Screen Printing Parameter Effect on the Surface Roughness Measurement of Printed Gold Paste on LTCC Tape

Sabrina Mohd Shapee, Rosidah Alias, Azmi Ibrahim, Zulkifli Ambak, Mohd Zulfadli Mohd Yusoff,

and Muhammad Redzuan Saad

Microelectronic & Nano Technology Program, TM Research & Development, TMR&D Innovation Centre,

Lingkaran Teknokrat Timur, 63000 Cyberjaya, Selangor, Malaysia.

Keywords: Screen printing, conductor, LTCC, gold paste.

ABSTRACT Effect of screen printing parameter such as squeegee speed was studied in this paper for the surface roughness of gold paste on the Ferro A6M LTCC tape. Roughness measurement was evaluated by 3D profilometer of the Ferro A6M LTCC tape co-fired with Au conductor paste. I#TRODUCTIO# One of the factors that affect microwave loss in the LTCC circuit is the surface roughness. Microwave loss or line loss comes from the following sources: ceramic, metal conductor, metal-ceramic interface, radiation and surface roughness. Surface roughness in the metallization process is related to the high frequency range where the electromagnetic wave does not penetrate deep into metallization. The current travel on the surface of the metallization is called the skin effect. So, the surface roughness of the conductor plays a very critical role in the microwave loss [1]. The surface roughness of the conductor increases the capacitance, resistance and inductance. The capacitance is increased because the effective thickness of the rough material between two conductors is less than the measured thickness. The roughness of the surface of the conductors can also affect the resistance due to the increased surface area [2]. Screen printing is the transfer of pseudoplastic paste (pastes that the viscosity decreases with increasing shear rates) through a fabric screen onto a substrate. The transfer occurs when the paste comes in contact with substrate surface and is pulled through the screen. The high shear action of the squeegee passing over the screen allows this transfer to occur. The paste is deposited in a pattern defined by the open areas in the emulsion of the screen [3]. The main aim during the printing is such that it should give uniform thickness of the print throughout the substrate [4]. The purpose of this paper is about to investigate the effect from some parameter used for screen printing process in the LTCC laboratory. The process parameter is the squeegee speed which is one of the critical parameter that will control the conductor surface roughness on the substrate. EXPERIME#TAL PROCEDURES Screen printing process was carried out on a KEKO P-200 Screen Printer Machine [4] using the FX33-025 Au conductor paste from Ferro with the viscosity around 1200 poise according to the Ferro Guidelines [5]. The printing were done in a temperature control room at 20oC. The effect of the printing parameter was assessed by printing a test pattern like an RFID antenna design. After the printing, the substrate was dried at 100oC for 10 minutes. The printed tape with conductor paste was then laminated with 5 “dummy” layers using isostatic lamination machine from KEKO [6]. The

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lamination condition used for this process is pressure of 21 MPa (3000 psi) and temperature of 70oC for 5 minutes. The laminated samples were then fired up to 850 oC by using Ferro firing profile. The fired samples were then measured for the surface roughness using a 3D profilometer. The surface roughness values were calculated from the measured surface profiles. The surface roughness number is the arithmetic average of deviation of the surface from the mean value. RESULTS A#D DISCUSSIO# Fig.1-2 shows the viscosity vs. shear rate data of the gold paste used in this experiment. The data of all the results is shown in Fig.3-5 below and the plot for the effect of surface roughness on the squeegee speed parameter of printed gold paste on Ferro A6M tape is shown in Figure 6. The Ferro A6-M tape was used in this study. The A6M tape of thickness 5 mils has a dielectric constant of 6 and very low dielectric loss (<0.002 @ 10GHz) [7].

