SOLDERLESS SMT ASSEMBLY NOW A HIGH
VOLUME COMMERCIAL REALITY Ken@ET-Trends.com
By Ken Gilleo, PhD, Sheldahl, inc., Northfield, Minnesota
Introduction
What is a circuit? It's really just an array of electrical
conductors supported on an insulator base. We can wonder
why the traditional manufacturing path to such a simple
product takes a drawn-out, inefficient and pollution-laden
detour!
Let's look at the conventional subtractive process from
the beginning. A foil manufacturer electroplates copper
onto a rotating metal roller in a continuous process to form
copper foil. The copper foil is then glued to a dielectric
board such as G- 10, FR-4 or flexible film. The circuit
manufacturer then goes through a dozen or so steps with the
goal of removing much of the copper to create the conductor array. A double-sided circuit requires several additional
steps to form plated through holes or copper "barrels". If
all of the steps are counted, perhaps 20 are used to make a
conventional double-sided circuit board. Regardless of the
complexity and questionable economics, the subtractive
process is firmly established.
A Short Circuit History
Metal strips and wires were the first electrical pathways
for emerging vacuum rube electronics. Early electronic
products were so power hungry that bucketfuls of electrons
were needed to feed the early components. Even the early
computers were watt-wasting monsters. The Eniac of the
1940's, with a performance considerably less than a laptop
computer, used 174,000 watts'. No wonder hefty metal
conductors caught on. Wires were to rule for some time as
the piping for the early electron plumbers.
Visionaries, of course, have always "seen" what could
be and what should be. Thomas Alva Edison, at the turn of
the century when the electricity revolution was being
sparked, suggested that wires could be replaced by cond uc tors produced insitu. When queried on wiring simplification by Frank Sprague, founder of the namesake electronics company, Edison invented additive circuitry. Edison's
simple idea was to apply adhesive (gum) and sprinkle it
with conductive powder (graphite) 2 . This is basically a
Polymer Thick Film (PTF) concept.
Many others after Edison, envisioned additive processing. During the 1930's and 1940's, patents were granted for
all kinds of additive circuit processes'. Thick film inks,
which could be painted or printed to achieve desired
electrical characteristics, were well known even then.
Polymer thick film materials, where the binder or primary
ink vehicle is based on organic polymers, actually preceded the ceramic type now used for hybrid circuitry. But,
decades of material science advancement would be needed
before PTF was ready for a bigger challenge.
PTF Materials
There are four distinct divisions of PTF materials:
• Conductors
• Dielectrics
• Resistors
• Bonding Agents
These are the basic building blocks from which almost
any electronic circuit and assembly can be built. They are
applied to a wide variety of base materials ranging from
rigid ceramics to thin, heat sensitive plastic films. Perhaps
the most significant characteristic of most PTF materials is
their mild processing conditions. All types of PTF materials can he processed at 100°C and even lower. Chemical
processing steps, widely used in traditional circuit making,
are not used for PTF. This is very significant in terms of
capital cost and pollution abatement.
Gentle processing conditions permit PTF to be used on
virtually any dielectric material. Low cost polyester and
even paper can be used. Total avoidance of chemical steps
in the process means that PTF is a safer, more
environmentally kind technology. Best of ail, assembly,
unlike traditional soldering, avoids high temperatures,
flux, chlorofluorocarbons (CFC's) and toxic lead. PTF has
already solved the CFC problem. PTF has already solved
the toxic lead (soldering) problem which will become the
circuit assembly crisis of the 1990's.
Conductors
Electrical conductors are typically made by filling
polymeric binder with specially milled silver flake. Silver
remains the preferred conductor because it retains high
conductivity under all commonly encountered environments. In fact, printed silver conductors generally improve
under all environmental aging conditions, especially
where heat is involved. Copper and nickel inks have not yet
achieved the required performance and reliability to gain
commercial acceptance. Metals, other than silver, may
come into use as metallurgy and chemistry solve the
EXPO SMT International '89 Technical Proceedings
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specification'.
