In-situ Characterization of Moisture Absorption and Desorptionin a Thin BT Core Substrate

Yi He and Xuejun FanIntel Corporation,

Assembly Test & Technology Development5000 W. Chandler Blvd., Chandler, AZ 85226, U.S.A.

yi.he(intel.com, 1-480-552-3154; xuejun.fan(intel.com, 1-480-554-1308

Abstract

Bismaleimide-triazine (BT) resin/glass fiber laminates arecommonly used as a substrate core material in microelectronicpackaging. Their moisture absorption and diffusion behaviorhave a significant impact on package reliability. Thetraditional method to determine moisture absorption relies ona weight gain measurement metrology with an analyticalbalance. This approach is generally not suitable for thin films.In this study, the moisture absorption-desorption behavior ofa thin BT core was characterized in-situ using a sorption TGAover a temperature range of 30 to 80°C, in an environment ofup to 80% relative humidity. From the experimental results,the moisture diffusivity and the saturated moisture contenthave been determined. Within the experimental temperaturerange, the diffusivity can be described by the Arrheniusequation and the activation energy can be computed. Theobtained results are compared with literature data. The impactof moisture diffusion in BT core on the reliability of ultra-thinstacked chip scale packages (UT/SCSP) will be discussed.

1. IntroductionIn the semiconductor industry, it has been long recognized

that moisture plays a key role in the reliability ofmicroelectronic packaging. Moisture absorbed by polymer-based packaging materials can cause substantial changes inmaterial properties, such as coefficient of thermal expansion(CTE), modulus, glass transition temperature (Tg), andviscoelastic behavior. In cured thermosets, moisture acts as aplasticizer, reducing the modulus and Tg, and changing thethermal expansion characteristics of the material [1-3]. Forexample, upon moisture saturation at 85°C/85% relativehumidity (RH), the Tg of a cured no-flow underfill decreasedby as much as 25°C, whereas its room temperature modulusdecreased by approximately 8% [4]. At elevated temperatures,moisture-induced hygroscopic swelling in packagingmaterials can cause a large increase in package stresses. Forexample, in some microelectronic packages encapsulatedusing commercially available molding compounds, the straininduced by the mismatch of the coefficients of hygroscopicswelling (CHS) is nearly twice as much as the strain inducedby the CTE mismatch over a temperature span of AT = 60 °C[5]. For other molding compounds, the hygroscopic mismatchinduced strain ranges from one to nearly four times of thestrain induced by the CTE mismatch over a AT of 45°C for T> Tg or over a AT= 100°C for T < Tg [6]. For some underfillmaterials, the hygroscopic swelling induced strain iscomparable to the thermal strain caused by thermal expansionover a temperature range of 100°C [7]. At solder reflow

temperatures, which are typically 230 - 260°C, vaporizationof residual moisture leads to a sharp buildup in vaporpressure, causing voiding, cracking, interfacial delamination,or "popcorn" failures in packages [8,9]. In addition, moisturecan cause degradation of adhesion strength, which leads to areduction of interfacial strength. For example, the adhesionstrength between the solder ball and some underfill materialscan decrease by more than 70% upon long time exposure at85°C/85% RH [10]. Many failures in microelectronicpackages can be traced back to moisture [11]. Therefore,characterization of moisture absorption-desorption anddiffusion in electronic packaging materials is essential forunderstanding moisture-induced failure mechanisms and formodeling reliability performance of the package. Once thathas been achieved, one can optimize the package, material,and process design to minimize or eliminate moisture-relatedfailure.

Bismaleimide-triazine, or BT, is a general term for thethermosetting resin obtained from additional polymerizationof two monomers, bismaleimide and triazine (a cyanate ester).The blending of BT and epoxy resin offers betterthermomechanical and electrical performance over standardepoxy systems [12]. The Tg of the BT resin is typically above185°C. In comparison, another popular resin used in substratelaminates, the standard FR-4 resin, has a Tg around 125-1350C, although high-temperature FR-4 materials with Tg-1800C are also available [13]. Thus, BT epoxy resin/glassfiber laminates are commonly used as a substrate corematerial in microelectronic packaging. Consequently, theirmoisture properties have a significant impact on packagereliability.

Recently, the development of ultra-thin stacked chip scalepackaging (UT/SCSP) technology has become essential toincreasing functionality and higher memory capacity withmore complex and efficient memory architectures in small-form factor packages. In these packages, the wafer must bethinned from the original 750 ptm down to as low as 50 ptm.The conventional die attach (DA) paste material and theassembly method cannot be applied to handle such thin dies.Instead, wafer-level thin adhesive films combined with thecorresponding lamination technique provides an alterativesolution. These die attach films (including wafer-level andpick-and-place) are usually very soft, with a tensile modulusless than 10 or even 1 MPa at solder reflow temperature.These small form-factor packages are quite sensitive tomoisture, and a new failure mode after preconditioning testhas been detected, i.e., cohesive failure within the DAmaterial located between the substrate and the die [14, 15],where the moisture is absorbed through the substrate, which

1375 2007 Electronic Components and Technology Conference1-4244-0985-3/07/$25.00 02007 IEEE

consists mainly of thin BT core and copper layers. Detailedfundamental studies and analysis of previous DOE datarevealed that the moisture properties of the thin BT core,including its temperature-dependent moisture diffusivity andsaturated moisture content, play a crucial role in modulatingthe observed failure in UT/SCSP [14]. Characterization ofsuch properties is the main objective of this study.

