Effect of Moisture on Thermal Properties of Halogen ... - IEEE Xplore

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IEEE TRANSACTIONS ON DEVICE AND MATERIALS RELIABILITY, VOL. 11, NO. 1, MARCH 2011

Effect of Moisture on Thermal Properties of Halogen-Free and Halogenated Printed-Circuit-Board Laminates Lili Ma, Bhanu Sood, Member, IEEE, and Michael Pecht, Fellow, IEEE

Abstract—Moisture plays an important role in the integrity and reliability of printed circuit boards (PCBs). The presence of moisture in a PCB alters its thermal performance and thermomechanical properties, thereby affecting overall performance. Due to the shift in market trends toward halogen-free products, halogen-free PCB materials have recently gained popularity. There are many studies on the behavior of halogenated PCBs exposed to moisture, but there are less data available on the use of halogen-free flame retardants in PCBs. Therefore, it is necessary to understand the moisture behavior of halogen-free PCB materials and the effect of moisture on material thermal properties when compared with halogenated PCBs materials. In the past, the Center for Advanced Life Cycle Engineering has conducted thermal-property measurements on halogen-free PCB materials. Measurements were conducted per industry-adopted test methods, including preconditioning of test samples. Some measurement results did not match manufacturers’ datasheets. This paper examines the dependence of out-of-plane coefficient of thermal expansion, glass-transition temperature, time to delamination, and decomposition temperature on the moisture content in halogenated and halogen-free PCB materials. Four types of PCB materials from two manufacturers, including two halogen-free and two halogenated, were tested in this paper. Furthermore, this paper investigates the suitability of IPC-TM-650 preconditioning steps for thermal-property measurements. Index Terms—Coefficient of thermal expansion (CTE), glasstransition temperature, halogen-free, laminate, moisture.

I. I NTRODUCTION

T

HE RELIABILITY of printed-circuit-board (PCB) laminates is strongly influenced by the presence of moisture. Moisture can be initially present in the epoxy glass prepreg, absorbed during the wet processes in the manufacturing of the PCBs, or diffuse into the PCB during storage [1]. Moisture may reside in the resins, resin/glass interfaces, and microcracks or voids due to defects. Moisture can cause internal shorts through metal migration, interfacial degradation resulting in Manuscript received April 19, 2010; revised July 27, 2010; accepted August 25, 2010. Date of current version March 9, 2011. This work was supported in part by the Center for Advanced Life Cycle Engineering Consortium (CALCE). The work of L. Ma was supported in part by the Chinese Scholarship Council and in part by CALCE. L. Ma is with the State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, China (e-mail: [email protected]). B. Sood and M. Pecht are with the Center for Advanced Life Cycle Engineering, University of Maryland, College Park, MD 20742 USA (e-mail: [email protected]; [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TDMR.2011.2106785

conductive-filament formation, and changes in dimensional stability [2]. Moisture also reduces the glass-transition temperature and increases the dielectric constant, leading to a reduction in circuit switching speeds and an increase in propagation delay times [3]. Due to the shift in market trends toward halogen-free products, halogen-free PCB materials have recently gained popularity. Halogen-free PCBs rely on less well-understood flame retardants compared with halogenated materials. There are many studies on the moisture behavior of halogenated PCBs [1], [3], [5], [6] but less data available on the use of halogenfree flame retardants in PCBs. Demaree [7] evaluated the thermal stability of halogen-free laminated materials but did not characterize the effect of moisture on the thermal properties of halogen-free materials. Wong and Rajoo [8] reported that halogen-free materials absorb more moisture than halogenated materials; however, they did not characterize the effect of moisture on the thermal properties of halogen-free materials. Therefore, it is necessary to understand the moisture behavior of halogen-free PCB materials and the effect of moisture on material thermal properties when compared with halogenated PCB materials. In the past, the Center for Advanced Life Cycle Engineering (CALCE) has conducted material-property measurements on halogen-free PCB materials, including glass-transition temperature (Tg ), coefficient of thermal expansion (CTE), decomposition temperature (Td ), time to delamination (T-260, T-288), and water absorption. Some measurement results were not in accordance with manufacturers’ datasheets. In some control sets, Tg results were more than 5 ◦ C lower than those in the datasheets, and Td results were 20 ◦ C higher than those in the datasheets. In this previous CALCE study, the control sets were conducted per IPC methods, including preconditioning of test samples [9]. This paper examines the dependence of out-of-plane CTE (z-CTE), glass-transition temperature, time to delamination, and decomposition temperature on the moisture content in halogenated and halogen-free PCB materials. Furthermore, this study establishes the suitability of preconditioning steps for thermal-property measurements in IPC-TM-650 test methods. II. E XPERIMENTAL D ETAILS A. Materials Four PCB materials—two halogen-free (A and C) and two halogenated (B and D)—were tested. These laminates were