Chri" Cr,T.,t;nn
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a method of applying a polymer bonding agent to the
surface of the circuit, Surface Mount Technology is the
ideal format for Polymer Thick Film, PTF bonding agents
can be printed, stenciled or deposited onto circuit bonding
pads. The SMD's are placed onto the bonding paste with
standard pick and place equipment. Heating activates the
liquid adhesive, causing it to polymerize, or "set", locking
the component in place.. Unlike solder, there is now reflow
and solidification. The liquid polymer "solder" is converted to a permanent solid. There are important differences between solder bonding and PTF solderless bonding.
Solder, with its strong surface tension wetting, wicks up
the side of a component termination and forms a fillet. The
surface tension is so strong that components are moved by
the molten solder, A reasonably well placed component is
often oriented and centered as the surface tensional forces
attempt to equalize in all directions. Conversely, a poorly
placed or a difficult to wet component is made worse by
these same surface tensional forces. A chip component,
wet on only one end, tips up (tombstones) as the surface
tension is equalized.
PTF bonding assembly can be summarized by "What
you see is what you get" Tombstoning never occurs.
Solder balls or their equivalent can't happen. Reflow
bridging is unknown. Conversely, misaligned parts stay
that way. True filleting does not occur. Is PTF bonding
easier? Yes! One extremely important difference for P7F
is the complete elimination of flux from the process. No
flux, no defluxing. PIT offers an important solution to the
CFC (chlorofluorocarbon) problem right now while other
processes still seek answers. No flux means that problems,
like corrosion, associated with residual flux are eliminated.
No cleaning means that the process line is simpler and
capital investment is less.
What about assembly? Is set up and placement easier?
SMT-PTF Assembly Methods
Liquid isotropic Conductive Adhesive
Materials
It's somewhat of a standoff. New advantages are cancelled
Screen printing can be used to deposit most conductive
adhesives, A relatively course mesh, usually made with
stainless steel, is commonly used. The stainless steel
screen mesh will provide better printing accuracy than
polyester. Screen emulsion should be 100 microns or even
thicker to provide the desired 4-6 mils printed adhesive
height. Specially made screens can be used where the mesh
is actually etched away to provide thicker deposits.
Stencils, typically made of etched brass, are also popular for conductive adhesive, just as they are for solder pas tc.
Stencils from 4 to 8 mils can be used. Adhes'. es that screen
print well may have the wrong rheology for stenciling.
Material vendors often supply adhesive in viscosities to
accommodate printing and stenciling. Higher viscosity
materials usually stencil best. Stencil printing provides the
by new problems. No tombstoning, or other parts moving
phenomena, but no self alignment. Components need to be
placed with a little more care. Downward placement pressure and speed are critical parameters. Too i -pid placement
can trap air under a component termination and "splash"
the bonding agent. Too great a pressure can displace most
of the adhesive resulting in a weaker bond. However, these
parameters are easily controlled with today's equipment.
We have not experienced any control problems, only
learning curve penalties. We have found yrF assembly to
he more difficult with mechanical register placement
equipment. Vision systems work well and equalize differences between PTF and solder assembly.
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Most isotropic conductive adhesives are blends of
silver powderand flake in an epoxy resin medium. Wetting
agents, surfactants and rheologica] modifiers may also be
present. Single component, or one-part adhesives also
contain catalyst or curing agents. The single component
adhesive is the preferred product for SMT assembly since
no catalyst mixing is required. Two-part adhesives not only
require. mixing in of a catalyst, but their useful working
time, or pot life, is often short. This can mean scrapping out
of expensive silver-containing product if all the material
cannot be used quickly. Two-part adhesives will often
show an upward visco. -..ity dri f t as the product is being used.
Any viscosity change will affect printing resulting in an
unstable process. Table l provides guidelines.
TABLE 1
Basic PTF Bonding Guidelines
• Design SMT mounting pads so that conductive
adhesive contacts the dielectric to enhance
mechanical bond strength.
• Apply only 4-6 mils (height) of PTF.
