Background information from one who participated in the development of the
equivalence factors for the Navy.
CLEANING SMT February 1998
IONIC TESTING: A PRIMER.
By William G. Kenyon
Industry users continue to ask about the ionic test method that was first
published in MIL-P-28809 and now appears in the J-STD-001B. As one of the
people involved in the development of the equivalence factors, I decided to write
this column as a “public service” for both experienced and new members of
our industry.
T. F. Egan of Bell Labs was looking for a way to determine if gold plated
parts were clean enough for spot welding. He devised a test method that
immersed a given number of such parts in deionized water, measuring the
conductivity of the water before and after the parts were introduced. Because the
parts were identical, limiting controlling the number of parts controlled the
surface area exposed to the DI water. Since this test was only concerned with
plating salt residues, the use of DI water, pre-equilibrated with air to
eliminate the effect of carbon dioxide, was a good choice. Egan looked at the
delta C in the conductivity before and after the test. If there was less than
1.0 micromho/cm increase for an area of 5 sq. in., the parts were clean
enough to go to the production line. If not, they were re-rinsed until they
passed, plus the rinsing steps in the plating line were checked to make sure
they met process standards, with corrective action taken as needed.
In the early 1970s, the military was encountering numerous failures in
radios and other conformally coated electronic hardware deployed in Southeast
Asia. After investigation, the Navy lab in Indianapolis found the cause to be
the ionic residues under the conformal coatings. These residues would react
with the water vapor that permeated the coatings, which turned to liquid if
conditions dropped below the dew point. The ionics, dissolved in liquid
water, could conduct current under the coating to cause the failures. Since the
organic conformal coatings are all permeable to water vapor, the key was to
eliminate, or significantly reduce, the ionic residues. The various phases of
this project were published as Material Research Reports (MRR). First, the
flux residue had to be dissolved away from the assembly surface so the ionics
could be measured. The Navy
group tested a number of 2-propanol/deionized water mixtures for their
ability to dissolve the solvent cleaned rosin flux residues found on military
hardware of that era. The best solution was a 75/25 volume/volume
2-propanol/deionized water mixture. Next, the team had to determine how to wash the test
boards and what pass/fail limits should be used. So this phase was based on
the Egan test. Thus a given volume of test fluid was applied via a wash
bottle per given area of PWA surface (10 milliliters per square inch). The
solution was purified by passage through a resin bed so the resistivity was higher
than 6 megohm-cm. After the washings had been collected in a clean vessel,
the final resistivity was measured. It had to be above 2 megohm-cm.
Titration of the test solution with standardized NaCl solution showed that ionics
equivalent to 1.26 micrograms of NaCl/sq. cm. of PWA surface would drop the
resistivity from 6 to 2 megohm-cm. The starting and final resistivity
numbers were developed using the levels on PWAs that performed satisfactorily in
field service. At this point the Navy had developed a standardized test
solution to remove a known flux residue, plus a test method to measure the ionics
and a pass/fail criteria based on field data (MRR 3-72). The method issued in
MIL-P-28809; Printed Wiring Assemblies, March, 1975. (The spec first issued
as MIL-P-XXXXX or “MIL-P-Quint X”, to use the verbal shorthand of the
time.)
The flux makers developed commercial instruments for automating the
cleanliness test found in MIL-P-28809. First the Ionograph™, providing data in terms
of conductivity, then the Omegameter™, using resistivity. At that time, all
military work was done within the military specification system, so any
change had to be authorized by a waiver or deviation. The military allowed a
single waiver for use of the Ionograph™ (see MRR 5-75) instead of the wash
bottle technique, then later a corresponding waiver was developed and issued for
the Omegameter™ (see MRR 10-76). Other military contractors wanting to use
these instruments would then cite this documentation in applying for their
waiver. The paperwork for waivers soon got so out of hand that the Navy lab in
Indianapolis had to develop a method to eliminate the waivers, thus allowing
contractors to use any of several methods to comply with the cleanliness test
requirements in their contracts.
