Materials and Manufacturing Processes, 21: 721–726, 2006 Copyright © Taylor & Francis Group, LLC ISSN: 1042-6914 print/1532-2475 online DOI: 10.1080/10426910600727833
Effects of Sintering Process Conditions on Size Shrinkages of Low-Temperature Co-Fired Ceramic Substrate Z. W. Zhong1 , P. Arulvanan2 , and C. F. Ang1 1
School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore, Republic of Singapore 2 Singapore Institute of Manufacturing Technology, Singapore, Republic of Singapore
The effects of sintering process conditions on the size shrinkages of low-temperature co-fired ceramic (LTCC) substrate were investigated. The process variables investigated were thickness of the stacked raw tapes, lamination pressure, lamination-pressure holding time, pre-heating time, debinding time, sintering dwell time, and sintering temperature ramp. Results revealed that the size shrinkage percentage of the LTCC samples in the lateral directions was always smaller than that in the thickness direction. The lateral shrinkage deviations were less than 2.1% for all the experiments conducted in this study. Pre-heating time, lamination-pressure holding time, debinding time, sintering dwell time and sintering temperature ramp had almost no effects on the lateral size shrinkage of the LTCC samples, and the average of the lateral shrinkage values was 15.3%, with a standard deviation of 0.17%. Lamination pressure and stacked raw-tape thickness had effects on the lateral size shrinkage of the LTCC samples, and empirical equations for calculation of the size shrinkage values were obtained by curve fitting. Keywords Debinding time; Lamination pressure; Lamination-pressure holding time; Lateral direction; Low-temperature co-fired ceramic; Manufacturing; Material; Microelectronics; Pre-heating time; Sintering dwell time; Sintering process conditions; Sintering temperature ramp; Size shrinkages; Thickness direction; Thickness of raw tapes.
avionics, and automotive areas, and in multichip modules for communication and computer applications [8]. The high-temperature co-fired ceramic (HTCC) technology was also developed to increase packaging density by building individual conductor/dielectric layers and then laminating them by firing at a high temperature under pressure. An HTCC module can have an almost unlimited number of layers, good thermal conductivity and low dielectric loss at high frequencies. However, it suffers from size shrinkage due to firing and a high dielectric constant [9]. LTCC is almost similar to HTCC but has a few distinct advantages. It is able to use low-resistivity conductors like silver, gold, copper, and alloys with palladium and platinum instead of tungsten and molybdenum. The separation between the two technologies is the firing temperature. HTCC uses temperatures of 1000 C and above, while LTCC uses 850 C to 875 C [10]. In more advanced integration, the use of ferromagnetic, dielectric, ferroelectric, and piezoelectric layers with highly conductive silver paste in LTCC applications would improve the diversity and performance of components with low production costs [11]. During the firing, LTCC tapes shrink by more than 10%. This size shrinkage could cause difficulties in controlling changes of bump pitches, which must be compatible with fine-pitch contact pads of the semiconductor ICs to be connected [12]. Defects such as camber, delamination, and cracks were caused by the mismatched sintering kinetic between the metal and ceramic. Different materials, irregular shapes, and asymmetric temperature in the furnace were the major causes of this mismatch. Mismatch sintering characteristics were observed in two ways: different size shrinkage values and different densification start-up times. The mismatch
Introduction Microelectronic products are used every day in our offices and homes. In these products, electronic packaging plays important roles such as supplying power to integrated circuit (IC) chips and distributing signals among microelectronic devices. As IC fabrication advances rapidly, electronic packaging faces more and more challenges [1–4]. The demand for higher-density packages has been increasing in the microelectronics industry. The lowtemperature co-fired ceramic (LTCC) technology has demonstrated great potential to meet the performance, processing, and cost requirements, especially in the highend radiofrequency (RF) packaging industry. High quality conductors and passive components are printed onto individual sheets in parallel, which improves yield and reduces cost and turnaround time [5]. The LTCC technology can be applied to the integration of passive elements into a monolithic, highly reliable, and robust LTCC module, which consists of several layers of the substrate material with integrated elements that are interconnected with 3D strip-line circuitry [6]. The ability to co-fire many layers simultaneously has the advantage of reducing both the process cost and process variability [7]. LTCC has excellent properties for packaging applications, rendering good conductors, low associated capacitance, simpler processes, and high layer count. It has been used for high reliability applications in military,
Received January 5, 2006; Accepted February 6, 2006 Address correspondence to Z. W. Zhong, School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Republic of Singapore; Fax: (65) 6791-1859; E-mail:
[email protected]
