film resistors are trimme upwars by removing the film material from certain portions of resistors an trimme ownwars by changing film material properties. One of the trimming techniques that has been use is base on the application of high voltage impulses. This technique has been applie so far to frit glass base thick film resistors [1 8]. In this paper we examine the possibility of trimming polymer thick film resistors (PTFR) using high voltage pulses. In what follows is a escription of the various experiments an the conclusions base on which the ownwar trimming for polymer thick film resistors is suggeste..experimental Work Polymer thick film resistors have been fabricate by screen printing route an are subjecte to high voltage pulses. The various steps followe in the fabrication an characterization of these polymer thick film resistors are escribe below..1 Preparation of Polymer Thick Film Resistors Polymer paste containing PVC an graphite has been prepare by first issolving PVC granules in cyclohexanone an then by blening graphite in to it. This paste has been use for printing resistors on alumina substrates with a screen printer supplie by DeHaart (Massachusetts, USA Moel SP SA 05 ). These printe resistors are processe using the usual thick film processing of polymer-base films. The heat treatment involves rying at room temperature for 15 minutes, followe by curing at C for four hours. Typical thick film resistor structure is shown in Figure 1.. High Voltage Behavior of Polymer Thick Film Resistors The setup use to apply high voltage pulses to the resistor is base on the one escribe by H. Delacy an is shown in Figure. It consists of a high voltage change over relay, a transformer, a variac an a function generator. The pulse with an the number of pulses are controlle through the high voltage change over relay an function generator. The pulse amplitue is controlle by the variac. In this metho an attempt has been mae to apply high voltage pulses to polymer thick film resistors an prouce a change in resistance. The trimming involves the application of high voltage pulses to polymer thick film resistors an measuring the value of resistance. After the application of each high voltage pulse, a variation in resistance has been observe an the variation is foun to be ecreasing an typical curves are given in Figures 4. From Figures 4, it may be seen that the resistance value of polymer thick film resistor ecreases with the application of high voltage pulses when the pulse amplitue is above a certain threshol value. Parts of the experimental results are alreay publishe in International Journals [9 1]. The ecrease in resistance of polymer thick film resistor with the application of high voltage pulses is explaine in the next section using a moel. 89

3 0.14" 0.0" 0.157" PVC - Graphite Thick Film Resistor 0.157" 0.47" Silver Contact Pa Figure 1. Typical Structure of a Polymer Thick Film Resistor Function Generator 40 VAC Variac Soli State Changeover Relay Setup Transformer Resistor Uner Test Figure. High Voltage Test Circuit 90

4 1.0 Normalize Resistance Grain size 45 microns Resistor imensions: L 8 mm, W 1 mm Composite by weight: Graphite 60%/PVC 40% Pulse uration: 0 ms Pulse amplitue: 175 V - A 10 V - B 40 V - C Number of Pulses A B C Figure. Variation of Normalize Resistance with number of Pulses for Different PulseAmplitues (Normalize with respect to initial resistance value) Normalize Resistance Pulse Duration 0 msec 0 msec 50 msec Grain size 45 microns Composition by weight: graphite 60% / PVC 40% Pulse Amplitue 50 V Resistor Dimensions: Length 1 mm, With 4 mm Number of Pulses Figure 4. Variation of Normalize Resistance with Number of Pulses for Different Pulse Durations. 91

