Wave Solder Process Parallelism
Frontiers in Wave Solder Optimization:
Measurement of Parallelism Will Transform Your Board Quality
Ingall and Nissim Sasson
extensive study ever conducted on wave solder process control, involving
384 wave machines worldwide, found 76% of all wave machines running
boards disparallel through the solder wave. 84% of the plants that
corrected their disparallelism problem reported what they condsidered
significant improvement in board quality the day the correction
A shorter version
of this study was published in the July 1998 SMT magazine.
Did you ever
want to kick your wave machine? Do you wonder why you still have
inconsistencies and rework despite doing "all the right things."
thermal profiles. You've tried no-clean. You have a maintenance
schedule. You tried a new solder. You bought an air knife. Steps
such as these can be important to effective wave soldering, but
there is more.
So what's wrong?
Why is wave soldering considered a weak link in your assembly line?
Why is it so frustrating for technicians to control the wave? Why
does management lower its quality, throughput and profit expectations
by accepting the high cost of rework of PCBs?
Inspection are Futile
and repair are due to production failures. Rework is expensive.
First is the large direct cost of the rework operation. Second,
the prevailing defect rate now considered "acceptable"
causes a throughput bottleneck at the rework stations. Third, rework
of defects results in joints with shorter lives than those successfully
made in the wave solder environment. Fourth, defects lead to field
failures that can only damage the reputation of the product and
eventually confidence in the assembly plant.
equally futile. The inspector is at best sampling the quality of
the joints on each board and, from the external appearance of the
joints, assessing the chances that a satisfactory result has been
achieved on all of them. The idea that an assessment can be made
of every joint is obviously ridiculous. The responsibility for quality
PCBs lies with the process, not the inspector.
85% of the PCBs
assembled today run through a wave machine as either mixed technology
or pure through hole. Clearly, improve your wave soldering and
you improve your entire assembly line. The study results presented
in this article show that this is easily achieved. You don't have
to live with your wave solder defect rate. Commanding progress can
be made right now.
Link: Board-Wave Interaction
conventional wave solder process control virtually ignored board-wave
interaction. By this we mean actual measurement of the physical
interaction of your board with the wave. This interaction has four
distinct, simultaneous facets, all of which can be directly and
accurately quantified: Parallelism, Dwell Time, Immersion Depth
and Contact Length.
A major conclusion
of the data you will see here: Your wave solder quality is vastly
improved when board-wave interaction is optimized and controlled.
Temperature and chemistry control are not enough. Board-wave interaction
occurs independently, having its own separate set of parameters
which are generally unaffected by temperature adjustments and chemistry
In the reflow
process, chemistry supports your board in its thermal experience
in the oven. Not true for the wave solder machine. In wave soldering,
chemistry and temperatures are supporting actors in delivering your
boards to the central event of the wave machine: Your board's interaction
with your solder wave. This has been the missing link in wave soldering
until recent years, when technology became commercially available
to accurately measure this event. Such technology permits a new
understanding of wave solder optimization.
Given all these
factors and the complexity of today's boards, it's common sense
to directly measure, optimize and control the interaction of your
boards with your solder wave. Why? Because seemingly small adjustments
- for example, a 24 mil change in immersion depth can have dramatic
quality consequences for today's boards. Human visual observation
(and reflex, if you're using stopwatches) are not reliable for detection
of what are, for your boards, highly significant variations.
measurement of board-wave interaction as the basis for adjustments
to your wave machine to ensure the highest quality product. Relying
on your wave machine settings alone leaves you in the dark, since
your boards themselves do not have a conveyor speed, pump speed
or solder pot height. Those are wave machine settings and, while
necessary, do not in and of themselves tell you what experience
you are delivering to your boards.
What you will
read here has never been published before, yet its principles have
already been implemented in hundreds of PCB assembly facilities
throughout the world. For this initial article we have chosen from
among our studies to focus on parallelism as an illustration of
these principles. Our purpose:
- Define parallelism
in a new way so that it is meaningful for wave solder optimization.
- Present data
and case studies drawn from numerous PCB assembly facilities.
is the most widely recognized parameter which occurs when a board
meets the solder wave - and the most misunderstood. Our industry
wide convention is to define parallelism as rail-to-wave parallelism
or conveyor-to-wave orientation. Both terms mislead the wave technician
and engineer disastrously.
term and practical intention should be board-to-wave parallelism.
This is because the alignment of your rail to your solder wave is
only one of many factors affecting your real goal, which is that
your board be parallel to your wave.
