Tutorial #115
Terence Collier
Summer 2011
A number of die suppliers are migrating towards electroless metal finishes on bond pads. Finishes of electroless nickel followed by immersion gold or immersion gold + palladium (ENIG and ENIPIG) help to improve both test and assembly yields, as well as long-term reliability.
A problem with pads shorting during electroless plating is a concern, and a solution is needed.
As part of the electroless processes, the top layers of aluminum, aluminum oxides, and other aluminum corrosion by-products are removed and replaced with strong, chemically bonded fresh metal layers that are robust and oxide-free.
Gold wire and copper wire adhere much better to oxide-free surface layers than corroded metal layers. Reliability concerns related to purple plague and Kirkendall voiding can also be reduced as a result of gold to gold bonding.
Contact resistance (CRES) is directly related to the degree of bond pad oxidation, contamination and corrosion. Reducing CRES improves both assembly yield and test yield. False failures can occur during testing when probe needles comes in contact with non-conductive CRES layers.
Penetrating these CRES barriers requires higher force on the probe needles, risking damage to both die and the test hardware. Since gold pads typically have no oxide, probe force is minimal and test yield increases.
One might ask, why not deliver die with the gold layer already on the aluminum pad?
Unfortunately, CMOS wafer fab processes don’t allow gold in the same place as silicon. Mixing the two can create bad intermetallics, causing mechanical stress on the silicon that can lead to eventual fracturing.
But having gold on the aluminum pad is a benefit. How can one treat the aluminum pad to reduce stress without sacrificing device reliability? This is where electroless nickel-immersion gold comes into play.
An ENIG advantage over standard electroplating is that ENIG is a maskless process, because electroless finishes adhere only to the target metals on the die. This eliminates the time and cost of creating and exposing masks to pattern the deposit.
The increasing cost of gold gives electroless finishes a growing cost over conventional sputtering and plating for typical electrolytic finishes.
One problem with electroless finishes, shared with electrolytic plated finishes, is that the deposited layers grow wider at the same rate as they grow higher. This gives a mushroom shape to the deposited material. Figure 1 shows the curved shoulders of a typical ENIG deposit. (The material on top is conductive adhesive for flip chip assembly).
Figure 1. An ENIG deposit, showing curvature from horizontal growth
This is a limitation for tightly packed bond pads at a time when closer packing is a major goal. The present method to work around this is depositing and curing organic insulators such as polyimide (PI) or polybenzoazole (PBO) prior to electroless processing.
The organic insulators, typically 4um thick, provide the isolation and barrier to prevent mushroom shorts during electroless plating. A 2um layer of Ni can then be plated on the bond pads.
The added cycle time and cost of PBO/PI processing almost eliminates the benefits of ENIG processing. PBO/PI materials are not cheap. PBO/PI processing may include up to two addition days of cycle time. PBO and PI also increase mechanical stress, which can lead to electro-mechanical failures, particularly on thinned die.
A further drawback is the high processing temperatures required for curing the PBO and PI, approaching 375 °C on some materials. These stresses and temperatures raise concerns for test failures that could mitigate any process improvements.
To avoid those costs and problems, CVInc has developed a proprietary process replacing the PBO/PI approach, with significant reductions in cycle time and cost. We have demonstrated up to 10um nickel thickness without bridging to adjacent devices.
This process also allows pad size reduction. As shown in Figure 2 , a NiAu island can be created in the middle of the pad with the aluminum serving as a solder barrier. Here the original array pad size is 75um with an 85um pitch.
The pads were resized to 60um and then plated with 2.5um of Ni followed by Pd then Au. When the solder bump is placed on this pad a more manufacturable, higher yield process results.
Figure 2. Smaller NiAu islands centered and surrounded by aluminum.
This a process promises many other benefits yet to be realized. For example, Figure 3 shows tall, closely spaced pillars of NiAu, not possible with standard ENIG processing because of the mushroom effect.
Figure 3. Deposited ENIG pillars. Heights are 12um; the gap between pillars is 8um. Test probe marks on top surface.
In summary, the benefits of electroless ENIG plating can now be extended to finer pitch, thicker, and taller depositions without the drawbacks of PBO/PI processing.
FOR MORE INFORMATION
CVInc.
850 S. Greenville, Suite 108
Richardson, TX, 75081
972-664-1568 (o)
www.covinc.com tqcollier@covinc.com