Making Better Contacts
by AMTECH
A Guide to Surface Mount Technology
Introduction
Powder and Alloys
Particle Size and Consistency
Solder Cream Chemistry
Printing
Reflow
Ordering and Technical Support
Introduction
Beginning with chip capacitors and resistors on hybrid substrates in the
1960's, the process of Surface Mount Technology (SMT) has become the state of
the art in electronics assembly. By placing the components on the board
rather than through the board, PCB manufacturers realize significant
production benefits and cost savings. Reduced component size and lead length
increase circuit density, resulting in more applications per unit area or weight
of board, which leads to lower production costs.
In the beginning, switching to SMT was an option for PCB manufacturers; the
concern over component availability and questions over cost savings delayed the
conversion for many assemblers. Today, SMT is not an option but a necessity,
since many components are no longer available in a through-hole package.
The shift to SMT has focused a great deal of attention on solder joints. Now,
in addition to providing the electrical pathway from the device to the board,
the joint must also provide the mechanical connection. As the device sizes and
lead pitches decrease, with a corresponding increase in lead count, the
mechanical and electrical properties of the solder joint are critical.
Defect-free assembly demands choosing the proper materials and equipment to
provide maximum control of the process.
Powders and Alloys
To make the best cream, you start with the best powder. Only the highest
quality solder powder is used in AMTECH products - solder powder manufactured by
Advanced Metals Technology, Inc. Solder powder quality is determined by purity
of the alloy, particle size, shape and distribution, oxide levels and lot-to-lot
consistency. All of the alloys manufactured by AMT exceed the applicable IPC,
QQS and J Standard specifications. AMT's proprietary processes allow the
tightest control over size, shape and oxide levels in the industry.
The degree of sphericity is related to the oxide levels on the powder -
generally, the more irregulars that exist, the higher the oxide level. Solder
alloys cannot exist in free-flowing powder form without a thin oxide coat, but
through a controlled production and classification process, AMT consistently
produces the lowest oxide powder available.
One of the properties of finer size powder is that, as the particle diameter
decreases, the ratio of surface area (and therefore oxide level) to mass or
volume increases. However, a major contributor to the higher oxide levels
typically found on smaller particles is the extensive processing most other
manufacturers need to perform to produce these finer sizes. Excessive processing
thickens the oxide layer. Through advanced technology, AMT makes ultra fine
pitch powder available with less concern over oxide levels.
Advanced technology means working to develop better, stronger and safer
alloys for the electronics industry. Our recent work with Hughes Aircraft has
led to the development of a Fatigue Resistant Solder that can prolong the life
of solder joints exposed to cyclic stress. Ongoing research and development
projects will continue to bring the latest in soldering technology to the
industry.
Partical Size and Consistency
Classification of powders has been a much disputed area. A good deal of the
controversy centers on the tools available for analysis. Until the advent of
computers, determining the particle size distribution was carried out through
the use of test sieves or screens. This technique was fairly quick and easily
carried out by personnel with little training. With particle size distributions
that ranged from 150µ - 15µ (-100/+625 mesh) the technique was adequate.
However, the demand for more restrictive size distributions pointed out the lack
of precision and resolution of test sieves. The development of supplemental
sizing techniques (e.g., laser diffraction, image analysis, and/or volumetric
resistivity modification) permits more precise characterization of powder
distribution. However, the labeling of a given powder distribution is still tied
to sieve designations. Unfortunately, these designations can be misleading since
test sieves have defined tolerances from sieve to sieve as well as tolerances in
the openings of a given sieve. It is much more prudent to specify a given
distribution by stating the actual particle diameters required.
Saying that a powder contains particles between 45µ and 25µ (-325/+500
mesh) still does not define the distribution. Two powder samples may have
the same average particle size (whether the average is calculated on a weight
percent basis or population basis) but have widely different properties.
Therefore, to properly specify a powder, it is necessary to recognize that it is
a population of particles, and it is necessary to characterize that population
in terms of the distribution of parameters that are important to the particular
application.
The demand for finer pitch printing and the development of no-clean pastes
require proper characterization of the powders used in solder pastes and creams.
Considering Stokes Law 
(where
F is the force on a spherical particle of radius a and velocity v
in a medium of viscosity  )
for the behavior of particles in a viscous fluid, it is seen that variations of
size distribution in a solder cream will have a significant impact on the
rheology with a corresponding effect on the paste's printing characteristics.
