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Cleanliness Testing on the Shop Floor
December 31, 1969 |Estimated reading time: 8 minutes
By Graham Naisbitt, Gen3 Systems
In Summary
One major threat to military electronics is the growth of dendrites and intermetallics that ultimately cause short circuits and seem to occur more frequently with lead-free electronics. Previously found only in the lab, cleanliness testers are migrating to the production line as manufacturers realize their potential as a process control tool for lead-free assembly.
Previously found only in the lab, cleanliness testers are migrating to the production line as manufacturers realize their potential as a process control tool for lead-free assembly. During the 1970s, the dramatic increase in electronics used in modern weapons was accompanied by a demand for higher reliability. Military planners realized, for example, that a submarine carrying a nation’s nuclear deterrent hardly does its job if it’s holed up in port for yet another round of electronics repairs.
One major threat to military electronics, then as now, is the growth of dendrites and intermetallics that ultimately cause short circuits. In a humid environment and in the presence of an electrical bias, excessive ionic contaminants (ICs) on an assembly can cause problems such as shorting between board traces due to electrolytic dendrite growth, erosion of conductors, or loss of insulation resistance (Figure 1).
Figure 1. Ionic contaminants can cause problems due to electrolytic dendrite growth, erosion of conductors, or loss of insulation resistance.
The solution is to eliminate ICs, since minimizing the humidity or electrical bias in an operating environment is virtually impossible. In practice, completely removing all contaminants is equally impossible, so, in the 1970s, military authorities put a “stake in the ground,” providing contractors with a “Go/No-Go” threshold of acceptable ICs remaining on the circuit. This requirement never changed, although the original threshold subsequently was lowered to today’s 1.5 μg/cm2 NaCl equivalence (IPC-J-STD001D Section 8.3.6.2). With modern densely packed assemblies, this level is almost certainly far too high. Consequently, around 0.2 μg/cm2 commonly is used.
The military standard was seized upon by high-reliability electronics manufacturers looking for a method to underwrite product integrity. However, measuring contamination was a challenge. What these manufacturers sought was a simple, automated tester to perform the measurement. The military developed equipment as a process control tool and used it for the original experiments. Brian Ellis commercialized the machine as the contaminometer.
The contaminometer has lived a quiet life since, hidden away in the lab at high-reliability manufacturers, diligently testing occasional samples to ensure contamination is below the threshold. But all that changed with the advent of lead-free soldering. Lead-free brings a dramatically narrowed process control window, and manufacturing engineers are searching for tools to help them maintain process equilibrium. One answer is the contaminometer.
Sources of Ionic Contamination
Before no-clean electronics assembly, fluxes contained rosin, and alcohol to keep the rosin liquid. The proportion of rosin used in these old fluxes was anything between 15% and 50%. Flux was neccessary to remove surface oxides from PCBs and components to promote correct alloy formation; and to act as a heat-transfer medium to ensure correct soldering temperatures. With a no-clean process, it was safe to leave flux residues on the assembly. But these safe levels couldn’t be met by the traditional aggressive, dirty high-solids fluxes. Flux manufacturers developed low-solids versions that, while demanding enhancements in process control, could be left on the board forever.
Flux is not the only source of contamination in the electronics assembly process. Common are etch, plate, tinning, or leveling residues; poor solder mask; undercured permanent or temporary solder masks; dust; moisture; oil pollution from fingerprints; component packaging materials; and machine maintenance oils (especially from wave soldering conveyors).
For high-reliability product suppliers such as those in the aerospace sector, medical, military no-clean regimes introduced a daunting challenge. While the manufacturers didn’t want to clean assemblies, they couldn’t risk exceeding the prescribed contamination threshold. Failure of high-reliability products could mean loss of life. Manufacturers solved this by periodically testing samples with a contaminometer. For manufacturers of products destined for normal use calculators, smoke detectors, electronic toys a contaminometer wasn’t required, as an occasional failure simply meant a warranty return rather than a lawsuit from a deceased user’s estate.
But the status quo changed with the replacement of tin/lead solder brought on by the need to meet the EU’s RoHS substances regulations.