• Control downward stroke to avoid "splashing" the bonding agent.
• Oxidized terminations must be cleaned or renewed. Fresh components don't require any
pre-conditioning.
Adhesive Application
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against the circuit pads while heat is applied, The adhesive
melts or softens allowing an adhesive bond to form and the
conductive particles to make contact with the component
pads. Bonding cycles range from many minutes down to a
few seconds. Table 2 gives guidelines.
TABLE 2
Basic Anisotropic Adhesive Bonding Rules
• Components should be reasonably light.
• Leads that are not precisely planar should be compliant.
• High lead count IC's should he packaged with TAB.
Success with anisotropic adhesive bonding has met
with large, but narrow commercial success. Work done by
Sheldahl and Hewlett Packard in the early 1980's, suggested that only small, light weight components could be
reliably bonded. Components with multileads also posed a
problem. SOIC's for example, typically have one or more
leads slightly or even significantly out of plane. This can
lead to connection loss as tensional force on the misaligned
lead causes an open between it and conductive particles.
Chip components, SOT packages and small SOIC's connect reasonably well. Their light weight reduces failure
during vibration and shock testing.
Intermittent opens are another problem. As ZAC
bonded components are temperature cycled, the differential thermal expansion between the metal conductive particles and the organic adhesive cause opens at higher
temperature and healing as the temperature is lowered. The
solution is to use a low expansion adhesive and the newest
materials have lower Temperature Coefficients of Expansion (TCE).
The problems of low mechanical bond strength and
intermittent opens can be solved with a mechanical assist
like the shrink film described previously. Potting material,
especially UV curable types, have been used on gull wing
and quad flat pack SMD's. Case designs with a spring type
action, can also be used.
TAB is the ideal Surface Mount package for anisotropic adhesive. TAB, consisting of a bare die on a thin,
compliant carrier, meets the necessary criteria of low mass
and compliant leads. TAB is actually a small flexible
circuit used as an SMD package. Extensive testing by
Sheldahl and others indicates that TAB components can be
reliably interconnected to copper and PTF circuits with
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ZAC technology, TAB is presently the optimum IC SMD
package to use with ZAC. The first disclosed use. of
anisotropic-bonded TAB was on an experimental HP calculator several yams ago. A large polyimide TAB package
was used. This product wilt be described in detail later,
Today, lower cost polyester TAB is bonded to PTF calculators with thermoplastic ZAC film. No bonded enhancement is required.
Generally, screen printed ZAC film provides the best
performance. The liquid ink is printed onto the circuit
bonding area and dried. Adhesion of the printed bonding
agent is better than for transferred dry film as could be
expected. Thickness of about 25 microns appears ideal.
TAB bonding is accomplished by aligning the outer leads
to the circuit and applying heat and pressure. Thin TAB
material (25 microns) allows rapid heat transfer.
SMT-PTF Products
Unrealized Potential
This section attempts to illustrate the value and versatility of PTF technology with examples of products built
over the past several years. Sheldahl's earliest work used
leaded devices configurc•l for surface mount since many
electronic packages were not available in SMD form.
Standard leaded Light Emitting Diodes (LED's) were
formed into a gull wing shape and bonded to the surface of
PTF circuits. Most of the following examples only reached
prototype stage, although the performance capability was
adequate, There has been a general reluctance in the U.S.
to embrace PTF assemblies as is often the case with
unproven technology. However, the most recent examples
using 300% SMD's are commercial, suggesting that the
long awaited SMT-PTF revolution is finally underway.
Yet, the revolution is a quiet one since most of the implementation is by captive manufacturers.
PTF has had its greatest application with membrane
switches. The switch has also been the test bed for PTF
assembly. The earliest attempts at PTF component assembly involved simple devices like LED's, resistors and
diodes. Early on, switch designers began incorporating
"indicator lights" into switches. The ideal indicator for the
PTF membrane switch is the low circuit LED. Most designs include a small key array, several LED indicators, a
graphic display overlay or housing and a flexible "tail"
serving as the interconnect cable. Figure 6 shows a fiveposition printer switch. Silver ink was used as the conductor and carbon ink resi slurs served to drop the voltage to the
LED's. The leaded LED's were bonded with silver filled
epoxy, Demonstration samples have survived 6 years of
"show & tell" abuse.
