10 Solder Tips for Electronic Manufacturing PCBA


10 Solder Tips
By Maureen Brown –Technical Service Engineer, Kester Solder
1) Start with the right stuff
a) Always use the best tools and assembly materials available.
There are always new product innovations to consider as well.
For example, the latest solder paste can make a SMT engineer’s
job easier. New product innovations can yield longer idle times
or relax/recovery times. Environmental resilience and extended
shelf life are other areas of improvement. Profile robustness and
high-speed printing have also been incorporated in new solder
paste products.
b) Do not hesitate to have a “show down” to compare products
or equipment. If you start out with the correct tools, then there
is a greater likelihood the end result will be favorable.

2) Storage and Handling
a) When you get the right stuff, treat it well. Current technology
allows unopened solder paste to be stored at room temperature
conditions. However, the best manufacturing practice is to refrigerate
solder paste upon receiving. Solder paste is a mixture of
paste flux and solder powder. Essentially flux is a chemical that
removes metal oxides and promotes spreading of the solder.
When flux and metal are mixed to make solder paste, the flux
will remove the metal oxides as described. To slow this reaction
and to extend the useable life of the solder paste, it should be
refrigerated.When the solder paste is needed, it should be allowed
to warm up to room temperature naturally. It is best to
remove the solder paste from the refrigerator 18-24 hours before
scheduled use. Expediting this process by placing the solder
paste on the reflow oven or in a warm environment is strongly
discouraged.Warming the solder paste quickly will alter the
physical properties and may promote defects such as slumping
and bridging. Lastly, when working with cartridges or syringes of
solder paste, store it vertically with the tips down.

3) Solderball Test
a) The solder paste was received and
was left on the receiving dock over a
hot weekend. Is it bad? The solder
paste was left out on the manufacturing
floor for an unknown time. Is
it good? One of the best performance-
related tests to judge the usability
of solder paste is a modified
solder ball test. This test is very simple
and reveals a lot of information
about the condition of the solder
paste. Dispense a small dot of solder
paste onto an unsolderable substrate then reflow. Unsolderable
substrates are ceramic, glass, and FR-4. The solder paste deposit
after reflow should form a large ball of solder with a pool of
flux. This indicates that the flux is active enough to effectively
remove the surface oxidization of the solder powder and allow
the solder to reflow and coalesce to form a single ball. If moisture
is trapped within, or the solder paste was improperly
stored and handed, the solder may form a ball with numerous
smaller balls at a fraction of the size in the pool of flux. This
indicates that if the solder paste is used for assembly then defects
are eminent. (See Figure 1.)

4) Profile
a) Profiling is not the most exciting
task in electronics assembly,
but it does assist in
reducing potential defects. All
reflow profiles consist of a
reheat zone, soak zone, reflow
zone, and cool-down
zone. Each of these zones
are important for a successful
solder joint to be
formed. Manipulation of any
of these zones will alter the
integrity and appearance of
a solder joint.
b) The preheat zone is defined
at the lower end by the
ambient temperature, and the upper end by approximately
140-150ºC. The main purpose of this is to drive off the solvent
carrier from the solder paste deposit. The ramp rate is fairly
quick and should not exceed the component manufacturer recommendations
of 2.5-3.0ºC/sec. If
the ramp rate is greater, the components
may be thermally shocked,
warpage of the board may be induced,
and the solder paste could
slump. As the preheat zone ends, the
soak zone begins. The soak zone is
commonly bounded by 150ºC and
the liquidus temperature of the alloy,
for Sn63Pb37 the liquidus temperature
is 183ºC. The soak zone
Figure 1: has two main functions. The first is
Solderball Test
As the industry continues to move away from through-hole
technology, surface mount technology (SMT) has become
the most popular mass-assembly technique in electronics.
SMT offers new challenges and adventures for engineers to
conquer. The industry has made available numerous
resources to successful soldering for SMT. Consider these
suggestions for overcoming obstacles.


Preferred
example
24 SEPTEMBER 2000 I N S I D E L I N E
to thermally equalize
the assembly, and the
second is to activate
the flux. Activating the
flux essentially allows
the paste flux to
“chemically scrub” the
surface oxidation of
the surface finishes on
the printed circuit
board and the component leads as well as the solder powder
to promote wetting. This is accomplished within approximately
60-90 seconds. Next is the reflow zone. The reflow zone is commonly
characterized when the solder powder changes phases
from a solid to a liquid (also known as the time above the liquidus
temperature of the alloy). The peak temperature is
around 30-40ºC above the alloy liquidus temperature. The main
purpose of the reflow zone is to form a metallurgical http://improvehearingnaturally.com bond between
the component leads, solder, and the lands on the printed
circuit board. Excessive time within the reflow zone may lead
to dark, dull, grainy joints. Insufficient time within the reflow
zone may result in weak interconnects. Last is the cool-down
zone. The cool-down zone begins as the alloy solidifies. The temperature
of the assembly should not decrease too rapidly; this
may lead to thermal shock of the components. Again, follow the
component manufacturer’s recommendations.


