and Immersion Depth Study Results
Parameters in Wave Solder Optimization:
Measurement and Control of Dwell Time and Immersion Depth
Improves Board Quality and Repeatability
Ingall, and Nissim Sasson
and process engineers have learned new insights to board-wave interaction,
leading to dramatic changes in wave solder procedures. This has
emerged only in the last several years, when technology was introduced
which could directly measure your board's experience in your solder
wave. Immediate and sharp improvements in board quality have been
the result, propelling widespread acquisition of such technology.
discusses concepts of board-wave measurement and reports study results
that employed commercially available technology to measure and respond
to the key parameters of wave soldering. The purpose of the study
was to determine the importance of dwell time and immersion depth
measurement in wave solder quality.
Parts of this
study were originally presented at NEPCON West 98. A shorter version
of this study was published in the April 1999 Circuits Assembly
of Board-Wave Interaction
Your wave machine
was built for only one purpose: To cause your boards to interact
with your solder wave. You know this to be completely true, because
when you look in your reflow oven you see no wave. In the reflow
oven, chemistry supports your boards as they experience temperatures.
However, this is not true for wave soldering.
In your wave
machine, chemistry and temperatures are only supporting actors as
they deliver your boards to your solder wave. That is why your process
window for temperatures in a wave machine is wide and forgiving
in comparison to that of the surface mount oven, and why precise
control of your board-wave interaction produces such large benefits.
Your leads are
in the solder wave for only a few seconds or less. Soldering is
supposed to be achieved in a single pass and emerge defect-free.
Since this event is so brief and todays boards are so complex,
your board must pass through your wave with precision. Thoughtful
engineers have learned that seemingly slight board-wave process
variations can cause large quality variations.
of Thermal Profiling
to adhere to the notion that wave solder process control is primarily
about temperatures, and therefore choose to rely strictly on temperature
stickers, pyrometers or thermal profiling. While temperatures are
important, they do not and cannot by their very nature address your
boards interaction with your solder wave.
without accurate board-wave data is a sure prescription for consistent
defects, production crises and downtime. Experientially, production
professionals understand this - they see the rework staff at their
workstations and bear the brunt of managements goals for throughput
This is true
despite Herculean efforts in thermal management, wonderful progress
in wave solder machine quality, and the continuous development of
flux and solder chemistries. Yet ask a manufacturing engineer from
where the majority of his assembly defects come and more often than
not hell point to his wave machines.
of rework staff work every day, every shift, solely to correct defects
off the production line. Rather than being viewed as compensatory
activity for production failures and therefore something to be exorcised,
current levels of rework are often deemed "acceptable"
as part of the production process itself. The net result, as we
shall see here, is significant exaggeration of production costs
and serious under performance in wave soldering.
adjusting your preheaters can never eliminate bridging caused by
too long a dwell time. Likewise for skipping caused by too shallow
an immersion depth. The study results presented here show that the
majority of existing wave solder defects can only be eliminated
through accurate, direct measurement and control of your boards
interaction with the wave.
your board is parallel (a parameter also requiring accurate measurement)
to your wave, board-wave interaction has three distinct, simultaneous
facets that can be directly and accurately quantified:
Time - The amount of time a lead is in the solder wave. This
needs to be controlled in one-tenth second increments.
- How deep your board immerses in the solder wave. Since the very
best waves have a wave height variation between 10 and 20 mil,
this parameter is optimally gauged by its passage through a process
window. The device used for this study employs 12 mil increments
for this purpose.
Length - The distance in which a lead passes through a wave.
on image for a larger view!
1 Impact of immersion depth.
Figure 1 illustrates
the interrelationship of immersion depth and contact length for
your boards, showing that in fact your immersion depth directly
determines contact length. This in turn directly affects dwell time,
= Contact Length x Conveyor Speed
What this means
to the wave solder engineer is that your conveyor speed setting
will not on its own control your dwell time in the wave. You must
in fact have a means of accurately measuring and controlling your
immersion depth as well.
Many of us have
experienced the frustration of running an assembly on two different
wave machines and seeing two very different board qualities emerge.
Why do your wave machines produce different results when both are
set at the same pump speed, conveyor speed, conveyor angle, solder
pot height, preheat and solder temperature, are using the exact
same chemistry, have the same maintenance schedules and show the
same thermal profile?
As an industry,
we have often retreated to accepting that "different wave machines
have different personalities." Others blame operators. Yet
the answer is often simple and measurable: All wave machines produce
waves that are different shapes.
on image for a larger view!
