POLYIMIDE COATING PROCESS AND MATERIAL

PROCEDE DE REVETEMENT PAR POLYIMIDE, ET PRODUITS UTILISES

English Abstract

Abstract
A protective layer composition, suitable for protecting
metal electrodes on components and other microelec-
tronic circuitry, comprises an organic thermoplastic
polymeric material, an organic solvent or solvents, and
a non-ionic fluorocarbon surfactant as a wetting/level-
ing/flow control agent. A typical formulation com-
prises an aromatic polymer which cures to form a poly-
amide-imide polymer, an organic solvent or a mixture of
organic solvents and a non-ionic fluorocarbon surfactant.

Claims

14
1. A process for forming a seal coating on electronic
circuitry comprising:
applying to said circuitry a coating material
which includes:
an aromatic polymer which, when cured, forms
a polyimide,
an organic solvent for said polymer,
a non-ionic fluorocarbon surfactant, and
curing the material to form a polyimide seal
coating on said circuitry.
2. The process of claim 1 wherein the aromatic
polymer is selected from the group consisting of
a polyamic acid and a polyamic acid-imide.
3. The process of claim 1 wherein the layer includes
from about 0 to 1% by weight of a silane adhesion
promoter.
EN 9-77-021

4. The process of claim 1 wherein the coating material
includes by weight: about 2 to 20% polymer, about
0.001 to 1.0% surfactant and about 80 to 98% solvent.
5. The process of claim 4 wherein the coating material
includes by weight about 4 to 8% polymer, about 0.001
to 1% surfactant and about 92 to 96% solvent.
6. The process of claim 1 wherein the fluorocarbon sur-
factant is selected from the group consisting of a
liquid non-ionic fluorinated alkyl ester surfactant
represented by the formula:
<IMG>
where Rf is a perfluoroalkyl group having from 3 to
12 carbon atoms and m is an integer from 2 to 12, and
R' is a low molecular weight polyoxyethylene, poly-
oxypropylene or polyoxyethylene-polyoxypropylene co-
polymer group;
N-polyoxyethylene substituted perfluorosulfonamides
represented by the formula:
<IMG>
in which R is a lower alkyl group having from 1 to 6
carbon atoms and n is an integer from 2-30; and
N-alkanol perfluoroalkanesulfonamides represented
by the formula:
RfSO2N(R')RCH2OH

where Rf is a perfluoroalkyl group containing 4 to 12
carbon atoms, R is an alkylene bridging group contain-
ing 1 to 12 carbon atoms, and R' is a hydrogen atom
or an alkyl group containing 1 to 6 carbon atoms,
16

16
7. A process for forming an aromatic seal coating
on metal conductor lines on the surface of a
substrate and the solder joints and pads form
ing the connection between the substrate and an
integrated circuit chip which is electrically
bonded thereto comprising applying to said surface
a coating material which comprises 2 to 20% by
weight of an aromatic polymer which, when cured,
forms a polyimide about 80 to 98% by weight of an
organic solvent sor said polymer, about 0.001 to
1% by weight of surfactant, and about 0 to 1% by
weight of silane adhesion promoter, and curing
material to form a polyamide seal coating.
8. A composition comprising:
An aromatic polymer which, when cured, forms a
polyimide,
an organic solvent for said polymer, and
a non-ionic fluorocarbon surfactant.
9. The composition of claim 8 wherein the aromatic
polymer is selected from the group consisting of
a polyamic acid and a polyamic acid-amide.
10. The process of claim 8 wherein the composition
includes about 0 to 1% by weight of a silane
adhesion promoter.
11. The composition of claim 8 wherein the coating
material includes by weight about 2 to 20%
polymer, about 0.001 to 1% by weight surfactant
and about 80 to 98% by weight of solvent.
EN 9-77-021
17

12. The composition of claim 11 wherein the coating
material includes by weight about 4 to 8% polymer,
about 0.001 to 1% surfactant and about 92 to 96
solvent.
13. The composition of claim 8 wherein the fluorocarbon
surfactant is selected from the group consisting of
a liquid non-ionic fluorinated alkyl ester surfactant
represented by the formula:
<IMG>
where Rf is a perfluoroalkyl group having from 3 to
12 carbon atoms and m is an integer from 2 to 12,
and R' is a low molecular weight polyoxyethylene,
polyoxypropylene or polyoxyethylene-polyoxypropy
copolymer group;
N-polyoxyethylene substituted perfluorosulfonamides
represented by the formula:
<IMG>
in which R is a lower alkyl group having from 1 to
6 carbon atoms and n is an integer from 2-30, and
N-alkanol perfluoroalkanesulfonamides represented by
the formula:
RfSO2N(R')RCH2OH
where Rf is a perfluoroalkyl group containing 4 to
12 carbon atoms, R is an alkylene bridging group con-
taining 1 to 12 carbon atoms, and R' is a hydrogen
atom or an alkyl group containing 1 to 6 carbon atoms
18

