1999 International Symposium on Advanced Packaging Materials
Initial Investigations into Low-Cost Ultra-Fine Pitch Solder Printing
Process Based on Innovative Laser Printing Technology
Anthony Walker
Daniel Baldwin, Ph.D.
Packaging Research Center
The George W. Woodruff School of Mechanical Engineering
Georgia Institute of Technology
Manufacturing Research Center
Atlanta, Georgia 30332-0405
Phone: (404) 894-4135 Fax: (404) 894-9342
e-mail: gt 1619b@,~rism.natech.edu,daniel.baldwin@,me.natech.edu
Abstract
Advances in electronics packaging and assembly technology are driving increased demand for ultra-fine
pitch solder deposition. In this work, innovative solder deposition techniques based on laser printing are being
investigatedfor low-cost ultra-fine pitch printing applications. This paper will investigate the feasibility of using
solder particles in of the shelf xerographic technology. The physics of the two development systems (dual
component and monocomponent) will be discussed. This inquisition will lead to a discussion of triboelectric
charging of the solder toners, coating the solder with thin dielectrics, and charge induction by an applied electric
field, The results @om these investigations are used to assessfeasibility and definefuture work.
Key words: Xerography, Laser Printing, Triboelectricity, Solder Printing
investigated for low-cost ultra-fme pitch printing
applications. This paper will present the optimal way
to charge the solder particles in order to use off the
shelf xerographic technology. The experiments
performed look at various ways to coat the solder
with thin dielectrics, triboelectric charging of the
solder toners, and charge induction by an applied
electric field. The results from these experiments are
used to assess feasibility and define h a r e work.
Introduction
Electronics manufacturing and assembly has
become increasingly challenging with the demand for
smaller, lighter, faster, and more reliable products.
The rapid progression of the electronics industry over
the last few years has produced a trend towards the
use of finer pitch and higher pin count packages.
Moreover, the move towards fine pitch BGAs, fine
pitch CSPs, and flip chip on board is driving
conventional solder paste printing processes to their
practical production limits.
For example, in many flip chip applications,
solder is electroplated on the substrate bond pads to
enable high yield processing. However, due to its
expense and negative environmental impact,
altemative ultra-fine pitch solder deposition
processes are being sought. Requirements for the
deposition technology include data-driven processing
where printed patterns can be software controlled,
area-based processing as opposed to point-to-point
processing, integratable with standard surface mount
lines, and low capital costs.
In this work, innovative solder deposition
techniques based on laser printing are being
Conventional Solder Deposition Technology
One of the conventional techniques for
applying fine pitch solder is stencil printing which
suffers from bridging, non-uniform print volumes,
and lack of repeatability for precision ultra-fine pitch
applications.
Another disadvantage of stencil
printing is the need to use a different stencil if the
printing pattem changes. A key advantage to stencil
printing is its high volume capability and short cycle
time. A more common technique for ultra-fine pitch
solder deposition is electroplating.
The
disadvantages of this technique include the high cost
associated with electroplating (compared to stencil
printing), the need to solder plate the entire substrate
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1999 International Symposium on Advanced Packaging Materials
into close proximity of the photoreceptor. Due to the
electric field created by the latent image, the toner is
attracted to the photoreceptor forming a real image.
Next, a substrate is fed into the system and comes
into contact with the photoreceptor. An electrostatic
charge is applied to the underside of the substrate by
a second corona that has the same polarity as the
photoreceptor. This second corona produces an
electrostatic charge that is much greater than the
charge on the photoreceptor. This charge serves to
strip the oppositely charged toner away from the
photoreceptor leaving an image on the substrate.
Next, the substrate moves to the fking station and
heated rollers melt or fuse the toner into the substrate.
Finally, the photoreceptor is cleaned and discharged.
At this point, the process begins again [4].
unless mask or lift-off techniques are used adding to
the cost, and the large scale environmental impact of
electroplating which is being subjected to increasing
governmental regulations significantly adding to the
cost. A third emerging technology is solder jet
printing (similar to ink jet printing) in which new
pattems can easily be accommodated since it is datadriven. Solder jet printing can be divided into two
categories: drop-on-demand and continuous mode.
The major drawback of drop-on-demand jet printing
is that it is a single point process and therefore suffers
from relatively low processing rates.
