Ceramic Substrates

Figure 3.4 Typical configuration of an eight-layer PWB

The AOI system is applied to inspect the process defects and to minimize failures of PWBS by taking immediate corrective actions or improving the PWB fabrication processes.^[3]

3.2.3 Application of PWB Substrates in MCM

A multichip module (MCM) is a specialized package where chip dies or CSP components are mounted on one substrate. The substrate can be a PCB, a thick/thin film ceramic substrate. or a silicon wafer with interconnected patterns.^[4]A MCM based on a multilayer laminated organic PWB of HDI is referred as on MCM-L, which is mainly used in electronics applications with frequencies below 30 MHz. An MCM-L usually has 6–8 layers or 10–12 layers, two of which are signal layers, and the rest are power layers, ground layers, soldering pads, or distributing layers. Lead pitch is usually within 70–120 μm. A via often has a diameter between 300 and 450 μm. A soldering pad typically has a diameter of 200–

300μm. Substrate materials mainly include FR-4 epoxy-glass, BT-epoxy, ployimide, and cyanate. Substrate fabrication is similar to the normal PCB fabrication process, except that MCM-L is required for etched conductive lines 75 μm wide and vias 150 μm in diameter through laser-drilling or tripping technology. A single polyester substrate together with a 100μm laminate should be controlled within 1500 μm in thickness. Wire bonding is done by thermosonic bonding, and leads pitch can be as narrow as 40μm.

Based on mature fabrication technologies, the advantages of MCM-L are they cost liffle, they are capable of mass production, and they provide copper conductive layers of various thicknesses. It can also be used for assembling components on both sides of the substrate.

However, the problems to be further resolved mainly include chip inspection for quality control, thermal conductivity of thin laminates, and coefficient of thermal expansion (CTE) mismatch between chips and PCB substrates.^[5]

3.3 Ceramic Substrates

Compared with organic substrates, ceramic substrates are superior for their resistance to heat, high thermal conductivity, proper CTE and being ease of use for finer wirings. There-fore, ceramic substrates have been widely applied in packaging for large-scale integrated circuits (LSIs) and hybrid integration circuits (HICs).

3.3.1 Classification and Fundamental Properties

Currently, most widely used materials in ceramic substrates mainly include Al2O3, BeO, SiN, mullite, AlN, and glass ceramics, which fall into the following two categories:

(1) Low permittivity substrates, such as, Al2O3, glass ceramic substrates and so on. They are easy to fabricate multilayers and mainly used in packaging high-speed devices.

(2) High thermal conductivity substrates, typical materials are AlN and BeO substrates.

They are mainly used in assembling power components.

42 Chapter 3 Substrate Technology To ensure high performance, ceramic substrates should have high thermal conductivity and a CTE similar to the chip material such as Si. With the increase of integrated density of power chips, power consumption is going up noticeably and chip size is also expand-ing. Therefore, electronic power devices usually prefer ceramic substrates of better thermal conductivity and proper size and cooling structure to meet the requirements for packaging large-scale and high-power chips. High thermal conductivity substrate will remove more heat from working devices, which ensures high reliability and long lifetime of the devices.

The CTE of the substrate should also be close to that of the silicon wafer, which is around 3×10⁻⁶/^◦C, so as to prevent failures resulting from excessive thermal stress between chips and substrates during packaging and assembling processes. This is especially essential for packaging LSI chips.

The basic requirements of properties for ceramic substrates are as follows.^[6]

(1) Electrical properties: low electrical permittivity, low dielectric dissipation, high in-sulation resistance, high breakdown voltage, and stability in a temperature and high-humidity environment.

(2) Thermal properties: high thermal conductivity, proper CTE close to that of the device to be mounted, and good heat resistance at high temperature.

(3) Mechanical properties: high mechanical stiffness, easy to process, good manufactura-bility for fine pitch and multilayer process, no distortion, no warp, and no flaws cracks and so on.

(4) Other properties include.

(I) good chemical stabilities and ease of metallization.

(II) low moisture absorption.

(III) nontoxic and nonpolluting.

(IV) low cost.

3.3.2 Fabrication of Ceramic Substrates

There are two typical processes to fabricate ceramic substrates as introduced below: tape casting and powder pressing.