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722 behavior induces thermal stresses and leads to defects in the co-fired system [13]. There are two approaches in searching LTCC compositions for sintering below 900 C. The first consists of adding a low-softening-temperature glass to the ceramic dielectrics. The second is the use of low-meltingtemperature oxides as sintering additives [14]. A typical LTCC tape after firing consists of the Al2 O3 phase, recrystallization products, remaining glass, and the phase for the modification of the dielectric properties. The role of the glass phase in commercial tapes is mainly in the enhancement of the sintering (and thus the mechanical properties) and through the recrystallization providing the phase with the appropriate dielectric properties [15]. Most of the known commercial ceramic materials need high sintering temperatures. To solve this problem, oxidetype powders such as B2 O3 , SiO2 , and Li2 O and low-melting glass frits are mixed with ceramic materials to reduce the sintering temperature to below 900 C [16]. The multilayer ceramic packaging technology consists of green tape preparation, metallization layout, lamination, and cofiring [17]. To say more in detail, the process steps in LTCC manufacturing are slitting, precondition, blanking, via forming, via filling, printing, lamination, co-firing, post-firing, and singulation [18]. Green tapes are punched, metallized, stacked, and laminated. After binder burn-out and cofiring, the final ceramic product is obtained. Lamination must result in homogeneous junctions between pure green tapes or between green tapes and metallized areas. The common method to join stacked metallized ceramic green tapes is thermo-compression based on joining binder phases of two adjacent green tapes at elevated temperatures and pressures. A holding time of 3–10 min at the peak temperature is needed to reach a homogeneous temperature profile in the entire stack. A thorough interpenetration of the powder particles in the tapes must be achieved. During this process, mass flow occurs, caused by the pressure, temperature, and the porosity of green tapes [19]. There are many process parameters that affect the size shrinkage of LTCC. It is necessary to determine which process parameters have the most/least effects on the size shrinkage. However, there is little information on the size shrinkage percentage of the ceramic substrate when there are changes in the sintering process parameters. In this study, experiments were carried out to investigate the effects of seven process parameters on the size shrinkage of LTCC. These parameters were: thickness of the stacked raw tapes, lamination pressure, lamination-pressure holding time, preheating time, debinding time, sintering dwell time, and sintering temperature ramp. Experiments The experiments were carried out in a clean room. The machines, devices, and materials used in the experiments included a punching machine, a debinding oven, an isostatic lamination machine, a firing furnace, a vacuum sealer, a coordinate measuring machine, a slider cutter, a collating jig, thermocouples, a height gauge, plastics bags for sealing, and raw tapes. The experiment steps were raw-tape sample
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preparation, profiling, vacuum sealing, isostatic lamination, debinding, co-firing, and measurement. Raw tapes, commonly known as green tapes, have different shapes and sizes. In these experiments, raw tapes from Ferro with sizes of 180 mm × 180 mm and a thickness of 0.25 mm (10 mil) were used. The tapes were cut to 150 mm × 150 mm. A round corner of the sample was used for aligning and stacking the samples into the collating jig. Holes were also punched so that they could be aligned on the collating jig. Table 1 shows the details of the experiments with seven process parameters investigated. For experiments 1–22, the stacked raw-tape sample thickness was 1 mm (40 mil), achieved by stacking four raw tapes with a thickness of 0.25 mm (10 mil). After firing, the thicknesses at four locations and four lateral distance values (two in the x direction and two in the y direction) of an LTCC sample were measured and the size shrinkage of the sample was calculated using the following equation: Shrinkage = 1 −
DS DO
(1)
where DO and DS are the dimensions (thickness or lateral distance values) of the sample before and after sintering, respectively. Subsequently, the average shrinkage values in the lateral x − y and thickness z directions were obtained. Results and discussion The effects of the seven process parameters on the size shrinkage of the LTCC samples fabricated are shown in Figs. 1–7, in which the measured and calculated shrinkage percentage values in the lateral x − y and thickness z are plotted against one of the experimental variables. As shown by these seven figures, the amount of the size shrinkage of the LTCC samples in the lateral directions was always smaller than that in the thickness direction. As shown by the deviations of shrinkage percentage values, the effect of any experimental variable on the size shrinkage of the LTCC samples in the lateral directions was also always smaller than that in the thickness direction. The lateral shrinkage deviations were less than 2.1% for all the 25 experiments conducted in this study. Furthermore, the shrinkage trends in the lateral directions were clear and the shrinkage values could be calculated using the empirical equations obtained by curve-fitting techniques. This allows the engineers to calculate the lateral dimensions such as pad pitches accurately at the design stage and set the process parameters properly at the manufacturing stage. Therefore, the discussion in this section is mainly based on the results of measured lateral size shrinkages. Figure 1 shows the size shrinkages in the lateral x − y and thickness z directions versus lamination pressure. The size shrinkage in the lateral x − y directions decreased as the lamination pressure increased, and the difference between the greatest and smallest lateral shrinkages obtained was 2.1%. The relationship
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EFFECTS OF SINTERING PROCESS CONDITIONS Table 1.—Details of the experiments with seven process parameters investigated.