In the
continuous mode, one of the major drawbacks of the
technology is that the fluid must be continuously
jetted even when there is no printing required,
resulting in significant waste [3]. An altemate
technique is xerographic printing based on laser
definition processes.
Uniqueness of Proposed Solution
The proposed laser printing process is
potentially a very low cost means of depositing ultrafine pitch solder onto substrates. High-end laser
printers typically cost two to three thousand dollars
and are standardized. Moreover, the solder particle
technology currently exists to produce particles on
the order of 15 microns or less. A major advantage
of the laser printing technology is that the solder
pattems are programmable directly for the circuit
board CAD data and do not require stencils or masks
resulting in significant setup time reductions and cost
savings. The proposed technology is expected to
yield prints on the order of one to three mils thick.
Another advantage of this process is its ability to
integrate directly into an existing surface mount line
as shown in Figure 2. It potentially could achieve
very fine pitch prints compared with stencil printing.
The basic concept is for a solder laser printer to
immediately proceed a stencil printer in a SMT line.
The laser printer deposits the ultra-fine pitch (<lo 20 mils) prints in the form of a fused solid brick and
a stencil printer is used to deposit the bulk solder on
the remaining bond sites.
Description of Xerography Process
Electrophotography is a relatively well
understood process involving six inter-dependent
steps. These steps are individual in operation, but by
changing any of them impacts the output print.
These six steps include charge, expose, develop,
transfer, fuse, and clean as shown in Figure
1.
( 1,
) Charge
-
( 2 ) Laser
Imaging
Optics
(6)
Cleaning
(5)
E
.
.
.
.
.
"
,
.
(3)
Developer
Station
I
(4) Transfer
Device
Figure 1. Diagram of Laser Printing
The f a t step in xerography is charging the
photoconductor drum or photoreceptor.
A high
voltage device, usually a corona, is charged to a high
voltage ionizing the surrounding air. This ionized air
is used to apply an even, uniform surface charge on a
photoreceptor, which is electrically isolated and nonconductive. Next, light, reflected from an image
(photocopier) or produced by a laser (laser printer) is
used to create a latent image by discharging areas of
the charged photoreceptor. The discharged area of
the photoreceptor now has a charge that is much
smaller than the remainder of the photoreceptor. The
third step is to bring electrostatically charged toner
Figure 2. Standard Surface Mount Line with In-line
Solder Laser Printer
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1999 International Symposium on Advanced Packaging Materials
Types of Development Systems
There are two main types of dry developing
systems for laser printing: dual component and
monocomponent. A dual component system is one
which uses both carrier and toner particles. An early
developer patent (U.S. patent 2,618,551 issued to
Walkup) presents the use of carrier material "to carry
and to generate triboelectric charges on the
electroscopic powders (toners). .."[7]. The advantage
of a dual component system is its ability to quickly
bring large amounts of toner to the photoreceptor.
Also, potentici problems with air discharge or sparks
within the machine are eliminated since materials are
triboelectrically charged. However, the toner and
carrier particles composition must be carefully
chosen for triboelectric charging characteristics. A
monocomponent system uses toner only. The
advantages of this system is more flexibility in the
material chosen due to the various wpvs to charge the
particles (triboelectric or induction &narging). One
major disadvantage to induction charging is the
potential of charging the air and creating an
electrostatic imbalance inside of the machine, which
may lead to dangerous, sparks.
Physics of Dual Component Development
The most common technique used to bring
the toner in close proximity of the photoreceptor is
magnetic brush development. This process was
invented by RCA in 1954. In this case, the carrier
has a magnetic core. Since the carrier is magnetic, it
is able to align itself along the magnetic fields created
by a magnetic located inside the roller as shown in
Figure 3. As the roller rotates, the doctor blade
breaks all carrier particle chains that are too long.
drum
There are three different development
theories available for dual component developing:
field stripping, powder, and equilibrium. However,
for solder toner printing, only two of these theories
are being investigated as shown in Figure 4: field
stripping and equilibrium.
field stripping
equilibrium
Figure 4. Development theories
In order for development to occur based on
field stripping theory, the force of attraction (FJ
between the toner and carrier (van der Waals and
electrostatic) must be less than coulomb force
(QtE&) from the latent image of the photoreceptor.
When the coulomb force is greater, the toner is
stripped from the carrier and developed onto the
photoreceptor.
Based on the equilibrium theory, the toner
contacts the photoreceptor and begins to develop.