Traditionally, the characterization of moisture propertiesof a material involves exposing the sample under a specifictemperature/humidity condition, then monitoring the sample'sweight increase as a function of the exposure time [16]. Thisprocedure requires removing the sample out of thetemperature/humidity chamber and then measuring the sampleweight intermittently with an analytical balance, until themoisture saturation is reached. For thick samples, this methodworks fine, but for thin specimens, it can have potentialproblems: the exposure of the sample at room temperature/humidity condition even for a short time can cause significantchanges in the moisture content of the sample. For example,for a 70 ptm thick film saturated with moisture and having atypical moisture diffusivity of l.x10-8 cm2/s, it is estimatedthat it only takes about 2.5 minutes for the sample to lose 50%of its initial moisture when it is placed in a dry environment[17]. For a 50 ptm thick film, that time reduces to less than 2minutes. Therefore, large error can be introduced duringmoisture absorption measurement using the traditionaltechnique. Another severe problem is that for a typicallaboratory balance, the sensitivity is only 0.01 mg, and inmany cases that is not enough to accurately determine themoisture uptake in a thin film. In addition, the measurementhas to be carried out manually. Based on these considerations,new techniques are needed for characterizing moistureproperties of thin films.

In this study, the moisture absorption-desorption behaviorof a 70 ptm thick BT core was characterized in-situ using aTA Instruments Q5000 SA Thermogravimetric Analyzer(TGA) at 30, 60, and 80°C, with the relative humidity cyclingbetween 0-60% or 0-80%, respectively. Based on theexperimental data, the diffusion constant and the saturatedmoisture density, Csat, were determined. We will demonstratethat when transitioning from a thin BT core to a thick sample,the glass fiber and its structure can have a large impact on themoisture diffusion behavior of the material. Based on theexperimental results, finite element modeling can be appliedto investigate the moisture distribution in UT/SCSP duringpreconditioning.

2. BackgroundMoisture diffusion in isotropic materials under constant

temperature and relative humidity conditions can generally bedescribed by Fick's second law [18]:

acD

a c a c a (1et= 0 C+ C+ i2Dt Oa2 ay 2 aZ2I

where C(x,y,z, t) is the moisture concentration inside thematerial and it has a unit of mg/cm3, D is the diffusionconstant, t is the time, (x,y,z) are the coordinates. BT/glassfiber laminates are clearly anisotropic, however, for thin

samples with large aspect ratios, one dimensional diffusionmodel is valid, and the diffusion equation can be simplified as

acat

a2cDax

(2)

For a thin plate sample with a thickness of h, one can setup the coordinate system so that the origin is in the center ofthe sample thickness, and the sample is bounded within -h2< x < hl2. The initial and boundary conditions are [3,18]:

C=Co, -hl2<x<hl2, t=0

C=Ci, x=-hl2, x=hl2, t>0 '9)

where C0 is the initial moisture concentration within thesample, Ci is the constant moisture concentration of theenvironment. For moisture absorption experiment, CO < Ci,and often C0 = 0; for a desorption experiment, CO > Ci, andoften Ci = 0. Under these initial and boundary conditions, thediffusion equation (2) can be solved using the standardmethod of variable separation, and the concentrationdistribution in the sample is given as [3,18]:

C(x, t) - Co-I=Ci -CO

4 E (_1)'nI O (2n+1) L

(2n + 1) 2i2TD1

h2

(2n + 1)iTxcos xh

(4)The change in sample weight due to moisture absorption

or desorption is given by integrating the moistureconcentration change at time t over the entire sample volume:

hl2Mt f (C -Co)dxfds,

-hl2 S(5)

where S is in-plane surface area of the sample, since diffusionfrom the sides is negligible. Substituting eq.(4) into eq.(5),and note that

I1 (2n + 1);T d 2h(- I)n(6h (2n + 1)i(

-hl2

We haveMt =(Ci -CO)Shx

!1 1exiI (2n+1)2/-T2D]2 E2 ep- 2 t/f n= 0(2n+1)2 L

h 2 j)(7)

In eq. (7), (Ci -CO)Sh= Mx is the ultimate change in

sample weight with t -* oc. For a moisture absorptionexperiment, M, is positive since Ci > CO; in a desorptionexperiment, M,. is negative since Ci < CO. Thus,

Mt =1-moo

8 x 1 F

nexpn/If n= 2 +1

(2n + 1)2f Dt]

1,t (8)

which is the familiar solution to ID diffusion equation [3,18].When analyzing the experimental data, D and M, areoptimized so that the difference between the experimentally

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determined M, and the one calculated using eq. (8) isminimized. When n reaches 50, eq. (8) converges very fast. Inthis study, n runs from 0 to 200. The saturated moistureconcentration in the material, Chat, is simply MJ/V, where V isthe sample volume. Alternatively,

Csat M p (9)where M is the initial sample weight and p is density of thesample. Here the change of sample volume caused bymoisture absorption-desorption can be ignored since it is quitesmall. Both Csa and D are critical in understanding moisture-related reliability issues, because Csat determines how muchmoisture can be absorbed by the material, and D determineshow fast the moisture can diffuse into and out of the sample.