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Fig. 6 PTF Lisplay Swath,
More complex examples of switch-displays were de- j
velopekl with the pseudo-SMD LED's. Figure 7 reveals a
multi-Mdicator circuit. Density requirements lead to a
double-sided circuit using printed through hole technology.
Fig.
8
: PTF D"PlaY
Fig. 9; VIE Display Circuit.
Fig. 7: PTF LED Display.
The most complex display-switch prototype built was
,
a 19 LED double-sided p-rr- assembly designed for a word
processor. Figure 8 shows the internal construction and
finished product.
Finally, surface mount LED's became readily available
in the mid 1980's. The 1206 package was ideal from a
bonding perspective but led to problems of orientation,
LED's must be correctly polarized since DC current is
tYPically used. The early packages required visual referencing — looking for small color dots on one end.
Later, the. mechanically polarizable SOT-23 would
solve this problem. Figure 9 shows a simple display circuit
made with silver ink conductors. Carbon resistor ink is
used to drop voltage and protect the silver conductors. A
screen printed one-part conductive adhesive was used to
bond the LED's.
The telephone industry has begun to produce telephone
circuits using the SMT-PTF concept. Virtually all telephones can use this technology. The low voltage requirements and benign environment suggest that SMT-PTF
assemblies will dominate. Figure 10 shows a low cost
polyester circuit MU& with silver ink. Three types of
SMD's arc bonded with isotropic conductive adhesives:
LED's, resistors and diodes_ As with tit::: calculator, the
keypad switch will eventually be incorporated into the
circuit. Memory IC's will be added next. By the early
1990's, most telephones will use SMT-PTF
barring unforeseen conservatism.
The calculator field has been the most adaptive and
most successful area for SMT-PTF development. We can
view the calculator as an extension of the switch since
virtually every calculator incorporates switch elements
into the circuit. The term "swircuit" has been suggested for
this hybrid integration.
Today, nearly every major brand of calculator uses a
PTF circuit. Typical designs are fully integrated switchcircuit-interconnect. Several strategies have been used to
connect SMD's. Simplicity is the hallmark for bonding.
During the 1980's, materials and processes were developed for "dry bonding." Heat activated PTF bonding
agents were designed and evaluated. Processes reached
commercial status.
In 1984, Hewlett Packard and Sheldahl, developed full
PTF calm, ,ator circuit and assembl y technology using new
concepts, including SMT an isotropic adhesive assembly.
A moderately high density double-sided circuit, with inte-
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Fig. 11. PTF Calculator Circuit.
Fig. 10: Commercial Telephone Circuit.
.
grated resistors, was used as the test vehicle. See Figure 11. Fig. 12: Assembled Calculat Or.
Anisotropic adhesive was used as the only bonding material for all components. LCD's, S MD 's, TAB (Tape Auto- tric. The thin copper and dielectric are compliant, allowing
mated Bonding) and a buzze., were all eiectimmechani- planarity to be achieved during bonding. This total complically connected to the circuit_ The Z-AxisTm adhesive was ancy allows the TAB outer lead bonding (OLB) area to be
printed onto the bonding zones and dried. Components pressed tightly against the circuit for intimate conductor
were attached using heat and pressure. Figure 12 shows the contact, Adhesive can be squeezed out from between the
fully assembled test calculator. Though an excellent con- TAB, conductors during bonding, creating enough tension
cept, the Z-Axisrm process of 1984 had to wait for im- to prevent intermittent discontinuity during thermal cyproved materials. Thermal mismatch between metal con- cling. More recently, a maiorlapanese calculator producer
ductive particles and the adhesive phase caused intermit- has implemented Z-Axis bonding of a TAB package to an
tent opens during temperature cycling. Non-planarity of all PTF circuit. Figure 13 shows a close-up of the TAB
SOIC leads were also a problem since even a slightly bent area.
lead eventually caused an open.
