Counting the costs of contamination
11 March 2010
For over 40 years, the electronics industry has been seeking ways to determine an answer to the thorny question of ‘how clean is clean?’ Graham Naisbitt, Managing Director of Gen3 Systems investigates.
Rather than addressing the question, some were plotting ways that they could monitor the quality of their production process with specific regard to the presence of ionics (salts) that conspire to cause circuit failure. They realised that using a blend of alcohol with de-ionised water would be an ideal medium to measure conductivity.
Alcohol was used because conventional rosin based fluxes used at that time were soluble in alcohol, and water was used because salt dissolves in water.
A mix of 75% propan-2-ol (IPA) with 25% de-ionised water with a mixed resin filter (a mixture of Cation, Anion and Chelate) could be used to strip out ionics as they pass through the medium.
By cleaning the test tank and its contents to a determined conductivity level, expressed as micro-Siemens (μS), the object to be tested can be placed in the tank and changes to the conductivity can be measured. If the results are extrapolated as an equivalence of NaCl – plain salt –a record of the amount of ionics a process leaves on, or puts on, the circuit assembly is achieved.
The names employed for this test are ROSE (Resistivity of Solvent Extract), or SEC (Solvent Extract Conductivity). Those seeking better process control found an ideal tool to be a measurement of ionics that might be present on a selected sample. Then, during the working day, changes in the level detected would be a good indicator that the process was in or out of control.
Ionics take several forms and from many and various process steps, some are more soluble than others. If they are present on a circuit and exposed to moisture in the presence of electricity, an electrolyte is formed, electro-chemical reactions occur, and that results in dendrites; inter-metallics between the cathode and anode that provide a path of lower resistance, leading to short circuits and/or circuit failure.
In the 1970s the US DoD considered that this might be a useful test to control cleanliness in production. It established a pass/fail at a level of 3.1μg/cm2 (20μg/inch2) of NaCl equivalence with dynamic testing, and 1.56μg/cm2 (10.06μg/inch2) of NaCl for static testing. However, this was not such a good decision for two reasons. Firstly, because, by logical extension, it indicated that it was possible to safely leave up to that amount of measurable ionics on assembly surfaces. Secondly, there is no difference between dynamic and static, as both test methods now require 1.56μg/cm2 NaCl equivalence. The dictum remains: ‘It is acceptable to leave that amount of salt on an assembly.’ But as many have found to their cost, it is not.
Test time is important because exposing any circuit assembly to a mixture of alcohol and water for 15 minutes or more significantly increases the risk of other ionics leaching out of the laminate and onto the surface (according to the Swedish Institute for Production Engineering Research’s IVF report of 1990).
Furthermore, it has been suggested that the test solution may be used when heated to 40°C or more. Apart from the fundamental changes in conductivity and its effect upon test accuracy, it significantly increases the risk of sub-surface ionic leeching as well as posing an explosion risk. It is important to note that the test solution flash-point is 19°C at ambient temperature.
So, static versus dynamic; what’s the difference? IPC-TM-650 2.3.25 & 184.108.40.206 permit the use of either static or dynamic test methods. Whilst both methods should yield the same result, good test methods should have only one variable; the item under test. The dynamic test method, by its very nature, has an additional variable in that it behaves more like a cleaner with the test solution passing the conductivity probe, the filter, and back into the tank. In this way, the test solution is being continuously cleaned during the test. By contrast, the static test re-circulates the test solution via the conductivity sensor but bypasses the ionic exchange filter, thereby removing this important variable.
Some talk about a saturation problem with static testing. Given that the test system may commonly be calibrated up to 30μg/cm2 of NaCl equivalence, and the pass/fail level is 1.56μg/cm2, it means that if a process was producing boards with a contamination level that was going to send the solution into saturation, it would be well above 30μg/cm2. Concerns should therefore be raised and the production process stopped, rather than worrying about the solution saturation.
Does size matter?
