Typically, the end application and technology requirements dictate the optimal surface finish and thickness tolerance. If the application requires a surface finish greater than what can be achieved with grinding, the next steps are lapping then polishing (if needed). Common applications where the additional precision of the surface finish and thickness are more critical are high frequency RF and microwave, high power, and optical applications including infrared, visible, and ultraviolet (UV). Other applications that require extreme surface finish requirements, thickness tolerance, and identical consistence from part to part are direct chip attach on boards (COBs), multichip modules (MCMs), and systems in package (SIP) technologies. The continual densification and miniaturization of monolithic microwave integrated circuits (MMICs), chip capacitors, chip inductors, chip resistors, filters, and other microwave and power components is also driving the need for tighter control over surface finishes and substrate thickness.
For much smaller dimensioned thin film applications, a surface finish tolerance of less than 5 micro-inches may be required. This is achievable by selecting 99.6% alumina as the base material with the added process step of polishing. For even finer finishes for thin film substrates, even more advanced polishing may be necessary. Methods such as Accumet’s micro-fine polishing process, can bring the surface finish for 99.6% alumina down to less than 1 micro-inch RA, and BeO to less than 4 micro-inch RA. For optical applications, fused silica substrates will need to be polished to achieve 60/40 scratch/dig tolerances.
Many market segments, from military and avionics to commercial and communications demand precision substrates. The surface finish, thickness, parallelism, and defect tolerances of the substrate material are all critical in the design of the end product. Lapping and polishing processes help to ensure that these substrates are machined to the precise metrics necessary to fulfill the demanding requirements of the space, defense, scientific and commercial technologies. When necessary, greater precision polishing techniques exist to enhance the surface finish and thickness tolerances to the grade necessary to meet the needs of the next-generation of highly advanced microelectronics.
To learn why you would consider lapping or polishing, please read the first part of this blog series "Why Lap or Polish Ceramic Substrates? " or learn even more by downloading our latest tech brief "A Pre-metallization Guide to Lapping and Polishing Ceramic Substrates"