The toner continues to be stripped from carrier until
the Coulomb force balances the force of attraction
between the toner and carrier. With the force of
attraction, the van der Waals and electrostatic forces
are canceled to fmt order since similar forces
between the toner and photoreceptor exists. The
major force of attraction is the net charge built up on
the carrier due to the toner being removed from the
carrier. The net charge increases as each toner
particle is removed form the carrier. Therefore, the
next candidate toner particle must overcome this
increased charge. Once the coulomb force is in
equilibrium with the force of attraction, development
ends [4].
Triboelectric Charging of Toner
I
gap
\
development
magnet
Figure 3. Magnetic brush development
The dual component system uses two
particles that must be carefully chosen, because they
require triboelectric charging characteristics.
Triboelectric charging is a type of electrostatic
charging. When two dissimilar materials contact
each other, electrons transfer from one surface to the
other resulting in one material having an abundance
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1999 International Symposium on Advanced Packaging Materials
of negative charge, other having an equal magnitude
of positive charge, as shown in Figure 5 [5].
and restricts the carrier particles.
measures the charge generated.
average charge per mass (Q/m) is
measuring the change in mass and
system.
A multimeter
The resulting
determined by
charge of the
Figure 5. Triboelectricity Diagram
Figure 6. Faraday cage
As a result of the charging, the toner electrostatically
attaches to the carrier particle [6]. In the printer, an
agitator located in the carrierhoner hopper induces
this rubbindcontact. Tribo-series tables are used to
determine the material systems that will result in
adequate triboelectric charges for printing.
The appropriate charge properties of the
toner are essential for an efficient xerography
process. If the charge per mass value is too large, the
resulting image wi11 be too light. In contrast, if the
charge per mass is too small, the image will be too
small. A typical range of a charge per mass for a
xerographic process is 10-25 pC/g [4]. The optimal
value varies for each machine. Therefore, it is
essential that this charge be measured in order to
design the material system.
Solder Toner Modifications Needed for Dual
Component System (Dielectric Coating)
In order for dual component systems to
work, there must be a triboelectric attraction between
the toner and carrier. It is possible to triboelectrically
charge metal-metal, metal-insulator, and insulatorinsulator. However, due to the conductivity of metals
which results in easy discharge, an insulator-insulator
system will be employed. In order for a metal to
"act" like an insulator, a dielectric must be applied to
the surface. The thickness of the dielectric must be
evenly applied. If a metal is exposed, there is a
chance for discharge. This coating must be thick
enough to withstand the contact involved in the toner
hopper mixing.
In triboelectric charging of
insulators, the electrons and protons are polarized.
Also, the strength of the attraction depends on the
coulomb force between the two materials. The
coulomb force increases as the charge increase. This
charge is directly proportional to the number of
electrons available. Therefore, the thickness of the
coating can be varied to supply the needed electrons
to produce adequate charges.
Triboelectrification Measurements
In order to measure the electrostatic charge
created by triboelectrification in xerography, a unique
method is employed. Both materials are placed
inside of a Faraday cage that has a screen on both
ends as shown in Figure 6. The average charge per
particle (QM)is measured by the Faraday cage.
This value is useful in determining the force of
attraction between the toner and photoreceptor. The
size of the screen mesh is between that of the size of
the carrier material and the smaller toner particles. A
jet of air is sent through the cage forcing the two
materials to move towards the screen. The coulomb
force between the carrier and toner keeps some of the
toner inside of the cage. However, the toner particles
that are strongly held are forced out of the screen.
The screen mesh lets the toner flow through with ease
Physics of Monocomponent Development
In using only toner in developing, there is much more
flexibility. In monocomponent development, the
toner can be conductive or insulating, magnetic or
insulating, and any combination of the four. The
magnetic brush development is the most widely used.
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1999 international Symposium on Advanced Packaging Materials
Ferric filings are injected into the toner in order to
make them magnetic. Monocomponent development
is typically done by the magnetic brush technique.
However, for insulating toner, the toner can be
cascaded or forced against the photoreceptor as
shown in Figure 7. In some systems, the toner does
not have to come into contact with the photoreceptor.
It can be just brought into close proximity of the
photoreceptor. The toner in either of these cases is
charged either triboelectrically by rubbing against the
toner hopper and rollers or by induction.
