基于環(huán)氧樹脂/鈦酸鋇/聚酰亞胺絕緣介質(zhì)的PCB 埋嵌電容的制作及性能研究

周國云何 為王守緒范海霞肖 強(qiáng)

1(電子科技大學(xué)電子薄膜與集成器件國家重點(diǎn)實(shí)驗(yàn)室 成都 610054)2(東莞電子科技大學(xué)電子信息工程研究院 東莞 523808)

基于環(huán)氧樹脂/鈦酸鋇/聚酰亞胺絕緣介質(zhì)的PCB 埋嵌電容的制作及性能研究

周國云1何 為1王守緒1范海霞1肖 強(qiáng)2

1(電子科技大學(xué)電子薄膜與集成器件國家重點(diǎn)實(shí)驗(yàn)室 成都 610054)2(東莞電子科技大學(xué)電子信息工程研究院 東莞 523808)

文中使用疊層技術(shù)制作了以環(huán)氧樹脂/鈦酸鋇/聚酰亞胺為絕緣介質(zhì)的 PCB 埋嵌電容器。制作的電容器容值與設(shè)計(jì)值之間誤差在 —4.0% 到 —6.0% 之間。通過將電容器面積增加 5%，電容器容值誤差降低到了 —1.1% 以下。為了檢測埋嵌電容器的可靠性，分別進(jìn)行了 260℃ 回流焊、高低溫冷熱沖擊、85℃/85% RH 及高壓擊穿測試。測試結(jié)果表明，以環(huán)氧樹脂/鈦酸鋇/聚酰亞胺為絕緣介質(zhì)的 PCB 埋嵌電容器有良好的環(huán)境可靠性，適合用于制作PCB 埋嵌電容器。

鈦酸鋇；埋嵌電容；印制電路板；可靠性

1 Introduction

Due to steadily increasing operating frequencies and the lowering of supply voltage in digital systems, simultaneous switching noise (SSN) is a serious concern, because it can affect the performance of high-speed systems[1,2]. Embedding capacitors in the inner layers of a circuit board is the preferred method to effectively decrease the SSN without increasing the size of product[3,4]. Accordingly, this method is of great interest to printed circuit board (PCB) manufacturers, especially to those who produce portable products with a wide range of operating frequencies[5-7].

The development of embedded capacitors should be divided into three parts∶ material, manufacturing process, and embedding reliability[7]. A polymer/ ceramic composite is one of the promising materials to fabricate embedded capacitors[8,9]. The epoxy/ BaTiO3is the preferred choice, because it has the combining advantages of dielectric constant of ceramic powders, good compatibility with PCBs, and excellent process ability of polymers[9]. Several approaches were employed toward the realization of embedded capacitor technology. The important techniques include sputtering, sol-gel, hydrothermal synthesis, anodization, screen printing, spin coating and roll coating[1]. For the mass-production, it is subjected to the build-up process using the copper clad laminate (CCL). The CCL contains the capacitor material deposited between the copper layers by the above-mentioned technologies[10]. The reliability of the embedded capacitors ultimately determines the breadth and success of their practical applications. The capacitance of an embedded capacitor can change due to various environmental stresses. The effect of various environment, such as thermal aging, temperature cycling, and temperature-humidity on the epoxy/BaTiO3were investigated by many capacitor reliability tests[1,11,12].

3M C-ply product performance test showed that the breakdown voltage of epoxy/BaTiO3substrate was around 1500 V with the thickness of 1/2 mil[13]. Capacitors with epoxy/BaTiO3dielectric experienced less than 10% decrease in capacitance after 1400 cycles at —55℃ and 125℃[11]. Lee et al.[14]fabricated the embedded capacitors aged at 85℃/85% RH for 24 h and reflowed three times at 260℃ for 60 s, respectively. The results showed that the capacitances were increased by 10% due to moisture absorption and decreased by 30% after solder reflow process. Polyimide (PI) has excellent thermal reliability with Tg up to 260℃ and low water absorption less than 1%[15]. The PI introducing in the epoxy/BaTiO3should improve the capacitive reliability of the as-embedded capacitor for PCB.

In this study, we have produced embedded capacitors in PCBs using multilayer PCB build-up process. A commercial epoxy/BaTiO3/PI embedded capacitor CCL was employed as the substrate. The reliability of as-fabricated capacitor was evaluated by various environmental tests, including thermal shock, high-voltage breakdown, thermal cycling and 85℃/85 % RH tests.

