Metal particles, polymers push connector performance

The trend of greater systems integration on microcircuits is accelerating. Where once were ASICs and microprocessors, developers are putting entire systems on single chips for fast synchronization. But with package input/output connections being pushed into several thousands, in close proximity, how do you connect effectively to these packages-- while keeping their temperatures down and electronic performance up?

In 1994, in response to increasing electronic-device circuit densities, Thomas & Betts (T & B) began looking into future connection requirements--a parallel effort based on stamped metal contacts and the promising technology of metal particles in a polymer base forming conformal contacts. Within two years, it was apparent that metallized particle interconnect (MPI) technology was the way to go. First production products were shipped in mid-1997.

Particular particles. The MPI concept consists of Angstrom-size spherical and jagged particles in a unique polymer blend developed by T & B. This mixture is formed into [email protected] columns having a slight taper. A 0.13-mm-thick Kapton® substrate, with the required grid of through holes, retains the columns in 1.5-mm centerline-pitch arrays of up to more than 5,000 positions. The substrate is placed between the chip contacts and the landing pads on a PCB. Upon compression mating, conduction goes from particle to particle. At the contact interfaces, the column widens slightly and the spherical particles indent the surface of the contact pads, further increasing contact area. The rougher particles pierce surface contaminants, lowering resistance.

The columns have a springlike force-deflection curve within a compression range of 0.3 mm, which can take into account any "land" irregularities. While a slight set does take place, the MPI connection can be used for 20 cycles at a minimum, with some having been used up to 80 times. Most applications will probably see only five to seven re-entries.

The polymer blend also seals the contact area against contamination. In a Battelle mixed-flow gas test done for resistance to corrosive gases, over 21 days, rather than the standard 10 days, contact resistance did not change. This was also the case for 3,000 hr at 145C (293F), rather than the usual 1,000 hr at 125C (257F) high-temperature test. Load testing produced similar results due to the polymer base absorbing shock and vibrations. These included 3D shocks to 75g and vibrations to 9.3g rms.

All contacts in an array are formed simultaneously. Contact cost in quantity production can be as low as one cent per contact--well below that of cartridge-type, pin-and-socket, or ball-grid array (BGA) interconnects. And the MPI columns minimize crosstalk and inductance and capacitance bridges between them, important for high-speed applications (see box). Thermal conductivity for the low-resistance contacts is such that some applications have seen a decrease in package bulk temperatures up to 5C (9F). Development is underway to produce contact arrays with 0.5-mm pitch by mid-1999.

Tradeoffs. An MPI connector is not a simple mate and unmate arrangement. A compressive load on the chip package or its integral heat sink must be maintained. For current MPI column dimensions, mating pads should be a minimum of 0.8 mm in diameter. And as a rule of thumb, traces on a board should not come within 0.15 mm of a pad. Vias 0.3 mm in diameter can be within a pad.

Developers more accustomed to metal connectors may have a natural reluctance to try the newer conductive-polymer-based MPI. But test data is encouraging many to change--even those who have had bad experiences in the past with carbon-filled elastomer contacts, where resistances ranged from 75-200 mV.

Thomas & Betts sees the MPI market approaching $60 million in 1999. Growth is projected at 25-30% annually for the following two years. MPI has been readily accepted in the workstation and high-end server market (plus at several microprocessor manufacturers), such that wherever we have demonstrated the technology, we now have a production or qualification program in place.

What MPI means to you:

MPI contact performance

DATA SUPPLIED BY THOMAS & BETTS