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Ionic Cleanliness Testing: Ultimate Process Optimization Tool
November 13, 2012 |Estimated reading time: 7 minutes
Editor's Note: This article was originally published in the September 2012 issue of SMT Magazine.The ROSE TestCleanliness testing of PCBs, once of critical importance during the reign of highly-activated fluxes, has become vital again with the conversion to lead-free soldering alloys and the threat of tin whisker and dendritic growth occasioned by the use of high-tin content solders.
Cleanliness of PCB assemblies is easily and accurately measured through the use of a contaminometer, a test machine that complies with IPC-TM-650 2.3.25, the resistivity of solvents extracted (ROSE) test. This quick and easy test of no more than 15 minutes, on average, can be directly used to control a process, because the test results will indicate that a process is either healthy, or veering out of control. The ROSE test method is designed to determine the proportion of ionic residues present on a circuit board, electronic component or assembly that are deleterious to the intended electrical performance. Thus, ROSE ionic cleanliness testing is a process optimization tool and a good way to ensure that electronic products will be robust and reliable in the field.
Why is ROSE Testing Needed?
Lead-free soldering, continuing circuit miniaturization, and evermore hostile operating environments for electronics conspire to demand cleaner assemblies. Typical service environments expose the circuit to humidity and, with the presence of electrical bias, excessive ionic contaminants on an assembly will cause problems such as shorting between board traces due to electrolytic dendrite growth, erosion of conductors, or loss of insulation resistance. Increased miniaturization means shorter spaces between component leads; tin whiskers have a shorter distance to grow, and thus cause failure of the circuit sooner. More compact assemblies with smaller clearances are tougher to clean, and tougher to inspect for residues such as entrapped flux.
If you want to know how robust your product is before it goes into the field, you need to know how clean it is. If it isn’t as clean as it should be, then there is a problem upstream in the manufacturing process that needs correction, so that high or unacceptable levels of ionic contamination are not present on an assembled PCB when it’s shipped.
Sources of Ionic Contamination
Common sources of ionic contamination include etching, plating, tinning or leveling residues, poor soldermasks, undercured permanent or temporary solder masks, dust, moisture, oil pollution from finger prints, component packaging materials, and machine maintenance oils (especially from wave soldering conveyors). And remember that when we talk about “cleanliness” testing, we’re only talking about ionic contaminants, not overall cleanliness of the board. Many other types of contaminants can be present on a board, such as surfactants and even dirt, that have no ionic reactivity.
Lead-Free Soldering
The most popular choice among manufacturers moving to lead-free soldering is Sn96.5Ag3.0Cu0.5. This ternary alloy melts at 219ºC--far higher than the 183ºC of eutectic tin/lead. The implications of this higher melting point are many, but in essence, the effect is to almost vitrify the undesirable residues and thereby increase the cleaning challenge. Because there is much less tolerance in the lead-free soldering process, dirty boards that could have once been soldered by using a more aggressive flux can no longer be tolerated. Measuring the cleanliness of bare boards with ROSE testing ensures that those entering the process have the best chance of soldering without problems. The test also provides good feedback of many other process parameters, such as how well the storage process is working. Test results will quickly identify trends in the manufacturing process that can be altered before they become a problem. For example, if flux composition begins to stray from the optimum, the residues on the board will begin to change. The sensitivity of the contaminometer is such that this change will be detected well before soldering is affected.
Keeping “No-Clean” Honest
ROSE testing is required by DOD/MIL/IPC/and most customer specs, but more importantly, it is a powerful process optimization tool because undetected ionics (salts) on PCBs can lead to adverse electro-chemical reactions. If you want to be sure your conformal coating remains attached, you must ensure board cleanliness. As Dr. Jack Brous summarizes in “Circuit Board Ionic Cleanliness Measurement: What Does It Tell Us?” [1], "This (ROSE) test can be used as a periodic check of the ability of the ‘no-clean’ process to leave residue amounts in a consistent range below levels that can seriously affect electrical characteristics. Significant increases of ionic levels, in a periodic testing program, would then indicate changes in the process which result in heavier residue levels and their associated effects on the electrical characteristics of the board surface."