5 . Moel In orer to estimate the changes in resistance with the application of high voltage stimuli, a moel for the structure of polymer resistive material is propose an the effect of high voltage stimuli have been evaluate. The following gives an account of this effort. Rajagopal an Satyam [14] have assume that small cubes of insulators are covere by the conucting granules that are in contact with each other forming close chains. Such elemental cells constitute the entire composite material. This moel is sufficient to explain the variation of the conuctivity in relation to composition an grain size. However, the moel oes not explain thermal trimming or high voltage trimming as was reporte earlier. Polymer films contain a large number of cavities. Murthy an Satyam [ 15] have introuce a more realistic picture of insulator with cavities in the elemental cells in orer to explain thermal trimming or thermal cycling effects in these composites. This moel seems to explain high voltage effects on these materials. In aition Murthy an Satyam have assume that all of the grains are connecte in the form of chains. A typical structure assume for the material is given in Figure 5. It may be seen that the material contains conucting graphite chains surrouning insulator material with cavities. For simplicity sake it is assume that there is unit microcell of the material that contains one cavity an this microcell repeats forming the entire material. Further it may be seen from Figure 5 that the conucting chains are close through out the material except at the cavity i.e. the conucting chains are broken on the surface of the cavities. When an electric fiel is applie to such a material, ielectrophoretic forces appear between the tips of the chains an the chains are subjecte to motion. If the forces exerte are higher than the threshol value etermine by the viscous force, ultimately these open chains coleslaw epening on the time for which the stimuli are applie. This results in reuction in effective cavity size with time resulting in reuction in resistance. Using the technique suggeste by Murthy an Satyam [14], the effective change in resistance is calculate. In what follows is a brief account of the effort that is put in to arrive at an iea of how the tips of chains move with time, how to evaluate the effective raius of cavity after the stimuli are applie an finally calculation of the change in resistance..1 Dynamics of Open Chain Motion For simplicity s sake it is assume that the motion between the tips of two chains is similar to the motion of two grains uner the stimuli. Therefore the motion of two isolate grains is given in the following Consier two spherical grains separate by a istance r an a raius r 0 as shown in Figure 6. When a electric fiel E is applie, a charge +q an a charge -q are introuce on these grains epening on the irection of the electric fiel. Because of the inuce charges, there is a force of attraction F between these two grains given by, F q 4ε( r + r0 ) (1) The potential ifference between these two particles is given by, 9

6 +Y Figure 5. Cross Section of a Microcell of a Composite Material x Ro r Ro -q +q Figure 6. Dynamics Between Two Spherical Grains 9

7 V F S r + r 0 r 0 q 4 ε x x r 4 ε q r 0 + r 0 ( r + r x) x Solving an substituting the limits, q r V ε r Solving Eq. () for Q, ( r + ) 0 r 0 ( r r0 ) V () ε r0 + q () r Substituting Eq. () into Eq. (1), the force of attraction is F r0 ( r + r0 ) ( r + r ) ε E V (4) r 0 This force is always attractive in nature between the granules an inepenent of the polarity. When this force of attraction excees the static friction ue to the polymer matrix on the graphite granules, it leas to the motion of graphite granules in the insulating polyvinyl chlorie. The net force acting on the graphite granules is F ma F E F V (6) where m mass of the graphite granule a acceleration of the graphite granule in the PVC meium Substituting Eq. (4) an Eq. (5) into Eq. (6), r0 ( r + r0 ) ( r + r ) ε m a V 6 r0 η ν r Let A ε r 0 V 0 B 6 r0 η Substitute A an B into Eq. (6) an ifferentiate twice with respect to time The viscous force, F V, (static friction of PVC on graphite granules) of polymer on graphite granules is given by 1 t r ( r + r0 ) B r ( r + r ) m t A m (7) r 0 F 6 r ν η (5) V 0 where r 0 raius of the granule in cm η viscosity of the meium in poise ν velocity of the granule in cm/sec A computer program has been evelope to solve the Eq. (7) base on the secon orer Runga-Kutta metho. This computer program computes the positions of the graphite granules with time, when they are uner initial 94

8 Time require for collapse (m sec) Grain size 10 microns Composition by weight: graphite 90% / PVC 10% Resistor imensions: Length 8 mm, With 1 mm Electric Fiel (V/m) Figure 7. Time require for collapse Electrical Fiel characteristics electric fiels. The time require for the granules to collapse uner ifferent applie electric fiels is plotte in Figure7.. Calculation of Chain Length As explaine earlier, the composite is assume to consist of chains of graphite particles an surroun by insulating meium as shown in Figure 5. The chain length is expecte to epen up on composition of graphite an the insulator. In fact one can guess that the chain length ecreases an the number of chains increase with increase in composition. In what follows is a metho of arriving at the chain length is a composite of a given composition. From Figure 5, it may be seen that each chain is share by four ajacent elemental cubes an therefore the amount of graphite in each chain is share by four cubes. Hence the graphite ue to one chain per cube is one fourth. However there are 1 chains per cube an therefore the graphite per cube (1/4 chains) is ue to the graphite present in the three chains. The graphite content in each chain, G e, is given by G e N V G where N Number of grains in a chain V G Volume of the graphite granule 95