2 Experimental

2.1 Materials

The used embedded capacitor CCLs were purchased from Mitsu Co. Ltd. (MC25L and BC12TM). The capacitive properties of CCL, including the dielectric constant (εr), the Dfand capacitance value, were listed in table 1. Note that the listed parameters intable 1 were tested at the frequency of 1 MHz.

Table 1. The properties of capacitor CCL BC12TM and MC25L

2.2 Design and Fabrication

The fabricated capacitors were embedded in the 10-layer PCB by CCL BC12TM and 4-layer PCB by CCL MC25L. Their geometries were designed into the rectangle structures with the size of 8.73 mm×24.575 mm and circle structures with the diameter of 10.21 mm, respectively. These two capacitor structures were embedded in the 4thand 5thlayers, 6thand 7thlayers for the 10-layer PCB, and the 2ndand 3rdlayers for the 4-layer PCB. The PCBs were assembled by the conventional build-up process. The build-up structures were shown in Fig. 1.

2.3 Characterizations

The capacitance values were measured by the LCR of HIOKI 3532-50 at the frequency of 1 MHz. Yangzi YD 9810 was employed to define the breakdown voltage of the capacitor insulating layer. Note that the voltage was increased from 0 V at the rate of 100 V/min. The reliability of embedded capacitors was investigated by thermal cycle during two temperatures, —55℃ to 125℃. The morphologies of embedded capacitors were characterized by Olympus optical microscope.

3 Results and Discussion

Fig. 1. Build-up structures of the multilayer PCB used to embedded capacitors

Prototype scale embedded capacitors in organic substrates were fabricated using conventional PCB build-up process. Fig. 2 illustrated the fabricatedcapacitor samples. The 10-layer PCB with embedded rectangle capacitor was showed in Fig. 2(a), and the Fig. 2(b) displayed the circle capacitor embedded in the 4-layer PCB. The as-plated holes were used as the points for capacitance testing. The cross-section images of the rectangle capacitor in 10-layer PCB were shown in Fig. 2(c) and 2(d). It can be seen that the capacitor consists of two-flat coppers as the electrodes and the very thin sandwiched materials as the dielectric layer. The capacitances of these embedded capacitors with different geometries and different CCL materials were detailed as shown in table 2.

Table 2 demonstrated the effects of embedded position and capacitor geometry on the capacitance tolerances. As seen from the data, the embedded position and geometry do not make obvious effects on the capacitance. All the capacitors were calculated in the range of —4.0% to —6.0% in capacitance tolerance. The tolerance of an electronic component is the quantitative magnitude for evaluating the value variation to meet practical needs. For the sake of uniformity, capacitors ranked with 5% (first grade), 10% (second grade) and 20% (third grade)tolerance to standard values, are generally accepted by electronic manufacturers[16,17]. Obviously, the asfabricated capacitors belong to the first grade or the second grade components.

As was shown in the table 2, the capacitance variances are lower than 5%, indicating these fabricated capacitors significantly concentrate on the average capacitances. We tried to decrease the capacitance tolerances by the geometry compensation. The copper panel area of the embedded capacitor was increased by 5%. Accordingly, the rectangle structure embedded in L4/L5 was revised to 9.00 mm×25.00 mm. Table 3 listed the capacitances of the compensated capacitors embedded in L4/L5. It showed that the capacitance tolerance was greatly decreased from —6.0% to —1.1%by geometry compensation, implying that these asfabricated capacitors were upgraded from the second grade to the first grade.

Table 2. The capacitances of these embedded capacitors with different geometries and different CCL materials

Table 3. The capacitances of the compensated capacitors

Table 4. The breakdown voltages of the embedded capacitors

The failure as a result of an abrupt drop in insulating resistance during voltage increasing was proposed to verify the insulation of the dielectric layer. Table 4 listed the breakdown voltages of the embedded capacitors in the 10-layer and 4-layer PCB, respectively. The voltage was elevated at a rate of 100 V/min. The data showed that the capacitors can withstand the voltage higher than 2000 V, which can significantly satisfy the requirements of 500 V or higher in the electronic circuit module.

Because embedded capacitors in PCB would undergo solder reflow process, the stability of electrical properties at solder reflow condition should be confirmed. The thermal shock stability of embedded capacitors was reflowed in the production line for 6 times (about 15 min for each time). The reflowing temperature was set at 260℃. Fig. 3 plotted the capacitances of the embedded capacitor in relation to the thermal shock times. It can be seen that the capacitance has a little decreasing less than 1% at the 1stor 2ndround thermal shock. At the further thermal shock, the capacitor was gradually kept a constant value. This verification demonstrates that the capacitors have very good thermal shock reliability.

