InSitu Characterization of Moisture Absorption and Desorption in a ...

In-situ Characterization of Moisture Absorption and Desorption in a Thin BT Core Substrate Yi He and Xuejun Fan Intel Corporation, Assembly Test & Technology Development 5000 W. Chandler Blvd., Chandler, AZ 85226, U.S.A. [email protected], 1-480-552-3154; [email protected], 1-480-554-1308 Abstract Bismaleimide-triazine (BT) resin/glass fiber laminates are commonly used as a substrate core material in microelectronic packaging. Their moisture absorption and diffusion behavior have a significant impact on package reliability. The traditional method to determine moisture absorption relies on a weight gain measurement metrology with an analytical balance. This approach is generally not suitable for thin films. In this study, the moisture absorption-desorption behavior of a thin BT core was characterized in-situ using a sorption TGA over a temperature range of 30 to 80°C, in an environment of up to 80% relative humidity. From the experimental results, the moisture diffusivity and the saturated moisture content have been determined. Within the experimental temperature range, the diffusivity can be described by the Arrhenius equation and the activation energy can be computed. The obtained results are compared with literature data. The impact of moisture diffusion in BT core on the reliability of ultra-thin stacked chip scale packages (UT/SCSP) will be discussed. 1. Introduction In the semiconductor industry, it has been long recognized that moisture plays a key role in the reliability of microelectronic packaging. Moisture absorbed by polymerbased packaging materials can cause substantial changes in material properties, such as coefficient of thermal expansion (CTE), modulus, glass transition temperature (Tg), and viscoelastic behavior. In cured thermosets, moisture acts as a plasticizer, reducing the modulus and Tg, and changing the thermal expansion characteristics of the material [1-3]. For example, upon moisture saturation at 85°C/85% relative humidity (RH), the Tg of a cured no-flow underfill decreased by as much as 25°C, whereas its room temperature modulus decreased by approximately 8% [4]. At elevated temperatures, moisture-induced hygroscopic swelling in packaging materials can cause a large increase in package stresses. For example, in some microelectronic packages encapsulated using commercially available molding compounds, the strain induced by the mismatch of the coefficients of hygroscopic swelling (CHS) is nearly twice as much as the strain induced by the CTE mismatch over a temperature span of ∆T = 60 °C [5]. For other molding compounds, the hygroscopic mismatch induced strain ranges from one to nearly four times of the strain induced by the CTE mismatch over a ∆T of 45°C for T > Tg or over a ∆T = 100°C for T < Tg [6]. For some underfill materials, the hygroscopic swelling induced strain is comparable to the thermal strain caused by thermal expansion over a temperature range of 100°C [7]. At solder reflow

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temperatures, which are typically 230 – 260°C, vaporization of residual moisture leads to a sharp buildup in vapor pressure, causing voiding, cracking, interfacial delamination, or “popcorn” failures in packages [8,9]. In addition, moisture can cause degradation of adhesion strength, which leads to a reduction of interfacial strength. For example, the adhesion strength between the solder ball and some underfill materials can decrease by more than 70% upon long time exposure at 85°C/85% RH [10]. Many failures in microelectronic packages can be traced back to moisture [11]. Therefore, characterization of moisture absorption-desorption and diffusion in electronic packaging materials is essential for understanding moisture-induced failure mechanisms and for modeling reliability performance of the package. Once that has been achieved, one can optimize the package, material, and process design to minimize or eliminate moisture-related failure. Bismaleimide-triazine, or BT, is a general term for the thermosetting resin obtained from additional polymerization of two monomers, bismaleimide and triazine (a cyanate ester). The blending of BT and epoxy resin offers better thermomechanical and electrical performance over standard epoxy systems [12]. The Tg of the BT resin is typically above 185°C. In comparison, another popular resin used in substrate laminates, the standard FR-4 resin, has a Tg around 125135°C, although high-temperature FR-4 materials with Tg ~ 180°C are also available [13]. Thus, BT epoxy resin/glass fiber laminates are commonly used as a substrate core material in microelectronic packaging. Consequently, their moisture properties have a significant impact on package reliability. Recently, the development of ultra-thin stacked chip scale packaging (UT/SCSP) technology has become essential to increasing functionality and higher memory capacity with more complex and efficient memory architectures in smallform factor packages. In these packages, the wafer must be thinned from the original 750 µm down to as low as 50 µm. The conventional die attach (DA) paste material and the assembly method cannot be applied to handle such thin dies. Instead, wafer-level thin adhesive films combined with the corresponding lamination technique provides an alterative solution. These die attach films (including wafer-level and pick-and-place) are usually very soft, with a tensile modulus less than 10 or even 1 MPa at solder reflow temperature. These small form-factor packages are quite sensitive to moisture, and a new failure mode after preconditioning test has been detected, i.e., cohesive failure within the DA material located between the substrate and the die [14, 15], where the moisture is absorbed through the substrate, which

