By Dr. Eric Bogatin
www.BeTheSignal.com
September 2007
The mantra of the electronics packaging industry used to be “faster, denser, cheaper, NOW!” These driving forces fueled the development and introduction of the five most important packaging technology revolutions of the last 10 years:
- Ball-grid array (BGA) packages
- Chip-scale packages (CSP)
- High-density interconnect (HDI) substrates
- System in Package (SiP)
- 3D stacked chip packages
Today, a new driving force is fueling the development and introduction of the next packaging technology revolution: the quest for green manufacturing, as dictated by the Restrictions on Hazardous Substances (RoHS) legislation.
Rising to the challenge, Verdant Electronics of Sunnyvale, CA, (www.VerdantElectronics.com) recently introduced a new packaging technology they say meets the existing requirements of faster, denser, cheaper, NOW! and provides a solution to green manufacturing by eliminating the need for solder, lead free or otherwise.
They propose building electronic assemblies in the reverse order of conventional approaches. Rather than assembling and soldering IC packages and passive components to a pre-fabricated interconnect board, their process—called The Occam Process and named after the 14th century philosopher William of Ockham who coined the title of this column—builds the interconnect assembly on top of a substrate created from the IC packages and passive components.
The pads of the IC packages and other components become the substrate upon which multiple layers of conventional micro-via, high-density interconnect layers are fabricated. The exposed pads of all the components become landing pads for copper metallization. Each layer’s interconnect is patterned using conventional lithography, and non-mechanically drilled vias connect one layer to the next.
Figure 1 shows an example of the process flow (illustration courtesy of Verdant Electronics).
Figure 1. Occam Process With Packaged Devices as the Substrate
The Occam process integrates the conventional HDI process for interconnect substrates, with the initial step of affixing the components to a surface, encapsulating them into a rigid substrate, and laser drilling openings to expose the pads.
“Products built by this process are expected to be more reliable than solder-free strategies as well as traditionally manufactured soldered assemblies,” Joe Fjelstad, the founder of Verdant Electronics, said. He went on to say, “The reverse order interconnect process embodies mature, low-risk, familiar core processing technologies in a novel sequence.”
Fjelstad is no stranger to innovative microelectronic technologies, with over 170 issued or pending patents. He is also the founder of Silicon Pipe, which won the 2004 Innovative Packaging Award from Advanced Packaging Magazine for its “over the top” packaging architecture.
Fjelstad is the first to say his approach to building the interconnect substrate on top of the chips is not new. In the early 1990s, GE pioneered this architecture, often termed, “chips first,” for satellite and aerospace applications where weight and size are at a significant premium.
Bare die were mounted to a metal or ceramic substrate with their pads sticking up. A planarizing polymer encapsulated the chips, and their pads were exposed by laser drilling. Multiple layers of interconnect were then built on top of the chips. The initial polymer layer provided mechanical stability and environmental protection and the substrate provided thermal management and EMI shielding.
More recently, in 2001, Intel introduced its BBUL (Bumpless Build-Up Layer) technology, a chips-first approach for microprocessor packaging. The features were essentially the same as the GE process, though on a finer pitch. The driving force for Intel was the elimination of the C4 pads and an increase in the number of I/O to adjacent chips. In July, 2006, Intel said the BBUL process would be crucial for implementing a 32 core processor module.
What’s different in this implementation, Fjelstad says, is that the Occam process uses packaged devices that have already been burned in and tested. “The encapsulation step can dramatically increase the reliability of conventional tin-plated leaded packages by inhibiting tin whisker growth.” He adds, “Prior efforts have been to build microelectronics using micro vias in an IC fab-like environment. The Occam process is focused on assembling macro electronics and interconnection of macro-vias produced in a printed circuit shop environment with no solder in the final assembly.”
It’s not just packaged devices and passives that can be integrated in the solderless interconnect substrates. To go to the next interconnect level, flex pigtails, connectors, and even sockets might be integrated into the substrate.
Fjelstad started Verdant Electronics as a vehicle to help increase awareness of the possibilities the Occam process opens up for enabling new products, cost-reducing current products and providing a new option to lead-free assemblies. Verdant Electronics is hosting a conference on the Occam process in mid October in San Jose, CA. Registration is available on their website at www.VerdantElectronics.com.
This and other signal integrity topics are covered in Eric’s public classes and online lectures, available from his website, www.BeTheSignal.com. Send your signal integrity technical questions to DoctorIsIn@BeTheSignal.com.
Bio: Eric is president of Bogatin Enterprises, whose mission is to set the standard for signal integrity training. He is the author of Signal Integrity - Simplified, published by Prentice Hall. Check out his public signal integrity classes posted on www.BeTheSignal.com. He can be reached at eric@BeTheSignal.com.