Secondly, with no-clean creams, it is imperative that solder balling be
nonexistent. This requirement demands that the printed solder cream come to the
reflow temperature uniformly. This can only occur if the individual solder
particles are heated at the same rate, which requires that they have nearly the
same specific area. Hence, the size distribution must be precisely specified,
and specification must be uni-modal with a small standard deviation. Under these
conditions the cream will have a consistent rheology and the oxide content of
the powder will be tightly controlled.
A beneficial side effect will be metallurgical uniformity in the powder
particles themselves, which will promote a metallurgically sound solder joint.
Of course, these specifications will preclude irregular particles and
multi-modal distributions.
Solder Cream Chemistry
Solder cream (sometimes refered to as paste) is made up of the powdered alloy
and the flux binder system. As the name implies, one of the system's duties is
to act as a flux.
Lead finishes are normally covered by thin films of tarnish, which can be
described as two layers, differentiated by the way they are bound to a surface.
Located directly on the metallic substrate and chemically bound are the layers
of oxide, sulfide and carbonate, as well as products from any preceding
production steps. On top of this layer is a physically bound absorption layer.
Absorptive forces attract residues including water, gases and residues from
preceding reactions, which will collect on the first layer. Therefore, the
specific requirements for a flux are:
- The dissolution of the outer absorption layers.
- The displacement of the chemically bound reaction layer.
- The dissolution of some of the substrate molecules to enable the formation
of intermetallic compounds.
The flux binder system has to perform many additional tasks during the life
of the cream. It must:
- Suspend the powder so it does not separate in the container.
- Protect the powder from oxidation without reacting with the surface at
storage temperatures - to promote a long shelf life.
- Give the cream thixotropic properties to aid in the printing and releasing
from the stencil.
- Provide enough tack to allow for processing time between printing and
component placement and between the placement and reflow.
- Perform the fluxing actions to promote coalescence of the molten solder.
- Protect the freshly cleaned surfaces from re-oxidation prior to reflow.
- Leave a residue that can be easily cleaned in the selected process, or if
a leave-on, be non-conductive, non-corrosive and not interfere with or detract
from visual or mechanical testing of the assembly.
RMA creams were the major type used for SMT applications. Rosin, along with
the proper solvent, activator system and rheology modifiers, was able to meet
all of the above criteria. The residue could be left on most assemblies without
concern or could be easily removed with the then readily available
chlorofluorocarbons (CFCs).
The adoption of the Montreal Protocol has phased out the production and use
of CFCs with total elimination not too far away. The elimination of CFCs as a
cleaning option presented a problem to manufacturers: customers had become
spoiled with sparkling clean assemblies and did not want to settle for less.
Drop-in replacements for CFCs were not perfected, so new formulations were
required for customers who still demanded cleaned assemblies; water-washable
assemblies were thereupon developed. The other alternative was to modify the
RMAs to leave a less tacky and less noticeable, cosmetically acceptable residue.
Today, there are two main options for post reflow residues: cleaning them off or
leaving them on. Presently, a different flux binder must be chosen for each
option.
The benefits of cleaning include better looking boards, easier visual and
bed-of-nails inspection, better adhesion for conformal coatings and removal of
residues and contamination from other manufacturing processes. Cleaning also
allows the use of more aggressive fluxes to widen the process window, with less
concern over the solderability of boards and components. Another concern has
been raised over the effect residues will have on high frequency circuits.
The first water-washable creams had very short working lives, measured in
minutes, and also left residues that had to be removed almost immediately to
avoid corrosion problems. Developments in formulation technology have resulted
in significant improvements in the succeeding generations of water-washable
formulas. Today, formulations are available which offer excellent working life
and activity, with reduced concern for corrosiveness prior to cleaning.
The main benefits of leave-on formulations are that they save manufacturing
operations, saving on equipment and the associated expenses of labor,
maintenance, power, chemicals and waste treatment. Many new leave-on
formulations offer RMA activity and process window in a no-clean product. Newer
formulations also have reduced residue levels, so the cosmetic problem also
decreases. The value of visual inspection has raised concerns in that
potentially damaging rework is being done on joints that were merely
cosmetically unacceptable, and defective joints often pass. Also, as board
complexity increases, the practicality and reliability of visual inspection
decreases. With the increased use of J leads and BGAs, unaided inspection is
impossible; the assembler must make the leap of faith that if his process is in
control, the joints will be acceptable.
In developing a leave-on formulation, the ultimate goal is no residue. The
drive for less residue is accomplished in two ways. The first is to increase the
percent metal. A small increase in weight percent makes a large difference in
volume percent. The second way is to reduce the total non-volatiles in the flux.