The temperature dependence of the diffusion constant canbe described by the Arrhenius equation [19]:

D =Dexpy -Ed) (10)

where Do is a pre-factor, Ed is the activation energy,k= 1.38x10-23 J/K is the Boltzmann's constant, Tthe absolutetemperature. In some published papers, the minus sign insidethe exponential function was omitted and a negativeactivation energy was used.

3. Experimental3.1. Material

The material used in this study was a 70 ptm thickBT/glass fiber laminated substrate core material. The generalproperties of this material have been characterized by us andare listed in Table 1 [20]. The dynamic mechanical propertieswere determined by DMA experiments, which wereconducted under tensile mode with a dynamic frequency of 1Hz and a heating rate of 3°C/min. The fiber content wasdetermined by thermogravimetric analysis. The detaileddescription for these measurements will not be discussed here.

Table 1. General properties of the 70 ptm thick BT coresubstrate used in this study.

Density 1.79 g/cm3Fiber Content 56.46 wt.%CTE (0-50°C) 16x 10-6 1/°CCTE (150-2000C) 18.5x 10-6 1/°CStorage Modulus (@250C) 14.65 GPa

Loss Modulus Peak 2070CTan 6 peak 215.60C

3.2 InstrumentThe instrument used in this study, TA Instruments Q5000

SA TGA, is a high sensitivity thermogravimetric analyzerwhich enables sorption/desorption analysis of materials undercontrolled temperature and humidity conditions [21]. Figure 1is an illustration of the instrument. The heart of thisinstrument is a high performance thermo-balance, which ismaintained at a constant temperature of 40°C for increasedthermal stability. The balance has a signal resolution of 0.01

fig, and a sensitivity of 0.1 pIg. The humidity chamber is awell insulated tri-level aluminum block containing de-ionizedwater, which can be refilled as necessary. The humiditysurrounding the sample is controlled and maintained by a pairof mass flow controllers. By adjusting the amount of "dry"and "wet" gases flowing through the controller, the softwareis capable of maintaining a desired relative humidity level.The temperature control is done by four thermoelectricdevices in conjunction with a thermistor in a closed-loopsystem. The actual temperature/humidity condition around thesample can be verified using deliquescence point of certainsalts.

The typical dimensions of the sample used in sorption-TGA experiments are 7mmx7mm with a weight of about 7mg. In this study, moisture absorption-desorption experimentswere performed at 30, 60, and 80°C, respectively. At eachisothermal temperature, two relative humidity levels werechosen: 60% and 80% RH. During an isothermal experiment,the relative humidity was cycled between 0 and 60% (or 0 and80%) twice, while the temperature stability is maintained tobe better than ±0.050C. At each temperature/moistureconditions, the sample was held for up to 600 min. In suchexperiments, quartz bowls were used as the sample pan andthe reference pan, and the sample was placed inside or on topof the sample bowl, allowing the moisture to diffuse into thesample from both surfaces. Since both the sample and thereference bowls experience the same temperature andhumidity conditions, the net effect on the sample weightchange is from moisture absorption or desorption.

3alance system

Reference pan Sample pan Moisture chamber

Figure 1. Illustration of a TA Instruments Q5000 SA TGAsystem (provided by TA Instruments. Used with permission.).

4. Moisture Absorption-Desorption4.1 Moisture Diffusivity

Figure 2 shows an example of moisture absorption-desorption experiment, where the temperature was kept at600C and the RH level was cycled between 0 and 60%.During the first 60 minutes, a 0% RH condition was imposedto drive any residual moisture out of the sample. This wasfollowed by a 200 min of moisture absorption at 60% RH,then a 200 min desorption at 0% RH. This sorption-

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desorption cycle was repeated one more time, as shown inFia ?

Figure 2. Moisture absorption-desorption experimentconducted using a sorption TGA at 60°C with the RH levelcycled between 0 and 60%. The initial BT core sample weightwas 6560.455 ptg.

Based on the results from Fig. 2, several points are clear:(1) during the first 60 min, the moisture was not completelydriven out of the sample, a longer time was needed toaccomplish that; (2) the subsequent absorption-desorptioncycles were repeatable, i.e. the sample reached approximatelythe same saturated moisture level during sorption and it lostthe same weight upon drying. This indicates that there is nochemical reaction between the water molecules and thematerial; (3) the saturated moisture level is about 0.33%, thus,the saturated moisture content, Csat, is about 5.91 mg/cm3.

