1 foreword

 

Twinkling changes voltage restrains implement (TVS: Transient Voltage Suppressor) it is to solve twinkling of electronic equipment voltage to change to defend with surge problem and a kind of when the design goes out high-powered electron circuit protects parts of an apparatus, twinkling changes voltage restrains diode, basically use at undertaking to circuit transient state is protected. When the high energy that provides two end to endure the moment when TVS is pounded, it can turn into the impedance between two end with extremely high rate low impedance, absorb a big electric current, on the numerical value that books its voltage clamp down on between two end in thereby, protect the circuit original from the back to because transient state is tall,voltage is pounded and do not damage. Product of domestic explicit electron and country are high the application in reliable equipment is very general. As TVS scope of application and the addition of use amount, the dependability of TVS oneself gets attention fully, because TVS dependability is the problem of TVS itself not only, still matter to by the utilization reliability of protective electron circuit. The dependability that studies TVS needs the knowledge that has development to the failure mode of TVS and failure mechanism. Document shows the failure mode of TVS has short circuit, open a way and electric character to degrade. Among them, short circuit be no longer in force is most familiar, and most serious to the influence of circuit. Current, home lacks depth to the research of failure mechanism of homebred TVS short circuit, insufficient system, accordingly, the research that has thorough, system to failure mechanism of homebred TVS short circuit is very necessary.

 

2 Sample of invalidation of TVS short circuit and invalidation analyse a process

   

In home the support of main TVS manufacturer falls, collect the sample of invalidation of the short circuit in be chosen about homebred TVS and be being used and choose the failure data such as stress condition or use condition, undertake to these sample electric parameter checks, cap, go protecting glue, canal core and electrode depart, go solder and microscopical the measure such as observation, find out invalidation place, the immanent quality factor that the analysis causes invalidation of TVS short circuit or use element, and the happening of invalidation process. Among them immanent quality factor is designed with parts of an apparatus namely, material, craft, assemble, enclose the relevant element that causes invalidation of parts of an apparatus; Use element removes the causes invalidation of parts of an apparatus factor beyond immanent quality factor namely, use with gravitation passing report, electrostatic discharge, overload, mistake normally etc relevant.

 

3 Cause the immanent quality factor of invalidation of TVS short circuit and failure mechanism

 

TVS parts of an apparatus basically is mixed by system of chip, electrode 3 parts make envelope. Among them chip is core, use diffusion technique to form on single crystal silicon chip normally. Become the chip that has diffused nickel of plating of mesa shape, chip through corroding, burn-in, Tu Bao protects glue, those who seal the working procedure such as tin-plating of the down-lead on cap, spot welding, terminal to finish TVS parts of an apparatus make. If TVS workmanship process is medium control is undesirable, cause the inherent defect of TVS parts of an apparatus possibly, make TVS n and dependability are reduced, easy in choose or be used invalidation. TVS chooses analysis of sample of short circuit invalidation and statistic to make clear, TVS causes the immanent quality factor that chooses short circuit to have a lot of, the scale of each element is shown 1 times like the graph. These elements also are the factor of main and immanent quality that invalidation of TVS short circuit causes in using.

 

Graph 1- causes TVS to choose the immanent quality factor of short circuit invalidation to distributing

 

Interface of 3.1 chip felt is empty

 

The most typical cause that causes TVS short circuit is a canal agglomeration of sheet copper of core and lead component, base is undesirable, in agglomeration interface occurrence bedding face is accumulated empty, if pursue,2 are shown.

 

 

Because solder is inhomogenous,empty likelihood is or each material profanes felt interface, oxidation makes solder touchs embellish undesirable, cause the solder when burn-inning and chip or metal electrode to solder without good fusion those who cause. When empty area is larger, electric current gathers together around weld point, tubal core comes loose hot difficulty, cause pyroelectricity stress concentration, produce local heat, serious when cause heat to run quickly, make burn down of parts of an apparatus. Undertake dissection an analysis to the parts of an apparatus of these burn down, can see have chip local deeper melt; When empty area is lesser, can quicken solder to heat up exhaustion, make solder layer can produce fatigue chap, cause thermal resistance of parts of an apparatus to increase, bring about burn down of overheat of parts of an apparatus finally.