When one or both of these modifications are used, all of the cream parameters
are affected. In the very low residue formulations, the flux in no longer able
to prevent re-oxidation of the cleaned surfaces prior to reflow. To combat this
situation, many ovens are being designed with a nitrogen option. The nitrogen
displaces the oxygen and reduces both the potential for re-oxidation and the
charring of any remaining residue. The user must decide if the added expense in
nitrogen and process modifications is justified for his particular product.
The R&D team at AMTECH is continually developing formulations that will give
manufacturers the widest possible process window. These ideal formulations will
allow for a manufacturer's shift changes or other down time, without the cream
drying out or absorbing moisture. They would also have activation robust enough
to compensate for poor solderability of parts or boards, while remaining benign
under any conditions the product may see. It would also not require any
modification or additions to the present state of manufacturing, such as inert
atmosphere. Each new formulation is evidence of our commitment to this goal.
Printing
The primary method for en masse depositing of solder cream on a
circuit board is through the printing process. The increased complexity of board
design has led to tighter tolerances for the printing process. This progression
has resulted in the near-total shift from screens to stencils and the increasing
popularity of polymer-coated metal blades.
The consistent deposition of solder cream is the first step in controlling
the SMT process. Having the proper amount of cream will enable the formation of
a joint with the proper fillet geometry, which will determine the thermal and
mechanical properties of the joint. Too little solder cream can result in
opens or mechanically and metallurgically weak joints. Too much cream can
lead to bridging. Also, excess solder will make the joint less compliant
and more prone to cracks due to component/substrate thermal coefficient of
expansion (TCE) mismatch.
The printing process involves using a squeegee to roll the cream across the
stencil surface. The cream fills the apertures corresponding to the pads, and
releases onto the pads as the stencil is separated from the board. Many factors
combine to determine the success of the printing process. These factors include
the printer itself, the stencils or screens, the squeegees, the operators and
their training, the environmental conditions, the board characteristics and the
solder cream. Optimizing these is the key to printing consistently.
AMTECH has found excellent results in the lab and in the field using 4-6
mil
metal stencils with metal blades in the on-contact printing mode. Squeegee
pressure should be just enough to wipe the stencil clean, with speed in the
10-50 mm/sec
range.
The design, quality and accuracy of the stencil becomes increasingly
important; as the openings and spacings decrease, tolerances become much
tighter. Recent improvements in the fabrication of stencils have resulted in
less variation in apertures, less roughness in the walls, and more uniform
prints.
An often-overlooked cause of print variation is the preparation of the bare
boards. Uneven tinning on fine pitch pads can lead to voids or insufficient
solder. This is one reason for the increased interest in organic solderability
preservatives (OSPs) as an alternate to hot air solder leveling (HASL).
Solder cream factors for printing include powder size and shape, percent
metal and viscosity of the cream. For applications down to 25 mil,
AMTECH recommends our -325/+500 mesh powder. For 20 mil
and below, the -400/+500
mesh powder gives increased resolution without excessive fines. For
stencil printing the range of 89-91 percent metal is
recommended, as well as a viscosity range of 180-340 Malcom (700-1400
Kcps Brookfield).
The conventional instrument for viscosity measurement has been the spindle
type viscometer. AMTECH has found the spiral pump viscometer to be more
repeatable and more representative of the shear actually experienced in the
printing process. We currently test at 5, 10 and 20 rmp, with the 10 rpm reading
being the reported measurement, also expressed in kcps.
Reflow
Soldering can be defined as the joining of two metals by material heated
above its melting point but below the melting points of the materials to be
joined. The bond is formed in one of two ways: by the formation of intermetallic
compounds, which is an irreversible chemical process, or by diffusion or
absorption, which is a physical process.
When joining 63Sn/37Pb and other high tin alloys with copper, two
intermetallic compounds are formed. On the copper side is Cu3Sn
and on the solder side, the relatively rough and irregular Cu6Sn5.
The total thickness of the intermetallic layers (IL) is usually 0.5-0.7
µm.
The intermetallic compounds of copper and tin form crystalline grains, the
structure of which is determined by the length and intensity of the thermal
interaction. Short reaction times form fine equiaxed grains, which promote good
solderability and solder joint strength. Long reaction times result in coarse
grains, and a thick intermetallic layer. A thick IL gives poor solderability and
poor joint strength, both in t = o shear
and long-term reliability. The thickness of the IL depends on the temperature
but will continue to grow even at ambient temperatures (which on the absolute
(°K) scale approaches 60% of the 63/37 eutectic point). This is particularly
important when parts or boards are solder coated or pre-tinned. Upon prolonged
or improper storage, these ILs can grow through the surface, severely affecting
the solderability.