The moisture diffusivity for the 70 ptm thick BT core at60°C / 60% RH can be calculated from the second moistureabsorption curve shown in Fig. 2. To determine the diffusivityD and the saturated weight gain (M,), the least-square fittingtechnique was used. In this approach, the sum of the square ofthe differences between the experimental weight gain and the

k 2calculated one, (AM)2 = expMt'7'), was calculated

based on eq. (8), using some initial estimated values ofD andMt. M eip was the i-th point of the experimentally

determined weight gain at time t, while Mcal was thecalculated one based on eq. (8), and k is the total number ofpoints used in calculating (AAM)2. Then, D and M, were varieduntil (AAM)2 was minimized. The obtained D and M, are takenas the diffusivity and the saturated moisture content of thesample for that particular temperature/humidity conditions.

Figure 3 shows the experimentally determined as well asthe calculated weight gain as a function of time for a 70 ptmthick BT core sample at 60°C / 60% RH. Curve 1 wascalculated using D = 1.85x10-8 cm2/s and M = 21.1 rtg, andcurve 2 was calculated with D = 1.65x10-8 cm2/s and M =

21.2 ptg. Both curves are in reasonable agreement with theexperimental data.

The diffusivity data calculated from the desorptionexperiment at 60°C / 60% RH was in the same range as the

one derived from the absorption experiment, as shown in Fig.4. Similar calculations were performed based on experimentaldata obtained under other temperature/humidity conditions,and the results will help us to determine the temperaturedependence of the moisture diffusivity, as shall be discussedlater.

From Figs. 3 and 4, one can notice that the calculatedabsorption and desorption curves can fit the experimental datareasonably well but the fit is less than ideal. One reason forthis is that the calculation is based on the assumption that thematerial is isotropic. In reality, the BT core is a polymermatrix woven composite, and the effect of the glass fiberweave structure and fiber density has to be considered tocompletely elucidate the moisture diffusion behavior of theBT core [22].

Figure 3. Experimentally measured and calculated weightgain for a 70 pim BT core laminate. The experimental datawas from the second absorption curve under 60C / 60% RHshown in Fig. 2. The initial sample weight at t= 0 sec (thebeginning of the second absorption cycle) was 6555.472 ptg.

4.2 Saturated Moisture ContentThe saturated moisture content in the BT core, or Ca,, is

another key material property, because Chat is directly relatedto the vapor pressure inside the material voids during hightemperature reflow [23]. While the temperature dependenceof diffusivity is well established, the same is not true for Chat[24]. For many materials, Chat is independent of temperatureand it depends only on the RH, although exceptions have alsobeen reported. Bao and Yee pointed out that the saturatedmoisture concentration is related to the heat of moistureabsorption [24]:

Csat Csat,ref exp -AHrbsr f (1 1)

where Csat,ref is the saturated moisture content at a referencetemperature Tref, AHab, is the heat of moisture absorption, Tthe absolute temperature and R the universal gas constant. Inthe absence of chemical reactions between water and thepolymer matrix, AHab, iS small. In addition, for practicalreasons, the temperature range of most moisture diffusionstudies is restricted to between 25°C and 90°C. Therefore, theexponential term in eq. (11) is close to 1 for most polymers

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that do not react with water, making Chat nearly temperatureindependent.

Figure 4. Desorption curves of a 70 ~tm BT core. Solidline represents the calculated weight loss vs. time curve usingD = 1.65x10-8 cm2/s.

Figure 5. Saturated moisture content as a function oftemnerature for two different RH levels.

Figure 6. Chat as a function of relative humidity at 30°C.

Figure 5 plots the saturated moisture content in the 70 ptmBT core as a function of temperature for two different RHlevels. These data were obtained from moisture absorption-desorption experiments. It clearly shows that Chat in BT coreis essentially temperature independent.

Figure 6 shows the Csat obtained from the RH step scanexperiment. It reveals that at a fixed temperature (30°C in thiscase), Csat is proportional to the RH level.

Because of their relatively weak molecular interactionsand large free volume, saturated moisture content inpolymeric materials is much higher than that in ceramicmaterials, Most of the trapped moisture in the free volume orvoids of the polymer has to condense into the liquid form.Otherwise, a simple estimation reveals that the internalpressure inside the voids can reach an unreasonably high levelof more than 150 times the atmospheric pressure [25]. Suchhigh pressure can easily cause internal rupture of thepolymeric material. During re-flow process, the condensedwater expands rapidly during vaporization. If the vaporcannot escape freely and quickly from the sample, then thebuilt-up internal vapor pressure can cause all kinds of failuresin the packages [14,15].