1. Tape Casting

One process of making superior ceramic substrates, known as tape casting, starts with continuously pouring paste tape in uniform thickness on a moving baseband under a scraping blade, after vacuum deformation, which creates a suspension paste made of ceramic powder, plasticizing agent, solvent, and dispersant. After drying, the soft tape, known as a green sheet, will become high-quality ceramic substrate through precutting, organics burning out, and sintering steps. The tape casting is widely used for large-scale fabrication of ceramic substrates in LSI packaging and HICs because of its efficiency in producing multilayer struc-tures. Figure 3.5 shows the process of tape casting in making a green sheet for substrate fabrication.^[6]

Preparation of the ceramic slurry is essential during the tape casting process. Granularity of the material is critical to the structure and performance of the ceramic slurry: Smaller particles will benefit from sintering a uniform and compact substrate by improving the osculant reaction probability among particles, the plasticity, and moulding of the slurry.

In addition, the conformation of particles guides the thickness of the cast sheet.^[7,8] To meet various requirements in different applications, ceramic powder needs to be mixed with 0.5% to 8% additives. For Al2O3 ceramic, common additives include oxides such as MgO, SiO2, and CaO. These additives will help to lower the sintering temperature and make the metallization easier. Furthermore, a certain quantity of organic binder and solvent should

3.3 Ceramic Substrates 43 be added for making the slurry via the ball milling process. In order to ensure the slurry flows well, surfactants should also be added.

Ceramic powder

Figure 3.5 A fabrication process of ceramic substrates by tape casting

Ceramic slurry passes through the blade of the tape casting machine onto an organic substrate at a stable speed to form a tape with uniform thickness. After drying, a soft green sheet is formed. After cutting the green sheet and punching vias and cavities, various kinds of substrates can be fabricated according to the three processes as follows.

(1) Lamination, hot pressing, burning-out organics, printing the circuit patterns, and sin-tering. Circuits can be made by thick-film or thin-film techniques. This process is very flexible, with adjustable parameters, noble metal of low melting point, and no special re-quirement for sintering atmosphere. This process is mostly used in packaging LSI and HICs.

(2) Lamination, printing the circuit patterns, hot pressing burning-out organics, cofir-ing. Since the ceramic substrate and circuit patterns are cofired at a high temperature, for example, the sintering temperature for Al2O3 is between 1500^◦C and 1600^◦C, it re-quires conductor metals of high melting point, such as Mo and W. To prevent oxidization, sintering should be conducted in a protective and deoxidizing atmosphere such as nitrogen or hydrogen. Generally, the electrical resistance of metal sintered will be higher.

(3) Printing the circuit patterns, lamination, hot pressing, burning-out organics, cofiring.

The sintering atmosphere should be the same as stated in (2). This process is currently one of the major methods for fabricating multilayer ceramic substrates.

2. Powder Pressing^[9]

Powder pressing, also known as mould pressing or isostatic pressing, involves processing ceramic powder into components or roughcasts to be sintered to certain sizes, shapes, density, and stiffness.

As shown in Figure 3.6, powder pressing mainly involves compacting the metal powder into the desired shape with the axial movement of multiple plungers in the mould cavity.

The pressure applied generally is between 350 MPa and 700 MPa. High pressure leads the power to mechanically interlock, followed by cold welding to get the roughcasts, which are then fired into the shape of the final products under high temperature (1120−1200^◦C) for 30 minutes to 120 minutes.

44 Chapter 3 Substrate Technology

Figure 3.6 Process of mould pressing

There are some limitations to the powder pressing process:

(1) The axial movement of plungers limits the shape of casts expected.

(2) The existence of friction may lead to non uniform density of the casts.

(3) Nonuniform density of roughcasts may cause size changes after sintering.

(4) The mechanical property of the sintered components may be influenced by partial density changes.

3. Pattern Formation on Ceramic Substrates^[6]

Circuit patterns are formed by the metallization of the surfaces of a ceramic substrate, which facilitate interconnections between carriers and I/O terminals. Ceramic substrate metallization techniques include:

(1) Thick-film processing, during which circuits and soldering pads are obtained by print-ing conductive and resistive pastes on a ceramic substrate with screen-printprint-ing process and then cofiring them.

(2) Thin-film processing facilitates metallization on a ceramic substrate by deposition of a metal layer in a vacuum, such as vacuum evaporating and sputtering. In principle, any metal can be deposited on any substrate through these vapor deposition techniques. In multi-layer structures, those thin films that cohere to the ceramic substrate directly are generally made of metals with strong reactivity and cohesion, such as Ti, Zr, Cr, Mo, and W. Then ductile metals, such as Cu, Au, and Ag, are plated, since these materials feature high electrical

3.4 Introduction of Typical Ceramic Substrates 45