LTCC Substrate Fabrication Process

Dielectric materials of LTCC mainly consist of glass/ceramic mixed with other organics such as organic binders and organic solvents. By means of the tape casting process, they are fully mixed and fabricated as high-density green sheet with an accurate thickness. After forming holes on the green ceramic sheet by laser drilling or mechanically punching, metal paste is used to fill interconnections between layers. Then the conductor patterns are printed on each layer by the screen-printing process. After stacking, thermal laminating, or hydro-static laminating in a sealed bag, laminates are cofired to obtain the multilayer substrate.

Figure 3.7 shows the process flow of LTCC fabrication.

1. Tape Casting

The green sheet obtained from tape casting should be flat and of high density. The width of the green film should be not less than 110 mm, and it should be reasonably stiff. The key technologies of tape casting include the mechanical equipment, material composition, and the control of various parameters.

2. Via Fabrication and Filling

Vias can be formed by drilling, punching three to five per second can be holes can be formed by drilling and holes can be over 0.25 mm in diameter with a tolerance of±50 μm.

The drill bit can be easily broken while drilling small holes. The smaller the drill bit is, the more expensive it is. So it is not cost effective to drill holes less than 0.25 mm in diameter with this method. Eight to ten holes per second can be formed by punching, and they can be as small as 0.05 mm in diameter with a tolerance of ±10 μm. Through laser drilling, 250−300 holes/s can be formed, and they can be as small as 0.1 mm in diameter with a tolerance of ±25 μm. For the LTCC manufacturing process, the preferred size of via holes is about 0.15−0.25 mm in diameter, to balance the improvement of wiring density with the process of via metallization. If the size of via holes is0.3 mm or 0.15 mm, it is difficult to form blind vias with existing metallization approaches, which may decrease the production efficiency and the product reliability.

Via filling is a critical part of LTCC substrate fabrication.^[21] Vias can be filled through the following techniques: screen-printing or mask-printing. The work bench of a printer designed for low-temperature processes is made of lacunose ceramic plates or metal boards with a locating pin which is around 1.5 mm in diameter, at each of the four corners, which matches the locating holes on the green sheet. Under the work bench, a vacuum pressure

52 Chapter 3 Substrate Technology of about 665−886.4 Pa is provided. Screen-printing should use a stainless steel screen with over 250 mesh or a nylon screen with a large coverage of holes.

Contact printing is preferred, with a paste of around 30 μm. A brass or stainless steel mask 25−30 μm thick may be used. A piece of filter paper is placed under the green sheet to prevent the paste from leaking to the work bench through vias. After the printing is completed, the green sheet is taken away together with the filter paper for 5−10 minutes’

drying at a temperature of 70−100^◦C before removing the filter paper. The paste for via filling should have good liquidity and proper viscosity, which should be adjusted to the size of the vias. Generally the viscosity is about 1000−2500Pa · s. Otherwise, it is difficult to form blind vias. After printing, it is important to check the vias under the microscope and to repair imperfect vias.

Glass/ceramic powder+organics

Tape casting

Green sheet

First layer

Via hole fabrication

Filling

Metallization

Second layer

Via hole fabrication

Filling

Metallization

Lamination and hot-pressing

Cut

Oganics burning-out and cofiring

Testing

Multilayer substrates

nth layer

Via hole fabrication

Filling

Metallization



Figure 3.7 Process of LTCC multilayer substrate fabrication

3.5 LTCC Substrates 53

3. Positioning

Positioning refers to the alignment between the screen and the green sheet during printing and between green sheets during laminating. If the positioning tolerance is too wide, open or short circuits may occur. Commonly used tools, such as locating holes or fiducial marks, are used to meet the different requirements in positioning accuracy. For example, when using fiducial marks, it is worth paying attention to the different tolerance requirements in line width, line spacing, via diameter, and via spacing. The positioning accuracy of normal wiring density is about±50 μm.

Factors that affect the positioning accuracy include drilling tolerance, lithograph mask tolerance, and visual tolerance during manual operations on printers. In order to increase the wiring density, those tolerances must be improved. New printers with automatic vision alignment systems, which can greatly increase the positioning accuracy, are already on the market.^[22]

4. Wiring Design and Metallization^[23,24]

During wiring design with the CAD tool, the line width and spacing and other param-eters must be designed according to the requirements of electrical paramparam-eters, positioning accuracy, and sizes of vias. Normally, with the help of LTCC technology, very fine pitch wiring and small spacing can be fabricated, but its process cost is high. A multilayer LTCC substrate with wider wirings is cost effective. During wiring design, both the technology and cost should be taken into consideration. For substrates applied to high-frequency and high-speed circuits, fine wire width and fine pitch should be used. To reduce production cost, it is preferable to choose multilayer structures with wider wire and larger spacing while ensuring the product quality.