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MA et al.: EFFECT OF MOISTURE ON HALOGEN-FREE AND HALOGENATED PCB LAMINATES

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TABLE I T EST M ATERIAL P ROPERTIES (DATASHEET )

TABLE II M EASUREMENT M ETHODS

acquired from two manufacturers (A and B from manufacturer I, and C and D from manufacturer II). The manufacturers were chosen from different geographic locations, with manufacturers I and II from Taiwan and Japan, respectively. The laminate properties are shown in Table I according to their datasheets. B. Test Methods The test methods and equipment used for measuring the properties are listed in Table II. The measurement procedures for each of the properties are discussed in the following sections. 1) CTE: The CTE of a laminate system is the fractional change of linear dimensions with temperature. z-CTE is dominated by the resin system, whereas in-plane CTE is dominated by the glass fabric. z-CTE influences failure mechanisms, such as barrel cracking and delamination, and in-plane (warp/fill) CTE affects shear failures of solder joints. Three samples of each material, each approximately 6.35 mm × 6.35 mm in size, were tested in this study. The samples were polished to ensure parallel edges, and the copper cladding was etched off using sodium per sulfate solution. The CTE of laminate materials along the z-CTE direction was measured using a Perkin–Elmer thermomechanical analyzer (TMA) (Pyris TMA 7). The TMA heated the sample from 30 ◦ C to 200 ◦ C at a 10 ◦ C/min ramp rate. The TMA measured z-CTE by monitoring the sample’s change in thickness. The laminate had one z-CTE below its Tg and another above its Tg , each of which were measured from the TMA results. A typical z-CTE measurement plot is shown in Fig. 1. 2) Glass-Transition Temperature (Tg ): The glass-transition temperature (Tg ) of a resin system is the temperature at which a material transforms from a rigid and glasslike state to a rubbery and compliant state due to the reversible breakage of Van der Waals bonds between the polymer molecular chains. Certain properties, such as thermal expansion, Young’s modulus, heat capacity, and dielectric constant, undergo a change at around

Fig. 1. z-CTE measurement plot.

Tg . Therefore, it is very important to determine the glasstransition temperature under ambient and moisture conditions in order to define the temperature range in which these systems can be used without decreasing their properties at the service temperature. Three samples from each material type weighing 15–30 mg each were tested. The edges were smoothed and burrs were removed by sanding, and the copper cladding was etched off using sodium per sulfate solution. Tg was measured using a Perkin–Elmer differential scanning calorimeter (DSC) (Pyris 1 DSC). The specimens were subjected to a temperature scan of 30 ◦ C to 200 ◦ C at a rate of 20 ◦ C/min. At Tg , the heat capacity of a material changes, and this is captured by a step transition in the DSC measurement curve. Tg is identified as the midpoint of the step transition (across which the heat capacity of the material changes) in the DSC measurement plot (Fig. 2). 3) Decomposition Temperature (Td ): Decomposition temperature (Td ) is the temperature at which a resin system undergoes irreversible physical and chemical degradation with thermal destruction of the cross links, resulting in weight loss of the material. Two samples from each material type weighing 10–20 mg each were tested. The edges were smoothed and burrs were removed by sanding, and the copper cladding was etched

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Fig. 2. Glass-transition-temperature measurement plot.