This was the beginning of the equivalence factor work, which has caused a
great deal on confusion. At the time, the Navy, realizing that component area
was quite significant, developed component area measurements for all the
components used for military work. The user would then just measure the length
and width of the PWB, multiply to get the area of one side, double it to get
the area of both sides and then add the sum of the component areas to get the
full surface area of the PWA to be tested. The Navy lab developed a series
of test boards and two lab fluxes that would simulate RMA and RA fluxes for
use in a test plan to qualify the various commercial test instruments for
general use. An expert group, including the inventors of the commercial
instruments, was invited to Indianapolis to help review and interpret the resulting
data. The activator level in the RMA flux was really in the RA range, so we
were faced with a single data set instead of the planned two sets. The
instruments included two resistivity/conductivity bridges (Beckman and Markson),
the Ionograph™, the Omegameter™ and the non-commercial Ion Chaser. Since the
military assemblies of the time included examples of all the different
components, we pooled the data for each instrument to determine the average
contamination level for all test boards for a given instrument. Next, the averages
for the two bridges were averaged, so we now had four averages. The average
for the bridges was chosen as the standard, since the bridge was used as the
original measurement instrument. The data sets were then normalized by
dividing the average for each instrument by the grand average for the bridges.
This gave the “equivalence factors” for each instrument, with the bridges set
equal to 1.0. Next, each equivalence factor was multiplied by 1.56
micrograms NaCl/sq. cm. to give the pass/fail limit for that instrument equal to
the Mil-P-28809 limit of 2 megohm-cm. Then, the resulting numbers were
multiplied by 6.452 to convert them to the values needed to express the pass/fail
limit or instrument acceptance limit” when the area was measured in square
inches. (The higher 1.56 micrograms NaCl/sq. cm. number instead of the
original 1.28 came into use when I modified the electronics in an OmegaMeter 300 to
give a 3X scale expansion on the digital resistivity meter. This showed that
the clean solution was really much higher than the 6 megohm-cm used to get
the 1.28 micrograms NaCl/sq. cm. value.)
The details of the equivalence factor study can be found in MRR 3-78. The
report concludes: “All of the instruments tested are suitable for
determining ionic contaminants remaining after solvent cleaning of post-soldering
rosin-flux residues as described in MIL-P-28809 providing the interim equivalence
factors are employed.” Meanwhile, W. B. Wargotz of Bell Laboratories had
been studying the deleterious effect of NaCl residues on telecom boards and
subjecting them to the 40 year life test. Wargotz reported at the Fall, 78 IPC
meeting that levels of less than 1 microgram NaCl/cm2 passed the 40 year life
test, while levels of 7 micrograms NaCl/cm2 or higher would fail. The 1-7
micrograms NaCl/cm2 range was a grey area. Above 7 was a guaranteed failure
area. However, the correlation of this laboratory data with the empirical Navy
data validated the pass/fail limit established in MRR 3-72 and confirmed in
MRR 3-78.
At this point the industry had a valid test method for measuring ionics
from solvent cleaned post-soldering rosin-flux residues. The question of
validity of the interim equivalence factors was addressed by an experimental study
conducted by the makers of the Omegameter™. The study concluded the
equivalence factors were applicable to newer models of this instrument. At this
point the equivalence factors became fixed.
Northern Telecom investigated the validity of using ionic testing for
water-soluble fluxes. Their findings, presented at the IPC by NT’s T. Duyck in a
report on their Deltameter, showed a dramatic difference in release rate of the
residues of rosin fluxes and water soluble fluxes from the surface of the
test boards. In the typical 10-15 minute test period, rosin flux residues
were about 90% percent extracted, while it took about two hours to achieve the
same level of extraction with water-soluble fluxes. Thus, using the
standard ionic test method with water-soluble flux residues gave numbers that were
very low, but not representative of the residues that were really present.