4.3 Temperature Dependence of DiffusivityBased on moisture absorption-desorption experiments and

RH step scan experiments performed at various temperatures,one can determine the moisture diffusivity in the BT corematerial as a function of temperature. The results were plottedin Fig. 7. It can be seen that the moisture diffusivity has anArrhenius temperature dependence with Do 6.61 x 10-3 cm2/s,and Ed 0.368 eV. Knowing these parameters, one canextrapolate the moisture diffusivity to other temperatures,assuming the diffusion mechanism will not change - which isusually true for T < Tg. Therefore, based on our data, thediffusivity at a reflow temperature of 260°C is estimated to be2.18x10-6 cm2/s, which is about 121 times the diffusivity at60°C. In reality, the diffusivity could be even higher since thereflow temperature is above Tg of the BT core. It is estimatedthat at 2600C, it takes less than 1 sec for a 70 ptm thicksaturated BT core to lose 50% of the total moisture, and lessthan 5 sec to lose 90% of the total moisture. If the total BTcore thickness increases by introducing multiple core layers orby increasing the thickness of each core layer, the time neededfor the same amount of moisture to escape from the BT corewill increase in such a way that t x h2, where h is the totalthickness of the BT core [17]. This will have a significantimpact on package and materials design for optimizedpreconditioning reliability performance.

5. Comparison with Literature DataUsing the standard procedure which involves exposing the

specimen to a specific temperature-humidity condition andmonitoring the change in sample weight over time, Liu et alstudied the moisture diffusivity in neat BT resins and inBT/glass fiber laminates [26]. Their sample thickness was0.035 inches (0.89 mm) for the neat resins and 0.030 inches(0.76 mm) for the laminates. Their results showed that for BTneat resin, the diffusivity is 1.29x10-8 cm2/s at 500C / 80%RH, and it increases to 2.45x10-8 cm2/s at 70°C / 80% RH. At

1379 2007 Electronic Components and Technology Conference

900C / 80% RH, it becomes 5.82x10-8 cm2/s, as shown in Fig.7. Using the Arrhenius equation, the diffusivity of the BT neatresin can be described using Do = 1.02x10-2 cm2/s and Ed =

8.72 Kcal/mol or 0.378 eV. For BT laminates with 280 E-glass fabrics, the diffusivity is reduced by nearly 50% underall three temperature/RH conditions, with Do = 1.50x 10-3cm2/s and Ed= 7.93 Kcal/mol or 0.344 eV. In their study, theresin content for the laminated samples was measured usingthermogravimetric analysis (TGA), but the results were notdirectly reported in the paper. However, based on reportednormalized maximum moisture uptake, it can be estimatedthat for BT/280 laminates, the resin content wasapproximately 62.5%. Their data was the first evidence tosuggest that for a relatively thick BT laminate, the addition ofglass fiber and its topological or weave structure can have ahuge impact on the moisture diffusion behavior of thematerial.

In another study, Pecht et al studied the moisture diffusionin BT/glass fiber laminates [13]. In their study, all of thelaminates were woven E-glass fabric with thickness of either0.038 or 0.053 cm. The reported diffusivity of BT laminateswas 1.22x10-8 cm2/s at 500C / 50% RH, and 1.65x10-8 cm2/sat 500C / 85% RH; and it becomes 4.75x10-8 cm2/s at 850C /50%RH and 3.03x10 8cm2/sat 850C /85% RH. These valueswere also plotted in Fig. 7.

In Galloway and Miles' paper [9], the moisture diffusionconstant was reported to follow the Arrhenius equation, withDo = 1.2x10-4 cm2/s and Ed= 0.295 eV based on absorptiondata, and the temperature dependence of the diffusivity wascalculated using these values and plotted in Fig. 7. Based ondesorption, Do = 6.0x 10-2 cm2/s and Ed= 0.465 eV. In theirexperiments, the sample thickness ranged between 0.155 to1.05 ± 0.003 mm, but the exact thickness of the BT epoxywas not given.

Wong and Rajoo studied the moisture diffusion behaviorof a BT core laminate [27]. Their sample thickness was 0.4mm, and the in-plane dimensions were much larger than thethickness, so that one dimensional diffusion can be assumed.They concluded that for the BT core, the temperaturedependence of the transverse moisture diffusivity follows bythe Arrhenius equation with Do = 3.33x 10-4 cm2/s and Ed =

0.32 eV, as shown in Fig. 7. Thus, at 300C, D 1.6xlO-9cm2/s, at 600C, D 4.8xlO-9 cm2/s. These results agree verywell with Galloway et al's data on BT epoxy, but again aremuch lower than the diffusivity data reported in BT neat resin[26]. In addition, the reported Chat was 4.83 mg/cm3 at 300C /60% RH, which is about 18% lower than the result obtainedfrom this study (Fig. 2). This is most likely caused by higherglass fiber density in their material, although the exact glassfiber content was not reported [27].

In addition to the 70 ptm BT core material, using theconventional method with an analytical balance, we have alsomeasured the moisture absorption behavior of two differentthick BT substrates with thicknesses of 0.72 mm and 0.812mm at 850C / 85% RH, and the diffusivity results wereplotted in Fig. 7.