off. The Td was measured using a thermogravimetric analyzer (TGA) (Shimadzu TGA 50). The specimens were subjected to a temperature scan of 25 ◦ C to 550 ◦ C at a rate of 10 ◦ C/min. The change in weight of the sample was obtained as a function of temperature, and Td was recorded at 2% and 5% weight loss compared with the sample weight at 50 ◦ C. 4) Time to Delamination (T-260, T-288): Time to delamination is the time it takes for a laminate material to delaminate (defined as the separation between layers of prepregs and copper clad cores in a multilayered structure) when exposed to a constant temperature. A temperature of 260 ◦ C (T-260) is used in the industry as a metric for assessing the leadfree process compatibility of laminates. The T-288 delamination time provides a more appropriate level of performance given the process temperature required for lead-free soldering. Both parameters were tested in this study. Four samples, each 6.35 mm × 6.35 mm in size, were tested using a TMA (Pyris TMA 7). The samples were subjected to a temperature scan of 25 ◦ C to 260 ◦ C or 288 ◦ C at a ramp rate of 10 ◦ C/min and then held at 260 ◦ C or 288 ◦ C until an irreversible change in thickness of the sample was observed or 30 min elapsed, whichever came first [10]. Time to delamination was determined as the time between the onset of isotherm (260 ◦ C or 288 ◦ C) and the onset of delamination. C. Experiments Copper-cladded materials that were cut from some laminates were etched to remove copper from the laminate surface. The samples were placed in a solution of water and sodium per sulfate on a hotplate until all the copper was etched from the surface; they were then cleaned under running water. The concentration of the solution and the operating temperature were noted on the package of the sodium per sulfate. After etching, the samples were dried in an air-circulating oven for 2 h at 105 ◦ C. Then, four coupons of each material were prepared by cutting the laminate material into 20 mm × 20 mm squares for studying the moisture absorption and desorption behavior. 1) Moisture Absorption and Desorption Experiments: Moisture-absorption and moisture-desorption experiments were conducted on four coupons of each PCB material. These experiments were conducted in order to characterize the moisture absorption and desorption behavior of the samples at different exposure times. The test started with the laminate from off

Fig. 3.

Rate of moisture absorption and moisture desorption of test materials.

the shelf. Two coupons were exposed to 85 ◦ C and 85% RH, and another two coupons were baked at 105 ◦ C in an air-circulating chamber from room-storage conditions. The moisture content of each laminate was measured at increasing time intervals. The weight of each laminate sample was measured with an analytical balance (Mettler AE100) having a resolution of 0.1 mg. After measuring the weight, the test coupons were put back into their chambers within 5 min. The moisture absorption and desorption results of the four PCB laminates are graphically shown in Fig. 3. Weight gain and weight loss were expressed as a percentage of the laminate’s initial mass Mt , which was determined by Mt (wt%) =

mt − m0 × 100% m0

(1)

where mt and m0 are the weights of a specimen at exposure time t and of the initial specimen, respectively. The results showed that the moisture absorption and desorption rates in the four laminates were fairly rapid in the early stages, and then the rates decreased with time. Based on the results from the absorption and desorption experiments, time intervals were selected for testing the z-CTE and Tg of the four laminates. The moisture diffusion rates of PCB laminates are affected by temperature and humidity [1], [3]. To accelerate the experimental process, the moisture absorption experiments were also conducted using boiling water. The results are shown in Fig. 4. All test materials were saturated after being immersed in boiling water for 240 h. 2) Impact of Moisture Absorption on z-CTE and Tg : Based on the results from the absorption and desorption experiments, ten sets of test coupons of each sample were prepared (Table III). One set was preconditioned and tested according to the IPC-TM-650 2.4.24 (TMA) [11] and IPC-TM-650 2.4.25 (DSC) [12] test methods as a control coupon. Coupon sets 2–4 were baked at 105 ◦ C for 24, 48, and 72 h, respectively, followed by cooling in a desiccator to room temperature before measurement. Coupon sets 5–10 were exposed to 85 ◦ C and 85% RH for 24, 48, 96, 192, 504, and 1200 h, respectively, to induce different moisture contents. Some of the coupons were tested directly without preconditioning, while the others were baked at 105 ◦ C for 2 h in an air-circulating oven followed by


Fig. 4.

Rate of moisture absorption of test materials in boiling water.

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Fig. 5. Normalized results of moisture absorption in boiling water.

TABLE III P RECONDITIONING P ROCESS OF T EST C OUPONS B EFORE z-CTE AND Tg M EASUREMENTS

Fig. 6. Rate of moisture desorption of laminates at 105 ◦ C.

