Integrated circuit assembly adhesive and method thereof

An integrated circuit assembly (16) embodied as flip-chip is encased in a plastic card (12) to form a smart card assembly (10). The flip-chip assembly has a substrate (20) with plated pads, and a semiconductor die (30) with contacts (32) to be connected to the plated pads on the substrate. An adhesive (38) is applied at selected locations less than an entire surface area between the substrate and the die for maintaining a fixed positional relationship between the substrate and the die. The selected locations, typically dots or a bead inside or around the perimeter of the die, have detrimental stress induced by differences in coefficients of thermal expansion between the die and the substrate before the contacts of the die are connected to the plated pads of the substrate during a reflow or cure process.

BACKGROUND OF THE INVENTION 
The present invention relates in general to integrated circuits and, more 
particularly, to an adhesive applied between a die and a substrate to 
maintain a fixed positional relationship therebetween. 
An integrated circuit assembly typically comprises a substrate that forms a 
physical and structural foundation for an integrated circuit die. The 
integrated circuit die is electrically connected to bonding pads on the 
substrate that allow communication between the die and external devices as 
is well known. 
One integrated circuit application is the smart card where an integrated 
circuit module is disposed in a plastic card, similar to a credit card. 
The integrated circuit module includes a microcontroller and memory device 
such as EEPROM of sufficient size to store large amounts of personal 
information. For example, the module is capable of storing a person's 
complete medical history from birth. When a person receives medical care, 
the medical staff need only read the card to obtain a medical history of 
the patient including name, address, birth date, primary care physician, 
medical insurance provider, religious preference, current medications, 
allergies, known conditions such as diabetes, prior operations, and so on. 
In other applications, the smart card module can store financial 
information, security information, frequent flyer mileage, and many other 
forms of personal and business data. The smart card can also operate as a 
debit card or a telephone card with a stored monetary value that is 
updated with each transaction. 
An important consideration for the smart card is its overall thickness. The 
assembly should be keep as close as possible to the thickness of an 
ordinary credit card for easy of carrying and use by consumers. If the 
smart card becomes too thick and cumbersome, it will be less accepted by 
consumers. In addition, thicker plastic cards are more expensive to 
manufacture. 
In the prior art, the smart card includes a plastic card with a pocket or 
cavity milled out of the card for the integrated circuit module. The 
module includes a die disposed over a substrate and electrically connected 
with gold or aluminum wire bonds. The wire bonds typically rise vertically 
from the die and bend over to a bonding pad on the substrate. The height 
of the wire bonds increase the thickness of the module. The cavity in the 
plastic smart card needs to be cut deep to avoid a noticeable bulge in the 
card. The deep module cavity results in either an overall thicker card 
which is undesirable as described above, or the backwall between the 
bottom on the cavity and the opposite side of the smart card becomes 
unacceptable thin. A thin backwall easily cracks under the normal stress 
of use and bending. A thin backwall also tends to ripple which is 
aesthetically undesirable in showing the module pocket or cavity. 
Hence, a need exists for smart card that is acceptable thin in the minds of 
consumers while maintaining sufficient thickness in the backwall to avoid 
cracking.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, a side-view of smart card assembly 10 is shown 
including a plastic card 12 having a pocket or cavity 14 milled out of one 
surface of the card to house integrated circuit module assembly 16. Module 
16 is a flip-chip assembly that uses ball contacts to join the substrate 
and die together. The overall height of flip-chip assembly 16 is about 
450.0 microns which is less than a conventional wire bond type of assembly 
because the flip-chip does not use conventional wire bonds. 
A backwall 18 is formed between the back surface of cavity 14 and the 
opposite surface of plastic card 12. The flip-chip assembly 16 allows 
backwall 18 to be about 250.0 microns thick to avoid cracking or rippling 
while still using a standard thickness (e.g. 800.0 microns) for plastic 
card 12. Flip-chip assembly 16 should not come in contact with backwall 18 
when smart card 10 is fully assembled. The remaining 100.0 microns is 
taken up as buffer space between the top of flip-chip assembly 16 and the 
inner surface of backwall 18. 
Flip-chip assembly 16 also allows a thinner backwall of say 125.0 microns 
which in turn allows the overall card thickness to be reduced to 650.0 
microns with 75.0 mircons of buffer space. The thinner card is a more 
desirable end-product for consumers. 
Module assembly 16 includes a multilayer substrate 20 having a dielectric 
layer 22 overlaying a metallization layer 24 which are joined together by 
an adhesive layer (not shown). Substrate 20 operates as a physical, 
structural, and electrical foundation for semiconductor die 30. Dielectric 
layer 22 is made from polyimide, epoxy glass, or other suitable dielectric 
material. Metallization layer 24 is made of copper with nickel-gold 
plating and provides electrical conductors between die 30 and devices 
external to plastic card 12. Gap 28 provides electrical isolation between 
the individual conductors in metallization layer 24. 
Wells 26 are formed in dielectric layer 22 by punching or laser cut. 