1375 2007 Electronic Components and Technology Conference

consists mainly of thin BT core and copper layers. Detailed fundamental studies and analysis of previous DOE data revealed that the moisture properties of the thin BT core, including its temperature-dependent moisture diffusivity and saturated moisture content, play a crucial role in modulating the observed failure in UT/SCSP [14]. Characterization of such properties is the main objective of this study. Traditionally, the characterization of moisture properties of a material involves exposing the sample under a specific temperature/humidity condition, then monitoring the sample’s weight increase as a function of the exposure time [16]. This procedure requires removing the sample out of the temperature/humidity chamber and then measuring the sample weight intermittently with an analytical balance, until the moisture saturation is reached. For thick samples, this method works fine, but for thin specimens, it can have potential problems: the exposure of the sample at room temperature/ humidity condition even for a short time can cause significant changes in the moisture content of the sample. For example, for a 70 µm thick film saturated with moisture and having a typical moisture diffusivity of 1.0×10-8 cm2/s, it is estimated that 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 µm thick film, that time reduces to less than 2 minutes. Therefore, large error can be introduced during moisture absorption measurement using the traditional technique. Another severe problem is that for a typical laboratory balance, the sensitivity is only 0.01 mg, and in many cases that is not enough to accurately determine the moisture uptake in a thin film. In addition, the measurement has to be carried out manually. Based on these considerations, new techniques are needed for characterizing moisture properties of thin films. In this study, the moisture absorption-desorption behavior of a 70 µm thick BT core was characterized in-situ using a TA Instruments Q5000 SA Thermogravimetric Analyzer (TGA) at 30, 60, and 80°C, with the relative humidity cycling between 0-60% or 0-80%, respectively. Based on the experimental data, the diffusion constant and the saturated moisture density, Csat, were determined. We will demonstrate that when transitioning from a thin BT core to a thick sample, the glass fiber and its structure can have a large impact on the moisture diffusion behavior of the material. Based on the experimental results, finite element modeling can be applied to investigate the moisture distribution in UT/SCSP during preconditioning. 2. Background Moisture diffusion in isotropic materials under constant temperature and relative humidity conditions can generally be described by Fick’s second law [18]: ⎛ ∂ 2C ∂ 2C ∂ 2C ⎞ ∂C ⎟, = D⎜ + + (1) ⎜ ∂ x2 ∂ y2 ∂ z2 ⎟ ∂t ⎝ ⎠ where C(x,y,z,t) is the moisture concentration inside the material and it has a unit of mg/cm3, D is the diffusion constant, t is the time, (x,y,z) are the coordinates. BT/glass fiber laminates are clearly anisotropic, however, for thin

samples with large aspect ratios, one dimensional diffusion model is valid, and the diffusion equation can be simplified as ∂ 2C ∂C . =D ∂t ∂ x2

(2)

For a thin plate sample with a thickness of h, one can set up the coordinate system so that the origin is in the center of the sample thickness, and the sample is bounded within –h/2 < x < h/2. The initial and boundary conditions are [3,18]: C = C 0 , − h / 2 < x < h / 2, t = 0 , (3) C = C i , x = −h / 2, x = h / 2, t ≥ 0

where C0 is the initial moisture concentration within the sample, Ci is the constant moisture concentration of the environment. For moisture absorption experiment, C0 < Ci, and often C0 = 0; for a desorption experiment, C0 > Ci, and often Ci = 0. Under these initial and boundary conditions, the diffusion equation (2) can be solved using the standard method of variable separation, and the concentration distribution in the sample is given as [3,18]: ∞ ⎡ (2n + 1) 2 π 2 D ⎤ C ( x, t ) − C 0 (−1) n 4 =1− exp ⎢− t⎥ π n = 0 (2n + 1) Ci − C0 ⎢⎣ ⎥⎦ . h2 (2n + 1)π × cos x h (4) The change in sample weight due to moisture absorption or desorption is given by integrating the moisture concentration change at time t over the entire sample volume:

∑

h/2

Mt =

∫

∫

(C − C0 )dx ds ,

−h / 2

(5)

S

where S is in-plane surface area of the sample, since diffusion from the sides is negligible. Substituting eq.(4) into eq.(5), and note that h/2

∫

−h / 2

cos

(2n + 1)π 2h(−1) n , x ⋅ dx = h (2n + 1)π

(6)

We have M t = (C i − C 0 ) Sh × ∞ ⎛ ⎡ (2n + 1) 2 π 2 D ⎤ ⎞ 1 ⎜1 − 8 exp ⎢− t⎥ ⎟ , ∑ ⎜ 2 2 2 ⎢⎣ ⎥⎦ ⎟⎠ h ⎝ π n = 0 (2n + 1) (7) In eq. (7), (Ci − C0 ) Sh = M ∞ is the ultimate change in

sample weight with t → ∞. For a moisture absorption experiment, M∞ is positive since Ci > C0; in a desorption experiment, M∞ is negative since Ci < C0. Thus, ∞ ⎡ (2n + 1) 2 π 2 D ⎤ Mt 8 1 t ⎥ , (8) = 1− exp ⎢− M∞ ⎢⎣ ⎥⎦ π 2 n = 0 (2n + 1) 2 h2

