Aging Test Results for High Temperature TRIACs During Power Cycling

Transcription

1 Aging Tet Reult for High Temperature TRIAC During Power Cycling Sébatien JACQUES (a),(c), Nathalie BATUT (a), René LEROY (b) and Laurent GONTHIER (a),(c) (a) Power Microelectronic Laboratory (LMP), Tour Univerity, France (b) Mechanical Laboratory (LMR), Tour Univerity, France (c) Application and Sytem Engineering, STMicroelectronic, Tour, France Tel.: +33 / (0) Fax: +33 / (0) ebatien.jacque@t.com Abtract Thi paper deal with the functional reliability tudy of a new 16A 600V high-temperature TRIAC family, ubjected to power cycle, imulating the component in harh real operation condition. The targeted application i a vacuum cleaner (1800W 230V 50Hz). In thi kind of application, one of the major iue for TRIAC, which lead to high mechanical tree for the aembly, occur when the witch turn-on in jammed nozzle operation, i.e. when the tube i blocked. In that cae, the TRIAC junction temperature reache at mot 180 C, higher than the maximum value pecified by the manufacturer (i.e. 150 C). The aim of thi tudy i to evaluate the TRIAC lifetime under thee operation condition. The thermal tree generate local temperature variation and then, ome mechanical tree (aembly degradation). For TRIAC, the junction-to-cae thermal reitance (R th(j-c) ) increae i the ignature of uch a damage. The lifetime ha been tudied and fitted with a Lognormale ditribution. The component damage, due to the mechanical tree, have been explained thank to ome qualitative two-dimenional thermo-mechanical imulation uing finite element (ANSYS ). I. INTRODUCTION The ue of power device require to adapt the technological choice to the application requirement (ambient temperature, power cycling), in order to atify reliability and lifetime [1]. In power cycling, device are potentially vulnerable to thermal fatigue becaue of temperature cycling due to device elf-heating [2], [3]. In thee condition, component lifetime could be impacted, and could limit the application ue. In a lot of AC application, power component, uch a TRIAC, are ubjected to high temperature wing, due to harh operation condition, epecially in houehold appliance (vacuum cleaner, cooker, or coffee machine). The failure can mainly affect the immediate ilicon die environment (metallization, bonding), or the package (older joint). The aim of thi tudy i to etimate the lifetime of a new high-temperature 16A 600V TRIAC family ubjected to power cycling. In particular, a failure analyi of the power device, after the tet, will provide an overview on the failure mechanim. Finally, we will perform ome numerical imulation uing finite element (ANSYS ) in order to ae the thermomechanical tre ditribution through the TRIAC during power cycling tet. II. POWER CYCLING TEST SET-UP A. TRIAC application leading to high thermal tre The targeted application i an 1800W 230V 50Hz vacuum cleaner (cf. Figure 1), where the uction control i carried out thank to a univeral motor peed variation. The motor peed i et-up with a phae control circuit. The TRIAC i directly connected to a microcontroller unit (MCU) and doe not require any buffer circuit if everal output pin are ued in parallel (cf. Figure 1). 230V 50Hz 16A 600V TRIAC Univeral motor M I RMS = 8 A Vacuum cleaner: 1800W 230V 50Hz Figure 1. Vacuum cleaner baic circuit +V CC MCU For vacuum cleaner, the wort operating condition occur when the tube i blocked. Thi operation, which i called jammed nozzle operation, doe not lead to a higher current. Quite the revere, a there i no air-flow any more, the motor torque i lower. Therefore, the motor RMS current can decreae. The tre come in fact from the heat-ink thermal impedance increae a there i no more cooling air-flow. In jammed nozzle operation, a the air-flow i topped, the thermal reitance of the heatink increae greatly, thu leading to a high junction temperature (T j ), typically in the range of 180 C here (cf. Figure 2).