III. R ESULTS AND D ISCUSSION A. Moisture Absorption and Desorption

TABLE IV T HICKNESS AND G LASS /R ESIN R ATIO OF F OUR M ATERIALS

cooling in a desiccator to room temperature per IPC method before measurement. 3) Impact of Moisture Absorption on Decomposition Temperature and Time to Delamination: Four sets of test coupons of each material were prepared. Set 1 and Set 2 were preconditioned and tested according to the IPC-TM-650 2.4.24 (time to delamination) [10] and IPC-TM-650 2.4.24.6 (decomposition) [13] test methods, respectively, as controls. For accelerating the moisture-absorption process, Set 3 and Set 4 coupons were immersed in boiling water for moisture absorption. Then, the test coupons were preconditioned per IPC test method before measurement to study the impact of moisture absorption on decomposition temperature and time to delamination.

The moisture concentration in a laminate increases with exposure time and approaches equilibrium after several days when exposed to a humid environment. The time to reach equilibrium depends on the thickness of the laminate and the ambient temperature. The laminate samples were assumed to be relatively thin. Thus, diffusion from the laminates’ edges was negligible. Moisture diffusion in laminates follows a 1-D Fickian equation. For Fickian diffusion in a laminate with thickness exposed on both sides to the same environment, the moisture content Mt at time t is given by the expression Mt 4 = M∞

Dt π

12 (2)

where M∞ is the equilibrium moisture content and D is the diffusion coefficient, or diffusivity, given in square of the length per unit time [3]. Moisture diffusion will only occur through the epoxy, as the reinforcement glass does not readily absorb moisture. Epoxies can also inherently absorb different moisture contents; also, the varying glass/resin ratios between different PCB constructions will result in varying moisture concentrations and maximum moisture-uptake values. As a result, the moisture absorption

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Fig. 7. z-CTE values of samples with various moisture contents and datasheet regions.

Fig. 8. CTE curves of sample A after exposure to 85 ◦ C and 85% RH for 24 h. One part was preconditioned per IPC-TM-650 2.4.24 test method before measurement compared with no preconditioning part and a control set.

characteristics of PCB materials will vary by epoxy type, construction, and epoxy/fiber content [14]. A TGA was used for measuring the glass/resin ratio for each material. Based on past experience, the specimens were subjected to a temperature scan of 25 ◦ C to 540 ◦ C at a rate of 10 ◦ C/min then held at 540 ◦ C for 60 min to make the epoxy resin decompose entirely. The calculated results are shown in Table IV. As we can see from Table IV, Fig. 3, and Fig. 4, materials A and B have the same glass/resin ratio, but material A absorbed more moisture since it has a different halogen-free epoxy-resin system. Material D absorbed more moisture since it has more epoxy resin in its system. After normalizing the glass/resin ratio, moisture absorption results were plotted and shown in

Fig. 5. From Fig. 5, we can see that halogen-free materials A and C absorbed more moisture in boiling water than did halogenated materials B and D. Moisture desorption experiments were conducted on the coupons exposed to 85 ◦ C and 85% RH for 2112 h and on the coupons prepared from room-storage conditions (Fig. 6). During ten days of moisture-desorption experiments, the moisture content decreased rapidly in the first 24 h. As shown in Fig. 6, halogen-free materials A and C released more moisture than did halogenated materials B and D in the desorption process starting from the room-storage conditions. In other words, halogen-free materials A and C absorbed more moisture in room-storage conditions. Neither of the room-storage specimens lost weight after baking for 72 h nor did any of the specimens that were exposed to 85 ◦ C and 85% RH for 2112 hrs after baking for 120 h. After the baking process, the final weights of the specimens that were baked after being exposed to 85 ◦ C and 85% RH for 2112 h were greater than the specimens baked from room-storage conditions. The water molecules absorbed into the epoxy can be classified as either bound water or free water [15], [16]. Bound water is trapped at polar sites and is usually bonded to hydroxyl groups in the epoxy network. Free water is clustered in the free volume or voids inside the epoxy. The free volume of the polymeric resin is defined as the volume of the resin without the volume of the polymer chains or the volume due to thermal vibrations of the polymer chains. The bound water that was bonded to hydroxyl groups in the epoxy network was not released in the baking process. As a result, the moisturecontent values of coupons subject to 85 ◦ C and 85% RH were higher than those of the coupons in room-storage conditions after the baking process, as shown in Fig. 6.