 

3.2 mesa blemish

   

The be no longer in force that TVS mesa blemish creates often of batch sex. The cause that chip mesa loss causes in TVS workmanship process basically has two:

 

1) chip shapes in acid etching when, because recipe of mixed liquor of hydrofluoric acid, nitric acid passes thick or temperature is exorbitant and reaction is severe;

 

After burn-inning, undertake alkaline corrode when cleaning, corrode fluid chroma too big, temperature is exorbitant and cause alkaline corrode clean overweight. Observe below microscope alkaline corrode clean overweight mesa, can see mesa has similar the trace that is eroded, if the graph is shown 3 times, because silicon chip is in,this is (1, 0, 0) brilliant to by alkaline corrode the rate that cleans reagent to corrode the fastest. 0) brilliant to by alkaline corrode the rate that cleans reagent to corrode the fastest..

 

 

3- is alkaline corrode clean overweight mesa

   

The loop of temperature of course of TVS parts of an apparatus of mesa blemish or injury and Qian after the screening test such as concussion, when undertaking electric parameter checks, normally expression is short circuit or puncture character is unusual, thereby by eliminate. But the TVS parts of an apparatus that slight mesa injures is discovered not easily when parameter of the report after choosing checks, the likelihood is labelled fine is tasted leave factory. These TVS parts of an apparatus are endured in use process long after the action of the stress such as hot, report, machinery, mesa blemish aggravate, form in blemish place carry stream child produce compound center, make retrorse leakage current raises the appearance greatly. Big surface is retrorse leakage current makes Pn writtens guarantee brim temperature lifts, generation pyroelectricity is integrated effect, bring about Pn finally to written guarantee temperature of brim semiconductor material crosses tall burn down.

 

3.3 surfaces are strong accumulate a layer or turn over by force model layer

 

Mesa of chip of parts of an apparatus of even if TVS is in good condition, invalidation of TVS short circuit is incidental also in the surface. As a result of,this is of crystal structure periodic in apparently interrupt, add semiconductor surface to often be put in a lot of grind segment, polish, gush the arenaceous, section crystal lattice defect that cause, the chemical that when adsorption is corroded, remains, gas or other contaminant, can make semiconductor surface electrified. Exterior charge is protected glue passivation, the freedom inside body carries adsorptive or repellent semiconductor stream child, in Pn knot brim forms a watch to the face accumulates layer, depletion layer or be turned over model the layer of exterior space charge such as the layer. Below the action of extra voltage, accumulate a layer or turn over by force by force model the layer makes Pn writtens guarantee intensity of brim electric field is more than body inside, if pursue,4 are shown. Accordingly, critical electric field is achieved below the voltage that Pn writtens guarantee brim part is comparing depress of rated puncture report and happen carry stream child times synergism should, cause Pn to written guarantee brim electric current is centered, power density is too big, temperature is exorbitant and burn down.

 

Graph 4- expresses area tired layer to be opposite below action of adscititious and retrorse voltage the action of extinct area inside body

 

3.4 chip crackle

 

Chip crackle is the factor of another important and immanent quality that causes invalidation of TVS short circuit. It may be by grind segment, polish, gush the remain after the rudimental stress such as arenaceous, section and agglomeration is out of shape wait for an element to cause, because glue and electrode system are protected to heat does not match stress and be caused to chip when temperature change,also may be. Subtle crackle circulates in high low temperature, pulse is pounded or the meeting below mechanical and oscillatory action increases. If crackle is in Pn knot place, cause TVS parts of an apparatus retrorse leakage current increases; If crackle is near Pn knot, will bring about crackle place to carry stream child compound, carry near crackle stream child amount decreases, addle of Pn knot character. These two kinds of circumstances can make TVS parts of an apparatus retrorse power load ability drops.

 

3.5 impurity diffuse inhomogenous

 

The chip of TVS is the P that leads in certain resistance normally model or those who form undertake boron diffuses after advanced travel phosphor diffuses on N silicon chip. If diffusion technique process is medium axial of rate of resistor of occurrence silicon chip or radial are inhomogenous, foreign matter chroma is inhomogenous, the blemish inside body (wrong, layer fault, tiny flaw) or Pn knot surface is rough whole wait for a circumstance, will cause silicon chip adulteration is inhomogenous, make silicon chip everywhere puncture voltage is different, make core electric current is in charge of to distributing when puncture of parts of an apparatus thereby inhomogenous, for many times the bureau after surge is pounded

Ministry burn down.

 

The 4 use elements that cause invalidation of TVS short circuit and failure mechanism

 

4.1 stress passing report

 

Exceed TVS to be able to bear when energy of transient state pulse the meeting when energy causes TVS parts of an apparatus to pass electric stress loss, achieve TVS to be able to bear when energy of transient state pulse especially of energy multiple when can bring about TVS parts of an apparatus to cross electric stress burn down directly, failure mode expression is short circuit. The TVS chip of invalidation of short circuit of stress passing report issues observation in scanning report lens, of the silicon chip inside the melt extent that can discover brim of Pn knot surface or body go up the surface and the shading of next surfaces.