Whether or not
your board will be parallel as it passes through your solder wave
is determined by numerous critical factors, among them: Loose, bent
or broken fingers (often so slightly bent as to be difficult to
spot with the human eye but severe enough to cause pervasive bridging
or skipping), dross clogging part of your nozzle, unlevel solder
pot, unparallel rails and improper placement of the board on the
fingers. And then there's the hidden criminal: a crooked solder
ramp causing your wave to collapse more quickly on one side. So,
measuring parallelism by putting a carpenter's leveler on your rail
is both helpful and at the same time completely inadequate.
on image for a larger view!
1 Example of board disparallelisn in the wave. Although
full contact with the solder is achieved across board width,
slight skewing can cause skipping on the shallow side and bridging
on the deeper side.
using glass to check "full coverage" by solder of the
width of your board. Such visual assessment leads to defects, as
it relies on the human judgement of a moving object passing over
a wave for only a few seconds. Figure 1 shows how a PCB can be dramatically
disparallel and still make full contact with the solder across its
width. Defects are inevitable because one side of your PCB experiences
a much shallower immersion depth as a result of the disparallelism.
This often causes persistent skipping on that side of your board
and bridging on the other.
element of our approach was the performance of a baseline study
which identified 0.2 seconds as a threshold measurement for parallelism.
This preceded our larger study involving 384 assembly plants, and
was achieved by collection of the data shown in the accompanying
graph. Three separate board types were run through the same wave
machine with no changes to the fluxer, preheat, conveyor speed,
solder pot height and pump speed settings.
For both our
baseline study and subsequent larger study, a commercially available
device was used which accurately simulates the user's own circuit
boards. Unique contact sensors which directly measure the physical
experience of the leads in the wave were employed. Do not confuse
this with previous attempts to extrapolate from temperatures obtained
from a thermal profiler or visual observation with a glass plate.
The actual parallelism
measurement of this device is the difference between the dwell time
in the solder wave of leads on the left and right side of the device.
If your board is parallel to your wave, then the dwell time of the
leads on its left side will be equal to those on its right hand
After each set
of runs, rails were adjusted to cause changes in parallelism readings
in 0.1 second increments, from 0.0 seconds to 0.7 seconds. Defect
rates for each board were recorded on each run, with their collective
range shown on the accompanying graph.
study data tells us a few things, some familiar, some not.
on image for a larger view!
2 How defect rates increase in concert with degree of board
disparallelisn (alignment across solder wave). The 0.2 sec level
is crucial, but dwell time, immersion depth and contact lenght
are equally important parameters of board quality.
a significant jump in disparallelism-caused defects occurs when
your parallelism reading is over 0.2 seconds.
defect rates increase as the extent of disparallelism increases.
As we shall see, the 0.2 second benchmark arrived at from our baseline
study proved extremely reliable in the larger study. Certainly there
are boards which demand a tighter parallelism window, say 0.1 seconds
or less, just as there are boards which well tolerate disparallelism
at even 0.4 seconds. It is your own board which holds the answer
for itself: At what parallelism reading does your own board quality
start to decline?
The third lesson
takes the form of an admonition: Even if your board is parallel
to your solder wave, you're still left with other, equally critical
challenges of board-wave interaction. Your dwell time can be too
brief or too lengthy, immersion depth too deep or shallow and contact
length too long or short. Different board types have varying causes
of defects along with different quality issues, parallelism being
only one among them. On the other hand, trying to ameliorate disparallelism-related
defects by adjusting temperatures is completely futile, as one has
no relationship to the other. So the larger subject of board-wave
interaction must always be a top priority.
observation: If your board is not parallel, you cannot attribute
to it a dwell time or an immersion depth. Put differently, if one
side of your board is deeper in the wave than the other, your board
is experiencing a different immersion depth every micro-inch across
its width. Therefore, parallelism is the prerequisite for control
of the other three critical facets of board-wave interaction, namely
dwell time, immersion depth and contact length.
baseline study, we proceeded to collect parallelism data from 384
PCB assembly facilities over a nineteen month period. With such
a large sampling, this study cuts across virtually the entire North
American PCBA community - OEMs and contract manufacturers with high
and low volume and high and low mix alike, performing pure through
hole and mixed technology, with single and multiple wave machines
and locations, with and without ISO 9000 certification. Data was
obtained from users of virtually all wave machine configurations
and chemistry types.
all of these plants were already exercising some form of process
control on their wave machines, including thermal profilers, profilers
on carrier boards, temperature stickers, glass plates, stopwatches
and wave height measurers. The device used in this study, its technology
and patented design had not been previously used by any of the study
of four separate runs through the wave machine without any adjustment
of the wave machine or its settings was required for inclusion in
our study. Multiple measurements were performed easily since data
was displayed on the device's liquid crystal display without delay
upon exit from the wave machine. Equally critical was that runs
could be made immediately in sequence with no affect on the parallelism
readings, since the direct contact sensors have no relationship
to and are unaffected by temperatures. Without such product features,
a study of this scope would simply not have been possible.