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Fig. 9. CTE curves of the four samples after exposure to 85 ◦ C and 85% RH for various times and later preconditioned per IPC-TM-650 2.4.24 test method before measurement.

Fig. 10. Tg results of the four samples after exposure to 85 ◦ C and 85% RH for different durations.

B. Effect of Moisture on z-CTE The z-CTE values (below Tg and above Tg ) of the four PCB laminates with different moisture contents are plotted and

shown in Fig. 7. The datasheet regions are also illustrated. Fig. 8 displays the CTE curves of sample A with different moisture conditions. After exposing sample A to 85 ◦ C and 85% RH

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Fig. 11. Moisture absorption and Tg results of samples A and B in boiling water.

for 24 h, one part was preconditioned per IPC-TM-650 2.4.24 test method and then the CTE was measured. The other part was tested without preconditioning. The two results compared with a control set are plotted and shown in Fig. 8. Fig. 9 shows the CTE curves of the four samples that absorbed moisture at 85 ◦ C and 85% RH for various lengths of time and were then preconditioned per IPC-TM-650 2.4.24 test method before measurement. There was no obvious trend in the z-CTE values calculated from the data below or above the glass-transition region, as shown in Fig. 7. However, there was appreciable deviation on the z-CTE curves around the glass-transition region, as shown in Figs. 8 and 9. Preconditioning per IPC test methods can reduce the effect of moisture on z-axis expansion. However, the swelling in the glass-transition region increased with the increase of moisture content, particularly in halogen-free material A, which absorbed more moisture than the other materials in the moisture-absorption experiments. It can be inferred from the CTE curves in Fig. 8 that there were three phases, each showing different thermal expansion rates. In phase 1, z-axis expansion was dominated by the resin, and hence, the CTE values below Tg were close to that of epoxy resin (∼ 60 ppm/◦ C). In phase 3, the PCB material expanded because an increase in temperature led to greater thermal vibration of the atoms in the material, and hence, there was an increase in the average separation distance of adjacent atoms. Therefore, the PCB material had a large expansion rate and became more viscous above Tg . In phase 2, there were two factors that caused appreciable deviation in the z-CTE values. First, moisture caused swelling in the PCB laminate above 100 ◦ C and increased in the z-axis expansion below the original Tg point. Second, water acted as a plasticizer in the epoxy system, increasing the viscosity of the epoxy resin and resulting in the reduction of Tg . Hence, the expansion rate was large below the original Tg point. As a result, the total z-axis expansion of the laminate increased as the moisture content in the laminate increased. C. Effect of Moisture on Tg Previous work has found that the absorbed water in epoxy materials leads to a decrease in the glass-transition temperature due to the plasticizing effect of water [17]–[20]. DSC results

Fig. 12. Td results of four samples after being saturated in boiling water compared with the control set.

Fig. 13.

Typical TGA curves for the four laminates.

from our experiments proved that the Tg of the epoxy samples did indeed decrease with a moisture increase in the early stages (until around 192 h, except for sample D), as shown in Fig. 10. However, Tg increased in the later stages (post-192 h except for sample D) after exposure for a long time to 85 ◦ C and 85% RH. In order to verify that the Tg results obtained from this experiment were also influenced by temperature exposure, Tg tests were repeated on specimens A and B, which were immersed in


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TABLE V T IME TO D ELAMINATION R ESULTS FOR F OUR S AMPLES A FTER A BSORBING M OISTURE IN B OILING WATER C OMPARED W ITH THE C ONTROL S ET