 

The experiment makes clear, happening in knot surface brim to cross invalidation of electric stress short circuit is normally by duration extremely short (Ns class) be caused by of pulse of high energy transient state, for example: The pulse that EMP, ESD produces; Electric stress invalidation passing inside body is normally grow a bit by duration (μs class above) be caused by of high energy pulse, for example: Report is fast twinkling changes, the pulse that thunder produces. If duration of pulse of high energy transient state uprights,mix at Ns class between μs class, criterion short circuit may happen in knot brim surface, happen possibly also inside body. The melt temperature that this one result can conduct rate, silicon and electrode metal through heat gets explanatory. If the graph is shown 5 times, pn knot brim conducts heat to what heat sinks grow inside method comparing body, conduct heat relatively inside body slow, because this writtens guarantee,brim temperature compares style inside tall. Duration is extremely short (Ns class) pulse of high energy transient state makes brim temperature lifts quickly, cause brim thermal breakdown and burn down. And duration is longer (μs above) pulse makes brim quantity of heat has enough time to be passed to chip center all round. As chip temperature elevatory, melt passage produces all round chip center. When melt the one surface from chip is outspread arrive when another surface, silicon chip temperature exceeds 1400 ℃ , be in melt and blame brilliant condition, become conductor, form electric access, make chip short circuit. If pulse duration reachs Ms level, for example, the pulse that thunder produces, still can make plumbic stannum solder achieves above of 700 ℃ and produce melt.

 

Surface of graph 5-TVS chip conducts heat method

 

4.2 high temperature

 

When TVS parts of an apparatus working temperature exceeds his the biggest when allowing working temperature, short-circuit easily invalidation and happen in Pn knot surface normally. Because,this is, in high temperature condition the job falls, the amount with movable and ionic surface increases greatly, exterior electric current also increases subsequently, exterior power density and temperature compare style inside tall, make Pn writtens guarantee the brim writtens guarantee Wen Chao crosses 200 ℃ , local district crystal lattice suffers the brim the attaint of mortality. TVS slants instead in high temperature count of case of invalidation of the short circuit in choosing makes clear: Tall puncture voltage (150 V above) TVS parts of an apparatus is more incidental short circuit invalidation. Because be in the TVS series of same rated power,this is, in bear when same power, temperature rise of chip of TVS of tall puncture voltage is faster.

 

4.3 work for long consume

 

Have surge life test to choosing the TVS parts of an apparatus of qualification, discover TVS parts of an apparatus is passed by tens of thousands pulse of second standard index (can frequency of susceptive surge pulse and quality grade are relevant) the invalidation after concussion, failure mode is short circuit normally. After undertaking dissection to invalidation sample, chip observes below scanning report lens, discovery writtens guarantee phenomenon of brim happening melt and structure of knot brim solder produced change, and knot most brim place is most serious. Failure mechanism is likely knot brim solder forms metallic compound and fragile change, make be in charge of core and base heat to sink to depart gradually, of knot brim medicinal powder heat energy force is reduced, work for long the knot is lukewarm increase continuously bring about overheat burn down.

 

5 last words

 

The mechanism that immanent quality factor causes invalidation of TVS short circuit basically is TVS workmanship process the chip defect that cause or injury make TVS is in bear local electric current centers the chip when pulse is pounded, cause chip local overheat and burn down. The use element that causes TVS short circuit basically has had report to use waste for long stress, high blandly. Use actually in TVS in, invalidation of TVS short circuit may be the result that collaboration of all sorts of element put together use.

     

Want to reduce be no longer in force of TVS short circuit, should strengthen the control of TVS workmanship process above all, be opposite especially burn-in, mesa shapes, alkaline corrode clean, the control of the process such as adulteration, with decrease or eliminating the inherent flaw of TVS. For example: Use on international burn solder craft advancedly to already can be empty area control under 10 % , use ionic infuse adulteration to be able to have better control to impure process, these raised the dependability of TVS greatly. Next, accomplish the correct type selecting of TVS and installation, had better undertake to TVS the forehead is used falling, can make TVS susceptive power lesser so, utilization reliability increases greatly. In addition, to make TVS short-circuits when invalidation arrive to be being fallen by the influence of protective electron equipment lowest, can string together the fuse that receives to match to it before TVS normally.