2 Junction temperature ( C) T ON T jmax = 180 C T OFF T j0 = 68 C Time () Figure 2. 16A 600V TRIAC junction temperature evolution during jammed nozzle operation Thi T j evolution i obtained by the following three main tep. Firt, we need a literal expreion of the device junction-to-ambient thermal impedance veru time (Z th(j-a) (t)). Thermal impedance give the watt denity capability of power device, i.e. the temperature drop acro the tack for each watt of heat flow. In our calculation, we have ued a mathematical expreion obtained by fitting the meaurement curve with a finite erie of exponential term a reported in equation (1). A t Z th( j a) th( j c) th( j a) τ 1 = 0.18, A= 0.32, τ2 = 44.54, B = 1.21 ( t) = R 1 e τ 1 + ( R R ) B t ( ) 1 e τ th j c 2 The econd tep conit in calculating the device power diipation (P di ), which depend on the ON-tate RMS current I RMS and the TRIAC tatic parameter (threhold voltage V t0 and dynamic reitance R d ) a reported in equation (2). Finally, the T j evolution i baed on the thermal Ohm law which i defined in equation (3). (1) Pdi = Vt0 I RMS + Rd I π RMS (2) I RMS = 8A, Vt0 = 0.85V, Rd = 25mΩ T j = Ta + Pdi Zth( j a) ( t) (3) - T a = 68 C: ambient temperature i.e. before blocking the tube of the vacuum cleaner. B. Equivalent reliability tet et-up Thi operation profile enabled u to define an equivalent functional reliability tet in order to etimate the TRIAC lifetime under thee harh operation condition. The tet wa performed on 30 non-inulated 16A 600V high temperature TRIAC and 30 one with inulated package. All TRIAC were ubjected to the ame current, which lead to a 155 C junction temperature wing ( T j ). Power cycling tet were performed by ubjecting the device to cyclic AC power injection with 9 power-on duration. The total RMS current i 16A. After reaching the defined heat-ink temperature, i.e. 156 C for the non-inulated TRIAC and 138 C for the inulated one, the load current wa turnedoff and forced air cooling wa turned-on for 111. The heat-ink temperature wa meaured with a thermocouple fixed on the heat-ink centre. The difference of the meaured heat-ink temperature value, between the noninulated package and the inulated one, i due to the junction-to-cae thermal reitance (R th(j-c) ) value which i more important for the inulated TRIAC than for the non-inulated one (2.1 C/W for the inulated TRIAC v. 1.2 C/W for the non-inulated package). The advantage of chooing the heat-ink temperature, a a control parameter, i the excluion of the cooling mechanim influence and at the ame time, the incluion of effect caued by change in thermal reitance [4]. Several thermal and electrical parameter were monitored during the aging tet. Particularly, thermal parameter, uch a the R th(j-c) parameter, wa meaured. Some electrical parameter, uch a the forward voltage drop (V TM ) and the leakage current (I DRM and I RRM ), were alo recorded. Among all the meaured parameter, only the R th(j-c) ha a ignificant evolution. The failure criterion, which decide a TRIAC reached it end-of-life, i a junction-to-cae thermal reitance increae of 20% with repect to the initial value [5]. Thi failure criterion i baed on the AEC-Q101 tandard, which pecifie that component fail in any cae of the following criteria: - Device exceed the allowable hift value within ± 20% of the initial reading with exception. - Device no longer meet the device pecification requirement. Figure 3 how a comparion of the normalized junction-to-cae thermal reitance (R th(j-c) ) for the noninulated 16A 600V TRIAC and for the inulated one. Thee variou repreentation how a linear evolution of the R th(j-c) parameter according to the number of power cycle, whatever the type of package. However, the noninulated TRIAC failed more quickly than the inulated one. Indeed, at power cycle, there i a 57% average R th(j-c) increae for the non-inulated power device, compared to their initial value. Concerning the inulated TRIAC, thi evolution only repreent 35%. Thu, at power cycle, the non-inulated power component which failed repreent 90% of the thirty device under tet. Converely, it only repreent 60% for the inulated TRIAC. N orm alized R th(j-c) Non-inulated package Inulated package 20% Number of power cycle Figure 3. Normalized R th(j-c) evolution during power cycling tet ( T j = 155 C) Comparion between non-inulated and inulated TRIAC