∑

which is the familiar solution to 1D diffusion equation [3,18]. When analyzing the experimental data, D and M∞ are optimized so that the difference between the experimentally


determined Mt and the one calculated using eq. (8) is minimized. When n reaches 50, eq. (8) converges very fast. In this study, n runs from 0 to 200. The saturated moisture concentration in the material, Csat, is simply M∞/V, where V is the sample volume. Alternatively, M C sat = ∞ ρ , (9) M where M is the initial sample weight and ρ is density of the sample. Here the change of sample volume caused by moisture absorption-desorption can be ignored since it is quite small. Both Csat and D are critical in understanding moisturerelated reliability issues, because Csat determines how much moisture can be absorbed by the material, and D determines how fast the moisture can diffuse into and out of the sample. The temperature dependence of the diffusion constant can be described by the Arrhenius equation [19]: ⎛ E ⎞ D = Do exp⎜ − d ⎟ , (10) ⎝ kT ⎠ where Do is a pre-factor, Ed is the activation energy, k = 1.38×10-23 J/K is the Boltzmann’s constant, T the absolute temperature. In some published papers, the minus sign inside the exponential function was omitted and a negative activation energy was used. 3. Experimental 3.1. Material The material used in this study was a 70 µm thick BT/glass fiber laminated substrate core material. The general properties of this material have been characterized by us and are listed in Table 1 [20]. The dynamic mechanical properties were determined by DMA experiments, which were conducted under tensile mode with a dynamic frequency of 1 Hz and a heating rate of 3°C/min. The fiber content was determined by thermogravimetric analysis. The detailed description for these measurements will not be discussed here.

µg, and a sensitivity of 0.1 µg. The humidity chamber is a well insulated tri-level aluminum block containing de-ionized water, which can be refilled as necessary. The humidity surrounding the sample is controlled and maintained by a pair of mass flow controllers. By adjusting the amount of “dry” and “wet” gases flowing through the controller, the software is capable of maintaining a desired relative humidity level. The temperature control is done by four thermoelectric devices in conjunction with a thermistor in a closed-loop system. The actual temperature/humidity condition around the sample can be verified using deliquescence point of certain salts. The typical dimensions of the sample used in sorptionTGA experiments are 7mm×7mm with a weight of about 7 mg. In this study, moisture absorption-desorption experiments were performed at 30, 60, and 80°C, respectively. At each isothermal temperature, two relative humidity levels were chosen: 60% and 80% RH. During an isothermal experiment, the relative humidity was cycled between 0 and 60% (or 0 and 80%) twice, while the temperature stability is maintained to be better than ±0.05°C. At each temperature/moisture conditions, the sample was held for up to 600 min. In such experiments, quartz bowls were used as the sample pan and the reference pan, and the sample was placed inside or on top of the sample bowl, allowing the moisture to diffuse into the sample from both surfaces. Since both the sample and the reference bowls experience the same temperature and humidity conditions, the net effect on the sample weight change is from moisture absorption or desorption. Thermally isolated balance

Balance system

Table 1. General properties of the 70 µm thick BT core substrate used in this study.

Density Fiber Content CTE (0-50°C) CTE (150-200°C) Storage Modulus (@25°C) Loss Modulus Peak Tan δ peak

1.79 g/cm3 56.46 wt.% 16×10-6 1/oC 18.5×10-6 1/oC 14.65 GPa 207°C 215.6°C

Reference pan

Sample pan Moisture chamber

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

3.2 Instrument The instrument used in this study, TA Instruments Q5000 SA TGA, is a high sensitivity thermogravimetric analyzer which enables sorption/desorption analysis of materials under controlled temperature and humidity conditions [21]. Figure 1 is an illustration of the instrument. The heart of this instrument is a high performance thermo-balance, which is maintained at a constant temperature of 40°C for increased thermal stability. The balance has a signal resolution of 0.01