The examples presented may suggest that SMT-I'1
One answer to the intermittent open problem was to can only play a role in simple, low cost commercial
apply mechanical force to the components. Various manu- applications. How about something more challenging? A
facturers use different approaches. The simplest approach major computer manufacturer asked that question. Just
is to design a "spring action" into the plastic housing. The how far could SMT-PTF technology be pushed? Could a
need to put different assemblies into the same housing and PC computer be produced with only PTF technology? In
the converse made this concept impractical. Customer I mid 1980, an all-SMT computer was introduced in laptop
goals for the project removed the option of mechanical form. Even before the introduction, a working prototype
enhancement.
was built using a 4 layer PTF circuit with all components
Others have successfully applied auxiliary mechanical
attached using conductive adhesive. Figure 14 shows this
enhancement strategies. A "shrink wrap" concept has been complex product. However, test data suggested that a
used by Japanese manufacturers. The heat bonded compoproduct of this complexity needed material technology
nents are covered with a high shrink plastic film which is advances in both conductive ink and adhesives.
bonded to the circuit forming a tent over the components.
Heat then shrinks the film so that it exerts force against the SMT-PTF Performance Limitations
tops of the components. This constant compressional force And Remedies
provides good reliability.
An alternative process is the application of UV curable Conductors
Silver-based ink remains the dominant PTF material
adhesive. This approach has been primarily used to enfor
circuit traces. Ink conductivity ranges from as high as
hance the bonding of quad packs.
-4
The work done by HP and Sheldahl, suggested that 1.9 x 10 ohm-cm down to 2.5 x 10's ohm-cm for the best
TAB is the ideal SMT package for anisotropic adhesive inks. This is nearly two orders of magnitude higher than
bonding. TAB is really a specially designed flex circuit, etched copper. This level of conductivity is very adequate
consisting of thin copper (35 micron) or. a flexible, dielec- for many commercial electronics applications. The fact re-
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EXPO SMT international '89 Technical Proceedings
Positive and negative conductors can be routed to keep
them from close proximiFy. Moisture may be excluded by
packaging design or use of sealing. The silver source can he
removed by covering with dielectric or carbon ink.
Isotropic Conductive Adhesives
Fig. 13: PTF Calculator with TAB.
mains that silver conductive ink has too high a resistance
for many applications and traditional circuit artwork must
be modified for PTF.
Electrometal migration, the formation of metallic dendritic growths under certain power conditions, is the most
noted problem associated with silver ink and adhesive.
Silver is a very active metal and easily ionizes, especially
in the presence of moisture. Silver ions quickly migrate
from a positive source to a negative pole where unacceptable silver dendrites form. Three conditions are required
for the silver migration phenomenon to occur:
1, Silver ions must be produced which means water or
moisture comes in contact with the silver.
2. DC current with positive pole on silver source.
3. Close proximity of positive silver-hearing source
and negative pole.
Figure 14: A 11 PTF computer hoard
Most conductive adhesives are also based on silver and,
therefore, have the same problem with electromigration.
The same causative conditions apply for adhesive as for
ink. Adhesive joints car, be covered with conformal coating or the enclosure can he sealed. Since most IC's are
designed with power and ground at opposite ends of the
package, it is easier to keep DC polarized lines apart.
Copper adhesive has some merit. While thin PTF copper ink tends to oxidize over time, we have found copper
adhesives to have long term reliability. PTF inks are
typically only 12-25 microns (0.5-1 mil) thick. Oxygen
permeation eventually converts the copper flake to nonconductive oxide. Copper adhesive is applied in tich thicker
than ink and only the copper near the surface will oxidize
if antioxidant is incorporated into the binder. Copper conductive adhesive is not subject to clectromigration and
should he preferred over silver. However, copper PTF
materials are not as widely available as silver-based products.