3 III. STATISTICAL ANALYSIS The TRIAC lifetime ha been fitted with a Lognormale ditribution. Thi law i commonly ued to repreent failure rate variation with time for unit whoe failure mode are linked to a fatigue mode [6]. Since thi include the mot of mechanical ytem, if not all, the Lognormale ditribution can have widepread application [7]. Therefore, it i a good companion to the Weibull ditribution when attempting to model thee type of unit. Figure 4 how the adjutment, realized with the Weibull++ oftware, for a power cycling tet. It i poible to extract the average (µ) and the tandard deviation (σ) from the Lognormale ditribution probability denity function, according to equation (4). f (t ') = 1 t ' µ 2 exp 2 σ σ 2π 1 (4) - t' = ln(t). - µ: Mean of the natural logarithm of the Time To Failure. - σ: Standard deviation of the natural logarithm of the Time To Failure. The µ and σ Lognormale parameter, fitting our tet reult, have been obtained uing the Maximum Likelihood Etimation method (MLE) [8]. Thi method work by developing a likelihood function baed on the available data and finding the value of the parameter to maximize the likelihood function. The confidence bound, whoe limit are repreented on both ide line, how that the Lognormale ditribution give a good approximation. The tandard deviation value (σ1 and σ2 parameter lower than 1) indicate the power component wear-out failure. The inulated device characteritic lifetime (µ 2 = 8270 power cycle), at which 50% of the unit failed, i 33% better than the non-inulated component (µ 1 = 5570 power cycle). Thi reult could be due to the le important heat gradient for the inulated package, becaue of a higher number of layer. Additional experimental power cycling tet hould be performed in order to undertand if the acceleration factor depend only on the junction temperature wing ( Tj), or if it i alo a function of the pecific related profile (impact of ramp time and dwell time). In the next part of thi paper, we have been only intereted in the non-inulated failure mechanim, becaue of the mot important number of the TRIAC failed. IV. FAILURE ANALYSIS Thee harh tet condition of power cycling have epecially affected the aembly level of the TRIAC rather than the ilicon die. The failed power device were inpected by Scanning Acoutic Microcopy (SAM) analye to determine the amount of delamination urface under the ilicon die. We have hown a delamination proce for the older joint die attach (Figure 5 (a)). Figure 5 (b) how a cro-ection for a non-inulated TRIAC, failed after power cycle at a 155 C Tj. A crack ha been oberved between the ilicon die and the clip. The Coefficient of Thermal Expanion (CTE) mimatch between the clip and the chip (for the copper layer α = 16.8 ppm/k v. α = 2.6 ppm/k for the ilicon) generate a ignificant thermo-mechanical tre during power cycling. Another interface, uch a the older joint between the frame and the die, can alo be damaged. However, it i more difficult to how crack becaue of the inpected urface width. SAM analyi of the older joint between the ilicon die and the clip for a noninulated TRIAC without power cycling SAM analyi of the older joint between the ilicon die and the clip for a noninulated TRIAC after power cycle at Tj = 155 C 99 Confidence bound (Non-inulated package) (a) 50 µ σ1 = 0,39 < 1 µ1 = 5770 cycle Scanning Electron Microcopy σ FIT non-inulated TRIAC FIT inulated TRIAC T1650H-6T (Non-inulated package) Rth(j-c) increae: 51 % power cycle σ σ2 = 0,34 < 1 µ 2 = 8270 cycle 10 Cro-ection Unreliability (%) Confidence bound (Inulated package) 5 Clip Die Crack Solder joint Die / Clip Solder joint Heat-ink / Die Heat-ink Number of power cycle Figure 4. Power cycling tet reult at a 155 C Tj - Comparion between non-inulated and inulated TRIAC (b) Figure 5. Solder joint degradation between the die and the clip for a non-inulated TRIAC failed after power cycle at a 155 C Tj: SAM analyi (a) and optical microcopy after a cro-ection (b)