Our study involved
four steps on the part the manufacturing or process engineer:
- Before taking
any measurements, identify the defect rate/quality of the boards
which are being run.
- Without making
any changes to the wave machine, perform four measurement runs.
The runs performed by each assembly facility easily determined
whether or not their wave machine was delivering their boards
parallel to the solder wave. The criteria was simple and came
from the baseline study results: When all four runs showed a parallelism
reading of 0.2 seconds or less, successful parallelism was recorded.
When all four runs showed a parallelism reading of 0.3 seconds
or greater, disparallelism was recorded.
whose data showed a disparallelism attempted to correct the problem
and then performed an additional set of parallelism measurements.
This showed if the maintenance and/or troubleshooting done on
the wave machine actually fixed the disparallelism.
- After parallelism
had been successfully achieved, production of the PCBs through
the wave machine was resumed. Their PCB defect rate/quality was
then compared to the defect rate/quality observed prior to ever
taking a parallelism measurement (step 1).
of the engineers to respond quickly to data was aided by important
features of the device used: Electronic identification of whether
a disparallelism is to the left or the right, along with a visual
depiction of the disparallelism. As a result, practically all facilities
completed the above four steps in less than 1 hour.
on image for a larger view!
3 Results of corrections taken by 283 facilities to achieve
a parrellelism reading from -0.2 seconds to + 0.2 seconds results
in quality improvement at 84 percent of the sites.
in all respects are astonishing and demonstrate that vast improvement
in wave solder performance is readily available to the majority
of PCB assembly plants:
- Of the 384
PCB assembly facilities, 291 (76%) found their board to be disparallel
in their solder wave the very first time a set of measurement
runs was taken.
- Of the 291
facilities which found their board to be disparallel in their
solder wave, 186 (64%) were able to successfully correct their
disparallelism in the first round of maintenance/troubleshooting,
while 105 (37%) did not successfully address their disparallelism
in the first round of maintenance/troubleshooting.
at virtually all of these 105 facilities stated that without the
data from the second set of parallelism measurements, they would
have restarted production.
- Of these
105 facilities, 97 (92%) were able to readily obtain a successful
set of parallelism measurements after additional maintenance/troubleshooting.
- This means
that 283 (97%) of the 291 facilities which were disparallel at
their initial set of measurement runs were able to correct the
problem to obtain a successful set of parallelism readings at
0.2 seconds or less.
- These 283
facilities then resumed production of the same board for which
they had identified the defect rate/quality before taking any
- Of these
283 facilities, 238 (84%) reported what they considered significant
improvements in board quality that very same day, thereby validating
the preceding baseline study.
There is a critical
distinction between defects caused by board wave interaction versus
defects caused by temperatures. For example, disparallelism can
cause bridging on one side of your boards. No amount of temperature
adjustments can remove such a defect. This is because the defect
is caused by bad board-wave interaction, namely disparallelism.
You can adjust your temperatures forever and you will never get
rid of this defect; your company will always bear its expense and
you its consequences.
More than 85%
of the companies which ran the board-wave interaction device used
in this study had been using a very different technology known as
a thermal profiler for years. Yet, despite excellent thermal profiling
and harmonization of temperatures with their flux type, they still
were experiencing persistent wave solder quality problems and inconsistencies.
Our studies, including that on parallelism presented here, strongly
suggest that most wave solder production failures are due to lack
of optimization of the four board-wave interaction parameters.
adjustment of parallelism cannot remove defects caused by temperatures.
For example, the cracking of components caused by a too high maximum
preheat slope will never be solved by adjustment of your parallelism.
So, just as temperature measurement is essential to the control
of temperature-related defects, measurement of board-wave interaction
is essential to the control defects related toparallelism, dwell
time, immersion depth and contact length.
of this principle only comes with the ability to perform direct
board-wave measurements. That's why 84% of the plants in this study
were so easily able to improve their wave soldering: Previously,
they were not able to make such measurements and therefore could
not react accordingly.
of board-to-wave parallelism is vital to wave solder optimization.
Ensuring that PCBs are run only when direct measurements confirm
board-to-wave parallelism causes significant improvement in board
quality. Learning to obtain such data requires little or no training
and can be performed in minutes. When data shows a disparallelism,
corrective action resulting in significant improvements can be made
facilities which do not perform such measurements have a 76% likelihood
that their boards are not parallel to their solder wave. That's
the bad news. The good news is that 84% of these facilities can
improve their wave soldering immediately simply by ensuring parallelism
through direct measurement of board-wave interaction. When presented
with the proper tool, this study shows that board quality improvements
are significant, ubiquitous and immediate.