Polyimide Cure for
Wafer-Level Packaging (Redistribution Layers)
Smartphone, tablet and laptop computer consumers are depending on their electronics to be small, lightweight and fast. Wafer Level Packaging (WLP) allows these products to be handheld sizes with high-quality graphics instead of enormous and poor quality. WLP will enable the electronics industry to make mobile devices smaller and smaller.
With processes being performed at the wafer level instead of later with wire bonding, companies are seeing results of smaller circuits and bigger profits. The most efficient way to complete wafer level packaging is with Redistribution Layers (RDL).
The need for a consistent quality cure is now critical for WLP/RDL.
However, for Wafer Level Packaging, accurate performance is required on every polyimide layer. A consistent repeatable cure is critical in achieving uniform properties and excellent electrical properties.
Yield Engineering manufactures tools to alleviate wafer level packaging issues:
- Polyimide cure
- Copper anneal
- BCB cure
- Low-k dielectric cure
Polyimide curing systems have several distinct advantages over conventional baking systems:
- Vacuum / N2 cycles create an oxygen-free environment for the curing. Incorporation of oxygen in the polyimide can lead to a dark brittle film.
- Partial vacuum. The vacuum draws the solvent out of the film of polymer leading to a faster more complete cure.
- Programmed temperatures with controlled ramp rates lead to low stress films. This is especially critical when employing multiple layers of polyimide. After the first metallization layer, temperature control of the cure is even more important. Improper curing can lead to the first polyimide layer softening and wrinkling the metal traces due to imparted stress.
- Efficient use of chemical.
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.
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