boiling water. Boiling water was used to accelerate the sample moisture-absorption rate. The results are shown in Fig. 11. The same trends were found in samples A and B when they were immersed in boiling water. The results indicate that the Tg of a water-saturated epoxy depends strongly on exposure time and temperature. At the time when the hygrothermally exposed materials almost reach saturation, the depression of Tg is greatest. However, Tg begins to gradually recover after saturation. A higher immersion temperature and longer immersion time induces a greater degree of recovery of Tg . A decrease in the Tg of an epoxy system is usually associated with entrapped plasticizers. Water typically acts as a plasticizer in epoxy systems, resulting in the reduction of Tg . An increase in Tg is associated with an increase in cross-linking density. Heat facilitates the chemical reactions that result in the cross linking of polymers, thus increasing Tg . According to the Tg results from both the 85 ◦ C and 85% RH conditions and boiling water, we can see that during the early stages, the test samples absorbed more moisture, Tg decreased rapidly, and moisture dominated the trends. In the later stages, the samples were nearly saturated, Tg increased with time under thermal conditions, and temperature dominated the trends. This kind of behavior can also be explained by the findings of Zhou and Lucas [21]. They have claimed that absorbed water molecules forming double hydrogen bonds would cause an increase in Tg . According to their findings, water molecules bind with epoxy resins through hydrogen bonding. Two types of bound water were found in epoxy resins. The binding types are classified as Type I or Type II bonding, depending on differences in the bond complex and activation energy. These two types of bound water have quite different influences on Tg variation. Type I bound water disrupts the initial interchain Van der Waals force and hydrogen bonds, resulting in increased chain-segment mobility and acting as a plasticizer, thus decreasing Tg . In contrast, Type II bound water contributes to an increase in Tg in water-saturated epoxy resin by forming a secondary crosslink network. Experimentally determined Tg values represent the combined effect of the two mechanisms.

to IPC-TM-650 2.4.24.6, the Td for the specimen that was saturated in boiling water did not show noticeable variation compared with the control set. In other words, moisture had no obvious effect on Td after preconditioning per IPC standard for all of the materials in this study. Further, typical TGA curves for the four test materials are plotted and shown in Fig. 13. The halogenated materials (B and D) started to decompose at 370 ◦ C and 320 ◦ C, respectively, and subsequently experienced a rapid degradation. Materials B and D underwent degradation from 2% to 5% within a narrow temperature range, which was not the case with halogen-free materials (A and C). The halogenfree materials (A and C) experienced a slope degradation, which indicated that the halogen-free materials had better thermal stability than the halogenated materials. The possible reason is that the halogen-free and halogenated materials have different epoxy-resin systems as they use different flame retardants. The mechanism is not yet clear. E. Effect of Moisture on Time to Delamination Two sets of samples were prepared in this test. One set was prepared and tested per IPC-TM-650 2.4.24 as a control. The other set was immersed in boiling water for ten days. The weight of each laminate was measured before and after immersion with an analytical balance. The laminate thickness was measured with a digital vernier caliper with a 0.01-mm least count. This test ramped the sample from 25 ◦ C to 260 ◦ C or 288 ◦ C at 10 ◦ C/min and held the sample at 260 ◦ C or 288 ◦ C until an irreversible change in thickness of the sample was observed or 30 min elapsed, whichever occurred first. The results are shown in Table V. The time to delamination is a measure of the ability of a dielectric bond line to absorb stresses. As we can see, the time to delamination decreased obviously when materials A and D were immersed in boiling water and absorbed around 1% moisture. The T-260 and T-288 of material B decreased slightly after absorbing 0.5% moisture. Material C did not show any delamination within 30 min in the control set or in the moisture absorption set. Degradation in the time-to-delamination results showed that moisture absorption had affected the bond line.

D. Effect of Moisture on Td Decomposition temperature measurement was conducted by exposing the specimens to a temperature scan of 25 ◦ C to 550 ◦ C under a flow of nitrogen at a heating rate of 10 ◦ C/min. The decomposition temperature measurement results for the four materials corresponding to 2% and 5% weight loss are plotted and shown in Fig. 12. After preconditioning according

IV. C ONCLUSION This paper has examined the effect of moisture on z-CTE, glass-transition temperature, time to delamination, and decomposition temperatures. Four types of PCB materials were studied from two manufacturers, including two halogen-free and two halogenated. Furthermore, this study investigated the

