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Reliable and Cost EffectiveSolution without BaseplatePower modules are used for a wide range of different applications. For some applications the powerdensity, the efficiency or the price is one of the key factors. But for all of them reliability is the major factor.This article describes the difference between modules with and without a baseplate and how reliability andthermal conductivity is affected. Patrick Baginski, Field Application Engineer, Vincotech,Unterhaching, Germany

Massive baseplates were used for powermodules in the past. These can be madeof Copper or Aluminum Silicon Carbide(AlSiC). Nowadays where costs play animportant role, also modules without anadditional baseplate can be found. Figure1 shows a flow 0 module without and aflow 2 module with baseplate. Here, forthe left picture, just the direct bondedcopper (DBC) is mounted with a thermalinterface material to a heatsink.

State of the artDBC substrates have been proven formany years in power electronicapplications. The advantages of DBCsubstrates are high thermal performance,Silicon matched coefficient of thermalexpansion (CTE), high current capability,high voltage isolation and low capacitance

between front and backside.Aluminum Oxide (Al2O3) DBCs are often

soldered to an additional baseplate whenusing more than one DBC. For somebigger modules with a rectifier, brake andan inverter part the first two mentionedparts are soldered to one substrate andthe inverter IGBTs and freewheeling diodesto another substrate.AlSiC baseplates are often used for

tractions applications where plates madeout of copper usually find place into powermodules for all other applications. Thedifferences of both materials that belong tomodules are the physics. Especially the CTEand the thermal resistance are of interest.

Thermal conductivityThermal conductivity is the property of amaterial describing its ability to conduct

heat. It is measured in watts per meter-kelvin [W/mK]. A high thermal conductivityis necessary to keep the averagetemperature of the die low. Also thetemperature ripple is influenced by theconductivity of the material. Figure 2shows a cross section of DBC andbaseplate modules without housing andalso without soft gel.These materials have different thermal

resistances. Therefore the temperature dropfrom the die to the case is not linear.Conductivities are between approximate 20W/mK and 400 W/mK as shown in Table 1.Clearly, DBC modules with an AlN

ceramic conduct heat much moreeffectively, so the heat transfer from thetop to the bottom is far better. What’smore, a comparison of modules showsthat the copper baseplate’s thermalresistance is lower than that of AlSiCbaseplates. It follows that a material withhigher thermal conductivity helps decreasethe die’s junction temperature.

Coefficient of thermal expansionThe section above described the thermalconductivities of materials commonly usedin power modules. The CTE is also criticalto a power module’s reliability. Thermalexpansion is the tendency of a material’svolume to change in response totemperature change. The CTE for allmaterials described below is measured in[10-6/K]. Copper, for example, has a higherCTE than Silicon. Given the sametemperature increase for both materials,copper expands about six times more thansilicon. Table 2 shows key materials usedin power modules.Values of soft gel, housing and also

bond wires are not discussed here. Itshould be mentioned only that also theCTE of the bond wires makes acontribution to the life time of modules.The CTE of Al2O3 for example is closer to

the Silicon compared to copper. Althoughthe CTE of AlN is closer to that of Si, whichreduces stresses in die attach materials,

Figure 1: DBC based flow 0 power module (left) and flow 2 with copper baseplate

Figure 2: Cross section of DBC and baseplate module

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the stresses are actually higher in the jointbetween the copper baseplate and DBC.This is due to the increased CTE differencebetween the net CTE of the AlN DBC andCu baseplate, which leads to a largerbending of the power module.Typical substrate and baseplate

material’s CTE may be several times that ofSilicon and other semiconductors. Thisdifference causes thermal stresses in thedevices, solder interconnections, andsubstrates because the mismatches arefrozen during the assembly process of themodule at high temperatures, especiallyduring the soldering process.These stresses can cause mechanical

and fatigue failure, or changes in operatingbehavior. Mismatched CTE is not the onlysource of thermal stress in the device. Theshear strength and stiffness of the jointmaterial and the joint area are also factors.In IGBT power modules, the joint betweenthe DBC and baseplate is much larger thanthe joint between the semiconductor andDBC, so it is more prone to failure broughton by thermal cycling. Consequently, theDBC may delaminate from the baseplate,which can cause thermal resistance toincrease, temperature to rise, and crackpropagation. Finally, a module’s compromised ability

to remove heat may culminate in thermalrunaway. Residual thermal stresses in theDBC-baseplate stack can also cause a

bimetallic effect that bows the module.The deformation is concave because thecopper baseplate’s CTE is greater than thatof the DBC. This creates a gap betweenthe module and the heatsink, whichincreases this interface’s thermal resistanceeven after thermal grease is applied. Thebimetallic effect is proportional totemperature. The grease may be squeezedout if a thermal compound is applied andthe module is mounted to a heatsink. Thisis known as the pump-out effect. Pre-bent,convex baseplates such as those used inflow 2 modules can compensate for thispackaging-induced phenomenon.