Test Solutions
Contaminometer systems employ a test solution that is a mixture of isopropyl alcohol (IPA) and deionized water polished via a mixed-resin filter bed comprising chelate-cation-anion resins. The resistivity of the test solution is measured before, during and after the test. The results are calculated to an equivalency factor of salt expressed as: < x μg/cm2 Ξ NaCl. The test solution temperature and resistivity value at start should be tared (zeroed).
Alcohol and deionized water is employed because salts dissolve in water and alcohol dissolves substances that are not readily water-soluble (such as rosin-based fluxes). The ratio is essentially 75% propan-2-ol (IPA) with 25% deionized water. But there are arguments in favor of a 50-50 ratio of IPA to water.
ROSE Tester Operation
While the science behind contaminometers is complex, operation needn’t be. This is especially important if the equipment is going to be used as a process monitoring tool. In this situation, the machine likely will be operated by unskilled personnel. Modern machines are designed so that the only manual task is to insert the PCB at the beginning of the test and to remove it at the end. All other test cycle operations are automated.
Typical testing starts with tank fill and solution preparation. The solution is pumped through a mixed-bed ion-exchange column until it reaches ultra-low conductivity. It is then homogenized. A mixed-resin filter strips out ionics as they pass through the medium. The test tank and its contents are cleaned to a determined conductivity level, expressed as microsiemens (μS).
To test, an operator inserts the test piece, causing a volume of solution to overflow into a calibrated tank for measurement; the solution is pumped across the test piece via the measuring cell and the rise in conductivity is monitored. The test ends either at a preset time limit or when the conductivity level rises less than 1% of the absolute value over a period of 48 seconds. Results are processed and analyzed via onboard computers.
The dissolved ionic substances alter the conductivity of the test solution; the test equipment precisely measures the change and expresses it as μg/cm2 Ξ NaCl equivalence. The measurements are made in accordance with IPC/ANSI-J-STD001D and UK DEF-STD, and other international specifications.
A contamination test system uses either a static or dynamic test method, but the terminology “open loop” versus “closed loop” would be more appropriate. Static or open-loop testing takes a predetermined volume of solution to carry out the test. Dynamic or closed-loop testing recirculates the total volume of solution to a given surface test area. In operation, the tester automatically repurifies the solution each time a new test is run, using a regeneration or deionizing cartridge.
Some machines use a solid gold measuring cell, ballistic amplifier, and a vigorous pumping system to achieve measurement accuracy at low conductivity values. The machine is designed to avoid any polarization effects between electrodes that might otherwise occur when using DC test currents. Error signals caused by DC and AC currents are eliminated.
Conclusion
A contaminometer using ROSE testing is accurately and reliably able to measure contamination levels on bare boards and assemblies quickly. Information is presented graphically, and it can be used for statistical analysis and process optimization. The degree of contamination directly correlates to the likelihood of a bare board successfully soldering or an assembly being soldered at less-than-optimum process parameters. Test results also can indicate whether the assembly is likely to suffer a field failure when exposed to conditions that promote growth of dendrites.
In an age where adherence to legislation, stringent process controls, high throughput of quality products, and low consumer tolerance of failures pressure manufacturing, the contaminometer is a reliable tool that makes this job easier. More should be done to further develop both the test and the testing system to meet emerging challenges.
References: 1. Dr. Jack Brous, “Circuit Board Ionic Cleanliness Measurement: What Does It Tell Us?” 1994. 2. Graham Naisbitt, “Cleanliness Testing on the Shop Floor,” SMT Magazine, March, 2008. 3. Graham Naisbitt, “How Clean Is Clean? Cleanliness Testing Moves from Lab to Shop Floor.” 4. Randy Allinson, Ascentech LLC, “Help With Ionic Cleanliness Testing – 101.”Gregory Alexander has spent 15 years in electronics design and manufacturing for military and medical instrument companies. For the past 20+ years he has been in sales and marketing roles for Tektronix, Mentor Graphics, and, most recently, Probot as director of sales. He is currently a partner and CTO for Ascentech LLC, and is an active member of IPC standards committees for solderability, SIR and CAF test, and ionic contamination test.