2 Impact of wave shape.
Figure 2 shows
the impact of wave shape on contact length. A wider wave will mean
a longer contact length - and therefore dwell time - at the same
of Machine Set-up
What this all
means is that in order to control your wave process you need to
directly measure what your board actually experiences in the wave.
Wave machine settings can never assure repeatability. Your board
does not see a conveyor speed; it does experience a dwell time.
Likewise, your board does not know your pump speed; it experiences
an immersion depth. Also, your wave machine settings do not tell
you the wave machines variability. Therefore, parameters for
wave soldering must be based primarily on guidelines for board-wave
interaction, not wave machine settings.
need no longer blame their wave solder machines, flux or personnel
when their real challenge is the wave solder process itself. Your
wave machine does not even purport to measure your board-wave interaction.
Good equipment does not compensate for uncontrolled process. The
best wave solder equipment in the world still requires a sound approach
to process optimization and control.
Baseline for Study
A major consumer
electronics company tasked one of its North American facilities
to perform a month-long study to assess the significance of dwell
time optimization and repeatability. The assembly with the greatest
volume, representing 19% of all the boards produced at that location,
was selected for the study.
For this purpose,
an electronic device was used with direct board-wave contact sensors.
Capable of performing four runs in a row, the device offered the
convenience of taking multiple readings before downloading the data
to a PC. Also, the device's LCD display allowed the reading of data
immediately upon its exit from the wave machine. Another important
capability: Direct measurement of immersion depth. The following
steps were performed:
Parallelism was measured and established.
Measurement of the boards current dwell time, which was
Measurement of the boards current immersion depth, which
was 24 mil.
Assessment of board quality, showing a defect rate at 312 ppm,
a level considered normal at the facility despite the amount of
rework being performed, and excellent by industry standards.
Steps 1 through 3 were easily performed for three shifts in a
row, twice per shift, since all data was obtained in a single
run of the device through the wave machine. Step four was performed
at the end of each shift.
If, prior to
running the assembly, measurements showed a disparallelism, or a
dwell time more than 0.1 second away from 1.0 seconds, or an immersion
depth other than 24 mil, adjustments to the wave machine were made
and additional measurements were taken to confirm that the desired
board-wave experience was occurring. Areas unrelated to board-wave
interaction were maintained throughout. These include, for example,
flux types, preheater settings and solder temperature.
At the end of
each shift, a ppm tally was made. Ppms for this board were
consistently at the 312 range. Hence, repeatability of board quality
on image for a larger view!
3 The first study's dwell time and defects results.
The next goal
was to determine if the boards defect rate was affected by
running it at different dwell times. This was achieved by running
the same board type at different dwell times in half-second increments,
from 0.5 seconds to 5.0 seconds, and correlating each dwell time
with the defect rate it produced. Steps 1 through 5 above were followed
at each dwell time.
The blue line
of figure 3 shows the results. It was determined that the dwell
time which produces the lowest defect rate for this particular board
was between 2.5 and 3.0 seconds. Further study of this board was
then performed. It was determined that defects are consistently
lowest at 2.8 seconds. As a result, this board is now only run at
the 2.8 second dwell time with 24 mil immersion depth. Machine settings
are now incidental regardless of the wave machines being used. Optimization
of dwell time was achieved, as was the flexibility to assemble the
board on virtually any wave machine with predictable quality.
For the consumer
electronics company which preformed this study, this has meant meaningful
wave solder process documentation, clearer instructions to operators,
greater flexibility since the board can now be run reliably through
any wave machine, fewer spikes in the process control chart since
measurements are made before boards are run, less downtime, higher
throughput, reduced pressure on process engineers and happier management.
As we all know, these are the natural results whenever specific
process improvement procedures are successfully implemented.
- What was
discovered to be the optimal dwell time was significantly different
than that which had been occurring but was heretofore unmeasured.
- Defect rates
vary sharply with different dwell times.
immersion depth was critical to this study, since varying immersion
depths mean varying contact lengths and, as a result, uncontrolled
Prior to this
study, yield loss was tracked on a monthly basis as a measure of
the cost of production failures. Production volume for the board
studied was 11,000 per month.
- With the
implementation of optimal dwell time procedures using the described
device, yield loss went from 3.0% (330 boards) to 1.6% (176 boards)
in the first month of daily use.
- This meant
a reduction in yield loss of 154 boards per month. For a 30 day
month, this means 5.13 boards per day.