The majority of parts and boards used today come to assemblers pre-tinned
with the ILs already established. However, alternative lead finishes and
passivated copper pads are becoming more common. These new lead finishes rely on
the solubility of the metal in liquid Sn-Pb to form the bond. For some
metals, e.g., Pd, Pt and Ni, the dissolution rate is very slow,
requiring temperatures above the normal soldering temperatures and/or dwell
times much longer than needed for normal soldering. The result is a joint that
does not appear like the traditional solder fillet, causing concern during
visual inspection.
The objective of the reflow process is to achieve high quality solder joints
on all of the component leads of a particular assembly, and do to this
consistently. The process involves heating the leads, pads and cream above the
melting point of the alloy so that the solder on the leads and pads, and in the
cream, reflows into a homogeneous fillet. Consistency in the process depends on
the ability to control the application of heat and the variation of heat both
across the board and from board to board. This controlled heating is called the
profile. The typical profile includes a preheat, drying or soak, and reflow
or spike zone. The preheat zone brings the assembly up to temperature uniformly,
generally at a rate of 2°C/second or less. This will minimize
the potential for thermal shock on the components due to varying heat
capacities. The preheat zone also begins the volatilization of some of the
solvents added to the cream for printing and releasing. The second zone
continues the drying process to prevent out-gassing and possible spattering of
the cream. This zone, sometimes called the soak
zone, is also where the flux begins to remove the oxides from the surfaces of
the leads, pads and the powder itself. The resins and/or higher boiling solvents
remain as a cover to prevent the reoxidation that would readily occur at the
elevated temperatures. In the reflow or spike zone, the temperature is
quickly raised 20-40°C above the melting point of the alloy. It is here
that the solder wets the surfaces and forms the intermetallic bonds. The period
of time above reflow is called the dwell time, typically 30-60 seconds.
The dwell should be long enough to allow for all of the joints to reach
temperature and form the bonds. Too long a dwell time can lead to excessive
intermetallic formation. Both of the intermetallics are brittle, and if they
make up a large portion of the fillet, can lead to premature failure of the
joint.
The recommended profile is not a line but a zone or band. The width of this
band is defined by the upper and lower temperatures that will still give
satisfactory results for the particular cream. This band is also referred to as
part of the process window; the larger the band, the larger or more forgiving
the window.
It would be very easy to profile an oven if you only had to reflow one component
type on a uniform board. In the real world, almost every assembly has variation
across the board due to different components or component densities. Variations
in the board itself can lead to large differences in thermal mass. If you were
able to plot the profile of each joint, you would get a band corresponding to
the variation across the board. A proper profile will have the board's variation
band completely inside the process window.
Besides variation across the board, you can also have variation across the
oven. This is sometimes caused by the heat sinking of the conveyer system, air
flow variations near the sides, or non-uniformity across the heating element.
Another source of variation is the ability of an oven to hold temperature and
recover after a board passes through. This is called the load factor of
the oven. This will vary from oven to oven, but a starting point would be
between one half and one board length between boards.
The actual method of heating is not as important as the ability to control
the heating in a repeatable manner.
Ordering and Technical Support
Our customer service representatives are ready to discuss your SMT needs. We
offer a very short turnaround time on orders and can ship to meet your
just-in-time requirements.
To help us meet your particular needs, just provide us with the information
on your alloy, flux type, powder size and packaging preferences.
Alloy
For most SMT applications, the eutectic 63Sn/37Pb is recommended. Other
common alloys include 62Sn/36Pb/2Ag, 96.5Sn/3.5Ag, 95Sn/5Ag,
and 95Sn/5Sb.
Flux Type
This is mainly dependent on your cleaning methods: water-washable, semi-aqueous
or leave-on.
- 100 Series: R Type
- 200 Series: RMA Type
- 300 Series: RA Type
- 400 Series: Water Washable
- 500 Series: No Clean.
Particle (Mesh) Size
For printing down to 25 mil pitch, we recommend our type 3 (-325/+500)
powder. For applications where higher resolutions are desired, or for 20 mil
pitch
and below, we recommend our type 4 (-400/+500) powder.
Packaging
We offer a variety of standard and custom packaging options to fit your
processing needs. Standard packages include foil packs, jars, syringes,
cartridges and FreshMix Kits.
If you are not sure which combination is best for your process or have any
questions regarding soldering techniques, procedures or products, one of our
technical support staff will assist you.
AMTECH is committed to bringing you the most advanced formulations available
and we back that up with highly trained staff to provide for all of your present
and future assembly needs.
Advanced SMT Solder Creams
A Division of Advanced Metals Technology, Inc
The World Leader in Solder Powders
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