There are other package substrate and dielectric isolation materials in use. One important material is polyimide. Manufacturers use this material extensively in redistribution layer and bump processes. It is easy to deposit, has a high glass transition temperature, a reasonable dielectric constant, and can be easily patterned. However, it does have a high coefficient of thermal expansion. Another important material is benzocyclobutene. It is also easy to deposit and has a low dielectric constant, making it good for high frequency applications. We discuss materials properties of these materials and others a little later in this presentation. However, it also has a high coefficient of thermal expansion. Another material in use is bismaleimide-triazine, or BT. It is also easy to process and has a low dielectric constant, and it has a relatively low CTE of 14 parts per million per degree centigrade.
BT epoxy resin is used commonly with laminates. Engineers can create build-up layers and plated through holes on the finished laminate. BT resin epoxy laminates make an excellent choice for BGA substrates since the BT epoxy allows for higher frequencies, and the higher glass transition temperature allows for higher temperature use. Furthermore, BT substrates can be processed in strip format for high volume manufacturing.
Polyimide can be used not only for isolation in redistribution layer technology, but also as a substrate material. DuPont makes polyimide materials that go by the trademarks Kapton and Pyralux that can be used for flexible substrates. Amkor also uses polyimide in one of their BGA packaging processes. fleXBGA is Amkor’s term for their fine pitch BGA substrate technology. They process this as either a 2-layer tape without adhesive and wet-etched holes, or a 3-layer tape with adhesive and punched holes for connections to the solder bumps.
BCB polymers are also used for microelectronic packaging and interconnect applications. During the 1990s, they gained commercial status in applications including the fabrication of gallium arsenide integrated circuits, bumping and redistributing GaAs chips, and for planarization and isolation in flat-panel displays. More recently, Dow Chemical and others have developed techniques to B-stage or partial cure the BCB material, and then spin deposit it onto a wafer or other substrate. Engineers then use the BCB as an isolation layer between dice in a multi-die package, as an isolation layer for interconnect in a redistribution layer, and other configurations. One can etch BCB with dry etch techniques, or one can formulate photosensitive BCB resins for lithography applications.
Another common Board Level Reliability (BLR) test is the Bend Test. The Monotonic Bend Test is a 4-point bend of the test board to failure, with an examination of how the board fails. This test can be useful to assess the impact of the backgrinding roughness. The Cyclic Bend Test is a 3-point test to introduce non-uniform strain distribution. One can also run the Cyclic Bend Test as a 4-point test to provide uniform strain distribution and better repeatability. IPC 9702 describes the Monotonic Bend Test, while JEDEC JESD22-B113 describes the Cyclic Bend Test.
Unlike other product qualification tests, BLR is a test to failure. In order to establish confidence in the distribution parameters, engineers will typically test to 50% failure. However, it is not necessary to get to 50% cumulative failures if one can establish slope and intercept. The test cycle time varies from job to job. Once the engineer obtains the data, they perform a Weibull analysis on the data to determine β (the characteristic lifetime) and η (the Weibull slope or spread in the data). Once the engineer calculates these values they can estimate the time to an arbitrary failure rate (typically one of interest to the customer), generate life estimates, and perform warranty analysis. Engineers analyze the failures to determine the failure mode and mechanism associated with the failures, and then performs root cause analysis to see if we need to make changes to our products, changes to the specifications, or changes to our customer application notes.
Q: Why do we use different materials in Solid Immersion Lenses (SILs)?
A: It has to do with the wavelengths we want to pass through the SIL. For example, a Si SIL works well for longer wavelengths like 1300nm (used for techniques like TIVA/OBIRCH), a GaAs SIL works well for mid-wavelengths like 1064nm (used for techniques like LADA), and a GaP SIL works well for shorter wavelengths (like those down in the visible range).
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