- At $300 for
the cost of each board, cost reduction based on improved yield
loss alone was $46,200 per month, which annualizes to $554,400.
- That means
that the return on investment on the device used in the study
was less than five days.
A seemingly small daily improvement in wave solder quality meant
fast, very large, measurable monthly and annual cost savings. These
figures do not even account for the valuable savings from benefits
like reduced rework and field failures, less downtime and increased
throughput, each of which in its own can be more valuable than the
rapid savings on yield loss.
Just as each
board type enjoys its own thermal profile in your surface mount
oven, each board type also enjoys its own board-wave parameters
in your wave machine. Hence, the above study was also performed
for a second board type.
are recorded as the red line in figure 3. The optimal dwell time
for that board was found to be 3.6 seconds, in contrast to 2.8 seconds
for the first board. As you can see, the "dwell time profiles"
of the two boards are different. This process resulted in dramatically
lower defect rates for the second board studied (which had also
been previously run at 1.0 seconds), although never quite as low
as the new baseline which was attained for the first board. This
strongly indicates the presence of sources of defects unrelated
to dwell time, for example non-optimal immersion depth or design
on image for a larger view!
4 The second study's immersion depth and defects results.
previously and shown in Figure 1, changing your immersion depth
changes your contact length and dwell time. This makes the direct
and accurate measurement of immersion depth critical. Your pump
speed produces a wave height (although this can diminish as your
solder pot empties of solder), but the actual immersion depth of
your boards depends on several factors, including solder pot height,
how they sit in the fingers, if your fingers are bent, broken or
crooked, the angle of your conveyor and whether or not pallets are
your immersion depth - measuring it and keeping it consistent -
is only one piece of the puzzle. Another is: At which immersion
depth is your board quality optimized? This point is illustrated
by figure 4. See that the defect rate of the board represented by
the blue bars is optimized over a different range (48 mil, or even
36 to 60 mil) than the board represented by the yellow bars (at
24 to 36 mil). So, different board types benefit most from different
Prescription for Optimization
of board-wave optimization are significant, and call for a board-by-board
assessment when determining wave solder guidelines. Direct measurement
and management of what your boards are actually experiencing is
the key. Using the same wave machine settings for all boards can
never produce optimal wave solder results across a range of assembly
types, and reliance on wave machine settings does not ensure repeatability
of board-wave interaction.
requires adjustments that are correlated with the actual defects
on the board. Just recording the machine setting will not produce
the results you want; neither will exclusive focus on your board-wave
data. As we already know, wave machines are not necessarily repeatable.
If they were we would have already found the right settings and
have no defects or even spikes in defect rates.
is simple. It will only take literally a few minutes and produce
information that will help you immediately:
- Set your
wave machine as you always would for the board with which you
- Once you
have measured and established board-to-wave parallelism, make
note of your dwell time and immersion depth readings.
- Run one of
your boards and make note of its wave solder quality.
- As a first
step toward identifying your optimal dwell time, decrease your
conveyor speed 0.75 feet per minute and then measure your new
dwell time reading.
- Run one of
the same board types again and make note of its wave solder quality.
If your board
quality improved that means that you have identified a dwell time
that is superior to that at which your have been running your boards.
You can now take dwell time readings every day prior to each run
of this board type, to ensure that your wave machine is delivering
the desired, superior experience to your boards. Hence, you use
the data to assure and document BOTH repeatability AND optimization.
If your board
quality became worse, then increase your conveyor speed and follow
the same procedure. You will quickly identify your optimal board-wave
interaction parameters. To assess the impact of immersion depth
on your wave solder quality, vary your pump speed and otherwise
perform the same procedure.
aspect to understand is process windows. All wave machines have
their own normal range of data variance and repeatability. This
can only be identified by repeating direct measurement of dwell
time and immersion depth while the machine is kept at each setting
you use. Understanding your wave machines process windows
for dwell time and immersion depth (and parallelism for that matter)
will help you to optimize your wave solder process for each board
If you are a
high mix shop, start with your most common or most troublesome boards.
For high volume, low mix operations, you have the opportunity to
optimize each board you run. For both types of facilities, there
is the added flexibility of reliably moving production of a particular
board between different wave machines and even plant locations.
have combined commercially available technology with simple procedures
to optimize their boards dwell times, control immersion depth
and enjoy true repeatability of their wave solder process. For facilities
which want to cut costs quickly and keep pace with industry norms
for wave solder quality, the technology and procedures reported
here are worth investigating.