Semiconductor Back-End Process: The Eight Steps of Assembling Conventional Packages
Step One: Backgrinding
The backgrinding process ensures a wafer is processed with the optimal thickness for its package’s characteristics.
Step Two: Wafer Sawing/Dicing
Wafer sawing is the process of cutting along the scribe lanes2 of a wafer in order to break it into chips or dies. Also referred to as the dicing process, wafer sawing is a necessary procedure of the packaging process for chips or dies.
Step Three: Die Attach
Die attach is the process of picking up the chips that have gone through the wafer cutting process from the mounting tape and attaching them to a substrate or leadframe that has been coated with an adhesive.
Step Four: Interconnection
Interconnection refers to the electrical connections between chips, chips and substrates, and other combinations within a package.
Step Five: Molding
Once the chip is wire bonded or flip chip bonded, it needs to be encapsulated to protect the structure from external impact. Such protection processes include molding, sealing, and welding, but only molding is used for plastic packages. The process of molding encloses EMC, which mixes thermosetting resin10 with several inorganic materials, around parts including chips and wires to protect them from physical and chemical external impacts and to create the desired package size or shape.
Step Six: Marking
Marking is a process of engraving product information such as the semiconductor type or manufacturer, in addition to patterns, symbols, numbers, or letters requested by the customer, on the surface of semiconductor packages. This proves to be important when a semiconductor product fails to operate after it is packaged as the markings can assist in tracing the cause of the product’s failure. Markings can either be engraved by burning materials such as EMC with a laser or by embossing using ink.
For plastic packages, they need to be molded before the requested information is displayed on the surface. Since laser marking is simply the act of engraving, a black EMC is usually the preferred choice as it increases the legibility of the markings. This is because color cannot be applied to the engraved characters or symbols, so it is more visible to have engravings on a black background. The remaining two steps will cover the final stages of packaging substrate packages. This is where the difference lies between the processes of substrate and leadframe packages.
Step Seven: Solder Ball Mounting
Solder balls in a substrate package do not only serve as an electrical pathway between the package and external circuitry, but they also provide mechanical connections. Solder ball mounting is the process of attaching a solder ball to a substrate pad. In the first step of the process, flux11 is applied to the pad and then the solder balls are placed on the pad. Then, the reflow process melts and attaches the solder balls before the flux is washed and removed. The role of the flux here is to remove impurities and oxides from the surface of the solder balls during the reflow process. This allows the solder balls to melt uniformly and provides a clean surface. When these melted solder balls flow into a stencil on the substrate, they fill each hole in the stencil. Then, the substrate and stencil are separated but the solder balls remain on top of the substrate due to the adhesion of the flux. As there will be flux that has already been applied to the pad, the solder balls will be temporarily adhesive and attach to the pad.
Step Eight: Singulation
Singulation is the final process of creating a substrate package. The process involves using a blade to cut the finished substrate strips into individual packages. Once the singulation process is complete, the packages are placed on a tray for package testing and the rest of the process steps.
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