ELECTRONIC ASSEMBLY HAVING A DIE WITH ROUNDED CORNER EDGE PORTIONS AND A METHOD OF FABRICATING THE SAME
BACKGROUND OF THE INVENTION
1). Field of the Invention
[0001] This invention relates generally to an electronic assembly of the kind
having a die with an integrated circuit formed thereon, and more specifically to
prevention of cracking of the electronic assembly due to differences in coefficients
of thermal expansion of the die, an underfill material below the die, and a package
substrate.
2). Discussion of Related Art
[0002] Integrated circuits are formed in rows and columns on semiconductor
wafers, which are subsequently "singulated" or " diced" by directing a blade of a
saw through scribe streets in x- and y-directions between the integrated circuits.
Resulting dies have conductive interconnection members that can be placed on
contact terminals of a package substrate, and be soldered to the contact terminals.
[0003] A package substrate typically has a coefficient of thermal expansion
(CTE) which is higher than that of the die, which creates stresses on the
interconnection members when the electronic assembly heats up and cools down.
An epoxy underfill material is often applied to the package substrate, flows into a
space between the package substrate and the die under capillary action, and is
subsequently cured at a high temperature. The stresses on the interconnection
members are redistributed to the solidified underfill material.
[0004] The underfill material typically has a CTE which is even higher than that
of the substrate, which creates stresses on certain areas of the die when the
assembly cools down after the underfill material is cured. These stresses are
particularly high at corner edge portions of the die where side edge surfaces
thereof meet, and may cause cracking in the die, the underfill material, or in the
package substrate at or near the corner edge portions of the die.
[0005]
[0006] BRIEF DESCRIPTION OF THE DRAWINGS
[0007] The invention is described by way of example with reference to the
accompanying drawings, wherein:
[0008] Figure 1 is a plan view of a semiconductor wafer having a plurality of
integrated circuits formed thereon;
[0009] Figure 2 is a cross-sectional side view of a portion of the semiconductor
wafer which is mounted on a support layer;
[0010] Figure 3 is a plan view of the wafer after the wafer has been singulated
into individual dies;
[0011] Figure 4 is a view similar to Figure 2, illustrating a blade of a saw as it
travels through the wafer;
[0012] Figure 5 is a view similar to Figure 4, further illustrating a laser that is
used to remove portions of the dies;
[0013] Figure 6 is an enlarged plan view illustrating a region of one of the dies
where a corner edge portion thereof is removed;
[0014] Figure 7 is a top plan view of an electronic assembly that includes one of
the dies;
[0015] Figure 8 is a cross-sectional side view of the electronic assembly on 8-8 in
Figure 7; and
[0016] Figure 9 is a cross-sectional view on 9-9 in Figure 7.
DETAILED DESCRIPTION OF THE INVENTION
[0017] Figures 1 and 2 of the accompanying drawings illustrate a semiconductor
wafer 10 which has been attached to a supporting layer 12, typically made of
Mylar®, for purposes of sawing the wafer 10. The wafer 10, as will be commonly
understood, has a plurality of identical circuits 14 that are replicated in rows and
columns across a circular area of the wafer. Scribe streets 16 are defined in x- and
y-directions between the circuits 14. A respective rectangular guard ring (not
shown) surrounds each respective circuit 14.
[0018] As illustrated in Figures 3 and 4, a blade 18 of a saw is directed through
the wafer (10 in Figure 1) so that the wafer is singulated into individual dies 20.
The blade 18 is not intended to cut through the supporting layer 12, but may cut it
partially. The dies 20 are attached to the supporting layer 12, and the supporting
layer 12 maintains the dies 20 in their original position of Figure 1. The blade 18 is
directed through the scribe streets (16 in Figure 1) and between the guard rings so
that the circuits 14 are protected by the guard rings. Each die 20 includes a
respective one of the circuits 14.
[0019] Figure 5 illustrates further processing of the dies 20, wherein a laser 22 is
used to remove portions of the dies 20. The laser 22 is preferably an Excimer
laser, because an Excimer laser beam does not transfer heat to an object that is
being ablated. The laser 22 is positioned above the dies 20, and a laser beam 23 is
directed by the laser 22 onto one of the dies 20. Laser is preferred over grinding
and milling because of the possibility to produce higher volumes. Laser is also
preferred over etching because of tighter control over dimensional tolerances.