4. Moisture Absorption-Desorption 4.1 Moisture Diffusivity Figure 2 shows an example of moisture absorptiondesorption experiment, where the temperature was kept at 60oC and the RH level was cycled between 0 and 60%. During the first 60 minutes, a 0% RH condition was imposed to drive any residual moisture out of the sample. This was followed by a 200 min of moisture absorption at 60% RH, then a 200 min desorption at 0% RH. This sorption-


desorption cycle was repeated one more time, as shown in Fig. 2. RH

Weight

100.3

60 C Weight (%)

100.2

60 50 40 30

100.1

20

100.0

10 99.9

0 0

200

400

600

800

Reference Sensor RH (%)

o

1000 1200

Time (min)

one derived from the absorption experiment, as shown in Fig. 4. Similar calculations were performed based on experimental data obtained under other temperature/humidity conditions, and the results will help us to determine the temperature dependence of the moisture diffusivity, as shall be discussed later. From Figs. 3 and 4, one can notice that the calculated absorption and desorption curves can fit the experimental data reasonably well but the fit is less than ideal. One reason for this is that the calculation is based on the assumption that the material is isotropic. In reality, the BT core is a polymer matrix woven composite, and the effect of the glass fiber weave structure and fiber density has to be considered to completely elucidate the moisture diffusion behavior of the BT core [22].

Figure 2. Moisture absorption-desorption experiment conducted using a sorption TGA at 60oC with the RH level cycled between 0 and 60%. The initial BT core sample weight was 6560.455 µg.

k

(

cal calculated one, (∆M ) 2 = ∑ M texp ,i − M t ,i i =1

) , was calculated 2

based on eq. (8), using some initial estimated values of D and M∞.

M texp ,i

was the i-th point of the experimentally

was the determined weight gain at time t, while M tcal ,i calculated one based on eq. (8), and k is the total number of points used in calculating (∆M)2. Then, D and M∞ were varied until (∆M)2 was minimized. The obtained D and M∞ are taken as the diffusivity and the saturated moisture content of the sample for that particular temperature/humidity conditions. Figure 3 shows the experimentally determined as well as the calculated weight gain as a function of time for a 70 µm thick BT core sample at 60°C / 60% RH. Curve 1 was calculated using D = 1.85×10-8 cm2/s and M∞ = 21.1 µg, and curve 2 was calculated with D = 1.65×10-8 cm2/s and M∞ = 21.2 µg. Both curves are in reasonable agreement with the experimental data. The diffusivity data calculated from the desorption experiment at 60°C / 60% RH was in the same range as the

o

60 C / 60% RH 20 Weight Gain (µg)

Based on the results from Fig. 2, several points are clear: (1) during the first 60 min, the moisture was not completely driven out of the sample, a longer time was needed to accomplish that; (2) the subsequent absorption-desorption cycles were repeatable, i.e. the sample reached approximately the same saturated moisture level during sorption and it lost the same weight upon drying. This indicates that there is no chemical reaction between the water molecules and the material; (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 µm thick BT core at 60oC / 60% RH can be calculated from the second moisture absorption curve shown in Fig. 2. To determine the diffusivity D and the saturated weight gain (M∞), the least-square fitting technique was used. In this approach, the sum of the square of the differences between the experimental weight gain and the

25

15 10 Experimental Data Fit 1 Fit 2

5 0 0

1000

2000

3000

4000

Time (sec)

Figure 3. Experimentally measured and calculated weight gain for a 70 µm BT core laminate. The experimental data was from the second absorption curve under 60oC / 60% RH shown in Fig. 2. The initial sample weight at t = 0 sec (the beginning of the second absorption cycle) was 6555.472 µg. 4.2 Saturated Moisture Content The saturated moisture content in the BT core, or Csat, is another key material property, because Csat is directly related to the vapor pressure inside the material voids during high temperature reflow [23]. While the temperature dependence of diffusivity is well established, the same is not true for Csat [24]. For many materials, Csat is independent of temperature and it depends only on the RH, although exceptions have also been reported. Bao and Yee pointed out that the saturated moisture concentration is related to the heat of moisture absorption [24]: ⎡ ∆H ⎛ 1 ⎞⎤ abs ⎜ − 1 ⎟⎥ , Csat = Csat , ref exp ⎢− (11) R ⎜⎝ T Tref ⎟⎠⎥ ⎢⎣ ⎦ where Csat,ref is the saturated moisture content at a reference temperature Tref, ∆Habs is the heat of moisture absorption, T the absolute temperature and R the universal gas constant. In the absence of chemical reactions between water and the polymer matrix, ∆Habs is small. In addition, for practical reasons, the temperature range of most moisture diffusion studies is restricted to between 25°C and 90°C. Therefore, the exponential term in eq. (11) is close to 1 for most polymers


that do not react with water, making Csat nearly temperature independent.

Weight Change (µg)

0

Experimental 2

Fitted (D = 1.65 µm /s)

-5 -10 -15 -20 -25

0

2000

4000

6000

8000

Time (sec)

Figure 4. Desorption curves of a 70 µm BT core. Solid line represents the calculated weight loss vs. time curve using D = 1.65×10-8 cm2/s.

10

3

Csat (mg/cm )

8

6

4

60% R.H. 80% R.H.