Based on all the data plotted in Fig. 7, it is clear that themoisture diffusivity of 70 ptm BT laminated core is much

higher than the values reported in refs. [9] and [27], but ourresults are in good agreement with the diffusivity of BT neatresin [26] or BT laminates reported in [13]. These differencesmay be attributed to the effect of glass fibers in thick BTlaminates. In BT/glass laminates, it is expected that the glassfibers absorb essentially no moisture, all moisture will beabsorbed by the BT resin. In a thick BT core, the woven glassfibers form a three-dimensional fabric structure, whichhinders the diffusion of moisture. In a thin BT core, however,the woven glass fibers more or less form a 2-D grid structure,and the diffusion of moisture is less hindered, leading to ahigher diffusivity. This explains why in the 70 ptm thick BTcore, the observed moisture diffusivity is the same as the onereported in BT neat resin, but for thick BT core, it is muchless.

100

cnNE0

C)

10

* This workLinear FitNeat BT (IBM)

a BT Laminates (Maryland)A Thick BT Core 1 (this work)A Thick BT Core 2 (this work)

Wong and Rajoo---Galloway and Miles

I~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~_,, _S,0

0.0028 0.0030 0.0032 0.0034

1/T (K-1)

Figure 7. Moisture diffusivity of BT core materials obtainedfrom absorption experiments as a function of temperature.Solid circles: this work. Solid line: linear fit of logIOD vs 1ITbased on diffusivity data obtained from this work. Opendiamonds: moisture diffusivity of neat BT resin, as reportedin [26]; open squares: diffusivity of BT laminates reported in[13]; dashed line: calculated diffusivity as a function of 1ITusing the pre-exponential factor and the activation energyreported in [27]; dash-dotted line: calculated D vs 1IT basedon the values of Do and Ed reported in [9]; open and filledtriangles (in-house data): diffusivity of thick BT substrateswith thicknesses of 0.72 mm and 0.812 mm, respectively.

6. Application - One-Dimensional Model for a Thin DAFilm/Substrate Structure

As discussed earlier, when the packaging materials aresaturated with moisture, a large percentage of it condensesinto the liquid phase in the voids or free volumes of thematerial. During reflow process, condensed water inside thesepores, if could not escape fast enough, expands and vaporizesrapidly from the liquid to the steam state, generatingtremendous vapor pressure within the material. Based on theobtained moisture diffusivity data for the 70 ptm BT core andthe die attach films, finite element modeling has been appliedto investigate the moisture and vapor pressure distribution in

1380 2007 Electronic Components and Technology Conference

I I~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

O

.-A-- ,

..., 11

1

the UT/SCSP packages [28]. In the following, a one-dimensional model for the thin DA film/substrate structure isdeveloped to understand the moisture loss in the die-attachfilm layer between the die and the substrate. Since themajority of the moisture diffuses out of the DA film throughthe BT substrate ( which has a total thickness of a fewhundred microns) rather than from the mold compound side(which has a thickness of up to or more than a thousandmicrons from the die attach edge to the mold compoundedge), the model considers the moisture diffusion through thesubstrate only. Following assumptions are made: (1) Thesubstrate thickness is much smaller comparing to its in-planedimensions, thus one-dimensional moisture diffusion model isvalid; (2) The die-attach film thickness (typically about 25ptm) is an order of magnitude smaller than the total substratethickness, thus the substrate on the die-attach side is insulatedin our model; (3) The moisture content in the substrate andthe DA film are fully saturated before reflow, thus the initialmoisture distribution in the DA film is uniform. In addition,since the DA film has a higher diffusivity than that of the BTand its thickness is much smaller than the substrate thickness,therefore, the moisture concentration in the DA film, denotedas Cfilm, can be considered uniform at any time; (4) Thesaturated moisture concentration is independent oftemperature (as we have discussed in section 4), thus theinterface continuity Cfilm/Csatfilm Csb/Csatsb holds all thetime; (5) A step change in temperature during reflow isassumed and the effective diffusivity D of the substrate athigh temperature is used according to the Arrheniusrelationship. Under these assumptions, the moistureconcentration in the DA the film as a function of time duringreflow can then be derived as:

film 00Dth

cfilm Csat 2. (-1)ne nDt h2(12)sat n=O n

with

in= 2n ') (13)n 2where D is the substrate moisture diffusivity, h the substratethickness, and t the reflow time. With the assumed value of an'effective' diffusivity of 5.Ox10-6 cm2/s for the substrate,Figure 8 plots the ratio of moisture concentration over thesaturated moisture concentration in the die attach film. Thisassumption about the effective diffusivity is based on theconsideration that during reflow, the moisture diffusion is notonly driven by Fick's law, but also by vapor pressure.Therefore, if we still use he Fick' s law for diffusionmodeling, we need to obtain an 'effective diffusivity' toaccount for the secondary moisture transport effect. We takethis number based on the approximation that the substrate isdried out about 75% according to the limited total weight gaindata after reflow.

From Figure 8, it can be seen that in the DA film/substratestructure, if the substrate thickness is 220 ptm or less, about80% of the saturated moisture within the DA film will be lostin 6 minutes during reflow. There also exists a criticalresidual moisture concentration (horizontal dashed line in Fig.8), above which cohesive delamination within the DA film

will occur. Based on the equation (12), the diffusivity D,substrate thickness h, and reflow time are three criticalparameters to control the residual moisture level in the DAfilm. Fig. 8 also shows that a significant difference on theresidual moisture concentration exists for two slightlydifferent thicknesses. This implies that the package is verysensitive to substrate thickness and copper structure layout.The experimental data [29] correlated well with our analysis.It should be noted that Fig. 8 is based on the measureddiffusivity data for thin BT core, which is an order greaterthan the data from the thick BT core measurement. Ifwe wereto use the diffusivity data for the thick BT core, calculationwould have suggested that the moisture loss in the DA filmwas negligible during reflow even if the substrate thickness ismuch reduced.

