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Ionic Cleanliness Testing
GEN3 CM Series cleanliness tester.
By Gregory Alexander, CTO, Ascentech, LLC
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 ROSE Test (Resistivity Of Solvents Extracted) is a quick and easy test that takes no more than 15 minutes, on average, to perform. This test can be directly used to control the cleaning process, because the test results will indicate, quickly and easily, 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 upon a circuit board, electronic component or assembly that can adversely affect the intended electrical performance. Thus, ionic cleanliness testing (ROSE) is a process optimization tool and a good way to ensure that electronic products will be robust and reliable in the field.
Lead-free soldering, continuing circuit miniaturization, and ever more hostile operating environments for electronics conspire to demand cleaner assemblies. Typical service environments expose the circuit to humidity and, with the presence of an electrical bias, excessive ionic contaminants on an assembly will cause problems such as shorting between board traces from 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 that much sooner. More compact assemblies with smaller clearances mean that they 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.
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 wavesoldering conveyors). And remember that when we talk about "Cleanliness" testing, we're only talking about ionic contaminants, not overall "cleanliness" of the board. There are many other types of contaminants, such as surfactants and even dirt, that can be present on a board that have no ionic reactivity.
The most popular choice among manufacturers moving to lead-free soldering is Sn96.5Ag3.0Cu0.5. This ternary alloy melts at a temperature of 219°C — far higher than the 183°C 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 soldering process, dirty boards, that could have once been soldered by using a more aggressive flux, now can't be tolerated.
PC board being lowered into the cleanliness tester.
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.
No Clean Issues
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 electrochemical reactions. If you want to apply a conformal coating and you want to be sure it remains attached, you must ensure board cleanliness. In "Circuit Board Ionic Cleanliness Measurement: What Does It Tell Us?" Dr. Jack Brous states: "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."
Contaminometer systems use a test solution that is a mixture of IPA and de-ionized water polished though use of a mixed resin filter bed comprising chelate, cation, and 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/cm
≡NaCl. The test solution temperature and resistivity value at start should be tared (zeroed).
Alcohol and de-ionized water are used, 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 percent propan-2-ol (IPA) with 25 percent de-ionized 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. Today's 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 then is 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 micro-Siemens (µS).
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's design avoids polarization effects between electrodes that might otherwise occur when using DC test currents. Error signals caused by DC and AC currents are eliminated.
A contaminometer using ROSE testing is able to measure contamination levels on bare boards and assemblies quickly, accurately and reliably. Information is presented graphically and 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 whether an assembly has been 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 manufacturing is pressured by the need to adhere to legislation, stringent process controls, the need for high throughput of quality products — coupled with a low consumer tolerance of failures, the contaminometer is a reliable tool that makes the job easier. And yet, more work needs to be done to further develop both the test and the testing systems to meet emerging challenges.
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