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suitability of IPC-TM-650 preconditioning steps for thermalproperty measurements. The results showed that the glass-transition temperatures of all the test materials were affected by both moisture content and exposure temperature. Tg decreased with an increase in the moisture content in laminates; on the other hand, Tg increased with a longer duration of exposure to thermal conditions. In the early stages, moisture dominated the trends before the laminate materials reached the saturation point, while temperature dominated the trends in the later stages. The results showed that exposure to humidity had no obvious effect on z-CTE values (below 100 ◦ C and above Tg ). However, moisture caused swelling in PCB laminates between 100 ◦ C and the Tg point, and moisture also increased z-axis expansion. Further, Tg and z-CTE results varied with different moisture contents, although the tests were conducted exactly per IPC test methods. The preconditioning steps outlined in IPC-TM-650 2.4.24 and 2.4.25 could not account for the varying moisture contents. The time to delamination of the laminates was degraded by moisture absorption. Moisture had no obvious effect on Td after preconditioning per IPC test methods. However, the halogenated materials underwent degradation from 2% to 5% within a narrow temperature range compared with halogen-free materials.

[13] Inst. Interconnecting Packag. Electron. Circuits, Decomposition Temperature of Laminate Material Using TGA, Bannockburn, IL, Apr. 2006, IPCTM-650 2.4.24.6. [14] P. Hamilton, G. Brist, B. Guy, Jr., and J. Schrader, “Humiditydependent loss in PCB substrates,” in Proc. IPC Printed Circuit Expo, Feb. 2007. [Online]. Available: http://www.ipc.org/ContentPage.aspx? pageid = IPC-Honors-Best-Papers-at-IPC-Printed-Circuits-Expo-APEXand-the-Designers-Summit [15] H. Zhao and R. K. Y. Li, “Effect of water absorption on the mechanical and dielectric properties of nano-alumina filled epoxy nanocomposites,” Composites: Part A, Appl. Sci. Manuf., vol. 39, no. 4, pp. 602–611, Apr. 2008. [16] C. Maggana and P. Pissis, “Water sorption and diffusion studies in an epoxy resin system,” J. Polym. Sci. A, Polym. Chem., vol. 37, no. 11, pp. 1165–1182, 1999. [17] P. Moy and F. E. Karasz, “Epoxy–water interactions,” Polym. Eng. Sci., vol. 20, no. 4, pp. 315–319, Mar. 1980. [18] S. Luo, J. Leisen, and C. P. Wong, “Study on mobility of water and polymer chain in epoxy and its influence on adhesion,” J. Appl. Polym. Sci., vol. 85, no. 1, pp. 1–8, Jul. 2002. [19] E. L. McKague, Jr., J. D. Reynolds, and J. E. Halkias, “Swelling and glass transition relations for epoxy matrix materials in humid environments,” J. Appl. Polym. Sci., vol. 22, no. 6, pp. 1643–1654, Jun. 1978. [20] E. S. W. Kong and M. J. Adamson, “Physical ageing and its effect on the moisture sorption of amine cured epoxy,” Polym. Commun., vol. 24, pp. 171–173, 1983. [21] J. M. Zhou and J. P. Lucas, “Hygrothermal effects of epoxy resin part I: The nature of water in epoxy,” Polymer, vol. 40, no. 20, pp. 5505–5512, Sep. 1999.

