Great info. Thank you.
Regards,
Ramon
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From: TechNet [mailto:[log in to unmask]] On Behalf Of [log in to unmask]
Sent: Wednesday, October 26, 2005 12:36 PM
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Subject: Re: [TN] Omegameter to IPC-TM-650, Method 2.3.25 cross reference
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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