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Powders and
Alloys
To
make the best solder paste, you start with the best solder powder. Only the
highest quality solder powder is used in AMTECH products
—
solder powder manufactured by Advanced Metals Technology, Inc., (AMT).
Solder powder quality is determined by alloy purity, 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 of 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 possible. A property of 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 require to produce these finer sizes.
Excessive processing thickens the oxide layer. Through advanced technology,
AMT makes ultra fine pitch powder without excessive 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.
No-Lead Solder
Development
Current industry work on developing lead-free alternatives to the standard
63Sn/37Pb and 62SnI36Pb/2Ag
eutectic solders has focused on altering
the composition of the Sn-Ag eutectic solder. The various proposals center
on two approaches:
1. Substituting a Sn-Cu Eutectic for the Sn-Ag
eutectic.
2. Modifying the Sn-Ag Eutectic with small
additions of other elements such as Sb, Cu, Bi, or Ni.
In the first instance, the rationale of the
Sn-Cu eutectic is to reduce the leaching of Cu from the pads of the circuit
board and thereby reduce the formation of the Sn-Cu intermetallics and the
resulting embrittlement of the solder joint.
In
the second instance, the additions lower the melting point of the alloy by a
few degrees, and by reducing the Ag content, lowers the cost of the
resulting alloy.
However, it is the position of AMT that both of these approaches introduce
factors that have not adequately been addressed:
1. Data on the Sn-Cu eutectic is extremely
limited. The melting point of the
99.3SnIO.7Cu alloy is reported as
227 C, which is higher than the96.5Sn/3.5Ag eutectic of 221 C, for which
there is plentiful data. Although the introduction of Cu into the alloy will
reduce the concentration gradient of the Cu between the pad and the solder
joint, the net effect will be to lower the diffusion rate of the Cu; it will
not eliminate the diffusion completely. Consequently, the resulting joint
will be in a non-equilibrium state, and the actual properties of the joint
will be largely determined by the cooling rate following reflow. Finally,
the higher melting point will require peak temperatures in the reflow oven
that will seriously affect the components currently available.
2. The multiple component variations of
96.5SnI3.5Ag
only lower melting temperatures by 3-4
degrees. The slight lowering of the melting point is insignificant when
compared to the ability of reflow ovens to maintain precise temperature
control, or the ability to accurately profile complex circuit boards. In
addition to the lack of long term reliability data, these multi-component
systems have other problems. Due to the number of components, the
thermodynamics (Gibbs phase rule) indicates that there will be a large
number of phases present in the equilibrium state.
Unfortunately, the standard reflow conditions do not permit equilibrium
conditions to exist. Consequently, the solder joint formed immediately after
reflow will not be the solder joint that exists some time later. In
addition, the distribution of phases following reflow will be largely
determined by cooling rates, implying there can and will be wide variations
in solder joint properties, even across a single board.
It
is AMT’s position that for the immediate future, the most predictable
lead-free solder joint is the Sn-Ag eutectic. Its properties are well
documented and its shortcomings are no greater than currently proposed
alternatives. In its own development work, AMT is working on a number of
alloys with melting points in the neighborhood of the standard Sn-Pb and
Sn-Pb-Ag eutectic. Getting the right melting point is only part of the
problem. AMT is utilizing the same principles utilized in the development of
the Fatigue Resistant Solder (see FRS paper
-
Hughes AircraftlAMT) to get the desired metallurgical properties in
no-lead.
Meanwhile, AMT and its subsidiary, AMTECH Solder Products, Inc. can and will
provide powder and paste to customers who want the currently proposed
non-lead alternatives, currently
99.3SnI.7Cu; 95.5SnI4.OAg/.5Cu;
96.5 Sn/3.5Ag; and 95Sn/5Sb. |