ACKNOWLEDGMENT The authors would like to thank M. Zimmerman for his help in copyediting. R EFERENCES [1] P. C. Liu, D. W. Wang, E. D. Livingston, and W. T. Chen, “Moisture absorption behavior of printed circuit laminate materials,” in Proc. Adv. Electron. Packag., 1993, vol. 4, pp. 435–442. [2] B. Rudra, M. Pecht, and D. Jennings, “Assessing time-to-failure due to conductive filament formation in multi-layer organic laminates,” IEEE Trans. Compon., Packag., Manuf. Technol. B, vol. 17, no. 3, pp. 269–276, Aug. 1994. [3] M. Pecht, H. Ardebili, A. A. Shukla, J. K. Hagge, and D. Jennings, “Moisture ingress into organic laminates,” IEEE Trans. Compon. Packag. Technol., vol. 22, no. 1, pp. 104–110, Mar. 1999. [4] E. H. Wong and R. Rajoo, “Moisture absorption and diffusion characterisation of packaging materials—Advanced treatment,” Microelectron. Reliab., vol. 43, no. 12, pp. 2087–2096, Dec. 2003. [5] M. Akay, S. K. A. Mun, and A. Stanley, “Influence of moisture on the thermal and mechanical properties of autoclaved and oven-cured kevlar49/epoxy laminates,” Composites Sci. Technol., vol. 57, no. 5, pp. 565– 571, 1997. [6] J. Lundgren and P. Gudmundson, “Moisture absorption in glassfiber/epoxy laminates with transverse matrix cracks,” Composites Sci. Technol., vol. 59, no. 13, pp. 1983–1991, Oct. 1999. [7] R. Demaree, “Halogen-free systems in current PCB practice,” in Proc. IPC EXPO, 2001, pp. S11-3-1–S11-3-4. [8] E. H. Wong and R. Rajoo, “Trends and challenges of environmentally friendly laminates,” PC FAB, Mar. 2003. [9] R. Sanapala, B. Sood, D. Das, and M. Pecht, “Effect of lead-free soldering on key material properties of FR-4 printed circuit board laminates,” IEEE Trans. Electron. Packag. Manuf., vol. 32, no. 4, pp. 272–280, Oct. 2009. [10] Inst. Interconnecting Packag. Electron. Circuits, Time to Delamination by TMA, Northbrook, IL, Dec. 1994, IPC-TM-650 2.4.24.1. [11] Inst. Interconnecting Packag. Electron. Circuits, Glass Transition Temperature and z-Axis Thermal Expansion by TMA, Northbrook, IL, Dec. 1994, IPC-TM-650 2.4.24. [12] Inst. Interconnecting Packag. Electron. Circuits, Glass Transition Temperature and Cure Factor by DSC, Northbrook, IL, Dec. 1994, IPC-TM650 2.4.25

Lili Ma received the B.S. degree in materials science and engineering from Harbin Engineering University, Harbin, China, in 2003 and the M.S. degree in material science from the University of Electronic Science and Technology of China, Chengdu, China, in 2006, where she is currently working toward the Ph.D. degree in material science. Her research interests are failure analysis of electronic components and packaging and reliability of halogen-free printed-circuit-board materials.

Bhanu Sood (M’08) received the M.S. degrees in advance material processing and materials science from National Technical University of Ukraine and The George Washington University, respectively. He was with the U.S. Naval Research Laboratory in the areas of embedded electronics, micropower sources, laser-assisted microfabrication, characterization of electrically conductive polymeric formulations, and advanced materials. He is the Director with the Test Services and Failure Analysis Laboratory, Center for Advanced Life Cycle Engineering, University of Maryland, College Park. His technical publications include papers on embedded electronics, energy storage systems, and instrumentation for fatigue studies. His research areas include electronic-material characterization, failure analysis of electronic components and assemblies, printed-circuit-board materials, and conductive-filament formation.


Michael Pecht (S’78–M’83–SM’90–F’92) received the M.S. degree in electrical engineering and the M.S. and Ph.D. degrees in engineering mechanics from the University of Wisconsin, Madison. He is the Founder of Center for Advanced Life Cycle Engineering (CALCE), University of Maryland, College Park, which is funded by over 150 of the world’s leading electronics companies. He is also a Chair Professor in mechanical engineering and a Professor in applied mathematics with the University of Maryland. He is currently a Visiting Professor in electronic engineering at City University, Hong Kong. He has written more than 20 books on electronic-product development, use and supplychain management, and over 400 technical articles. He has consulted for over

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100 major international electronics companies, providing expertise in strategic planning, design, test, prognostics, and IP and risk assessment of electronic products and systems. Prof. Pecht is a Professional Engineer, an ASME Fellow, and an IMAPS Fellow. He has served as Chief Editor of the IEEE T RANSACTIONS ON R ELIABILITY for eight years and on the advisory board of IEEE Spectrum. He is Chief Editor for Microelectronics Reliability and an Associate Editor for the IEEE T RANSACTIONS ON C OMPONENTS AND PACKAGING T ECHNOLOGY. He was the recipient of the European Micro- and Nano-Reliability Award for outstanding contributions to reliability research, the 3M Research Award for electronics packaging, and the IMAPS William D. Ashman Memorial Achievement Award for his contributions in electronics reliability analysis. He was also the recipient of the highest reliability honor, the IEEE Reliability Society’s Lifetime Achievement Award in 2008.