Flexible BGA hurdles

cost barrier

By using wire bonding to attach die, a patented tape-based BGA package

is cost competitive with plastic BGAs. The substrate's fine-pitch design

yields the high performance and I/O counts approaching those of ceramic

BGAs.

3M's Electronic Products Div. (Austin, TX) has introduced a tape ball-grid-array
package that uses wire bonding to attach dies in place of the thermocompression
bonding or flip-chip attach methods used by other tape BGAs. Called the
Microflex Wire Bond Tape Ball Grid Array (WB TBGA), the package uses existing
leadframe packaging processes and equipment.

The package overcomes the high costs previously encountered in implementing
flexible-tape BGA technologies. According to 3M, the new package provides
improved reliability, assembly throughput, and electrical and thermal performance
over plastic QFPs and plastic BGAs while offering higher lead counts. At
the same time, the Microflex WB TBGA costs less than ceramic BGAs and PGAs,
while providing competitive electrical and thermal performance.

3M offers the Microflex WB TBGA either in a cavity-down configuration
with a two-piece copper leadframe stiffener or in a cavity-up configuration
with a plastic overmold (see diagram ). The cavity-down package can
incorporate from 350 to 700 I/O bumps with a spacing of 1 or 1.27 mm, while
the cavity-up package can incorporate 100 to 300 I/O bumps with spacing
of 0.8 or 1 mm.

Offered as combining the best features of plastic and ceramic BGAs,
the Microflex Wire Bond Tape Ball Grid Array is available in both a cavity-down
configuration with a two-piece metal stiffener (top), or in a cavity-up
configuration
with a plastic overmold (bottom).

The package's high reliability results from the substrate's compatible
material set consisting of a Microflex circuit, a thin thermoplastic polyimide
adhesive, and–in the cavity-down versions–the leadframe stiffener. By
using a fine-pitch Microflex circuit having 50-µm or finer lines
and spaces, the package allows closely spaced wire bond pads and use of
a smaller die. This results in shorter wire lengths and lower overall self-inductance.

Measuring 2 mils thick, the polyimide adhesive allows the leadframe
stiffener to electrically couple with the circuit traces. This in turn
helps to reduce self-inductance and minimize crosstalk.

The adhesive is specially formulated to ensure a more robust, reliable
wire bond to the package. The metal stiffener, measuring 0.010 in.thick,
acts as a heat spreader to improve the heat dissipation of the die that
is bonded to it.

Typical electrical characteristics for the Microflex TBGA include a
self-inductance of 1.8 to 4.9 nH and a total capacitance ranging from 0.26
to 1.29 pF. At an airflow rating of 200 ft/min, the TBGA's junction-to-ambient
thermal resistance is 11.5°C/W.

The Microflex WB TBGA can be used for microprocessors, digital signal
processors, gate arrays, ASICs, and other products. The package is supplied
on leadframe strips with up to four circuits, minus the die and wire bonds,
for loading onto magazines of placement equipment. Pricing depends on the
configuration. For more information, contact the 3M Electronic Products Div.
at 888-845-3393, visit http://www.mmm.com/interconnects.