Synthetic Activated (SA) fluxes were tested for release rate and found to be as
fast or faster than rosin fluxes. The standard method was shown to be
applicable to SA and rosin flux residues, but not to water-soluble flux residues.
Later, when the ozone depleting solvents used in the electronics industry
were phased out, the IPC Cleaning & Coating Committee re-examined ionic
testing to determine the proper test solution and pass/fail limit appropriate for
residues from alternative soldering/cleaning processes, including no clean.
The reports of this task group are available from IPC.
The ionic test is still valid today for rosin flux residues, provided the
equivalence factors are used (J-STD-001C). Since many users employ the test to
comply with telecom requirements or to monitor their processes, the
applicability to modern water-soluble and no clean flux residues needs to be
established. Investigations carried out by IPC members showed that high loadings of
adipic acid, a major ingredient in most no clean fluxes, had little
deleterious effect on surface insulation resistance. The ionic test is difficult to
use in this case, since adipic acid is not very soluble in typical solvents
or in water. Advances in water-soluble flux formulations to eliminate the
polyethylene glycols that degrade SIR provide us with a new set of residues that
need to be assessed. The ionic test has been such a valued process
monitoring tool in addition to providing reliable electronics that the industry needs
to press for the research needed to keep it current with today’s soldering
and cleaning technologies.
[Had many requests for the full text of Feb98 column vs the shortened
version actually published, here is what was cut from that Feb. col.]
IONIC TESTING: PART 2.
By: William G. Kenyon
Many readers have asked for more information on the ionic testing primer
provided in the February column. I have expanded the text that was cut from
that column for space reasons, so between the two columns, a fairly complete
account is now available. I will also try to answer some of the questions that
have come up in the intervening months.
Q. Why do we use 75/25 alcohol/water?
A. The Navy Lab that developed the test looked carefully at the test
solution, testing 25/75, 50/50 and 75/25 alcohol/water mixtures. They found the
50/50 and 25/75 mixtures did not dissolve all the rosin matrix, thus not all the
ionics were released into the solution for measurement (MRR 5-77). This is
the reason 75/25 was selected as the official test solution.
Q. Why not 50/50 alcohol/water, since it is alleged to be “more sensitive”
?
A. Some workers, ignoring Q&A #1, looked at the larger numbers obtained with
50/50 compared with 75/25, and concluded it was “more sensitive.” In
reality, they were seeing the greater dissociation of incompletely dissociated
ionic materials. In other words, compounds like table salt are completely
dissociated into ions as long as there is minimum water present, while
incompletely dissociated flux ingredients dissociate into measurable ions depending on
the amount of water present. The more water in the test solution, the greater
the dissociation, and the higher the number; thus leading to the claim of “
greater sensitivity”.
Q. How do you comply with MIL-P-28809 with a 50/50 test solution?
A. Electronics makers in Europe often used ionic test equipment designed to
run on 50/50 alcohol/water, which gave significant problems when trying to
comply with MIL-P-28809 limits designed around 75/25. Since the values from
identical test boards run in 50/50 could be as much as 10x higher than the
values from 75/25, some electronics makers were desperate when achieving of only
” 2 micrograms/sq. cm., which in real numbers could be as low as 0.2
micrograms/sq. cm., when the pass/fail limit was 1.56 micrograms/sq. cm. Thus
many electronics makers were either over-cleaning or discarding valuable
electronics hardware that was really quite adequately cleaned.
[In Europe, the Contaminometer was used with 50/50 (ignoring the above) to
compensate for the rather insensitive Myron L conductivity probe used in it.
The probe should be 0.01 but the Myron L had a cheap 0.1 probe which was fine
for plating baths, but not for this work.]