70 µm BT core

30

40

50

60

70

80

Temperature (°C)

Figure 5. Saturated moisture content as a function of temperature for two different RH levels. 9

Experimental Data Linear Fit

8

3

Csat (mg/cm )

7 6 5 4 3

Slope = 0.11

2 1 0

0

10

20

30

40

50

60

70

80

90

Relative Humidity (%)

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

Figure 5 plots the saturated moisture content in the 70 µm BT core as a function of temperature for two different RH levels. These data were obtained from moisture absorptiondesorption experiments. It clearly shows that Csat in BT core is essentially temperature independent. Figure 6 shows the Csat obtained from the RH step scan experiment. It reveals that at a fixed temperature (30°C in this case), Csat is proportional to the RH level. Because of their relatively weak molecular interactions and large free volume, saturated moisture content in polymeric materials is much higher than that in ceramic materials, Most of the trapped moisture in the free volume or voids of the polymer has to condense into the liquid form. Otherwise, a simple estimation reveals that the internal pressure inside the voids can reach an unreasonably high level of more than 150 times the atmospheric pressure [25]. Such high pressure can easily cause internal rupture of the polymeric material. During re-flow process, the condensed water expands rapidly during vaporization. If the vapor cannot escape freely and quickly from the sample, then the built-up internal vapor pressure can cause all kinds of failures in the packages [14,15]. 4.3 Temperature Dependence of Diffusivity Based on moisture absorption-desorption experiments and RH step scan experiments performed at various temperatures, one can determine the moisture diffusivity in the BT core material as a function of temperature. The results were plotted in Fig. 7. It can be seen that the moisture diffusivity has an Arrhenius temperature dependence with Do ≈ 6.61×10-3 cm2/s, and Ed ≈ 0.368 eV. Knowing these parameters, one can extrapolate the moisture diffusivity to other temperatures, assuming the diffusion mechanism will not change – which is usually true for T < Tg. Therefore, based on our data, the diffusivity at a reflow temperature of 260°C is estimated to be 2.18×10-6 cm2/s, which is about 121 times the diffusivity at 60°C. In reality, the diffusivity could be even higher since the reflow temperature is above Tg of the BT core. It is estimated that at 260°C, it takes less than 1 sec for a 70 µm thick saturated BT core to lose 50% of the total moisture, and less than 5 sec to lose 90% of the total moisture. If the total BT core thickness increases by introducing multiple core layers or by increasing the thickness of each core layer, the time needed for the same amount of moisture to escape from the BT core will increase in such a way that t ∝ h2, where h is the total thickness of the BT core [17]. This will have a significant impact on package and materials design for optimized preconditioning reliability performance. 5. Comparison with Literature Data Using the standard procedure which involves exposing the specimen to a specific temperature-humidity condition and monitoring the change in sample weight over time, Liu et al studied the moisture diffusivity in neat BT resins and in BT/glass fiber laminates [26]. Their sample thickness was 0.035 inches (0.89 mm) for the neat resins and 0.030 inches (0.76 mm) for the laminates. Their results showed that for BT neat resin, the diffusivity is 1.29×10-8 cm2/s at 50°C / 80% RH, and it increases to 2.45×10-8 cm2/s at 70°C / 80% RH. At


higher than the values reported in refs. [9] and [27], but our results are in good agreement with the diffusivity of BT neat resin [26] or BT laminates reported in [13]. These differences may be attributed to the effect of glass fibers in thick BT laminates. In BT/glass laminates, it is expected that the glass fibers absorb essentially no moisture, all moisture will be absorbed by the BT resin. In a thick BT core, the woven glass fibers form a three-dimensional fabric structure, which hinders 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 a higher diffusivity. This explains why in the 70 µm thick BT core, the observed moisture diffusivity is the same as the one reported in BT neat resin, but for thick BT core, it is much less. This work Linear Fit Neat BT (IBM) BT Laminates (Maryland) Thick BT Core 1 (this work) Thick BT Core 2 (this work) Wong and Rajoo Galloway and Miles

100

-9

2

D (10 cm /s)

