03/01/2009, Irvine, California


Challenging Flip Chip Applications No Problem for New Underfill Technology

The benefits of flip chip technology for advanced device applications have been well proven. With advantages such as more efficient electrical interconnections, smaller device footprints and increased I/O counts, flip chip technology has gained favor among package designers and electronic product OEMs in recent years. While the reliability of flip chips in smaller footprint packages such as BGAs, CSPs and the like has been established, large die (20 mm and above) flip chips for applications such as ASICs, video chips and microprocessors are subjected to more stress.

Plus, these larger devices can be even more challenging when they are bumped with lead-free solder, as Pb-free materials tend to have less solder fatigue resistance. Newly developed underfill technology, however, addresses these challenges along with the hurdles of lead-free flip chips, helping device manufacturers realize all the benefits of this versatile technology without the problems often associated with large die flip chip processing.

The flip chip package is the first level packaging where the active side of the silicon chip is placed face down and connected to the substrate. The active side of the chips are bumped with eutectic, high-lead or lead-free solders and then reflowed to form the electrical and mechanical connections. Underfill is used to enhance the reliability of the solder bump joints by absorbing and evenly distributing any thermal-mechanical stress across the die. As one can imagine, the bigger the die, the larger the stress.

To overcome the challenges associated with large die flip chip applications and lead-free bumped chips, Henkel has developed a new Amine-based underfill system that delivers the robustness and reliability of smaller-die flip chip technology. One common problem with older generation underfills is their limited adhesion to Silicon Nitride (SiN), which is one of the more common die passivations utilized by modern packaging specialists. Many of the traditional underfill materials do not adhere well to SiN, subsequently causing problems – such as shorts and, thus, catastrophic device failure when the devices go through real life cycles. Henkel’s next generation Amine underfill system –Hysol FP4585 -- alleviates these issues by delivering excellent adhesion to SiN and polyimide and, when tested against competitive underfills, provided superior performance. This next generation underfill system is designed to deliver lower stress with the unique combination of thermal mechanical characteristics to prevent delamination, bump fatigue and UBM (Under Bump Metallization) failure. The formulation is suited for lead-free applications and also delivers outstanding flux compatibility and moderate Tg to address the lower solder fatigue resistance of lead-free bumped chips.

Certainly excellent adhesion is a key property requirement of underfills utilized by advanced device manufacturers, but the materials must deliver other high reliability
characteristics as well. Low moisture absorption is also critical, so that as the device goes through the reflow processes – both in package assembly and subsequent PCB assembly – it will remain void-free. Henkel’s new Amine-based Hysol FP4585 delivers these benefits as well as superior fracture toughness. In testing to JEDEC Level 3 plus 1000 cycles TCB, Henkel’s underfill system outperformed competitive materials by showing no UBM failure – proving the superiority of the material’s long-term integrity. Since large die flip chips are subjected to more mechanical stress because of the die’s larger DNP (distance from neutral point), the underfill material’s fracture resistance is crucial.

All of these longterm reliability underfill characteristics – optimal thermal mechanical characteristics, good adhesion, low moisture absorption and good fracture toughness – are vital for excellent in-field performance. But, of equal importance is the material’s manufacturability. An underfill’s flow characteristics, process window and cure properties dictate its ability to perform well – and at relatively fast speeds – within a manufacturing environment. In addition to the reliability characteristics of Hysol FP4585, the material also embodies the important manufacturing properties of a wide process window, faster flow and longer gel time, which enables self-filleting and eliminates the need for a seal pass. All of these benefits amount to superior performance, more efficient manufacturing and, ultimately, lower costs and higher reliability. Plus, this Henkel underfill technology is also suitable for overmold packages, giving electronics specialists even more manufacturing flexibility.

Large die flip chip packages with fine-pitch, lead-free bumps are arguably the most difficult devices for which to design robust underfill systems. Therefore, it is believed that if an underfill material can pass reliability testing on this package, then it can be incorporated successfully on other packages as well. Henkel’s new Hysol FP4585 and the fine-filler formulation, Hysol FP4583, have proven to deliver robust performance on both high-lead bumped flip chips and lead-free bumped flip chips. They are, indeed, the next-generation answer to flip chip underfill technology.

For more information on Henkel’s advanced underfill offerings, log onto www.electronics.henkel.com or call the company’s headquarters at 949-789-2500.