Substrate 20 and semiconductor die 30 come together to form flip-chip 
assembly 16. Die 30 includes ball or bump electrical contacts 32 and 
extended tips or tails 34. Semiconductor die 30 typically has 5-10 
contacts 32 with associated conductors and gaps in metallization layer 24 
for inputting and outputting electrical signals. A solder paste or 
conductive epoxy is placed in wells 26 to form an electrical contact 
between ball or bump contacts 32 and conductors in metallization layer 24 
during a reflow or cure process. At the bottom of wells 26 is a plated pad 
and conductor in metallization layer 24 to route the electrical signals 
from semiconductor die 30 to external devices. For example, when smart 
card assembly 10 is swiped through a card reader (not shown), the wipers 
in the card reader make electrical contact with the conductors in 
metallization layer 24 which in turn contact semiconductor die 30. 
The ball contacts 32 and extended tips 34 of flip-chip semiconductor die 30 
are lowered into wells 26 during the reflow or cure process. If a solder 
paste is placed in wells 26, then flip-chip module assembly 16 is reflowed 
at say 250.degree. C. to join contacts 32 and extended tips 34 through the 
solder to the conductors in metallization layer 24 at the bottom of wells 
26. If a conductive epoxy is placed in wells 26, then the flip-chip module 
assembly 16 is cured at say 175.degree. C. to solidify and join contacts 
32 and extended tips 34 through the conductive epoxy to the conductors in 
metallization layer 24 at the bottom of wells 26. 
During the temperature heating and cooling that occurs with the reflow and 
cure processes, differences in the thermal coefficient of expansion cause 
substrate 20 to move up to ten times more than die 30. The movement occurs 
in all three directions: x, y, and z. The difference in movement between 
substrate 20 and die 30 creates stresses on the contact point between ball 
contacts 32 and the conductive material in wells 26. The electrical 
connection between ball contacts 32 and the conductive material in wells 
26 can easily break, fracture, or otherwise separate in the presence of 
the thermal induced stresses causing a short or a void (electrical open). 
As part of the present invention, an adhesive epoxy 38 is added to the 
surface(s) of substrate 20 and/or die 30 before the reflow or cure step. 
The adhesive epoxy 38 can be applied as dots to dielectric layer 22 shown 
as a top-view in FIG. 2. Alternatively, adhesive dots 38 are to applied to 
die 30. The dots are typically 500-1000 microns in diameter and spaced 
about 1000-3000 microns apart or more depending on the die. The dots are 
applied with syringe, screen stencil, or pin transfer. Adhesive dots 38 
need not cover the entire surface area of substrate 20 and/or die 30 
because that is typically more coverage than is necessary to achieve the 
objective of holding the die and substrate together during temperature 
heating and cooling at reflow or cure. Instead, adhesive dots 38 are 
applied at selected locations of detrimental stress due to the differences 
of coefficient of thermal expansion between substrate 20 and die 30. The 
selected locations are typically along the inside perimeter of die 30 
where significant movement occurs, or at corresponding positions on 
dielectric layer 22. 
Adhesive dots 38 have good "green state" strength, i.e. gripping strength 
immediately upon contact even before complete cure. Adhesive dots 38 
typically cure before the conductive epoxy cures or the solder reflows for 
maximum gripping strength. Once adhesive dots 38 are applied to dielectric 
layer 22 and/or die 30, the two assemblies are pressed together to secure 
the bond and then extended tips 34 are placed in wells 26 for reflow or 
cure. The adhesive dots 38 operate to stabilize movement of substrate 20 
and die 30 to negate any mismatch of coefficients of thermal expansion. 
Therefore, any stress developed during temperature changes by differences 
in coefficient of thermal expansion between substrate 20 and die 30, i.e. 
all three dimensions, are taken up by adhesive dots 38 instead of contacts 
32 and the conductive material in wells 26. The electrical connection 
between contacts 32 and the conductive material in wells 26 remains intact 
during and after the reflow and cure processes. 
In an alternate embodiment, die 30 can first be placed next to substrate 20 
after which adhesive 38 is tacked or applied as a bead around one or more 
edges of the module. Again, adhesive 38 is a good contact cement to hold 
die 30 to substrate 20 during the reflow or cure process to stabilize 
movement of substrate 20 and die 30 and negate any mismatch of 
coefficients of thermal expansion. Any stress developed during temperature 
changes by differences in coefficient of thermal expansion between 
substrate 20 and die 30 are taken up by adhesive dots 38 instead of 
contacts 32 and the conductive material in wells 26. The electrical 
connection between contacts 32 and the conductive material in wells 26 
remains intact during and after the reflow and cure processes. 
After the reflow or cure process when semiconductor die 30 and substrate 20 
are securely joined as flip-chip module assembly 16, module 16 is 
underfilled with epoxy resin to provide an environmental seal. Module 16 
is placed in cavity 14 and secured with adhesive at lips 40 to complete 
smart card assembly 10. 
The present invention is applicable to other integrated circuit assemblies 
where it is desirable to maintain a fixed positional relationship between 
a die and a substrate during processing. 
While specific embodiments of the present invention have been shown and 
described, further modifications and improvements will occur to those 
skilled in the art. It is understood that the invention is not limited to 
the particular forms shown and it is intended for the appended claims to 
cover all modifications which do not depart from the spirit and scope of 
this invention.