Coating apparatus – Immersion or work-confined pool type – Mask or stencil
Reexamination Certificate
1997-08-05
2002-01-08
Lamb, Brenda A. (Department: 1734)
Coating apparatus
Immersion or work-confined pool type
Mask or stencil
C118S683000
Reexamination Certificate
active
06336973
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to achieving a level surface on an exposed surface of a viscous fluid. More particularly, the present invention relates to maintaining a level surface on a pool of adhesive material for applying the adhesive material to the lead fingers by contacting the lead fingers with the pool of adhesive material.
2. State of the Art
Higher performance, lower cost, increased miniaturization of semiconductor components, and greater packaging density of integrated circuits are goals of the computer industry. One way to reduce the overall cost of a semiconductor component is to reduce the manufacturing cost of that component. Lower manufacturing costs can be achieved through faster production and/or reduction in the amount of materials used in fabricating the semiconductor component.
One area where faster production and reduction in material usage can be achieved is in the area of lead frame attachment to semiconductor dice. U.S. Pat. No. 5,286,679 issued Feb. 15, 1994 to Farnworth et al. (“the '679 patent”), assigned to the assignee of the present invention and hereby incorporated herein by reference, teaches attaching leads to a semiconductor device with adhesive material in a “lead-over-chip” (“LOC”) configuration. The '679patent teaches applying a patterned thermoplastic or thermoset adhesive layer to a semiconductor wafer. The adhesive layer is patterned to keep the “streets” on the semiconductor wafer clear of adhesive for saw cutting and to keep the wire bonding pads on the individual dice clear of adhesive for wire bonding. Patterning of the adhesive layer is generally accomplished by hot or cold screen/stencil printing or dispensing by roll-on. Following the printing and baking of the adhesive layer on the semiconductor wafer, the individual dice are singulated from the semiconductor wafer. During packaging, each adhesive coated die is attached to lead fingers of a lead frame by heating the adhesive layer and pressing the lead fingers onto the adhesive. If the adhesive layer is formed of a thermoset material, a separate oven cure is required. Furthermore, the adhesive layer may be formulated to function as an additional passivating/insulating layer or alpha barrier for protecting the packaged die.
Although the teaching of the '679 patent is an effective method for attaching leads in a LOC configuration, it is difficult to achieve an adequate profile on the adhesive such that there is sufficient area on the top of the adhesive to attach the lead fingers. The process disclosed in the '679 patent is illustrated in
FIGS. 23-29
.
FIG. 23
illustrates a side, cross-sectional view of a semiconductor substrate
302
with a bond pad
304
, wherein a stencil or a screen print template
306
has been placed over the semiconductor substrate
302
, generally a silicon wafer. The stencil or screen print template
306
is patterned to clear the area around the bond pads
304
and to clear street areas
308
for saw cutting (i.e., for singulating the substrate into individual dice). An adhesive material
310
is applied to the stencil or screen print template
306
, as shown in FIG.
24
. Ideally, when the stencil or screen print template
306
is removed, adhesive prints
312
are formed with vertical sidewalls
314
and a planar upper surface
316
, as shown in FIG.
25
. However, since the adhesive material
310
must have sufficiently low viscosity to flow and fill the stencil or screen print template
306
, as well as allow for the removal of the stencil or screen print template
306
without the adhesive material
310
sticking thereto, the adhesive material
310
of the adhesive prints
312
will spread, sag, or flow laterally under the force of gravity after the removal of the stencil or screen print template
306
, as shown in FIG.
26
. This post-application flow of adhesive material
310
can potentially cover all or a portion of the bond pads
304
or interfere with the singulating of the semiconductor wafer by flowing into the street areas
308
.
Furthermore, and of even greater potential consequence than bond pad or street interference is the effect that the lateral flow or spread of adhesive material
310
has on the adhesive material upper surface
316
. As shown in
FIG. 27
, the adhesive material upper surface
316
is the contact area for lead fingers
318
of a lead frame
320
. The gravity-induced flow of the adhesive material
310
causes the once relatively well-defined edges
322
of the adhesive material to curve, resulting in a loss of surface area
324
(ideal shape shown in shadow) for the lead fingers
318
to attach to. This loss of surface area
324
is particularly problematical for the adhesive material upper surface
316
at the longitudinal ends
326
thereof. At the adhesive material longitudinal ends
326
, the adhesive material flows in three directions (to both sides as well as longitudinally), causing a severe curvature
328
, as shown in
FIGS. 28 and 29
. The longitudinal ends of the adhesive print on patch flow in a 180° flow front, resulting in blurring of the print boundaries into a curved perimeter. This curvature
328
results in complete or near complete loss of effective surface area on the adhesive material upper surface
316
for adhering the outermost lead finger closest to the adhesive material longitudinal end
326
(lead finger
330
). This results in what is known as a “dangling lead.” Since the lead finger
330
is not adequately attached to the adhesive material longitudinal end
326
, the lead finger
330
will move or bounce when a wirebonding apparatus (not shown) attempts to attach a bond wire (not shown) between the lead finger
330
and its respective bond pad
304
(shown from the side in FIG.
29
). This movement can cause inadequate bonding or non-bonding between the bond wire and the lead finger
330
, resulting in the failure of the component due to a defective electrical connection.
LOC attachment can also be achieved by attaching adhesive tape, preferably insulative, to an active surface of a semiconductor die, then attaching lead fingers to the insulative tape. As shown in
FIG. 30
, two strips of adhesive tape
410
and
410
′ are attached to an active surface
412
of a semiconductor die
404
. The two adhesive tape strips
410
,
410
′ run parallel to and on opposing sides of a row of bond pads
406
. Lead fingers
402
,
402
′ are then attached to the two adhesive tape strips
410
,
410
′, respectively. The lead fingers
402
,
402
′ are then electrically attached to the bond pads
406
with bond wires
408
. Although this method is effective in attaching the lead fingers
402
,
402
′ to the semiconductor die
404
, this method is less cost effective than using adhesive since the cost of adhesive tape is higher than the cost of adhesive material. The higher cost of the adhesive tape is a result of the manufacturing and placement step which are required with adhesive tapes. The individual tape segments are generally cut from a larger tape sheet. This cutting requires precision punches with extremely sharp and accurate edges. These precision punches are expensive and they wear out over time. Furthermore, there is always waste between the segments which are punched out, resulting in high scrap cost. Moreover, once punch out is complete, the tape segments are placed on a carrier film for transport to the die-attach site. Thus, there are problems with placement, alignment, and attachment with film carriers, plus the cost of the film carrier itself.
LOC attachment can further be achieved by placing adhesive material on the lead fingers of the lead frame rather than on the semiconductor substrate. As shown in
FIG. 31
, the adhesive material
502
may be spray applied on an attachment surface
504
of lead fingers
506
. However, the viscous nature of the adhesive material
502
results in the adhesive material
502
flowing down the sides
508
of the lead finger
506
and collecting on
Ahmad Syed S.
Chapman Gregory M.
Jiang Tongbi
Moden Walter L.
Lamb Brenda A.
Micro)n Technology, Inc.
TraskBritt
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