Metallization of LTCC substrates falls into two categories: inner surface metallization and outer surface metallization.^[25]Screen-printing and computerized direct plotting are the most widely used techniques for inner surface metallization. Besides these two, photolithography and thin-film deposition are also used in outer surface metallization. With the development of finer screens with larger mesh coverage and masks and pastes of higher resolution, lines as thin as 100−150 μm can be screen-printed. Some spacing (called screen spacing, off-substrate distance, or off-contact distance) is kept between screen and off-substrate, and paste is forced by squeegee at a given speed and pressure along the screen and then is printed on substrate below through the open areas of mesh to transfer images on it. Screen-printing can be classified into two main types, contact mode and non contact mode, by the ways in which the screen returns back to its original position after transferring. The former takes advantage of screen tension to divorce the screen from the substrate and return it to its original position. The latter makes use of mechanical methods to divorce the screen from the substrate, which clings to it during the printing process. In electronics packaging engineering, non-contact screen-printing technology is used most frequently^[26] If thinner lines and smaller spacing are required, thin-film deposition or thick-film photolithography can be used, in which lines as thin as 40−50 μm can be obtained. However, these two technologies can only be used for outer surface metallization, and they are quite expensive as well.

Computerized direct plotting is a versatile technique, for it needs no screen. However, it features low throughput, complicated operation, and expensive equipment.

With the increased density and pin numbers of chips attached to the LTCC substrate, smaller width and spacing of interconnection traces are required, as are smoother trace edges, which can hardly be fabricated by traditional thick-film printing technique. This led to the development of thin-film techniques. The process of thin-film metallization is shown in Figure 3.8, and detailed steps are given in the references^[23].

54 Chapter 3 Substrate Technology Before thin-film metallization, the surface of an LTCC substrate should be ground and polished. Then multilayer films are deposited by the magnetic sputtering process, and soldering pads and conducting traces can be obtained through the lithography process.

Finally, substrate products are obtained through heat treatment.

Grind and polish surface

of LTCC substrate Magnetic sputtering

Make mask

Products

Lithography process Heat treatment

Testing Cutting

Figure 3.8 Thin-film metallization process of LTCC substrates

5. Lamination and Hot-pressing

After via filling and metallization, the green sheet is shifted into the stack mold with locating pins that will match with the locating holes on the sheet, which ensures positioning accuracy. The best material for the stack mold should have enough hardness to prevent distortion in the process. The laminating and hot-pressing process calls for uniform pressure, which is closely related to the shrinkage of the substrate.

If the pressure is too high during hot-pressing, the substrate will delaminate because of air bubbles as a result of organic burning or out. Delamination will also-occur if the pressure is too weak, together with larger shrinkage and nonuniform shrinking rate of the substrate during sintering process. It is necessary to have in-depth experiments to optimize the pressure of hot pressing. The isostatic lamination machine can provide uniform pressure for idea lamination and equal shrinkage, which is conducive to the improvement of alignment accuracy between sections of soldering pads and between post fire conducting surfaces and through vias.

6. Organics Burning-out and Cofiring

The organics burning-out in a substrate can be carried out in an ordinary muffle furnace.

The rate is determined by the thickness of the substrate. A typical condition is that in which the temperature increases at a rate of 0.2−0.5^◦C/min and is maintained for 3−5 hours when it reaches 450^◦C. If the temperature rises too fast, the substrate will delaminate because of air bubbles coming out of the layers. The substrate should be place on a flat ceramic or quartz plate. Besides in a muffle furnace, sintering can also be conducted in a chain furnace with temperature increase at 8^◦C/min. When it reaches 850^◦C, the temperature should be kept unchanged for 10 min, and then decreased at the speed of 8^◦C/min. The key to sintering lies in the accuracy of the sintering curve and the temperature uniformity in a furnace, which is closely related to the flatness and shrinkage of the substrate after sintering.

If the temperature inside the furnace is not uniform, the shrinkage of the substrate will hardly be even. During the sintering process, temperature that increases too rapidly may lead to an unsmooth substrate surface and large shrinkage.

An experiment validated that the sintering temperature remained at 850^◦C in both cases.

In one case, the temperature ramp rate of 12^◦C/min led to flatness of 120 (μm/50 mm) and shrinkage of 13.8%, and in the other case the temperature ramp rate of 8^◦C/min resulted in flatness of 70 (μm/50 mm) and shrinkage of 13.5%. During sintering, there will be a difference between the temperatures of the conductor and the substrate. If the temperature increases too quickly, stress resulting from different levels of density will cause the substrate to warp. In addition, it may result in melting glass penetrating into the space between ceramic particles, which will increase the shrinkage of the substrate. If the fired substrates are not so flat, an effective way to flatten them is to place the substrate between two ceramic