5) Compatibility
a) Do not try to mix and match flux chemistries. Use all No Clean
products or all water-soluble products.Water-soluble flux
chemistries are very active by nature in comparison to No
Clean formulations. Due to this inherent difference, operators
may be drawn to water-soluble flux products. No Clean flux
chemistries are designed to have residues that are non-conductive
and non-corrosive, whereas water-soluble flux chemistries
are conductive and corrosive. If a water-soluble flux was used
on a No Clean assembly, this would be an important reliability
issue for the assembly and should be immediately addressed.
Any residue resulting from the use of water-soluble fluxes should
be completely removed from the assembly.

6) Printing Suggestions
a) Printing is the first process in surface mount assembling. Some
general printing suggestions are offered to reduce defects and
optimize yields. The bead diameter of the solder paste should
be approximately the same size in diameter as a cigar (about
0.5” in diameter).When the squeegee is gliding across the stencil,
the solder paste in front of the squeegee should be rolling.
This will assist in proper filling and leveling of the apertures
with solder paste in addition to achieving consistent solder
paste deposits. Traditional solder pastes required kneading to establish
a good roll, while current formulations do not possess
this attribute. Clogging of the apertures is another problem
within the printing process that is a common root cause for
many defects. Innovative solder paste formulas release cleanly
from the wall of the stencil apertures. Solder paste adhering
and drying in the apertures reduces the quantity of paste
transferred or deposited, resulting in a higher occurrence of defects
in the form of insufficients or opens.

7) Avoid Humidity and high
temperature situations
a) Solder paste can be a delicate blend of numerous chemicals to
yield a specific rheology and consistency. Harsh environmental
fluctuations will alter physical properties such as viscosity and
tack.When the humidity begins to climb above 60-70%RH, the
solder paste may entrap or absorb moisture, which will decrease
the tack and promote solderballing. A common indication will
be the solder paste sticking to the squeegee and forming a curtain
instead of releasing from the squeegee. On the other hand,
if the humidity is below 25-35%RH, then the solder paste will
dry out quickly and the stencil life will decrease, resulting in additional
scrap. If the solder paste dries out, the apertures in the
stencil are easily clogged as well. Activity, tack, and viscosity are
all properties of solder paste that are altered as the environmental
conditions go to extremes, and the inclination is to add
something to the solder paste to revive the material. This is
strongly discouraged. Solder paste does not react in the same
fashion and is unpredictable with additions. If the manufacturing
facility is prone to these harsh conditions, then make sure the
solder paste selected is robust; review Tip #1. Recent solder
paste formulations are designed to be environment-resilient.
8) Double Sided Reflow Applications
a) As the electronics industry is driven by end products that are
lighter in weight, faster, and less expensive, double-sided reflow
applications are increasing. This application can be simplified
with some useful tips. One common tip is to bias the topside
preheaters of the reflow oven slightly hotter than the bottomside
preheaters. This will direct more of the heat to the topside
of the assembly than the bottomside.When attempting this, the
temperature gradient should not exceed more than 15-20ºC, in
case the oven isn’t able to stabilize. If this tip is used, the same
alloy can be used for both sides of the assembly.While reflowing
the second side of the assembly, you may need to glue
components so they do not fall off the assembly.
9) Troubleshooting
a) As experience with a specific process increases, the defects will
be reduced and an expert will emerge. There are many ways of
finding the root cause of a defect or failure. List the possibilities
and eventually isolate the problem. Employ organized troubleshooting
techniques to quickly resolve the problem. Involve
other departments to facilitate the troubleshooting process. Engineering
and troubleshooting are typically well accomplished with
well-functioning teams.
10)Assistance
a) Do not be afraid to ask for assistance. There are several
sources for troubleshooting, problem solving, and general
question forums available. The electronics assembly community
is a helpful, very informative bunch. Solder vendors have
technical service engineers that are available to answer questions.
Another very good source is forums. The Internet-based
forums through Surface Mount Technology Association (SMTA)
and the Institute for Interconnecting and Packaging Electronic
Circuits (IPC) consist of industry professionals and consultants
that are eager to share experience.

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