9 96 The number of grains is given by L N Where L the length of the chain iameter of the graphite granule which is assume to be spherical The volume of each graphite granule is 8 4 V G an the volume of graphite in a cube is L L 8 4 V C (8) The number of elemental cubes in a unit microcell is expresse as C L 6 X N (9) where X length of the unit microcell The volume of graphite granules in a microcell is 6 X L V M (10) The volume of PVC in a microcell is given by P L 1 6 X V (11) Therefore, the weight ratio of graphite to PVC is given by P G P G L 1 6 X 6 X L W W ρ ρ (1) where ρ G ensity of graphite ρ P ensity of PVC Equation (1) reuces to P G P G 1 L 1 W W ρ ρ (1) It may be seen from this expression that the chain length L ecreases with an increase in graphite content an is inepenent of the microcell length or the volume of the microcell (X ). The effect of the cavity is only to reuce the volume of the material in the microcell by an amount equivalent to the volume of the cavity. 4. Calculation of Resistance The variation in resistance with change in iameter of the cavity has been calculate following the lines escribe by Murthy an

10 Satyam [15]. They have given variation of normalize resistance with cavity iameter for a two imensional system, which is reprouce in Figure 8. In our case the cavity is spherical in nature an the microcell is a cube. To compute the resistance of the cube, it is slice parallel to fiel lines or current lines. Each slice is a planar resistor with a circular cavity as escribe in reference [15]. As the cavity size of each slice varies, the normalize resistance of each slice with respective circular cavity is compute using Figure 8. These resistors are parallel an therefore the effective resistance of the cube with a given cavity size is obtaine. 5. Effect of Trimming on the Cavity size an Resistance When a particular voltage above the threshol value is applie to the resistor, some of the open chains close epening on time an the fiel that is applie. This essentially results in the formation of close chains in sie the cavity as shown in Figure 5. Assuming that the time neee for closing the chains is approximately the same as that involve in the collapse of free particles, as escribe in this section. After the application of the fiel for a known perio, we ientifie the chains that will be close. In the presence of newly close chains, an effective cavity iameter is compute graphically as follows. A continuous close line rawn through all the tips of the close chains an the average raius from original center is calculate. Using this new raius of the cavity the normalize resistance is once again calculate. This proceure is repeate assuming that the resistor is subjecte to voltages for ifferent urations. The calculate normalize cavity iameter an consequent change in resistance are given in Figures It may be seen from this Figures that the cavity iameter ecreases with the uration of the high voltage applie an the resistance ecreases. Further the general shape of the resistance variation with uration of the stimulus is similar to the variation of resistance with the number of pulses as seen from Figures This shows that the hypothesis presente here seems to explain the phenomena responsible for ownwar trimming of polymer thick film resistors with high voltage pulses. In actual practice the cavity iameter an microcell size are not known. Therefore an attempt has been mae to arrive at these parameters by trial an error proceure that will explain the observe resistance variations with various pulse parameters as escribe in Figures Verification of the Mechanism In the fabricate resistors it is observe that after a certain number of high voltage pulses, the resistance oes not vary much with further application of high voltage pulses. The value of resistances after the application of each high voltage pulse are normalise with respect to stable values of the resistors. In our calculations, the effective cavity iameter is taken as cavity iameter (Effective cavity iameter is efine as the iameter of the cavity that passes through all the tips of the close chains). The length of conucting chain is calculate using Eq. (1) for a particular composition of graphite an PVC. Assuming the size of the cavity, the change in resistance with the application of high voltage pulses is calculate for ifferent microcell lengths. The change in normalize resistance versus the number of high voltage pulses are plotte in all these cases. This proceure is repeate for ifferent values of cavity iameters with ifferent microcell lengths an results are 97

11 Normalize resistance Normalize cavity iameter Figure 8. Variation of Normalize Resistance with Normalize Cavity Diameter Normalize Resistance 1.0 Experimental Calculate Grain Size 10 microns Composition by weight: Graphite 90% / PVC 10% Resistor imensions: Length 8 mm, With 1 mm Pulse amplitue: 160 V Pulse uration: 0 msec Number of Pulses Figure 9. Typical Calculate Variation of Normalize Resistance with Number of Pulses 98

12 Normalize Resistance Experimental Calculate Grain Size 10 microns Composition by weight: Graphite 80% / PVC 0% Resistor imensions: Length 8 mm, With 1 mm Pulse amplitue: 160 V Pulse uration: 0 msec Number of Pulses Figure 10. Typical Calculate Variation of Normalize Resistance with Number of Pulses plotte in all these cases. It has been observe that there existe a curve that is very close to observe variation for each composition. The cavity size an microcell size corresponing to all these curves have been foun to be approximately same an are given in Table 1. This shows that the cavity size an ensities of cavities which are expecte to epen on polymer use an process conitions remains the same as long as the resistors are prepare with the same conitions. From this one can seen that ielectrophoretic moel propose here explains the behavior of PVC-graphite thick film resistors uner high voltage conitions (explains ownwar trimming mechanism with high voltage pulses). 7. Electrical Characteristics of Trimme Resistors The Current Noise an TCR of the trimme an untrimme resistors are measure an the measurements are escribe below Electrical Noise The noise inex measurements of these resistors have been carrie out as escribe below. The Quan Tech Moel 15C Resistor Noise Meter is use to measure the current noise inex in ecibels (B). The current noise inex is efine as the ratio of the rms noise voltage, in microvolts, to the applie DC voltage in 99

