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Transient induced failure of miniature resistors the substrate, and can cause local hotspots that reach the melting-point temperature of the track material. Predicting when and where such hotspots will occur require a resistor model that is more sophisticated than the traditional ‘ideal resistive layer’ model that has normally been used. For example, the equivalent Although a great deal of attention is given to protecting ICs from damage due to electrostatic discharge or current surges, many design engineers fail to realize that it is also possible for resistors to be damaged by such transients. Resistors are frequently concentrated in areas most likely to experience transients, such as those adja- For very short pulse durations in Region 1, which are typically the result of transients such as ESD or lightning strikes, all the heat remains in the resistive layer. There is no time for it to penetrate either the ceramic former or the outer protective lacquer. For such transients in cylindrical resistors, P peak is proportional to d/ti where d is the track layer thickness and ti the 10 pulse duration. 10 For transients last10 ing between 1µs and 10ms (Region 2), the 10 heat generated at a 10 Region I Region II Region III Region IV hotspot has time to pen10 etrate the cylindrical 10 substrate but not time enough to travel along 10 it. In this situation, 1 which is typical of dis10 10 10 10 10 10 10 10 10 10 10 1 10 10 10 charge pulses, P peak is proportional to 1/√ ti. Time (s) For longer pulses of Figure 1: Compared to traditional models, sophisticated models identify various types of hotspots between 10ms and 1s depending on the pulse duration. (Region 3), the heat has time to travel both cent to power supplies. The circuit model must take into into and along the substrate, small thermal mass of surface- account intrinsic inhomogene- and Ppeak becomes proportional mount miniature resistors often ity in the resistive layer and the to 1/ti. It is important to know the makes them unable to absorb f ield strength at the laser high-energy pulses without sus- trimmed end-points of the re- type of transients that a resistor might experience in order to taining damage. Even if the sistive material. transients do not cause catastrophic failure, they may result in an unacceptable shift in the V = 0V resistor’s ohmic value. Virtually all of today’s metalfilm cylindrical resistors, including those from BC Components, are fabricated using high aluminum content ceramic substrates that have high thermal conductivities. Because this allows heat to be transferred easily to the resistor’s end caps and out onto the PCB, these resistors tend to have less pronounced hotspots and a more uniform temperature disHelixing Resistive layer tribution than many older designs. For a given hotspot temV = 100V perature, the overall power rating of the resistor can therefore be increased. As a result, it is Figure 2: Hotspot generation through current crowding at resistor failure points. now possible to produce very This more sophisticated choose an appropriate power small resistors with fine-line resistance tracks that have con- model allows various types rating. Calculations based tinuous power ratings higher of hotspot phenomena to be solely on normal dc power ratidentified depending on the ings will often lead to compothan expected. The heat generated by very pulse duration and can be con- nent failure. In addition to inhomogeneshort transients, however, has veniently divided into four ities in a thin-film resistor’s heno time to dissipate itself into different regions. 8 7 6 Power (W) 5 4 3 2 1 10 9 8 7 6 5 4 3 2 1 2 3 lical tracking, there are also potential failure points where the helical track meets the end rings. At these points, current crowding can result in an uneven current density that leads to a hotspot. This same effect can be observed in thin-film chip resistors that utilize a meander pattern to increase track length, except that in these resistors, hotspots due to current crowding can occur at any point where the meander pattern changes direction. However, with careful track design, such as that used in BC’s MCA 1206 chip resistors, the curve radius of the meander pattern can be chosen to reduce or eliminate these current crowding effects. Nevertheless, different manufacturers still use different meander patterns, which means that equivalent resistors (in terms of their dc characteristics) may have different survival rates when subjected to similar voltage transients. Thick-film resistor performance The pulse performance of thickfilm resistors is considerably more difficult to predict than it is for thin-film types, due to the inherent inhomogeneity of current flow in thick-film materials and the less predictable nature of the manufacturing processes involved. Published curves for these resistors are therefore normally determined from direct measurement. As with conventional resistors, heat energy concentrates at the end of the laser groove, and the type of laser cut influences this behavior. For example, an L-cut gives a lower heat flux at these points than a single-cut. The effect of laser trimming is significant, with the result that trimmed resistors may have only one third or a quarter of the pulse load capabilities of untrimmed resistors. In general, thick-film resistors also have lower pulse-load ratings than cylindrical resistors. By Joachim Von der Ohe Applications Marketing Manager BC Components E-mail: joachim.von.der.ohe@ bccomponents.com