Ionic residues on electronic assemblies
What are ionic residues and what is their significance?
If the smartphone falls into the sink at home, there is a good chance that it will work again after drying. However, if it has fallen into the sea, it usually doesn't look good despite careful drying. The reason for this is the salt in the seawater, traces of which remain on the electronic components, leading to corrosion or even short circuits.
However, ionic contamination already occurs during the production of electronic assemblies such as printed circuit boards (PCBs) or printed circuit board assemblies (PCBAs), for example through soldering processes. Similarly, ionic residues are also of critical importance here and must be removed, as they can lead to premature failure if they remain on the assembly. This can manifest itself in faulty behavior of the control electronics due to excessive leakage currents or lead directly to a defect due to the slow formation of dendrites. In extreme cases, it can even lead to an immediate short circuit. It is therefore essential for the reliability of a series-produced electronic assembly to check whether ionic residues have been successfully minimized to an acceptable level.
The ROSE test is often the method of choice for determining ionic residues. In the following, we show the benefits and limitations of the test and present its useful range of application.
ROSE test: definition and application
Definition
The ROSE test (Resistivity of Solvent Extract) is a comparatively simple method that can be used to determine ionic contamination in the form of a sum parameter. It is standardized in IPC TM 650 2.3.25 for assembled PCBs (PCBAs) and in IPC TM 650 2.3.25.1 for bare PCBs. The conductivity of a solution obtained by extracting the ionic contamination from the surface of the assembly is measured. The output value (µg NaCl equivalents/cm²) reflects the total ionic contamination without providing any indication of its origin or composition. The output value is to be understood as the determined amount of sodium chloride (common salt) per cm² of the sampled surface of the part to be tested has an equivalent conductivity to the value produced by the ionic contamination.
It is therefore of fundamental importance to include the correct component surface area in the result. Determining the surface area of a bare PCB is simple: 2 x length x width. However, determining the surface area of a single-sided PCB without CAD data is not trivial. Therefore, IPC TM 650 2.3.25 allows a formula for estimating the total surface area: 2 x length x width + 0.5 x length x width. The additional "half, one-sided" area therefore represents a kind of "fudge factor" for the surface area of the mounted/soldered components, which is difficult to determine.
Application and limit value
In general, however, the ROSE test is less suitable for generating absolute and independent values without comparison. It has its strengths in continuous process monitoring, in which two parts in different states can be directly compared with each other under constant conditions.
Nevertheless, the manual "Requirements for Soldered Electrical and Electronic Assemblies" (IPC JSTD 001) contains a limit value of 1.56 µg/cm², which is often used as an assessment criterion. However, revision H from 2020 stipulated that this value can no longer be used as a qualifying measure for an i. O. or n. i. O. without objective evidence. O. assessment may be used. The limit values should therefore be determined in the customer-supplier relationship after appropriate qualification. One such qualified test is the SIR test (Surface Insulation Resistance). This involves recording the resistance values between a large number of measuring points on the PCB while it is in a climatic chamber at an elevated temperature and humidity. The extreme conditions promote corrosion processes and ionic impurities are mobilized. This leads to an increase in the measured currents between the measuring points and thus to a value being exceeded that is safe for operation. A circuit board in this condition is therefore too heavily contaminated and the value obtained in a subsequent ROSE measurement must be assessed as not OK (n. i. O.). Result to be evaluated. This provides objective evidence of a relevant limit value.
For interested readers, we would like to refer you to this excellent article on the subject of "Objective Evidence":
Ion chromatography: the elementary addition
In order to be able to initiate appropriate remedial measures, the source of the ionic contamination must be identified. The ROSE test is completely unsuitable for this purpose. Instead, ion chromatography according to IPC TM 650 2.3.28 or IPC TM 650 2.3.28.2 is particularly suitable for unpopulated PCBs. Here too, the contamination is extracted from the test part (static extraction). However, instead of the conductivity as a sum parameter, the ions contained in the extract are separated (keyword: chromatography) and characterized on the basis of their specific retention time and quantified using a conductivity detector.
IPC TM 650 2.3.28 contains a comprehensive list of relevant cations, anions and organic acids. It is possible to add specific ions that are not listed. The result, i.e. the type and concentration of the respective ion, enables a direct comparison with the ingredients of the process agents used and thus the initiation of corrective measures within the cleaning process.
Advantages and disadvantages of ion chromatography
The improved reproducibility enables a better comparison between different setups and laboratories. The high sensitivity of the method also enables spatially resolved extraction of ionic contamination from functionally critical components or particularly conspicuous areas on the PCB(A). This can be particularly useful when investigating the source of contamination.
If the results of ion chromatography are to be used as an evaluation criterion for "OK" or "not OK", these must be determined for each ion. O.", these must be determined for each ion. If the ion chromatography is used as a criterion for the "OK" or "not OK" state, these must be determined for each ion. Here too, an assessment must be made according to the above-mentioned criteria of "objective evidence" before limit values can be defined.
However, the additional information content contrasts with the higher technical, time, financial and personnel costs compared to a ROSE measurement.
The C3 method from Foresite, Inc.
The C3 method from Foresite, Inc. represents a middle ground that enables automatic measurement of local ionic contamination via a sum parameter. Although no information on the material composition of the contamination is obtained, a spatially resolved measurement can quickly and effectively investigate failures at specific points on the assembly.
The advantages and disadvantages of the ROSE test at a glance
| ➕ Advantages | ➖ Disadvantages |
|---|---|
| Quick and easy implementation | No statement about the type or origin of the contamination |
| Cost-effective compared to other methods | Only one sum parameter - no differentiation of individual ions |
| Well suited for process monitoring under constant conditions | Low significance for individual measurements without a reference for comparison |
| Standardized (e.g. IPC TM 650 2.3.25) | Surface estimation for populated assemblies inaccurate without CAD data |
| Can be used as a monitoring tool for quality control | Limit values must not be used across the board without objective evidence |
| Quick assessment of "OK" or "not OK" with known reference values | No spatially resolved analysis possible |
Conclusion
The ROSE test is a valuable monitoring tool for determining ionic contamination on printed circuit boards and assemblies. However, the determination of fixed values with simultaneous comparability between different devices, for example, is not a strength of the technology. Therefore, component-dependent limit values must always be determined using the "objective evidence" method. Ion chromatography is the preferred method for detailed analysis and root cause analysis.
CleanControlling offers both the ROSE measurement according to IPC TM 650 2.3.2. and the analysis via ion chromatography according to IPC TM 650 2.3.28. as well as according to relevant standards from the automotive sector.
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