90°C / 80% RH, it becomes 5.82×10-8 cm2/s, as shown in Fig. 7. Using the Arrhenius equation, the diffusivity of the BT neat resin can be described using Do = 1.02×10-2 cm2/s and Ed = 8.72 Kcal/mol or 0.378 eV. For BT laminates with 280 Eglass fabrics, the diffusivity is reduced by nearly 50% under all three temperature/RH conditions, with Do = 1.50×10-3 cm2/s and Ed = 7.93 Kcal/mol or 0.344 eV. In their study, the resin content for the laminated samples was measured using thermogravimetric analysis (TGA), but the results were not directly reported in the paper. However, based on reported normalized maximum moisture uptake, it can be estimated that for BT/280 laminates, the resin content was approximately 62.5%. Their data was the first evidence to suggest that for a relatively thick BT laminate, the addition of glass fiber and its topological or weave structure can have a huge impact on the moisture diffusion behavior of the material. In another study, Pecht et al studied the moisture diffusion in BT/glass fiber laminates [13]. In their study, all of the laminates were woven E-glass fabric with thickness of either 0.038 or 0.053 cm. The reported diffusivity of BT laminates was 1.22×10-8 cm2/s at 50°C / 50% RH, and 1.65×10-8 cm2/s at 50°C / 85% RH; and it becomes 4.75×10-8 cm2/s at 85°C / 50% RH and 3.03×10-8 cm2/s at 85°C / 85% RH. These values were also plotted in Fig. 7. In Galloway and Miles’ paper [9], the moisture diffusion constant was reported to follow the Arrhenius equation, with Do = 1.2×10-4 cm2/s and Ed = 0.295 eV based on absorption data, and the temperature dependence of the diffusivity was calculated using these values and plotted in Fig. 7. Based on desorption, Do = 6.0×10-2 cm2/s and Ed = 0.465 eV. In their experiments, the sample thickness ranged between 0.155 to 1.05 ± 0.003 mm, but the exact thickness of the BT epoxy was not given. Wong and Rajoo studied the moisture diffusion behavior of a BT core laminate [27]. Their sample thickness was 0.4 mm, and the in-plane dimensions were much larger than the thickness, so that one dimensional diffusion can be assumed. They concluded that for the BT core, the temperature dependence of the transverse moisture diffusivity follows by the Arrhenius equation with Do = 3.33×10-4 cm2/s and Ed = 0.32 eV, as shown in Fig. 7. Thus, at 30°C, D ≈ 1.6×10-9 cm2/s, at 60°C, D ≈ 4.8×10-9 cm2/s. These results agree very well with Galloway et al’s data on BT epoxy, but again are much lower than the diffusivity data reported in BT neat resin [26]. In addition, the reported Csat was 4.83 mg/cm3 at 30°C / 60% RH, which is about 18% lower than the result obtained from this study (Fig. 2). This is most likely caused by higher glass fiber density in their material, although the exact glass fiber content was not reported [27]. In addition to the 70 µm BT core material, using the conventional method with an analytical balance, we have also measured the moisture absorption behavior of two different thick BT substrates with thicknesses of 0.72 mm and 0.812 mm at 85°C / 85% RH, and the diffusivity results were plotted in Fig. 7. Based on all the data plotted in Fig. 7, it is clear that the moisture diffusivity of 70 µm BT laminated core is much

10

1

0.0028

0.0030

0.0032

0.0034

-1

1/T (K )

Figure 7. Moisture diffusivity of BT core materials obtained from absorption experiments as a function of temperature. Solid circles: this work. Solid line: linear fit of log10D vs 1/T based on diffusivity data obtained from this work. Open diamonds: moisture diffusivity of neat BT resin, as reported in [26]; open squares: diffusivity of BT laminates reported in [13]; dashed line: calculated diffusivity as a function of 1/T using the pre-exponential factor and the activation energy reported in [27]; dash-dotted line: calculated D vs 1/T based on the values of Do and Ed reported in [9]; open and filled triangles (in-house data): diffusivity of thick BT substrates with thicknesses of 0.72 mm and 0.812 mm, respectively. 6. Application – One-Dimensional Model for a Thin DA Film/Substrate Structure As discussed earlier, when the packaging materials are saturated with moisture, a large percentage of it condenses into the liquid phase in the voids or free volumes of the material. During reflow process, condensed water inside these pores, if could not escape fast enough, expands and vaporizes rapidly from the liquid to the steam state, generating tremendous vapor pressure within the material. Based on the obtained moisture diffusivity data for the 70 µm BT core and the die attach films, finite element modeling has been applied to investigate the moisture and vapor pressure distribution in


C film (t ) =

film C sat sub C sat

⋅

∞

∑

n =0

2 ⋅ (−1) n e

−λ2n Dt / h 2

λn

,

(12)

with (2n + 1)π , (13) 2 where D is the substrate moisture diffusivity, h the substrate thickness, and t the reflow time. With the assumed value of an ‘effective’ diffusivity of 5.0×10-6 cm2/s for the substrate, Figure 8 plots the ratio of moisture concentration over the saturated moisture concentration in the die attach film. This assumption about the effective diffusivity is based on the consideration that during reflow, the moisture diffusion is not only driven by Fick’s law, but also by vapor pressure. Therefore, if we still use he Fick’s law for diffusion modeling, we need to obtain an ‘effective diffusivity’ to account for the secondary moisture transport effect. We take this number based on the approximation that the substrate is dried out about 75% according to the limited total weight gain data after reflow. From Figure 8, it can be seen that in the DA film/substrate structure, if the substrate thickness is 220 µm or less, about 80% of the saturated moisture within the DA film will be lost in 6 minutes during reflow. There also exists a critical residual moisture concentration (horizontal dashed line in Fig. 8), above which cohesive delamination within the DA film