Figure 7. Cascade Development
Most of the physics for developing
monocomponent particles is very similar to the dual
component. Both field stripping and equilibrium
theories apply. In both theories, there is still a
coulomb force created by the latent image. In the
magnetic toner case, the main force that repels the
coulomb force in both theories is the magnetic force
of attraction between the roller magnet and toner.
For cascade development, primary difference is the
charge placed on the toner must have the correct sign
and magnitude to be attracted. When the toner
contacts the photoreceptor, the inertia force comes
into consideration.
fft+H-++++
electoststic
\positive
charge
Figure 8. Electrostatic Repulsion Created by Polarized
Toner
In induction charging, the solder particles
are placed between two metal plates and a voltage is
applied which creates an electric field E& as shown in
Figure 9. The uncoated solder will allow charge to
flow from the metal plate to the particle. Once the
solder particle is charged, it will have the same
polarity as the plate it is touching. It will then be
repelled by that plate and attracted by the oppositely
charged plate. It will remain here until it obtains the
charge of this plate and it will once again be repelled.
The conductive particles will bounce back and forth
between the two plates changing charge. However, if
a hole is applied in one of the plates, the particle will
be allowed to escape with the last charge it acquired.
It has been shown that the induced charge Q and the
charge-to-mass ratio Q/M for the conductive particles
are given by:
Q = (1.65E,)(4z.c0r2)
(1-1)
Q / M = 4.95.5,E,, Irp,
(1-2)
where r is the radius of the particles, pt is the density
of the particle, and eo is the permittivity of free space
[41.
metal plate
Charge induction
This method is one of the most
straightforward solder toner charging techniques. It
is possible to use either conductive or non-conductive
particles. As such, the process is flexible enough to
handle coated or uncoated particles. However, for
solder printing, only uncoated particles will be used.
In the case of coated solder, the dielectric will
polarize in the presence of the electric field. This is
not desired, because it will cause electrostatic
repulsion during the transfer step as shown in Figure
8.
-
\
metal plate
Figure 9. Charge induction setup
Charging Experiments
Qualitative induction charging experiments
have been conducted and have shown effective
charging of the powders.
These qualitative
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1999 International Symposium on Advanced Packaging Materials
experiments involved observing the behavior of the
particles before and after charging. When the solder
was brought near a material with a negative charge,
the particles were attracted to the material. However,
after charging negative, the particles were no longer
attracted to the negative material. In this case, there
was an observed force of repulsion between the two
materials.
[5] J.A. Cross, "Electrostatics: Principles, Problems,
and Applications", Adam Hilger, Bristol, p. 17,1987
[6] Donald M. Burland and Lawrence B. Schein,
"Physics of electrophotography", Physics Today,
May 1996, p. 47.
[7] John F. Cooper, An Introduction to Dry Toner
Technology, Toner Research Services, Black
Mountains, NC, p. 3-1, 1992.
Conclusion
The advantages of ultra-fme pitch solder
printing using laser printing technology include high
speed, flexible print patterns, and low cost. Based on
the initial investigations, the basic process appears to
be feasible. The initial results obtained from solder
toner charging experiments indicate that induction
charging may be the preferred charging method.
Using this charging process, there is no need to coat
the solder particles and process control is improved.
A disadvantage foreseen is the need to use a cascade
development instead of the magnetic brush.
By employing a monocomponent system
with particles that are non-magnetic, hence bringing
the particles in contact with the photoconductor poses
some challenges. If triboelectric charging proves
effective, a dual component system may be used with
magnetic carrier particles.
Future studies will focus on quantitatively
measuring the charge generated using induction
charging. Identifying dielectric coating materials of
the solder powder which are compatible with reflow
processes will also be investigated.
References
[11 Dr. Gary Marks and Dr. Jerry Sergent, "The
Electroformed Stencil: A Solution to Printing Solder
Paste for Fine Pitch Devices': http://AMTX.com,
1998.
[2] Richard Clouther, "Pinpointing the Processes for
Fine and Ultra-Fine-Pitch Printing"
http://www .smtmag.com/editoriaYwebxclus/webxclu
s2.htn-11, 1998.
[3] D. B. Wallace, "Automated Electronic Circuit
Manufacturing Using Ink Jet Technology", Joumal of
Electronic Packaging, Vol. 1 11, June 1989.
[4] L. B. Schein, "Electrophotographyand
Development Physics", Springer-Verlag, second
edition, Berlin, Chapter 2, pp.26-38, 81, 123-124,
187,188,1992.
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