Carbon PTF inks can serve a dual role as circuit conductor and bonding agent as suggested earlier. Application of
heat softens the thermoplastic ink binder allowing it to
serve as a hot melt adhesive. A surface mount component
can be forced against the. softened ink to form an electrical
connection. The mechanical bond strength, although relatively low, can be enhanced with one of the methods
described earlier. Both the "shrink wrap" and potting
approach are in use today. The inertness of carbon results
in a durable product. The high resistance of the bonding
agent has limited this technology to low end calculators.
Generally, bond strengths for conductive adhesives are
lower than for solder, and PTF bonds are much more prone
to cracking if the circuit is flexed. Packages with a large
bond area to total area are ideal. Various rectangular
packages used lot capacitors and resistors can be reliably
bonded with conductive adhesive. Bond strengths of over
1000 PSI are common. SOT packages also work well, as do
smaller SOIC's. Larger SO1C's and other dual in-line
packages are much more subject to failure in vibrational
tests. Quad packs are preferable to dual lead types for
higher lead counts.
Anisotropic Conductive Adhesives
Since all three conditions must simultaneously occur
for migration to occur, prevention requires eliminating any
one. Design is commonly used to solve the problem.
Anisotropic bonding films are well suited for TAB
packages for virtually any lead count.One excellent feature
of these adhesives is the ability to connect very fine pitch,
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Page 333
even down to 100 microns (4 mils). TAB made from thin
thermoplastic dielectric materials work best. Polyester
based TAB can actually thermoform during the bonding so
that the package mates perfectly with the circuit in a
stressless mode. Two-sided and tour-sided TAB packages
can be attached with high reliability. No mechanical assist
is required. The extreme lightness of bare IC provides
excellent vibration resistance. TAP„ bonded with Z-Axis
adhesive, is the preferred PTF-SMT configuration for
larger IC's.
Conclusions And Predictions
Polymer Thick Film is the simplest, most efficient
production method for manufacturing printed circuits. It is
a 100% additive, no waste concept. Although tens of
millions of membrane switches and simple interconnects
have been manufactured, the market has been reluctant to
embrace full assemblies. A major reason has been the
difficulty of assembly with teed through devices.
The arrival of SMT provided the ideal assembly concept for PTF. The availability of SMD's made the 100%
PTF assembly feasible, practical and economically desirable. An SMT-PTF revolution, fired and inspired by the
great success of SMT has already conquered some vary
high volume niches of the consumer electronics field.
Calculators, telephones and other low cost consumer electronics products are now manufactured by the tens of
millions. Virtually all use SMT exclusively. PTF-SMT
products will continue to move into the consumer electron-
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ics field and beyond as materials improve and long term
field reliability data accumulates. The arrival of completely new inks, based on innovative concepts, will thrust
SMT-PTF into new, more challenging areas. SMT-PTF
will continue to expand into and eventually dominate the
consumer electronics field.
References
I. A ugarten, S., Bit by Bit, Ticknor & Fields, NY, 1984.
2. Cadenhead. R. L. and DeCousey, D. T., - The History of
Microelectronics - Part One," The Interna tional Journal
of Microelectronics, Vol. 8, No, 3, pp. 14-30, Sep!.
1985.
3. Franz., E. E., U.S. Patent 2,014,524. Sept. 17, 1935.
4. Gillen, K., "Screened Through Hole Technology,"
Screen Imaging Technology for Electronics, pp. 18-22,
Feb. 1988.
5. G illeo, K., "New Multilayer Process," Electronic Packaging & Production, pp. 134-136, Feb. 1989.
The author, Dr. Ken Gillec, Director of Research at
Sheldahl, Inc., wax trained as an organic chemist and has
worked exclusively on new product development. The last
se ven years have been focused on electronic materials wi
emphasis on Polymer Thick Film technology. This work
includes development of SMT for flexible circuitry, especially all-PTF electronic assemblies. His present work
involves direct chip interconnect using polymer bonding.
EXPO SMT International '89 Technical Proceedings