Besides the promotion of the 50/50 as “more sensitive”, there were also
problems with some of the early versions of the test equipment, especially the
test cells. Initially, many of these were made of acrylate plastic and the
corresponding adhesives, which were not compatible with the more aggressive
75/25 v/v test solution. Once this compatibility problem was recognized, the
instrument makers quickly converted to other plastics and the problem
disappeared. [Note: MIL-P-28809 has been canceled. The ionic cleanliness
requirements can now be found in ANSI J-STD-001.]
Q. Some instruments use conductivity and some use resistivity. Why?
A. Conductivity, which is linear, is the reciprocal of resistivity, which is
logarithmic. The Ionograph™ used conductivity, which was also used by T.
Egan at Bell Labs. The advantage of conductivity was that a recent reading that
was twice what an earlier reading had been meant that your PWAs had twice as
much ionic residue on them. Resistivity values were not as easy to
interpret, since we don’t tend to think in powers of ten. The Omegameter™, which
used resistivity, did so in order to automate the original Navy manual ionic
testing procedure. Initially this caused much confusion in the industry, until
enough papers were published that the industry was comfortable with the two
systems.
Q. I’ve got an older Omegameter™ that uses a little slide rule to generate
the actual ionic cleanliness readings. The meter on the machine goes up to
20 megohm-cm, while the numbers on the slide rule go up to 30 megohm-cm. Can
I just modify the circuitry of my instrument to take advantage of the higher
reading capability of the slide rule?
A. A careful check of the slide rule will show that you can paste in some
log paper (if you can find it) to continue the log scale to higher values.
However, the electronics in the read-out module of your instrument won’t track
the log scale graph paper above 30 megohm-cm. If you are planning on doing a
scale expansion from 20 megohm-cm to 60 megohm-cm, then it is best to attach
a precision resistance box to the terminals on the back of the instrument
where the resistivity probe is usually attached. Then track the actual
resistance against the readings to generate a graph of observed readings vs. actual
values.
[This log paper patch worked quite well with my Omegameter™ 300. We did a
3x scale expansion on it as described, I still have a copy of the report with
the circuit design, which was about $200 to build and install. The precision
resistor box is not included in the price. The electronics in the
Omegameter weren’t linear above about 30 megohm-cm. ]
Q. In the February article, you explained “equivalence factors”. Is there a
tabulation of these factors available?
A. The equivalence factors are summarized in this table, taken from MRR
3-78, which concludes: “All of the instruments tested are suitable for
determining ionic contaminants remaining after solvent cleaning of post-soldering
rosin-flux residues as described in MIL-P-28809 providing the interim equivalence
factors are employed.”
Instrument
Average NaCl/in2
Equivalence Factor
Acceptance
Limit, micro-
grams
NaCl/cm2
Acceptance Limit, micrograms
NaCl/in2
Beckman bridge
7.47
1.56
10.06
Markson bridge
7.62
1.56
10.06
Combined bridges
7.545
1.0
1.56
10.06
Ionograph™
15.20
2.01
3.1
20
Omegameter™
10.51
1.39
2.2
14
Ion Chaser
24.50
3.25
5.1
33
Loose Ends
The Department of Defense, realizing that this work needed to be extended to
cover saponifier cleaned rosin flux residues plus water cleaned water
soluble flux, fusing fluid and hot air leveling fluid fluxes, proposed new studies
to determine the proper test solutions and pass/fail limits for the residues
from these technologies. This work was never funded and thus never done (see
Appendix 2, DoD Soldering Standardization Study).
[As I recall, this was published in 1978-1980 timeframe.]
W G. KENYON, an SMT Editorial Advisory Board member, is president of Global
Centre Consulting, 3336 Birmingham Drive, Fort Collins, CO 80526-2336. Tel:
(970) 207-9586; Cell: (970) 980‑6373;email [log in to unmask]
(mailto:[log in to unmask])
PULL QUOTES
The ionic test is still valid for rosin and SA flux residues, provided the
equivalence factors are used.” --End--
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