Up-Curve Log Plot

y = -0.1636x + 4.5136

R2 = 0.9649

4.2

4.25

4.3

4.35

4.4

4.45

4.5

0 0.5 1 1.5 2

Log Shear Rate

Log Viscosity

Fig. 1: Viscosity vs. shear rate up-curve plot for gold paste in the experiment

Down-Curve Log Plot

y = -0.2017x + 4.5893

R2 = 0.986

4.25

4.3

4.35

4.4

4.45

4.5

4.55

0 0.5 1 1.5 2

Log Shear Rate

Log Viscosity

Fig. 2: Viscosity vs. shear rate down-curve plot for gold paste in the experiment

486 Asian Ceramic Science for Electronics III and Electroceramics in Japan XII

Figure 3a: Surface profile of printed gold on Ferro A6M tape with squeegee speed parameter of 75 mm/s.

Figure 3b: Surface area of printed gold on Ferro A6M tape with squeegee speed parameter of 75 mm/s.

Figure 4a: Surface profile of printed gold on Ferro A6M tape with squeegee speed parameter of 100 mm/s.

Figure 4b: Surface area of printed gold on Ferro A6M tape with squeegee speed parameter of 100 mm/s.

Figure 5a: Surface profile of printed gold on Ferro A6M tape with squeegee speed parameter of 125 mm/s.

Figure 5b: Surface area of printed gold on Ferro A6M tape with squeegee speed parameter of 125 mm/s.

Key Engineering Materials Vols. 421-422 487

Surface Roughness of Printed Gold Paste on Ferro A6M Tape

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

70 80 90 100 110 120 130

Squeegee Speed (mm/s)

Surface Roughness , Rq (

µµ µµm)

Fig.6: Effect of the Surface Roughness on the Squeegee Speed Parameter of Printed Gold Paste on Ferro A6M Tape The squeegee speed is the speed with which the squeegee travels across the screen [8]. Squeegee speed is decided by the printing time and viscosity of the paste when printing. When the squeegee speed is very fast, the viscosity of the paste falls and its fluidity through the openings in the screen improves. From the graph (see Fig.6), it shows that the surface roughness of the gold conductor is decreased with the increasing of squeegee speed in the printing process. The co-fired gold conductor with higher squeegee speed (125 mm/s) showed a smoother surface than the low squeegee speed (75 mm/s). This indicates that the squeegee speed has considerable influence the surface roughness of the conductor. It is considered as a critical parameter in the printing process because paste needs time to roll into the aperture on the screen mesh. CO#CLUSIO#S The squeegee speed is the important value to optimize in order to get the smooth conductor surface on the LTCC tape. So, it is important to control the squeegee speed in the screen printing process parameter to make sure the roughness of the conductor surface is smooth so that it will not affect so much the microwave loss in the LTCC circuit. The changing parameter of screen printer during printing is critical in order to control the conductor surface roughness in improving the printing quality. ACK#OWLEDGEME#TS The authors wish to thank Telekom Malaysia for their funding support under project ComPACT (RDTC/080700). REFERE#CES [1] S. Vasudevan, ISHM 96 Proceedings, (1996), pp. 40-47.

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[2] J. Baker-Jarvis, M.D. Janezic, B. Riddle, C. L. Holloway, N.G. Paulter, and J.E. Blendell “Dielectric Conductor-Loss Characterization and Measurements on Electronic Packaging” in NIST Technical Note 1520, 2001.

[3] Heraeus Technical Information. [4] G. C. Dubey, “The Squeegee in Printing of Electronic Circuits” in Microelectronics and

Reliability, Vol. 14, (1975), pp. 427-429. [5] FERRO Design Guidelines. [6] “LTCC Production” in 2003 KEKO Research Newsletter [7] L. Chai and S. Turvey, “Ferro LTCC System for Wireless Packaging Applications” in Ferro

Design Guidelines. [8] W. Borland, “Ceramic Thick-Films Process and Materials” in Ceramic Materials for

Electronics: Third Edition, Revised and Expanded, edited by Relva C. Buchanan, Marcel Dekker Inc., New York, (2004), pp. 527-580.

e-mail: sabrina@tmrnd.com.my

Fax: +603-88839596

Key Engineering Materials Vols. 421-422 489

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