4 V. THERMO-MECHANICAL SIMULATIONS Thermo-mechanical imulation are commonly ued to determine diplacement, tree and train of power device, and epecially, for component with variou layer [9]. In thi paper, the TRIAC failure mechanim have been tudied uing two-dimenional qualitative thermomechanical imulation, uing finite element (ANSYS oftware). A. TRIAC aembly A many power device, a TRIAC i declined in two verion: one inulated, the other not. Thi inulation i generally performed by inerting a ceramic between the ilicon die and the back-ide of the cae (if thi one i conductive). The ceramic fulfil two main tak: - Electrical iolation of TRIAC terminal and package tab (to avoid any electrical hock). - Heat tranfer toward the mounting plate. Figure 6 and Table 1 give an overview of typical material propertie ued for a TRIAC. B. Finite element model Due to the geometrical and loading ymmetrie, we have choen to model only half a TRIAC (non-inulated package), which allow computation time to be reduced. The choice of the thermo-mechanical model ha been baed on the following aumption: - The main heat tranfer i thermal conduction. Natural convective heat tranfer ha been taken into account by fixing the value of the overall heat tranfer coefficient h = 5W.m -2.K -1 (cf. Figure 7). - The temperature influence on the thermomechanical parameter ha not been taken into account. Two temperature have been fixed (cf. Figure 7) o a to reproduce the thermal tree oberved in jammed nozzle operation: - In the middle of the ilicon die: 453 K (180 C). - In the back of the frame: 433 K (160 C). Concerning the mechanical load, we have blocked the X-diplacement for the ymmetry axi. All the diplacement have been blocked at the origin of the plan (cf. Figure 7). h = 5 W.m -2.K -1 Y Mould (ECN) X Clip (Cu) h = 5 W.m -2.K -1 Mold (ECN) Clip (Cu) Silicon die Heat-ink (Cu) Y X Mold (ECN) Clip (Cu (Cu) a1) Silicon Puce (Si) die Lead frame (KFC) Céramique Ceramic (Al (Al 2 O 3 2 O 96 3 %) ) Heat-ink (Cu) Y X Die (Si) T j = 180 C T Heat-ink (Cu) cae = 155 C h = 5 W.m -2.K -1 Solder joint (92,5Pb-5Sn-2,5Ag) Blocking of diplacement 92,5Pb-5Sn-2,5Ag older joint 92,5Pb-5Sn-2,5Ag older joint Figure 6. Baic 2D cro-ectional view for a TRIAC aembly (not to cale) TABLE 1. TYPICAL MEASUREMENTS AND RELEVANT DATA OF TRIAC MATERIALS Material Thickne λ 300K α 300K [µm] [W.m -1.K -1 ] [10-6 /K] 1) Heat-ink ) Lead-frame Cu ) Clip ) Ceramic Al 2O 3 96% ) Solder joint 92.5Pb-5Sn-2.5Ag ) Silicon die Si ) Mould ECN Young modulu E [MPa] Poion ratio υ 1) Heat-ink ) Lead-frame ) Clip ) Ceramic ) Solder joint T ) Silicon die ) Mould Figure 7. Thermo-mechanical model and loading condition We have conidered a non-linear tranient mechanical analyi. The model ha been mehed with 2D 8-node element (PLANE82), uing linear elatic propertie for all the material, except the older layer which have been mehed with VISCO108 element for highly nonlinear behavior, uch a creep [10]. There i no contitutive model unanimouly ued to decribe older joint behavior. Among the variou contitutive law, the Anand model win unanimou upport. Thi contitutive model ha the advantage of taking into account time dependence, material deformation hitory, and material hardening. Moreover, thi model i temperature enitive and take into account train velocity [11]. The Anand model conit of two coupled differential equation which link inelatic train rate dε p dt and deformation reitance velocity. Equation (5) give the inelatic train rate expreion.