Soldering Without Lead
The International Electrotechnical Commission (IEC) and IPC Association Connecting Electronics Industries recommend high-tin alloys to replace tin/lead solder. The most popular choice is SAC; this ternary alloy melts at 219°C far higher than the 183°C of eutectic tin/lead. This tightens the process window considerably. Manufacturing engineers now need improved process monitoring. The contaminometer can help, at the beginning of the process and after wave solder.
Dirty boards that would have taken more aggressive flux now can’t be tolerated. Measuring the cleanliness of bare boards with a contaminometer ensures that the best chance of soldering PCBs without problems. The test also provides good feedback on storage conditions.
At the end of the line, the contaminator quickly identifies trends in the manufacturing process that can be altered before they become a problem. For example, if flux composition starts to stray from optimal, board residues will change. The sensitivity of the contaminometer is such that the change will be detected well before soldering is affected.
A contaminometer gauges effectiveness of preheat profiles, a critical factor in lead-free soldering. Preheat is a delicate balance between ensuring the flux does its job and the board is at a sufficient temperature such that, when it hits the solder bath, thermal shock isn’t excessive. The temptation is to wind up the thermostat, but this detrimentally affects FR4 substrates.
If preheat is too hot, the flux vitrifies, nullifying its effect and coating the board with unacceptable contamination. The board expands dramatically, encouraging absorption into the laminate; the consequent risk of phenomena such as delamination becomes higher, as do subsurface reactions of residual contamination from the chemicals used in the assembly process. This can cause electrochemical reactions and reliability problems. A contaminometer will pick up on these overheating effects before they impact quality.
Contaminometers are useful measurement and documentation instruments for statistical process control (SPC) of the soldering process and also cleaning. By testing a predetermined number of samples per hour or per day, EMS providers can detect fluctuations in the level of ionic contamination on the assemblies a sign of process variation.
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 (Figure 2). All other test cycle operations are automated.
Figure 2. Contaminometers are easy to use, requiring only that the operator load and unload the board under test.
Typical testing starts with tank fill and solution preparation. The solution pumped through a mixed-bed ion-exchange column until it reaches ultra-low conductivity. It then is homogenized.
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; 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 sec. Results are processed and analyzed via onboard computers.
Conclusion
The lead-free manufacturing regime urges tighter process control. Whereas previously an engineer could tune the process by increasing flux activity, changing conveyor speed, or increasing preheat ad hoc, high-tin/high-melting-point alloys are a different matter. Slight changes from ideal can result in a flurry of failures detected at the automated test equipment (ATE) station or, worse, in the field where rework costs escalate a hundredfold. Several tools help SPC; the most overlooked is the contaminometer.
A contaminometer is an accurate tester useable by unskilled staff yet able to measure contamination levels on bare boards and assemblies quickly. Information is presented graphically and can be used for statistical analysis. 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 intermetallics.
In an age where adherence to legislation, stringent process control, high throughput of quality products, and low consumer tolerance of failures pressure manufacturing, the contaminometer is a reliable tool to make the job easier.
Graham Naisbitt is managing director, Gen3 Systems Limited, founded on Concoat Systems, and is a member of the IEC’s TC91 WG3. He may be contacted at Graham.Naisbitt@gen3systems.com.
Testing, Testing
The contaminometer uses a mix of alcohol and ultra-pure deionized (DI) water to dissolve flux residues and ionic salts. The dissolved ionic substances alter the conductivity of the test solution; the test equipment precisely measures the change and expresses it as μg/cm2 NaCl equivalence. Common names for the machines are resistivity of solvent extracted (ROSE) or solvent extract conductivity (SEC) testers. 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. Static testing takes a predetermined volume of solution to carry out the test. Dynamic 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. Machine 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 thermistor in the test cell measures temperature for feedback to the automatic temperature compensation; all measurements are related to the international standard of 20°C. Testers use a complex algorithm to compensate for ambient temperature, circuit volume, and atmospheric absorption of iogenic gases.
Analysis of contamination test data uses a complex curve-fitting routine to indicate ionic contamination on the circuit. Contamination is plotted against time and a curve automatically is extrapolated, producing meaningful data even for a short test.
It’s advisable to ensure the test data meets the requirements of international and military specifications as documented in MIL-P-28809, DEF standard 00-10/3, IPC-J-STD 001D & IPC-TM-650, and IEC specifications.