When selecting a test system, it is important to use the smallest possible tank size for the circuit under test. As outlined in IPC-TM-650 220.127.116.11:
6.10 There is some concern regarding ROSE tester cell size. Testing a 2cm x 2cm board in a 20,000mL cell causes such a severe dilution as to cause the signal to be lost in the noise. A recommended cell size is 5000mL or less. Smaller cell volumes will allow for a more measurable result. If a smaller cell, or running with a smaller test volume, are not an option, then the number of bare boards can be increased, all extracted separately, and the extract solutions all tested at once.
If the test system is working correctly, then during calibration a quantified amount of NaCl solution is placed into the test tank and the system should be capable of precisely recording the amount. If it doesn’t, then there is something fundamentally wrong with the machine.
According to IPC-TR-583, An In-Depth Look At Ionic Cleanliness Testing, good cleanliness test systems need to be accurate, reliable, repeatable, simple to use, and easy to maintain. They also need to reduce test time to a minimum, avoid polarisation effects between electrodes, and take account of temperature, circuit volume, and atmospheric absorption of iogenic gasses.
However, this is not the only way to answer the original question of ‘how clean is clean?’
It is quick and simple to check that a production process is under control, but it is also important to recognise that cleanliness testers are in fact ionic contamination testers and they do not inform about the presence of non-ionic contaminants, of which there are many.
An enormous variety of surfactant additives exist in various process chemistries:
• Solder resist
• Solder flux, wire, paste
• Cleaning chemistries
They are mostly used as wetting, levelling or de-wetting agents that can contribute to failure by adverse electro-chemical reactions, especially if the manufacturing process does not include cleaning.
So what are the alternative answers? Ion Chromatography (IC) is based on the use of specialised column packing for separation of ions that is able to separate, identify, and quantitate ions in a sample matrix, allowing the separation of ions and polar molecules based on their charge.
The advantages of this are numerous. It is highly accurate and can identify what kind contaminants are on the boards, and help to trace the root cause of the problem of each production process. It can be used as a quality control tool for goods inwards inspection on a sampling basis (such as solder mask cure), it boasts excellent species differentiation, and can be employed for localised contamination. However, the limitations are that it requires highly skilled operators, is expensive to run, and can take in excess of 15 minutes to run. Furthermore, although IC will specify exactly what is on the surface under test, it will not report whether the end product will be reliable.
FTIR (Fourier Transform Infrared Spectroscopy) is a measurement technique where spectra are collected based on measurements of the temporal coherence of a radiative source, using time domain measurements of the electromagnetic radiation or other type of radiation.
It involves collecting infrared spectra, but instead of recording the amount of energy absorbed when the frequency of the infra-red light is varied, the IR light is guided through an interferometer. After passing through the sample, the measured signal is the interferogram.
The advantages of this are that it is highly accurate and can identify exactly what polymers may be on the boards, but it is limited in other ways. It requires scientifically trained operators, is expensive to run, and just like IC, it will report exactly what has been found (polymer contaminant), but will not state whether the end product will be reliable with it’s presence.
However, SIR (Surface Insulation Resistance) Testing, involves an inter-digitated test pattern to which an electrical bias is applied. It then measures the degradation or changes to surface insulation resistance.
This method determines the effects of ionic and non-ionic contamination, and demonstrates the electro-chemical compatibility between all process materials. It can be used to monitor material trends, is a quantitative rather than a qualitative test method, and it works in conjunction with ROSE / SEC. Importantly, it also predicts whether the end product will be electro-chemically reliable, but does require dedicated equipment, skilled operators, and takes around 72 hours. Furthermore, this process will inform users if the end product will be reliable, but it will not confirm what is causing a failure; so IC or FTIR are required.
It is therefore recommended that users decide upon a preferred process material mix and run SIR qualification tests, analyse failures using IC or FTIR, use ROSE / SEC tests to maintain process control, and use SIR to monitor material quality by trend analysis. Finally, remember that the cost of repairing mistakes increases roughly by an order of magnitude at each stage.
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