0.8 -- 2 t

0.6 -

o0.4 -Fail

v

0.2 NPassCritical moisture concentration

00 60 120 180 240 300 360

Time (seconds)

Figure 8. Estimated moisture loss in DA film during reflowprocess based on two different substrate thicknesses.

7. ConclusionsMoisture absorption-desorption behavior of a 70 ~tm BT

laminate used as the UT/SCSP substrate core material hasbeen characterized in-situ using a sorption TGA equippedwith a moisture chamber. Based on moisture absorptionexperiments, the moisture diffusivity of this thin BT core hasan Arrhenius temperature dependence, D = Duexp(-Ed l),where Do :: 6.61X10-3 CM2Ss, and Ed= 0.368 eV. Themeasured diffusivity agrees well with reported value of BTneat resin, but it is much higher than that ofthe thick BT corematerials. This difference is attributed to the effect of glassfiber: in thick BT laminates, glass fibers forms a 3D networkstructure, which greatly hinders moisture diffusion. In thin BTcores, the glass fibers are mainly in a 2D structure, which hasa much less effect on moisture diffusion in the BT resin.

Experimental results revealed that the saturated moisturecontent of the 70dim BT core,Cis ,, has little dependence ontemperature, and it depends approximately linearly on relativehumidity. At 600o RH, Caremainly5.9 a6.1 Mg cmr; at80wo RH,Clat,;:~8.7 -8.9 Mg/CM3.

Based on literature diffusivity data obtained from the thickBT samples, one would conclude that the moistureconcentration in the die attach film in a stack die chip scalepackage would not change significantly during reflowprocess. However, based on the diffusivity data obtained forthin BT core in this work, we can show that a small change insubstrate thickness can result in a substantial difference in theresidual moisture concentration in die attach film, leading to

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very different reliability performance. Indeed, such reliabilityresults correlated well with the experimental data.

AcknowledgmentsWe are grateful to Drs. Paul Koning and Ibrahim Bekar

for stimulated discussions. We want to thank Dr. Steve Chofor carrying out moisture uptake experiments on two sets ofthick BT substrate cores, and Mr. Fred Cardona for measuringmoisture uptake on the third set of thick BT cores.

References1. R. B. Prime, in: Thermal Characterization of Polymeric

Materials, Volume 2, 2nd ed., edited by E. A. Turi, Ch. 6,pp.1702-1704, Academic Press, New York, 1997.

2. V. B. Gupta, L. T. Drzal, and M. J. Rich, "The physicalbasis of moisture transport in a cured epoxy resinsystem", J. Appl. Polym. Sci., Vol. 30, No. 11 (1985), pp.4467-4493.

3. Y. C. Lin, and X. Chen, "Moisture sorption-desorption-resorption characteristics and its effect on the mechanicalbehavior of the epoxy system", Polymer, Vol. 46, No. 25(2005), pp. 11994-12003.

4. Y. He, unpublished results, Intel Corporation, 2004; also,T. Caskey, G. Oskarsdottir, and Y. He, internalpresentation, Intel Polymer Workshop, Nov. 2004.

5. E. Stellrecht, B. T. Han, and M. G. Pecht,"Characterization of hygroscopic swelling behavior ofmold compounds and plastic packages", IEEE Trans.Comp. Packag. Technol., Vol. 27, (2004), pp. 499-506.

6. H. Ardebili, E. H. Wong, and M. Pecht, "Hygroscopicswelling and sorption characteristics of epoxy moldingcompounds used in electronic packaging", IEEE Trans.Comp. Packag. Technol., Vol. 26, No. 1 (2003) pp. 206-214.

7. Y. He and X. J. Fan, unpublished results, IntelCorporation, 2005.

8. I. Fukuzawa, S. Ishiguro, and S. Nanbu, "Moistureresistance degradation of plastic LST's by reflowsoldering", Proc. 23rd International Reliability Phys.Symp., pp. 192-197, 1985.

9. J. E. Galloway and B. M. Miles, "Moisture absorptionand desorption predictions for plastic ball grid arraypackages", IEEE Trans. Comp. Packag. Manuf Technol.A, Vol. 20, No. 3 (1997), pp. 274-279.

10. C. E. Park, B. J. Han, and H. E. Bair, "Humidity effect onadhesion strength between solder ball and epoxyunderfills", Polymer, Vol. 38, No. 15 (1997), pp. 3811 -3818.