Description

33~
POLYIMIDE CO~TING PROCESS AND MATERIAL
. _ . ~
Back~round of the Inven~ion
The present invention relates generally to protective
coatings and specifically ~o an imide polymer contain-
5 ing coating material which includes an non ionic fluoro-
carbon surfactant to more effectively hermetically top-
seal eiectronic circuitry, components, and integrated
circuit chip devices~rom hostile environments as well
as provide mechanical stress relief to extend funct-
10 ional electronic device package li~e.
In the manu~acturing of integrated circuit modules, it
is customary to seal the metal circuitry and tha con-
- necticns between the substrate and the integràted
circuit chip devices with a polymer layer. Typically,
15 a liquid topseal coating is dispensed over the devices
~ and then cured to protec~ the critical joints and pad
- areas against corrosion~migration, atmospheric con- ~
taminations, and moisture permeation. The coatings 7
also mechanically enhance the chip joint reliability
20 while serving as dielec~ric insulators. The topseal
coating can be applied by spin, spray, dip, or dispense
techniques. The method of application is optional
; dependent upon product requiremen~s.
EN 9-77 021
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~ .

Polyimide coatings have been used in the p~st to seal
electronic components and circuitry. Prior to the
present invention, valid concerns existed in the in-
tegrity of the coating coverage of joints which are
located underneath the chip devices. The need existed
for a more uniform protective coating over all pad
areas.
,
In addition, the increasing complexity of chip designs,
which may contain joints in the middle of the chips as
well as along the periphery of the chips, requires fur-
ther improvement in topseal wetting to penetrate be-
tween the chips and substrates and insure coverage of
the internal pads and solder joints.
Accordingly, it i5 an objective of the present inven-
lS tion to significantly reduce the surface tension of
top layer protective coatings. This greatly facili-
tates the ability of the coating to uniformly wet all
joints and pad areas when, for example, coating modules
where the integrated circuit chips are joined to the
substrate. The coatings penetrate under those chips having
design configurations that contain internal ioints/pads
` that, like perimeter joints/pads~ also require coa~ing
- coverage.
Brief Summary o the Invention
In accordance with this invention there is provided
a process for forming a seal coating on electronic
circuitry comprising:
.
- Applying to said circuitry a coating material
which includes:
An aromatic pol~mer which, when cured, forms
a polyimide,
. . .
EN 9-77-021
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3;~
an organic solvent for said polymer,
a non-ionic fluorocarbon surfactant, and
curing the material to form a polyimide
seal coating on said circuitry.
` A composition is provided which includes:
: .
An aromatic polymex which~ when cured, ~orms
a polyimide,
an organic solvent ~or said polymer, and
a non-ionic ~luorocArbon su~factant.
l0 Optionally, adhesion promotors such as silanes can b~ .
added to ~he compositionsO The term polyimide~as used
herein includes both polyimide and polyamide-imide
- polymers.
Description of the~Drawings
.
Fig. l is an elevational view, partly in section, show~
ing a stacked module.
Fig 2 is a schematic cross-sectional ~ew of a solder
joint between a chip and module with a polyme~ top-
sea1 coating.
Det~iled Description
. :
Ths present invention offers ~h advantage of con-
sistently providing a chip coating which is uniform in
- rilm thickness and free of pinhoIes, voids and blis-
ters.
Surface conditions, circuit metallurgy, and complex
- : , -,
EN 9-77-021
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33;3
,
pad conf igurations require improved wetting of the top-
seal protective coating to insure good film coverage.
Pinholes, voids, and blisters pose reliability problems
due to site corrosion and protrusion problems when
using low temperature solder alloys at the solder
joints.
Incomplete coverage or lack o~ covera~e o~ coatings
results in a shift ~n thermal stre~s~s affea~ing the
uncoated joints/pads creating stress cracking at and
early failure o the uncovered joints.
When dispens.ing protective coatings into stacked mod-
ules, the advantage o~ improved wetting to insure
coverage is even more important. As in the case of
planar substrates, complete coverage of all chip
joints~pads is needed to ensure adequate protection so
as to extend device life. Besides providing mechanical
enhancement of the joints the pxesence of a uniform
protective coating layer also serves to improve the
heat transfer characteristics. The modules coated by
the process of the invention exhibit a cooler area
between the chip and substrate so tha~ the chip remains
cooler during operation. Fig. 1 shows a stacked module
- 11 having connecting pins i3, and chips 15 mounted by
joints 17 on substrates 19 and a metal cap 21. Fi~. 2
25 is a schematic of an indi~idual solder jointipad 17, ;~
with solder 18 connecting pads 23 on chip 15 and pad 24
- on substrate 19, showing the protective layer 25
coverage required. The typical distance from the
substrate to the chip is about .004 to .005 inch.
The polyimide and polyamide-imide resins which are use-
ful in the practice of the invention are a class of
polymers finding use in electronic packaging as pro-
tective coatings due to their excellent chemical re-
sistance and high temperature stability properties. In
addition, this group of ~hermoplas~ics also exhibits
- EN 9-77-021 ,
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3~3~
good alectrical as well as mechanical atributes,
~` The polyamide-imide polymers are prepared from anhy-
drides and diamines, for example, trimellitic anhydride
and P~P'-diaminodiphenylmethane.
` Q O
S~ HO -C- ~ C~0 ~ -CH2- ~
TRIMELLITIC ANHYDRIDE P,P'-DIAMINODIPHENYI,MET~NE
This combination gives a polyamic acid-amide polymer
which cyclicizes or imidizes on curing, such as by
heating, t~ form a cured polyamide-imide.
H O O H H O 11 r
10 t ~ c o ~L~ t ~c
~ . POLYAMIC ACID-AMIDE POLYAMIDE-IMIDE
;.~
The polyimides are the reaction product of dianhydrides ~ a
and diamines J for example, benæophenone tetracarboxylic
acid anhydride and oxydianiline (4,4'-diaminodiphenyl-
15 ether).
\ ~ ~ / HzN ~ --NHz
:~ BENZOPHENONE TETRACAR- OXYDIANILINE 4,41-DI~MINO-
- BOXYLIC ACID ANHYDRIDE DIPHENYL ETHER ~ ~:
'
EN 9-77-021
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33'~
The followiny illustrates the cond~nsation reaction of
pyromellitic dianhydride and an aromatic diamine.
'
,
O O
Il ~ 11
~C--~¦_Cf '~H2N-R-NH2--
O O
PYROMELLITIC AROMATIC
ûIANHYûRID DIAMINE
~ O O ~
10~ C~C--N--R~ HE~T
_ H O O H - b
- POLYAMIC ACI DINTERMEDlATE
. - ~,
. - O o ~ ~
_
t "`~ C` ~ ~
o O n
CURED AROMATIC POLYM I DE
- ..
Other materials such as 4~4 ? -diamino-diphenylmethane
: may be added to obtain the desired properties required
for a specific application.
3~ NH2 ~ NH2
EN 9-77-021