13 Tab le 1. Cavity Sizes an Microcell Sizes for Various Compositions Composition by Weight Graphite: PVC Cavity Diameter (microns) Microcell Length (microns) 90% : 10% % : 0% % : 0% % : 40% volts, expresse in ecibels, when the banwith of measurement is 1khz an its frequency is geometrically centere at 1 khz. In the Quan Tech noise meter, the noise inex can be measure with these parameters uner varie conitions of current. The noise inex of each resistor is measure by varying the voltage across the resistor (current through the resistor). The resistors on which measurements are carrie out are prepare with 10%, 0%, 0% an 40% of PVC. Five resistors of each type are subjecte to these measurements an the average noise inex taken as noise inex for that resistor. Typical noise inex measurements are given in Table It is foun that for a given resistor uner ifferent current levels, the noise inex is approximately constant. However, the resistors after trimming (Downwars) have lower resistance values an also lower noise inices. Even these trimme resistors exhibit constant noise inex with current. Thus the effect of ownwar trimming may be seen to be reuction in noise inex an thus prouces lower noise output. 7. TCR The temperature coefficient of resistance (TCE) of a polymer thick film resistor is measure as follows. The resistors are kept in temperature-controlle ovens an the resistance is measure by using a Fluke Multimeter Moel 884 A at room temperature an 50 0 C. These measurements are carrie out on ifferent values of resistors fabricate from ifferent PVC graphite compositions. Table gives the temperature coefficient of resistance for untrimme resistors an for a10%, 0%, an 0% reuction in resistance of trimme resistors for graphite PVC (90%: 10%) thick film resistors. In orer to obtain the temperature coefficient of resistance of the samples, the resistance was measure at two temperatures, 5 0 C an 50 0 C, an the average temperature coefficient of resistance was calculate using the formula TCR R ( R50 R5 ) ( T T ) 5 The calculations were carrie out on samples prepare with ifferent compositions of PVC an graphite with a grain size of 45 microns. The range of compositions covere is graphite 60%: PVC 40% to graphite 90%: PVC 10%. Temperature coefficient of resistance has been measure both on untrimme an trimme resistor with ifferent compositions. The measure TCRs for ifferent resistors are given in Table

14 Table. Typical Current Noise Inex Measurements for Thick Film Resistors with 90% Graphite: 10% PVC Current ( ma ) Current noise inex ( B ) Table. Variation of TCR with Percentage Change in Resistance of Polymer Thick Film Resistors ue to Pulse Voltage Trimming Percentage Change in Resistance ue to High Voltage trimming Temperature Coefficient of Resistance (ppm/ o C) x Grain size: 45 µ Composition by weight: Graphite: 90%/ PVC 10% Resistor imensions: Length 8 mm, With 1 mm 401

15 Table 4. TCR s of Different Composition for Untrimme an Trimme Resistors Composition by weight TCR in ppm/ o C ( x 10 - ) Percentage reuction in resistance with trimming Graphite:PVC 0% 10% 0% 0% 60%:40% %:0% %:0% %:10% From Table 4, it may be seen that temperature coefficient of resistance is foun to be positive for all the polymer thick film resistors before an after high voltage trimming when the composition is varie from graphite 60%: PVC 40% to graphite 90%: PVC 10%. It is foun that temperature coefficient of resistance epens on the original composition an as well as the extent of trimming. When the composition is graphite 60%: PVC 40% an graphite 70%: PVC 0%, it is foun that the temperature coefficient of resistance ecreases with 10% trimming an comes to the approximately to the original value, when the trimming is 0% to 0%. However in the higher compositions, namely graphite 80%: PVC 0% an graphite 90%: PVC 10%, with 10% trimming the temperature coefficient of resistance oes not appear to be ifferent from the original value. However with 0% an 0% trimming, the TCR seems to be higher than the original value. High voltage trimming closes the open chains aroun the cavities that are present in the polymer thick film resistors. The effective change in resistance has been explaine through the change in effective cavity iameter. When the temperature is varie to a higher value, two types of phenomena s seem to be occurring in the material, the insulator (polymer) in the cavity expans an this is equivalent to an increase in cavity iameter. From the earlier iscussion, an increase in cavity iameter increases the resistance. The increase in resistance at the higher original iameter is more than with the lower iameter [15]. After trimming, since the iameter is small, one expects the increase in resistance for a given temperature change to be less than that of the resistor with out trimming. This is the case when the percentage of composition is lower (number of chains involve in trimming process is limite). With higher compositions or high level of trimming, the number of chains aroun the cavity involve in the trimming are larger an when heating takes place, some of the chains forme seem to be breaking at the elevate temperature giving rise to consierable increase in resistance with increase in temperature. This perhaps is the reason for having higher temperature coefficient of resistance after trimming for higher composition or for higher level of trimming. 8. Conclusions This paper reports that the resistance of polymer thick film resistors ecreases when high voltage pulses are applie to them. The 40