5 1 m dε p ξ σ Q = A inh exp (5) dt k T lead to high hear tree. Thee hear tree can mainly be explained by the CTE mimatch between the joined material. - A: Pre-exponential factor ( -1 ). - ξ: Multiplier of tre. - σ: Stre (Pa). - : Average iotropic reitance to macrocopic platic flow or deformation reitance (Pa). - m: Strain rate enitivity. - Q: Activation energy (J). - k: Ga contant ( J.K -1 ). - T: Abolute temperature (K). Clip Silicon die Max. Shearing Stre Solder joint die / clip Mould The equation for the internal variable i aumed to be of the imple form reported in equation (6). a = h0 1 ign 1 * n ε p Q * = exp A k T dε p, a > 1 * dt - h 0: Hardening/oftening contant (Pa). - a: Strain rate enitivity of hardening/oftening. From the above vico-platic model, there are nine material parameter: A, Q, ξ, m, h 0,, n, a, and 0. The lat one i the initial value of the deformation reitance, which i needed to determine the evolution of deformation reitance in equation (5). In thi paper, the TRIAC older joint (92.5Pb-5Sn- 2.5Ag) have been modeled uing the Anand formulation, which parameter are given in Table 2 [12]. TABLE PB-5SN-2.5AG ANAND PARAMETERS [12] Parameter Unit ANSYS deignation Parameter value 0 MPa C Q / k K C A -1 C ξ / C4 7 m / C h 0 MPa C MPa C n / C a / C9 1.3 C. Thermo-mechanical imulation reult Figure 8 how the hear tre ditribution through a non-inulated TRIAC. The maximum hearing i oberved on two older joint: the firt one, between the die and the clip, and the econd one, between the die and the frame. Thi reult confirm the analyi of experimental failed device (cf. Figure 5). On the one hand, the maximum hearing oberved i due to important tenion train in the older joint, on the other hand, the important dilatation train (6) - : Coefficient. - n: Strain rate enitivity for the aturation value of deformation reitance. Heat-ink VI. CONCLUSION The experimental reult how that in high temperature application, and epecially, in power cycling, the noninulated TRIAC fail more quickly than the inulated package. Particularly, power cycle tudie relate mainly to the TRIAC aembly level (older joint). However, additional meaurement will be performed in order to obtain an acceleration factor for thi failure mechanim. Particularly, the junction temperature variation ( T j ) and the dwell time impact will be analyzed. Further thermo-mechanical imulation will enable u to undertand the variou parameter influence previouly quoted, uing a more accurate model. For example, the aembly material (older joint) will be experimentally characterized and be taken into account in our imulation model. REFERENCES Max. Shearing Stre Solder joint heat-ink / die Figure 8. Shear tre ditribution for a non-inulated TRIAC [1] M. Held, P. Jacob, G. Nicoletti, P. Scacco, M.-H. Poech, Fat Power Cycling Tet for IGBT Module in Traction Application, Proc. IEEE Power Electronic and Drive Sytem Conf., Vol. 1, pp , [2] J.-M. Thebaud et al., Strategy for deigning accelerated aging tet to evaluate IGBT power module lifetime in real operation mode, IEEE tranaction on component and packaging technologie, vol. 26, n 2, pp , June [3] M. Bouarroudj, Z. Khatir, J.-P. Outen, L. Dupont, S. Lefebvre, F. Badel, Comparion of tre ditribution and failure mode during thermal cycling and power cycling on high power IGBT module, EPE 2007, Aalborg, Denmark, September [4] R. Amro, J. Lutz, Power Cycling with High Temperature Swing of Dicrete Component baed on Different Technologie, 35 th Annual IEEE Power Electronic Specialit Conference, Aachen, Germany, pp , [5] M. Held, P. Jacob, G. Nicoletti, P. Scacco, M.-H. Poech, Fat Power Cycling Tet for IGBT Module in Traction application, Proc. IEEE Power Electronic and Drive Sytem Conf., Singapore, Vol. 1, pp , [6] W. Kup, W.-T. K. Chien, T. Kim, Reliability, Yield, and Stre Burn-In: A Unified Approach for Microelectronic Sytem Manufacturing and Software Development, Kluwer Academic Publiher, January [7] W.B. Nelon, Accelerating Teting: Statitical Model, Tet Plan, and Data Analyi, John Wiley & Son, 1990.

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