i 33~
The polymers are co~ted onto the circuitry from a
solvent solution. Preferred solutions would i~clud2
about 4 to 12 weight percent of polymer solids dis-
solved in an organic solvent or a mixture of organic
solvents.
Table I lists examples of organic solvents suitable for
use as vehicles for the protective coating formulations
of the invention.
Table I
soL*
SOLVENT _ PARA _M.P. B.P. __ FL.P.
Dimethylacetamide 10.8M-20 166 70
N-methyl pyrrolidone 11.3M -24 202 95
15 l-formyl piperidine 11.5M -31 222
Dimethylsulfoxide 12.0M18 189 95
Tetramethylurea - 1 177
Tetramethylenesulfone 13.4M 27 285
* Solubility parameters and strength of hydrogen
bonding
~ ** at 3MM Hg
; In addition to the above organic solvents, mixtures of i`
solvents or a combination solvent~diluent are also
applicable. Polyamic acid intermediates for forming
polyimides may be dissolved, for example, into the
following combinations of solvent/diluent:
1/1 blend of N-methyl pyrrolidone (NMP~/Acet~ne
NMP/Cellosolve*
NMP/Xylene
NMP/toluene
(2-Ethoxyethanol) Cellosolve*/acetone
* Trademark of Union Carbide Corporation
:
~; For polyamide-imide forming polymers, Dimethyl aceta-
mide, Dimethyl formamide, Dimethyl sulfoxide, and N-
methyl pyrrolidone are known solvents.
EN 9-77-021
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8 ~ 3~
The solubillty range, by weight, of diluent concentrations
for the polymers as known in the ar-t, include:
40-50% Aromatics Itoluene~ xylene, cyclohexanone)
40-50~ Esters tethyl acetate, methyl propionate)
50-60% Ketones (acetone, methyl ethyl ketone,
80% cyclohexanone)
30-40% High boiling amides, aliphatic amides
containing 1-4 carbon atoms
(acetamide, propi~namide, butyramide)
40-50% Cellosolves* (2-ethoxyethanol)
*Trademark of Union Carbide Corporation
The preEerred polyimides and polyamide-imides useful in
the presen-t invention have molecular weights from about
13,000 to about 60,000. These materials are suitab;le for
purposes of the present invention since they possess high
temperature stability in coating formulas with good elec-
trical properties at temperatures > 200C after imidiza-
tion.
To this mixture, a non-ionic fluorinated alkylester sur-
factant is added to improve wetting, leveling character-
istics, and co.verage of the protective film coating. Pre-
ferred fluorocarbon surfactants are non-ionic types that
will reduce tha surface tension of the polyimide and poly-
amide-imide precursors to less than 25 dynes/cm at 25C
for a 0.1% weight percent solution. ; .
Non-ionic fluorinated surfactants have been found to be
unique in producing uniform coatings and in avoiding the
introduction of materials into the coating that can cause
corrosion of the metallurgy. Non-fluorinated hydrocarbon
surfactants have not been found effective in producing an
acceptable coating process~
:In the fluorocarbon chain of the non~ionic surfactants ~-
useful in the present invention, which can be cyclic or
non-cyclic, it is necessary that the hydrogens on
EN9-77-021