Wear-out failuresDifferent wear-out failures are observable.But only the failures due to CTE or in otherwords mismatches of the stack are takeninto account. This means that this failurecan occur between every material withdifferent temperature expansioncoefficients. Delamination starts from the edges of

the solder joint and works to the middle ofit (Figure 3). This is because of theabsolute movement of the materials. Smallchips and small solder joints do not havethat high delamination compared to bigsolder joints. A good solution could be toassemble two small semiconductorsinstead of one big. This needs a bit morespace but reduces the wear-out failure as

well as the thermal resistance. The samemethod can be considered when it isnecessary to solder DBCs to baseplates.Also here smaller DBCs helps to decreasethe delamination of solder layers. Bothfailures cause an increase of junctiontemperature and reduce the lifetime of themodule.Power modules’ reliability also depends

on the load profile. An uninterruptiblepower supply (UPS) furnishes a constantload so the average junction temperatureremains very stable. Also, a UPS works at50 or 60 Hz so the die’s rippletemperature remains low. All materials onlysee one cycle when the applicationpowers up. After a few minutes, the entireapplication will run at a constanttemperature. This is the best-case scenariofor all components. In other applicationssuch as welding, the power is switched onand off repeatedly. The devices generatelosses that heat up the system, whichcools down again while the power isswitched off. This exposes components tomany temperature cycles, which causessolder layers to delaminate.There are several ways to counteract this

effect. Smaller chips may be paralleled asdescribed above, or the module may beoversized so that it generates less loss andtherefore less heat. An efficient way ofsolving the problem is to use a modulewhose materials’ CTE are well matched.

Reliability of different conceptsAgain, reliability is a function of CTE valuesand the number of temperature cycles. Togain a better understanding of this weneed to look closer at thermal spreading asshown in Figure 4.The red lines represent thermal

spreading. It is obvious that the thermalspreading in each case depends on thenext layer below.Vincotech subjects each module to

battery of quality and reliability tests duringthe qualification process. Two differenttests assess the various materials’ thermalexpansion properties.One is the load or power cycling test. It

Table 1: Thermalresistance of modulematerials at 25°C

Table 2: CTE ofmodule materials at25°C

Figure 3: Delamination of solder layer

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places considerable stress on theconnection between the bond wire andsemiconductor, generating substantiallosses and a temperature drop from thesemiconductor to the module’s case. Theother test is the thermal shock test wherethe module is moved from a cold to a hotchamber for a certain time within atransition time of less than 30 seconds.Figure 5 shows reliability data

standardized to a DBC module with Al2O3

DBC. Test conditions for all three moduleswere equal. Also the chip size and number

of chips were the same. The temperaturedifference of each cycle was 100K, startingfrom 25°C to 125°C. The failure criteriawas an increase of Rth(j-c) of 20% due todelamination. This description of the CTE

phenomenon and how it relates toreliability would not be complete withoutmentioning heat-driven expansion. Thesolder joint must absorb the Silicon, DBC,and baseplate’s expansions without failing.The challenge is to design a solder jointthin enough to ensure a low drop in

temperature, yet thick enough to absorbthe movement of joint materials. Thetemperature drop from top to bottom alsohas to be taken into account. The highesttemperature is at the top where the dieresides and lowest at the bottom wherethe heatsink sits.

Influence of thermal spreadingAgain, each downward layer influencesthermal spreading. The Rth values stated inVincotech’s datasheets are measured witha water-cooled heatsink. It absorbs energyvery well, so very little thermal spreadingoccurs. Figure 6 illustrates thermalspreading in a water-cooled and in aconventional heatsink.The specifications for all modules are

given for a water-cooled heatsink. Thismeans the Rth values in the datasheetsare worst-case values. If an air-cooledheatsink is used instead, thermal spreadingis higher, which results in a better Rth(j-h)

value. The heatsink is not the onlycomponent to influence thermalspreading; the given thermal compound isalso a contributing factor.

ConclusionHaving examined different variants ofmodules, we can draw the followingconclusions: The longest component lifemay be achieved by keeping temperatureripple low. The load and environmentalconditions are key factors. The fewer thenumber and the lesser the extent ofthermal expansions, the greater thereliability. The CTE of materials shouldmatch. If thermal capacity is not an issue, amodule without a copper baseplate is theright choice.Modules with baseplates may be

necessary if brief spikes of high energy areexpected. Each downward layer of materiallayer influences thermal spreading. Rth

values stated in the datasheet aremeasured with a water-cooled heatsinkand indicate the worst-case scenarios.

Figure 4: Thermal spreading of DBC (left) and baseplate module

Figure 5: Reliability data standardized to a DBC module with Al2O3 DBC

Figure 6: Thermal spreading with low (left) and high conductivity heatsink (water cooled)

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