λn =

will occur. Based on the equation (12), the diffusivity D, substrate thickness h, and reflow time are three critical parameters to control the residual moisture level in the DA film. Fig. 8 also shows that a significant difference on the residual moisture concentration exists for two slightly different thicknesses. This implies that the package is very sensitive 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 measured diffusivity data for thin BT core, which is an order greater than the data from the thick BT core measurement. If we were to use the diffusivity data for the thick BT core, calculation would have suggested that the moisture loss in the DA film was negligible during reflow even if the substrate thickness is much reduced. 1 280 micro µm

0.8 C/Csat (film)

the UT/SCSP packages [28]. In the following, a onedimensional model for the thin DA film/substrate structure is developed to understand the moisture loss in the die-attach film layer between the die and the substrate. Since the majority of the moisture diffuses out of the DA film through the BT substrate ( which has a total thickness of a few hundred microns) rather than from the mold compound side (which has a thickness of up to or more than a thousand microns from the die attach edge to the mold compound edge), the model considers the moisture diffusion through the substrate only. Following assumptions are made: (1) The substrate thickness is much smaller comparing to its in-plane dimensions, thus one-dimensional moisture diffusion model is valid; (2) The die-attach film thickness (typically about 25 µm) is an order of magnitude smaller than the total substrate thickness, thus the substrate on the die-attach side is insulated in our model; (3) The moisture content in the substrate and the DA film are fully saturated before reflow, thus the initial moisture distribution in the DA film is uniform. In addition, since the DA film has a higher diffusivity than that of the BT and its thickness is much smaller than the substrate thickness, therefore, the moisture concentration in the DA film, denoted as Cfilm, can be considered uniform at any time; (4) The saturated moisture concentration is independent of temperature (as we have discussed in section 4), thus the interface continuity Cfilm/Csatfilm = Csub/Csatsub holds all the time; (5) A step change in temperature during reflow is assumed and the effective diffusivity D of the substrate at high temperature is used according to the Arrhenius relationship. Under these assumptions, the moisture concentration in the DA the film as a function of time during reflow can then be derived as:

220 micro µm

0.6

Fail

0.4

Pass

0.2 Critical moisture concentration

0 0

60

120

180

240

300

360

Time (seconds)

Figure 8. Estimated moisture loss in DA film during reflow process based on two different substrate thicknesses. 7. Conclusions Moisture absorption-desorption behavior of a 70 µm BT laminate used as the UT/SCSP substrate core material has been characterized in-situ using a sorption TGA equipped with a moisture chamber. Based on moisture absorption experiments, the moisture diffusivity of this thin BT core has an Arrhenius temperature dependence, D = Doexp(-Ed/kT), where Do ≈ 6.61×10-3 cm2/s, and Ed = 0.368 eV. The measured diffusivity agrees well with reported value of BT neat resin, but it is much higher than that of the thick BT core materials. This difference is attributed to the effect of glass fiber: in thick BT laminates, glass fibers forms a 3D network structure, which greatly hinders moisture diffusion. In thin BT cores, the glass fibers are mainly in a 2D structure, which has a much less effect on moisture diffusion in the BT resin. Experimental results revealed that the saturated moisture content of the 70 µm BT core, Csat, has little dependence on temperature, and it depends approximately linearly on relative humidity. At 60% RH, Csat ≈ 5.9 – 6.1 mg/cm3; at 80% RH, Csat ≈ 8.7 – 8.9 mg/cm3. Based on literature diffusivity data obtained from the thick BT samples, one would conclude that the moisture concentration in the die attach film in a stack die chip scale package would not change significantly during reflow process. However, based on the diffusivity data obtained for thin BT core in this work, we can show that a small change in substrate thickness can result in a substantial difference in the residual moisture concentration in die attach film, leading to