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the carbon structure be raplaced by fluorine to such a
degree that the fluorocarbon chain or "tail" portion of
the liquid surfactant be essentially a fluorocarbon.
This does not imply complete hydrogen replacement in
all instances, though such will, for ease of commercial
availability, generally be the case. Liquid non-ionic
fluorinated alkylester surfactants of the present inven-
tion contain the recurrent unit (CF2) and can generally
be represented by the Eormula:
Rf(CH2)n~OR
Where Rf is a perfluoroalkyl group having from 3 to 12
carbon atoms and m is integer from 2 to 12, and R' is a
1OWJ molecular weight (for example, about 15,000 centi-
poises at 25C), polyoxyethylene, polyoxypropylene or
polyoxyethylene-polyoxypropylene copolymer group.
These are a class of fluorocarbon surfactants that will
reduce the swrface tension of the amide-imide or imide
polymer solutions to less than 25 dynes/cm, for example
from 39.4 dynes/cm down to 24.2 dyne~s-/cm 90 that com-
;~ plete and uniform topseal coatings can be achieved.
Another non-ionic type of fluorocarbon surfactant useful
in the invention are N-polyoxyethylene substituted per- ~-
fluorosulfonamides of the formula:
- C8F17~O2-N-(c2H4o)n
in which R is a lower alkyl group having from 1 to 6 car-
bon atoms such as methyl, ethyl propyl, butyl, etc. and
n is an integer from 2-30.
Also useful in the practice of the invention are the
fluorocarbon compounds of the class consisting of N-
alkanol perfluoroalkanesulfonamides having the formula:
~.
RfS02N (Rl ) RCH20H
where Rf is a perfluoroalkyl group containing 4 to 12
EN9-77-021

38~
carbon atoms, R is an alk~lene bridging yroup contain-
ing 1 to 12 carbon atoms, and R' is a hydrogen atom or
an alkyl group containing 1 to Ç carbon atoms; and the
corresponding esters. An example of these compounds
would be N-propyl, N-ethanol perfloro-octanesulfonamide
`' having the formula:
C8F175O2N(c3~g)cH2c~2o
Mixtures of the above surfactants can also be used.
A silane or a mixture of silanes may be used to promote
bond adhesion,between the ceramic substrate, metallic
circuitry and the top seal coating. Silane adhesion
promoters having a preerred ~pecific gravity of 0.94
to 1.06 g/l at 25C can be added to the composition~.
Useful silanes include, for example, gamma-aminopropyl-
triethoxysilane; beta-3,4(epoxycyclohexyl)ethyl~ri-
methoxysilane; and gamma-glycidoxy-propyltrimethoxy-
silane. The silanes are optionally used in amounts up
to about 1% by weight of the total mixture.
Generally, the protective coating formulations of the
present invention comprise approximately by~weight:
2-20~ Polymer (Polyamic acid precursor of
polyamide-imide or polyimides)
0.001-l~ Non-ionic fluorocarbon surfactant(s)
0.0-1% Silane additive or a mixture of
silane additives
with the balance (80-98~) being an organic solvent or a
mixture of organic solvents and diluents.
Preferred protective coating formulations of the
present invention would be:
4-8% Polymer
92-96~ Organic solvent from the following
group or a mixture of solvents
-~ including dimethylacetamide, N-
methyl pyrrolidone, l~formyl -
' piperderic, dimethylsulfoxide,
EN 9-77-021
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1 1
tetramethyl urea and tetramethyl-
sulfone
0.001-0.1% Non-ionic fluorocarbons sur-
factant
0.0-1.0~ Silane additive (optional)
As described above, diluents can also be included in
combination with the above solvents and are useful as
~ bisolvents to a degree of solubility as known in the
art~
Specific formulas are:
Percent by Weight
(1) (2) (3)
Polyamide-imide pe~ursor powder 7.30 4.46 7.47 4,50
(AI-10 polymer of Amoco)
15 Dimethylacetamide 92.00 95.00 92.00 95.00
Silanes; 1:1 ratio of Beta,(3,4- 0.65 0.44 0.46 0.45
epoxycyclohexyl)ethyltrimethoxy-
silane/2-aminopropyltriethyxy- -
silane
20 Non-ionic alkyl-ester fluorocarbon 0.05 0.10 0.07 0.05
surfactant
5~
~FC430 brand surfactant of 3M Company) ~ -
100 100 100 100
Formulas 3 and 4 were used to measure the effect of
adding the surfactant. Both showed a reduced surface
tension to 24.1 dynes/cm from the formula (without
~ surfactant) of 39.4 dynes/cm.
- A change in the formula by replacing the polyamide-imide
forming polymer with a polyimide forming polymer produced
30 a coating mixture ~5) having a similar surface tension
reduction applicable to this invention and necessary to a
enhance wetting:
EN 3-77-021