16 change in resistance of polymer thick film resistors with the application of high voltage pulses is attribute to ielectrophoretic motion of graphite granules in PVC meium. Base on these investigations a ownwar trimming technique in which high voltage pulses are applie to polymer thick film resistors has been propose. Polymer thick film resistors trimme by high voltage pulses are free from hot spots an their power hanling capacity remains same with high voltage pulse trimming. References [1] R.P.Himmel, Thick Film Resistor Ajustment by High Voltage Discharges. Proceeings of Electronic Component Conference ( ECC), IEEE, pp , [] Y.Taketa an M.Haraome, Thick Film Resistors with improve Voltage Stability., IEEE Transctions on Parts, Hybris an Packaging, Vol.10, pp , [] H.De Lacy, The Assessment of Thick Film Resistor Performance by High Voltage Pulsing, Proceeings of Hybri Microelectronics ( ISHM ) Symposium Conference, pp. 19 7, [4] P.F.Carcia, S.E.Champ, an R.B.Flippen, High Voltage Stable Thick Film Resistors. Proceeings of Electronic Components Conference (ECC), IEEE, pp , [5] B.E Taylor an J.R. Larry, A New Thick Film Resistor Systems for High Voltage Applications. Proceeings of Electronic Components Conference (ECC), IEEE, pp , [6] E.H.Stevens, D.A.Gilbert an J.A.Ringo, High Voltage Damage an Low Frequency Noise in Thick Film Resistors. IEEE Transactions on Components, Parts, Hybris, an Packaging, Vol.1, pp.51 56, [7] A. Olivei, On the Sensitivity to High Voltages of Thick Film Resistors, Proceeings of Electronic Components Conference (ECC), IEEE, pp , 197. [8] T. Tobita an H.Takasago, New Trimming Technology of Thick Film Resistors by the Pulse Voltage Metho., IEEE Transactions on Components, Hybris an Manufacturing Technology (CHMT), Vol.14, pp , [9] Y.Srinivasa Rao an M.Satyam, Electrical Breakown in PVC Graphite Thick Film Resistors, Proceeings of the International Conference on Emerging Microelectronics an Interconnection Technologies (EMIT 96), pp , I.I.Sc, Bangalore, Inia, February 1 16, [10] Y.Srinivasa Rao an M.Satyam, The Effect of High Voltage Pulses on PVC Graphite Thick Film Resistors, The International Journal of Microcircuits an Electronic Packaging, Vol.0, No.1, pp.51 56, First Quarter, [11] Y.Srinivasa Rao an M.Satyam, Dielectrophoretic Moel of Conuctance Increase in PVC Graphite Thick Film During Electrical Breakown., The International Journal of Microcircuits an Electronic Packaging, Vol.0, No.1, pp.57 60, First Quarter,

17 [1] Y.Srinivasa Rao an M.Satyam, The Effect of High Voltage Pulses on Current Noise of Polymer Thick Film Resistors, Proceeings of Secon International Conference on Emerging Microelectronics an Interconnection Technologies (EMIT 98), pp , Bangalore, Inia, [1] Y.Srinivasa Rao, Ph.D Thesis entitle, High Voltage Trimming of Polymer Thick Film Resistors, Electrical Communication Engineering Department, Inian Institute of Science, Bangalore, INDIA, [14] C.Rajagopal an M.Satyam, Stuies on Electrical Conuctivity of Insulator Conuctor Composites, Journal of Applie Physics, Vol.49, No.11, pp , [15] K.S.R.C.Murthy an M.Satyam, Curing an Thermal Cycling Process in PVC - Graphite Thick Films, Journal of Material Science Letters, Vol.4, pp ,