Exp. no.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Stacked thickness (mm)
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1.5 2 2.5
Lamination pressure (MPa)
Pre-heating time (min)
Laminationpressure holding time (min)
De-binding time (h)
Sintering ramp ( C/min)
Sintering dwell time (min)
6.89 15.8 20.7 27.6 20.7 20.7 20.7 20.7 20.7 20.7 20.7 20.7 20.7 20.7 20.7 20.7 20.7 20.7 20.7 20.7 20.7 20.7 20.7 20.7 20.7
5 5 5 5 5 5 5 10 15 20 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5
15 15 15 15 5 10 20 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15
3 3 3 3 3 3 3 3 3 3 2 4 5 3 3 3 3 3 3 3 3 3 3 3 3
40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 10 20 30 50 40 40 40
15 15 15 15 15 15 15 15 15 15 15 15 15 5 10 20 25 30 15 15 15 15 15 15 15
between the size shrinkage in the lateral directions and the lamination pressure could be expressed by the power equation y = 203x−00952 , or approximately by the linear equation y = 174 − 00993x. The size shrinkage in the thickness z direction was largely affected by the lamination pressure. For ideal lamination, the boundary between two adjacent tapes should be undetectable after compression, which can only be achieved during the thermo-compression process if individual particles at the surfaces of the tapes in contact move and interpenetrate within a thin, nearsurface region, thus smoothing the micro-roughness of the tapes and forming a homogeneous structure. Increasing the lamination pressure promotes the particle movement, particle interpenetration, and the formation of a good
interface union between tapes [20, 21]. This also affected the size shrinkages of the LTCC samples, as shown by Fig. 1. Figure 2 shows the size shrinkages in the lateral x − y and thickness z directions versus lamination-pressure holding time. The size shrinkage in the thickness z direction was largely affected by the lamination-pressure holding time. The lateral size shrinkage–increase due to increasing of the time in lamination-pressure holding was small. The difference between the greatest and smallest lateral size shrinkages obtained was only 0.06% and the average lateral shrinkage was 15.2%. The relationship between the lateral size shrinkage and the laminationpressure holding time could be expressed by the linear equation y = 152 + 0004x.
Figure 1.—Shrinkages in the lateral (x − y) and thickness (z) directions versus lamination pressure.
Figure 2.—Shrinkages in the lateral (x − y) and thickness (z) directions versus lamination-pressure holding time.
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Figure 3.—Shrinkages in the lateral (x − y) and thickness (z) directions versus lamination pre-heating time.
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Figure 5.—Shrinkages in the lateral (x − y) and thickness (z) directions versus sintering dwell time.
