Datasheet

7
TANCAPTECHNOLOGYCO.,LTD.
Becausethecapacitancechangesrapidlyimmediatelyafterde‐aging,capacitancemeasurementsare
usually delayedfor at least 10 hoursafter the de‐aging process, which isoften referred to as “last
heat”.Inaddition,manufacturersutilizethe agingratestosetfactorytestlimitswhichwi llbringthe
capacitance within the specified tolerance at some future time, to allow for customer receipt and
use.Typically, thetestlimitsareadjustedso thatcapacitancewi ll bewithinthespecifiedtolerance
aftereither1,000hoursor100days,dependingonthemanufacturerandtheproducttype.
5. ThermalShock.
Multilayerceramiccapacitorsaresensitivetothermalshockduetodeviceconstructionconsistingof
interleaved layers of ceramic dielectric and metal electrodes with metal terminations for electrical
contact.
Thermal shock is mechanical damage caused by a structure’s inability to absorb mechanical stress
caused by excessive changes of temperature in a shot period of time. This stress is caused by
differencesinCTE(coefficientofthermalexpansion),δT(thermalconductivity)andtherateofchange
oftemperature.CTEandδTareafunctionofthematerialsusedinthecomponent’smanufactureand
therateofchangeoftemperatureisdependentonthesolderingprocess.
When the temperature rate of change is too great, thermal shock cracks occur. These cracks are
initiatedwherethestructureisweakestandmechanicalstressis concentrated.Thisisatornearthe
ceramic / termination interface in the middle of the exposed termination. Mechanical stress is
greatest at the corners where the chip is strongest but cracks tend to start where the  structure is
weakest. Whentemperature ratesof change areexcessive,as inuncontrolledwave soldering,large
visible cracksareformed.
Thermalshockhastwomanifestationsobviousvisiblecracksandthemoreinsidious,invisiblemicro
crack.Thesameforcesareinvolvedbutonasmallermagnitudesosmallercracksareformed.Again
itstartsinthemiddleoftheexposedsurfaceatorjustundertheceramic/terminationinterfaceand
propagates slowly with temperature changes or assembly flexure during handling. In a matter of
weeks a micro crack can propagate through the ceramic causing open, intermittent or excessive
leakagecurrents,atimebombduetoprocessing.
Thermalshockcracksarealwayscausedbyimpropersolderprocessingorclearing.Wavesolderingis
thebiggest culprit  becauseit hasthehighestheattransferrate (usingliquid metal)and thelargest
temperature
changeswithcausebothvisibleandmicrocracks.Vaporphasesolderinghasthesecond
highest heat transferrate andtemperaturechangesthat can inducemicrocrackswhen inadequate
preheatisused.Infrared(IR)reflowsolderinghasthelowestheattransferratesandthermalshockis
unheardofforthissolderingtechnique.
Assemblycleaningcannotbeignoredbecausethermalshock
canoccurduringheatingorcooling.Anassemblyshouldbeallowedtocooltolessthan60℃before
itissubjectedtothecleaningprocess.
It is important to avoid the possibility of thermal shock during soldering and carefully controlled
preheatisthereforerequired.Therateofpreheatshouldnotexceed4℃/secondandatargetfigure
2℃/second is recommended. An 80℃ to 120℃ temperature differential between the component
surfaceandthesolderingtemperatureispreferred.
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