[0020] Figure 6 illustrates a portion of one of the dies 20 after a corner edge
portion 24 thereof has been removed with the laser 22 in Figure 5. Before removal
of the corner edge portion 24, the die 20 has two side edge surfaces 26 that meet at
right angles to one another at a corner edge 28. After removal of the corner edge
portion 28, the die 20 has a rounded surface 30 that joins remaining portions of the
side edge surfaces 26. The corner edge portion 24 is thus bound by the rounded
surface 30 and extensions 32 of the side edge surfaces 26.
[0021] The rounded surface 30 may have a radius (R) of between 50 μm and 1000
μm. The corner edge portion 24 accordingly has an area of between 537 μm2 and
860000 μm2. The purpose for providing these ranges is merely to establish that
the intent is to differentiate over the tiny radii found on sharp, even knifelike
edges.
[0022] Referring to Figure 3, the process of removing a corner edge portion from
one of the dies 20 is repeated on all four corners of each one of the rectangular
dies 20. It can thus be seen that removal of the corner edge portions is automated
by removing the corner edge portions directly after the dies 20 are singulated, but
before the dies 20 are removed from the supporting layer 12.
[0023] Figures 7 and 8 illustrate an electronic assembly 34 that includes a
package substrate 36, one of the dies 20, and an underfill material 38. The
package substrate 36 includes a carrier substrate 40 and a plurality of contact
terminals 42 formed at an upper surface of the carrier substrate 40. The die 20
also has a plurality of contact pads 44 and a plurality of conductive solder ball
interconnection members 46, each attached to a respective one of the contact pads
44.
[0024] The die 20 is placed on the package substrate 36 so that each one of the
interconnection members 46 is on a respective one of the contact terminals 42.
The contact terminals 42 are in rows and columns forming an array, and the
interconnection members 46 have a pattern that matches the pattern of the contact
terminals 42. The entire assembly, excluding the underfill material 38, is then
heated in a reflow oven so that the interconnection members 46 melt, and is
subsequently allowed to cool. The interconnection members 46 are so soldered
and secured to the contact terminals 42.
[0025] The underfill material 38 is an epoxy that is applied in liquid form on the
package substrate 36 around the die 20. Capillary forces draw the liquid underfill
material 38 into a space between an upper surface of the carrier substrate 40 and a
lower surface of the die 20 between the interconnection members 46. The entire
volume between the die 20 and the carrier substrate 40 is substantially filled with
the liquid underfill material 38, and some of the underfill material 38 also forms
on side edge surfaces 26 of the die 20.
[0026] As illustrated in Figure 9, the rounded surface 30 has formed through an
entire thickness 50 of the die 20. The die 20 typically has a thickness of about
750 μm, and the rounded surface 30 thus also has a thickness of 750 μm. The
underfill material 38 is also formed on a lower portion of the rounded surface 30.
[0027] The entire assembly illustrated in Figures 7, 8, and 9 is then located in an
oven and heated to a temperature sufficient to allow the underfill material 38 to
cure. Curing solidifies the underfill material 38. The assembly 34 is then allowed
to cool. The underfill material 38 has a CTE of between 16 and 50 ppm/°C, the
die 20 has a CTE of approximately 4 ppm/°C, and the package substrate 36 has a
CTE of approximately 20 ppm/°C. The different coefficients of thermal
expansion creates stresses on the die 20 when the electronic assembly 34 is
allowed to cool after curing of the underfill material 38. These stresses are
particularly high at sharp edges. By removing the corner edge portion 24, these
stresses are reduced. Rounded corners, as opposed to, for example, faceted
corners, are particularly effective for reducing stresses. Dome-shaped corners
may be even more effective to reduce stresses than cylindrically rounded corners,
but may be more difficult to manufacture. By reducing the stresses, cracking of
any part of the electronic assembly is avoided in a region where side edge
surfaces thereof meet.
[0028] While certain exemplary embodiments have been described and shown in
the accompanying drawings, it is to be understood that such embodiments are
merely illustrative and not restrictive of the current invention, and that this
invention is not restricted to the specific constructions and arrangements shown
and described since modifications may occur to those ordinarily skilled in the art.