For a good homogeneous junction of the tapes, it is crucial to ensure good interpenetration of the powder particles [21]. Thus, a sufficiently long lamination process is needed to allow the necessary movement of these particles. However, as shown in Fig. 2, the “standard” lamination-pressure holding time of 15 min in these experiments, compared to the holding time of 5 min, did not make a significant change in the lateral shrinkage of the LTCC sample. Figure 3 shows the size shrinkages versus lamination preheating time at 70 C. The difference between the largest and smallest lateral size shrinkages obtained was only 0.13% and the average lateral shrinkage was 15.3%. Because 0.0003 in the linear equation y = 153 − 00003x was not much different from zero, the lateral size shrinkage was not affected by a longer pre-heating duration and the “standard” pre-heating time of 5 min in these experiments was sufficient. Figure 4 shows the size shrinkages versus debinding time. The maximum lateral shrinkage deviation was 0.45% and the average lateral size shrinkage was 15.5%. As shown in Fig. 5, the maximum lateral shrinkage deviation caused by changes in sintering dwell time was only 0.19% and the average lateral size shrinkage was also 15.5%. Because 0.0233 and 0.0039 in the linear equations in these figures were not much different from zero, there was no significant
difference in the lateral size shrinkage caused by the changes in debinding time and sintering dwell time. The effects of sintering temperature ramp on the size shrinkages of LTCC samples were studied by firing the samples using the temperature ramps of 10, 20, 30, 40, and 50 C/min measured from 100 to 800 C. As shown in Fig. 6, the maximum lateral shrinkage deviation was only 0.2% and the average lateral size shrinkage was 15.1%. This shows that the effect of sintering temperature ramp on the lateral size shrinkage is not significant. The effects of the thickness of the stacked raw tapes on LTCC size shrinkages were investigated by laminating different amounts of raw tapes. The LTCC sample with a 1 mm (40 mil) thickness was achieved by laminating 4 layers of green tapes, the samples with a 2.5 mm (100 mil) thickness was fabricated using 10 layers, and so on. As shown in Fig. 7, the 1-mm-thick substrate had the least lateral size shrinkage, at 15.2%, and the 2.5-mm-thick substrate had the greatest lateral size shrinkage, at 15.8%. The difference was 0.57%. The lateral size shrinkage increased when more layers of raw tapes were laminated. Therefore, when a large number of layers of raw tapes were used for fabrication of LTCC substrate, caution must be taken in calculating the amount of the lateral size
Figure 4.—Shrinkages in the lateral (x − y) and thickness (z) directions versus debinding time.
Figure 6.—Shrinkages in the lateral (x − y) and thickness (z) directions versus sintering temperature ramp.
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Acknowledgment The authors thank Mr. K. M. Chua of Singapore Institute of Manufacturing Technology for his cooperation. References
Figure 7.—Shrinkages in the lateral (x − y) and thickness (z) directions versus the thickness of the stacked raw tapes.
shrinkage, because this might pose a problem of mismatched pitches when conductor pastes are printed and this mismatch induces thermal stresses and leads to defects in the cofired system [13]. The relationship between the lateral size shrinkage and the thickness of the stacked raw tapes could be expressed by the linear equation y = 148 + 0367x. Pre-heating time, lamination-pressure holding time, debinding time, sintering dwell time and sintering temperature ramp had almost no effects on the lateral size shrinkage of the LTCC samples, and the average of these lateral shrinkage values was 15.3%, with a standard deviation of 0.17%. On the other hand, lamination pressure and the thickness of the stacked raw tapes had effects on the lateral size shrinkage of the LTCC samples, and the size shrinkage values could be calculated using the empirical equations obtained by curve fitting. This can help engineers to calculate lateral dimensions and features such as pad pitches accurately at the design stage and set the process parameters properly at the manufacturing stage. Clear trends about the effects of the seven process parameters on the LTCC thickness shrinkage were not obtained in this study. Although control of LTCC thickness might not be as important as control of lateral dimensions and features of LTCC substrate, further study is being carried out. Conclusions The size shrinkage percentage of the LTCC samples in the lateral directions was always smaller than that in the thickness direction. The effect of any experimental variable on the size shrinkage of the LTCCs in the lateral directions was also always smaller than that in the thickness direction. The shrinkage trends in the lateral directions were clear. The lateral shrinkage deviations were less than 2.1% for all of the 25 experiments conducted in this study. Pre-heating time, lamination-pressure holding time, debinding time, sintering dwell time, and sintering temperature ramp had almost no effects on the lateral size shrinkage of LTCC samples, and the average of these lateral shrinkage values was 15.3%, with a standard deviation of 0.17%. Lamination pressure and thickness of the stacked raw tapes had effects on the lateral size shrinkage of LTCC substrate, and empirical equations for calculation of the lateral shrinkage values were obtained by curve fitting.
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