In Assembly, the wafers are cut, and the die are removed, packaged, and prepared for final testing and shipping.
Using grindstone wheels, the thickness of the wafer is reduced to provide both a clean, uniform surface and a specified product thickness. Backgrind is capable of grinding each wafer down to approximately 0.3 millimeters—as thin as an average piece of paper.
Wafer Mount / Saw
This process attaches a wafer and a film frame to ultraviolet-sensitive adhesive tape. This tape holds the wafer and die in place during subsequent processing.
A diamond-edged saw blade, approximately the thickness of a human hair, cuts the wafer into individual die. The blade spins at 45,000 revolutions per minute and cuts at a speed of 8.9 centimeters (3.5 inches) per second. During the cutting process, water sprays on both the blade and the wafer to keep the temperature low and remove the debris. After the wafer is cut, a final high-pressure rinse washes it.
Using the wafer map created in Probe, the good die are identified, removed from the wafer, and placed with adhesive on a leadframe or "interposer" at a speed of up to 4,000 die per hour. To remove the die from the tape, needles push up from beneath the tape as a vacuum tip lifts the die from the top. The unqualified die are left on the adhesive as illustrated below.
The good die are then adhered to the interposer--or lead frame--and batches of interposers are cured in an oven to set the adhesive/epoxy.
Thin gold wire—99.9999% pure and thinner than a human hair—is attached to the die and the interposer. This wire provides the communication path (circuitry connection) between the die and the computer. Ultrasonic gold-ball bonding, a technique that combines ultrasonic energy, heat, and force, is capable of interconnecting the die bond pads to the interposer/leadframe bond pads.
During Encapsulation, the die and a small portion of the interposer/leadframe are covered with a hard plastic compound to protect the die. The equipment encapsulates the die by moving the interposers into a mold area, using force to inject heated compound into the mold cavities, and curing the compound. The mold is opened, and the interposers are pressed out and cleaned
Product must now go through either an Electroplating or Solder-ball Attach process. In electroplating, the exposed metal on a leadframe is covered with a conductive metal coating. While submerged in a tin and lead solution, leadframes are charged to attract the tin and lead ions. This results in a uniform plated coating that increases conductivity, keeps the leads from rusting, and provides a clean, even surface.
In solder-ball attach, solder balls and flux are placed on gold-plated pads located on the substrate. When heat is applied to the part, the solder balls adhere to the pads. Leads or solder balls provide the final interconnect between the component and the board application in the end-use product.
Trim and Form
In Trim & Form, leadframes are loaded into trim-and-form machines where the leadfingers are formed step by step until finally the chips are severed from the frames. An opens-and-shorts test is performed on each device, and the devices are sorted into good or reject trays or tubes.
The various positions and shapes of the leads and the package size and shape depend on the final application and the customer's packaging requirements.