Page 128 - System on Package_ Miniaturization of the Entire System

Stacked ICs and Packages (SIP) 103

The wire bonding process is particularly challenging when an upper die in a stack is
larger or overhangs a lower die. Bonding to an overhanging die can cause many problems,
including die cracks, loop damage, and inconsistent bump formation, due to die edge
bouncing. The maximal overhang length for a package depends on the application and
is determined by the die thickness, back-side die defect sizes, properties of the die
attachment layers, and the impact and bonding forces in the wire bonding process.

Die Adhesive Two types of die adhesives are used in stacking chips: nonconductive
epoxy (NCE) and film adhesive (FA) [27]. NCE is generally lower cost and involves
minimal capital investment because it is used with existing die bonders. The weaknesses
of NCE processing, however, include control of voids, fillet coverage, epoxy bond-line
thickness control, and die tilt—all critical issues for successful die stacking. In addition,
resin bleed can contaminate die-bond pads and make wire bonding difficult. The FA
technology, on the other hand, can address the above process concerns associated with
using NCE in die-stacking applications. Because resin bleed is a major concern when
stacked dies are of the same size, the FA is the only workable option. In addition, FA
provides a uniform bond-line thickness that is void-free, with 100 percent edge coverage.
The FA also acts as a stress absorber between dies. The FA technology, however, requires
an initial capital investment in wafer-back lamination and die-bonder modifications,
and involves higher materials cost. The increasingly higher demand for higher quality
of die-stacking applications can offset these additional expenses.

Spacer Technology Stacking of chips with varying die sizes requires a spacer between the
dies when the top die is either the same size or larger than the bottom die, to avoid damage
to its wires. Numerous spacer materials have been used, including silicon, adhesive paste,
and thick tape. Each presents advantages and shortcomings. Silicon is widely used because
of its acceptance, its infrastructure, and its cost-effectiveness. But it has more processing
steps. Epoxy with spacer spheres requires fewer process steps, but has more epoxy bleed.
Tape has no bleeding, but it is more costly. Epoxy with spacer spheres is preferred for a die
with a thickness < 100 μm, because it minimizes the overhanging span of the top of the die
and enables its wire bonding [28]. The use of spacers affects mold-cap thickness and total
package height. The process capability for controlling wire loop height and mold flow
dictates the spacer gap. A larger mold gap works against the trend to thinner package
height. Choosing a reasonable spacing gap is important for mold compound flow, since
turbulent mold compounds flow inside a mold cavity.

Molding Increased wire density and wire length in wire bonded chip stacking makes
molding the stack more difficult than conventional single-die packages. Different layers
of wire bond loops that are subjected to varying amounts of drag force can result in
differences in wire sweep. This increases the possibility of wire shorts. Further, the
variable gaps between various die components make it more difficult in the molding
process to achieve a balanced flow without voids free. Molding compound development
and selection, as well as gate design and wire layout optimization, are required to
achieve a better yield in molding. Low-viscosity compounds and compounds with
smaller filler sizes and slower molding transfer speeds show improved wire sweeps. A
lateral loop trajectory is known to be able to reduce the mold sweep by predeforming
the wire in anticipation of the sweep direction [26]. The change in gate design from
conventional bottom-gate to top-center mold gate can also reduce the wire sweep,
especially for long wire applications [29].