i333I;~
Percent
(5)
Polyimide percursor (solids-powder) 4.50
N-methyl pyrollidone 95.00
Silanes 1:1 ratio as in formulas 1-~ 0.45
- Non-ionic alkyl-ester fluorocarbon sur-
factant (F-430 brand surfactant of 3M Co.) 0.05
Other non-ionic types of Eluorinated surfactants such
as the N-polyox~yethylene substituted perfluorosulfon-
amides ancl compounds of the class consisting o~ N-
alkanol perfluoroalkanesulfonamides can be substituted
in the above Eormulas.
The ~luorinated surfactants used in this invention
specifically are non-ionic rather than cationic, anionic,
or amphoteric. This is to deliberately avoid using
ionic materials in the protective coatings.
Significant differences are apparent in comparing
fluorinated surfactants with hydrogen surfactant groups.
The fluorocarbons promote unusually low cohesive sur- ;
face tension and excellent solubility parameters which
were not obtained when using non-ionic hydrocarbon sur-
factant types such as, for example, ethoxylated alkyl-
phenols, ethoxylated aliphatic alcohols, ethoxylated
sorbitols and other polyethylene adducts.
.
Hydrocarbon surfactants exhibit higher coefficients of
friction as well as problems of miscibility (gel for-
mations) corrosiveness, hygroscopicity low temperature
decomposition and chemical interaction. Significantly
larger quantities of hydrocarbons are also required ad-
versely affecting final coating properties and their use
did not result in satisfactory topseal coatings, partic-
- ularly where internal connection pads are present in
the structure.
In practice, a liquid protective coating of 4-8~ polymer
solids is dispensed in quantities of 15-150 mg over a
chip device to coat the solder joints/pad areas. The
quality of mix dispensed depends upon the chip size and
EN 9-77-021
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number of chips present on the substrate. ~he exact
thickness of protective film coating on the solder
joint/pad is not critical so long as there ls complete
coverage. Preferrably, the film should be uniform and
from ~ 1 mil to several angstroms thick (Ref. Fig. 2).
Excessive filleting at the chip/substrate interface
should be prevented to avoid excessive thermal stress
at the chip interface resulting in cracks due to the
diffQrant coeEficients expansion of the polymer and
the SnPb solder. The coatings are cured by heating in
an oven per the following cure cycle, for example;
(a) 60 to 70 minutes at 110C ~ 5C
(b) 60 to 70 minutes at 150C i 5C
(c) 180 to l90 minutes at 170C ~ 5C. It should
be understood that~the curing times and t~mperatures -
would be optimized for any particular mixture of
polymers and solvents~
The use of the fluorinated surfactant enables the pro-
tective coatings to overcome surface contamination due
to chip/substrate handling, processing and inefficient
cleaning. With improved wetting~ more uniform coverage
is obtained aspecially in worst case conditions. The
coatin~ uniformity problems, which are encountered when
non-surfactant containing polyimide layers are used,
- 25 are alleviated by tha process and coatings of the
invention to an extent not previously achieved. The
use of other surfactants has not been found to be
effective in this respect and also may create other
problems such as corrosion and gel formation.
EN 9-77-021
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