11. M. Teo, S. G. Mhaisalker, E. H. Wong, P.-S. Teo, C. C.Wong, K. Ong, C. F. Goh, and L. K. Teh, "Correlation ofmaterial properties to reliability performance ofanisotropic conductive adhesive flip chip packages",IEEE Trans. Comp. Packag. Technol., Vol. 28, No. 1(2005), pp. 157-164.

12. Z. F. Li, "A review of BT/epoxy resin chemistry,composition, processing, properties, and applications",internal project report, Intel Corporation, July 1995.

13. M. Pecht, H. Ardebili, A. A. Shukla, J. K. Hagge, and D.Jennings, "Moisture ingress into organic laminates",IEEE Trans. Comp. Packag. Technol., Vol. 22, No. 1(1999), pp. 104-110.

14. Xuejun Fan, Ibrahim Bekar, Anthony A. Fischer, Yi He,Zhenyu Huang, and Edward R. Prack,"Delamination/cracking root cause mechanisms for ultra-thin stacked die chip scale packages", IntelManufacturing Excellence Conference (IMEC) 2006technical paper.

15. Zhenyu Huang, John Tang, Changmin Hu, MichaelWang, Mu Zhang, Bin Liu, Xuejun Fan, and EdwardPrack, "Moisture induced cohesive delamination in die-attach film in ultra thin stacked chip-scale package", IntelAssembly Test Tech. J., 2006.

16. ASTM D5229, Standard Test Method For MoistureAbsorption Properties and Equilibrium Conditioning ofPolymer Matrix Composite Materials, ASTM 1998.

17. The easiest way for such estimation is to use a simplified

expression of eq. (8): M = -exp[ 7.3K1i)j1

where h is the total thickness. See also, C. H. Shen, andG. S. Springer, "Moisture absorption and desorption ofcomposite materials", J Compos. Mater., Vol. 10, No. 1(1976), pp. 2-10; T. Ferguson and J. Qu, "Moistureabsorption analysis of interfacial fracture test specimenscomposed of no-flow underfill materials", J. Electron.Packag.: Trans. ASME, Vol. 125, No. 1 (2003), pp. 24-30.

18. J. Crank, The Mathematics of Diffusion, 2nd ed., OxfordUniversity Press, Oxford, 1990.

19. R. J. Borg and G. J. Dienes, An Introduction to Solid StateDiffusion, Academic Press, Inc., San Diego, 1988, p.60.

20. Yi He, unpublished results, Intel Corporation, 2006.21. Technical Brochure, TA Instruments Q SeriesTm Thermal

Analyzers; http://www.tainstruments.com/Default.asp.22. Xiaodong Tang, John D. Whitcomb, Yanmei Li, and

Hung-Jue Sue, "Micromechanics modeling of moisturediffusion in woven composites", Compos. Sci. Technol.,Vol. 65, (2005), pp. 817-826.

23. X. J. Fan, Moisture Related Reliability Issues inElectronic Packaging, ECTC short course, 2006.

24. Li-Rong Bao, A. F. Yee, Effect of Temperature onMoisture Absorption in a Bismaleimide Resin and ItsCarbon Fiber Composites, Polymer, Vol. 43, No. 14(2002), pp. 3987-3997, and refs. [11-14] cited in thispaper.

25. Consider Csat = 6 mg/cm3 at 60°C, and the volumefraction of the free volume or voids is 5% of the totalsample volume. The actual moisture density inside thevoids is then 120 mg/cm3. If the moisture is notcondensed into the liquid form but still in gas form, thenthe internal pressure inside the voids can be calculatedusing the ideal gas approximation: pV= nRT. For V= 1cm3, n = 0.12/18 = 0.0067 mole, R = 8.314 J/mol K is the

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universal gas constant, for T = 60°C or 333 K, p isestimated to be 18.549 MPa, which is -183 atm.

26. P. C. Liu, D. W. Wang, E. D. Livingston, and W. T.Chen, "Moisture absorption behavior of printed circuitlaminate materials, advances in electronic packaging",Proc. of the 1993 ASME International ElectronicsPackaging Conf, Vol. 1, 435-442, American Society ofMechanical Engineers, September 29-Oct. 2, 1993,Binghamton, NY. Also quoted in ref. 12.

27. E. H. Wong and R. Rajoo, "Moisture absorption anddiffusion characterization of packaging materials -

advanced treatment", Microelec. Reliability, Vol. 43, No.12 (2003), pp. 2087-2096. [Notice that Table 2 contains atypo: the units of Do should by cm2/s, not x10-9 cm2/s.Also, private communication with E. H. Wong, 2006].

28. Bin Xie, Daniel Shi, and Xuejun Fan, "Sensitivityinvestigation of reflow profile and substrate thickness onwafer level film failures in 3-D chip scale packages byfinite element modeling", IEEE Electronic Componentsand Technology Conference (ECTC), 2007.

29. Xuejun Fan, Daniel Shi, Ibrahim Bekar, Edward Prack,Jianfeng Wang, Zhenyu Huang, Yi He, and AnthonyFischer, "Dicing tape format die-attach film selectionmethodology for ultra-thin stacked die chip scalepackages", IEEE Electronic Components and TechnologyConference (ECTC), 2007.

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