very different reliability performance. Indeed, such reliability results correlated well with the experimental data. Acknowledgments We are grateful to Drs. Paul Koning and Ibrahim Bekar for stimulated discussions. We want to thank Dr. Steve Cho for carrying out moisture uptake experiments on two sets of thick BT substrate cores, and Mr. Fred Cardona for measuring moisture uptake on the third set of thick BT cores. References 1. 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 physical basis of moisture transport in a cured epoxy resin system”, J. Appl. Polym. Sci., Vol. 30, No. 11 (1985), pp. 4467-4493. 3. Y. C. Lin, and X. Chen, “Moisture sorption-desorptionresorption characteristics and its effect on the mechanical behavior 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, internal presentation, Intel Polymer Workshop, Nov. 2004. 5. E. Stellrecht, B. T. Han, and M. G. Pecht, “Characterization of hygroscopic swelling behavior of mold compounds and plastic packages”, IEEE Trans. Comp. Packag. Technol., Vol. 27, (2004), pp. 499-506. 6. H. Ardebili, E. H. Wong, and M. Pecht, “Hygroscopic swelling and sorption characteristics of epoxy molding compounds used in electronic packaging”, IEEE Trans. Comp. Packag. Technol., Vol. 26, No. 1 (2003) pp. 206214. 7. Y. He and X. J. Fan, unpublished results, Intel Corporation, 2005. 8. I. Fukuzawa, S. Ishiguro, and S. Nanbu, “Moisture resistance degradation of plastic LST’s by reflow soldering”, Proc. 23rd International Reliability Phys. Symp., pp. 192-197, 1985. 9. J. E. Galloway and B. M. Miles, “Moisture absorption and desorption predictions for plastic ball grid array packages”, 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 on adhesion strength between solder ball and epoxy underfills”, Polymer, Vol. 38, No. 15 (1997), pp. 38113818. 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 of material properties to reliability performance of anisotropic 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 ultrathin stacked die chip scale packages”, Intel Manufacturing Excellence Conference (IMEC) 2006 technical paper. 15. Zhenyu Huang, John Tang, Changmin Hu, Michael Wang, Mu Zhang, Bin Liu, Xuejun Fan, and Edward Prack, “Moisture induced cohesive delamination in dieattach film in ultra thin stacked chip-scale package”, Intel Assembly Test Tech. J., 2006. 16. ASTM D5229, Standard Test Method For Moisture Absorption Properties and Equilibrium Conditioning of Polymer Matrix Composite Materials, ASTM 1998. 17. The easiest way for such estimation is to use a simplified 0.75 ⎤ ⎡ Mt ⎛ Dt ⎞ ⎥, ⎟⎟ expression of eq. (8): = 1 − exp ⎢− 7.3 ⎜⎜ 2 M∞ ⎢ ⎥ h ⎠ ⎝ ⎣ ⎦ where h is the total thickness. See also, C. H. Shen, and G. S. Springer, “Moisture absorption and desorption of composite materials”, J. Compos. Mater., Vol. 10, No. 1 (1976), pp. 2-10; T. Ferguson and J. Qu, “Moisture absorption analysis of interfacial fracture test specimens composed of no-flow underfill materials”, J. Electron. Packag.: Trans. ASME, Vol. 125, No. 1 (2003), pp. 2430. 18. J. Crank, The Mathematics of Diffusion, 2nd ed., Oxford University Press, Oxford, 1990. 19. R. J. Borg and G. J. Dienes, An Introduction to Solid State Diffusion, 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 moisture diffusion in woven composites”, Compos. Sci. Technol., Vol. 65, (2005), pp. 817-826. 23. X. J. Fan, Moisture Related Reliability Issues in Electronic Packaging, ECTC short course, 2006. 24. Li-Rong Bao, A. F. Yee, Effect of Temperature on Moisture Absorption in a Bismaleimide Resin and Its Carbon Fiber Composites, Polymer, Vol. 43, No. 14 (2002), pp. 3987-3997, and refs. [11-14] cited in this paper. 25. Consider Csat = 6 mg/cm3 at 60°C, and the volume fraction of the free volume or voids is 5% of the total sample volume. The actual moisture density inside the voids is then 120 mg/cm3. If the moisture is not condensed into the liquid form but still in gas form, then the internal pressure inside the voids can be calculated using the ideal gas approximation: pV = nRT. For V = 1 cm3, n = 0.12/18 = 0.0067 mole, R = 8.314 J/mol⋅K is the


26.

27.

28.

29.

universal gas constant, for T = 60°C or 333 K, p is estimated to be 18.549 MPa, which is ~183 atm. P. C. Liu, D. W. Wang, E. D. Livingston, and W. T. Chen, “Moisture absorption behavior of printed circuit laminate materials, advances in electronic packaging”, Proc. of the 1993 ASME International Electronics Packaging Conf., Vol. 1, 435-442, American Society of Mechanical Engineers, September 29-Oct. 2, 1993, Binghamton, NY. Also quoted in ref. 12. E. H. Wong and R. Rajoo, “Moisture absorption and diffusion characterization of packaging materials – advanced treatment”, Microelec. Reliability, Vol. 43, No. 12 (2003), pp. 2087-2096. [Notice that Table 2 contains a typo: the units of D0 should by cm2/s, not ×10-9 cm2/s. Also, private communication with E. H. Wong, 2006]. Bin Xie, Daniel Shi, and Xuejun Fan, “Sensitivity investigation of reflow profile and substrate thickness on wafer level film failures in 3-D chip scale packages by finite element modeling”, IEEE Electronic Components and Technology Conference (ECTC), 2007. Xuejun Fan, Daniel Shi, Ibrahim Bekar, Edward Prack, Jianfeng Wang, Zhenyu Huang, Yi He, and Anthony Fischer, “Dicing tape format die-attach film selection methodology for ultra-thin stacked die chip scale packages”, IEEE Electronic Components and Technology Conference (ECTC), 2007.