Temperature cycling tests are expected to induce deformation, delamination or stress relaxation in the embedded capacitor, leading to an effect on the capacitance. Generally, the residual-stress relaxation took place during the initial 100 to 300 cycles. Therefore, thermal cycling test at —55℃/15 min and 125℃/15 min for 300 cycles was performed to illustrate the capacitance variances. Fig. 4 showed the results. As was indicated in Fig. 4, the thermal cycling made a very limited effect on the capacitance of the embedded capacitors. For the rectangle structure of 1395 pF, the 300-cycle processing decreased the capacitance less than 5%. The circle structure decreased about 4% in capacitance. The stressrelaxation for capacitor embedding did not result in obvious variance of capacitance, indicating thesefabricated capacitors showed very excellent thermal cycling reliability to eliminate the inner stress.

Fig. 3. The capacitances of the embedded capacitor in relation to the reflow times

Fig. 4. The capacitance of the embedded capacitorin relation to the thermal test cycles

Under humidity conditions, the dielectric constant of epoxy/BaTiO3/PI composite increases due to water absorption. It can be understood that absorbed moisture changes molecular dipoles. Polar group of water increases polarity of composite, and it results in increase of capacitance[18,19]. Fig. 5 showed thechanges of capacitance during 85℃/85% RH test for 336 h. We can see that the capacitances increased less than 5% compared to initial capacitance, indicating the moisture absorption did not obviously lead to the delamination and cracks in the dielectric.

Fig. 5. The capacitance variances after 85℃/85% RH test for 336 h

4 Conclusion

The embedded capacitors in PCB were successfully fabricated using commercial epoxy/BaTiO3/PI capacitor CCL in PCB mass-production line. The average capacitances of the obtained capacitors were deviated from the designed values in the range of —4.0% to —6.0%. With the geometry compensation, the tolerance of the revised capacitor was decreased from —6.0% to —1.1%. To evaluate the reliability of embedded capacitors fabricated by epoxy/BaTiO3/ PI composite, reflow process at 260℃, high thermal cycling, 85℃/85% RH, and high-voltage breakdown tests, were performed. There was no obvious electrical failure for embedded capacitors in the reliability tests. The capacitance changes of epoxy/ BaTiO3/PI due to the thermal and moisture effects are very small generally lower than 5%. Accordingly, the addition of PI in epoxy/BaTiO3showed a better environmental stability for this embedded capacitor in comparison to that of the pure epoxy/BaTiO3dielectric.

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Fabrication and Characterization of Embedded Capacitors in PCB Using Epoxy/BaTiO3/PI Capacitor CCL

ZHOU Guoyun1HE Wei1WANG Shouxu1FAN Haixia1XIAO John21
( State Key laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China,Chengdu 610054, China )2( Institute of Electronic and Information Engineering in Dongguan, UESTC, Dongguan 523808, China )

The embedded capacitors in PCB (Printed Circuit Board) were fabricated using commercial epoxy/BaTiO3/PI capacitor CCL in conventional PCB build-up process. Capacitance measuring demonstrated the tolerances of the obtained capacitors ranged from —4.0% to —6.0%, and special design to compensate the capacitor geometry significantly decreased the tolerance to —1.1%. Reflow process at 260℃, high thermal cycling, 85℃/85% RH and high-voltage breakdown tests were performed to evaluate the reliability of embedded capacitors. It is summarized that the fabrication of epoxy/BaTiO3/ PI composite embedded capacitors is successfully demonstrated using conventional PCB build-up processes, and their environmental reliability are evaluated to be excellent.

BaTiO3; embedded capacitor; reliability; printed circuit board

2014-09-03

TN 6

A

Foundation:Guangdong Innovative Research Team Program(201001D0104713329)

Author:Zhou Guoyun(corresponding author), Ph.D., Assistant Professor. His research interests include PCB manufacturing and its materials, E-mail∶zhougouyun2011@gmail.com; He Wei, Ph. D., Professor. His research interestsinclude PCB materials and their applications; Wang Shouxu, Master of Chemistry, Associate Professor. His research interests include PCB materials, PCB design and electronic component integrated technology; Fan Haixia, M.S. candidate. His research interest is electronic component integrated technology in PCB scale; Xiao John, Ph.D., Professor. His research interests include electronic component integrated technology and its application in functional miniature circuit.