Polyimide Cure for
Wafer-Level Packaging (Redistribution Layers)

Smartphone, tablet and laptop computer consumers are depending on their electronic products to be small, light, and fast. Wafer Level Packaging (WLP) allows these products to be handheld sizes with high-quality graphics, instead of large bulky devices. Advanced WLP will enable the electronics industry to make the next generation of electronic products be even smaller and more powerful.

The use of Redistribution Layers (RDL) is an integral part of WLP, in which processes are being performed at the wafer level instead of later with wire bonding. An important component of RDL is the polyimide layer protecting the WLP/ Redistribution Layers (RDL) circuits. The manufacturability of the polyimide layer depends on the curing process for the polyimide precursors.

For WLP, consistent performance is required on every polyimide layer. A consistent repeatable cure is critical in achieving uniform properties and excellent electrical properties.

The need for a consistent quality polyimide cure is now critical for WLP/RDL.

Yield Engineering manufactures tools specifically designed to address wafer level packaging issues:

• YES-VertaCure
• YES-PB Series

Applications:

• Polyimide cure
• BCB cure

Benefits:

YES Polyimide curing systems have several distinct advantages over conventional atmospheric baking systems:

• Uniform Solvent Evaporation – The vacuum efficiently draws the solvent out of the film of polymer leading to a faster more complete cure and to a solvent-free polyimide film.
• Optimal Temperature Control – Imidization rates can be better controlled when the casting solvents are efficiently evolving from the film. Controlled temperature ramp rates provide a larger process window for the proper curing of the polyimide film. This is especially critical when employing multiple layers of polyimide. Improper curing can lead to the first polyimide layer softening and wrinkling of the metal lines due to imparted stress.
• Vacuum / N₂ cycles create an oxygen-free environment for the curing – Incorporation of oxygen in the polyimide can lead to a dark brittle film.
• Cleaner Process – Laminar flow is achieved by pulling vacuum from one side of the oven and flowing in pre-heated nitrogen from the opposite side. The result is a Class 1 environment in which there is a laminar flow that is parallel to the orientation of the wafers.

Perhaps the bond pads are located down the middle of the die to allow for faster access, but you need them around the outside perimeter like the last generation. Maybe the chip was designed for wire bonded surface mount, but you need solder bumps and flip chip mounting. The answer is RDL or Redistribution Layer.

There are three primary uses for RDL. The first is to move the bond pads around the face of the die for flip-chip applications. It is important to spread the contact points around the die so that solder balls can be applied and the stress of mounting can be spread. Another important application of RDL involves die stacking. In this application, similar die can be mounted in a single package. In order to give each die a unique address, the address lines of each die can be placed in a unique location. A third application for RDL is simply to move the bond pads of a device to a position more convenient or accessible for subsequent bonding and packaging steps. This may include matching the layout of an old die that is no longer being supported and must be replaced by a newer design.

To see the full white paper with diagrams,  click here