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How clean do assemblies have to be?
Important standards on qualifying the cleanliness of electronic assemblies
How clean do assemblies have to be?
Many users are confronted with this question when assessing the cleanliness of their electronic assemblies. There is a wide range of standards, which deal with this issue and specify methods for analysing the surface cleanliness by means of different processes. However, these days hardly any process engineer user has the time to get a general idea and understand all the standards relevant for their applications. For this reason, the most important standards on qualifying the cleanliness of electronic assemblies will be presented in the following.
Various test methods for qualifying cleanliness of assemblies are described in the standards. Since most test methods are very time consuming and/or costly to complete, this article additionally describes alternative alternative test methods, which are inexpensive and can also be used for on-site sampling inspections.
When is high surface cleanliness required?
To ensure that the coating or wire bond adheres permanently on assemblies, an extremely clean surface is required. Assemblies, which are exposed to critical, alternating climatic influences, require a very high surface cleanliness as well. Otherwise, malfunctions due to impurity-related leakage current and electrochemical migration can arise.
Therefore, this article will focus on the following standards and test methods:
• Coating related standards
• Leakage current protection related standards
• Bonding processes related standards
To provide a short overview, all industry qualification standards related to cleanliness qualification including coating, leakage current protection and bonding will be presented.
Qualification Standards
All standards for qualifying the cleanliness of assemblies are listed below:
• IPC J-STD-001, (chapter 8): “Requirements for Soldered Electrical and Electronic Assemblies” describes the methods and requirements for electronic assembly manufacturing.
• IPC-A-610, (section 10.4): “Acceptability of Electronic Assemblies” is a visual document for specifying the quality requirements of electronic assemblies using best and worse cases.
• IPC TM 650: A collection of various test methods regarding electronic assemblies is summarised in IPC TM 650 “Test Methods Manual.”
• IPC TP 1113, (Technical Paper): “Circuit Board Ionic Cleanliness Measurement: What does it tell us?” describes the methods and limitations of the ionic contamination measurement and the influences of various flux systems.
• IEC – 68-2 Is a standardised method for testing climatic reliability of electronic assemblies.
• The GfKORR guidelines: “Leitfaden fur Anwendung und Verarbeitung von Schutzlacken” (guidelines for the use and processing of protective coatings) is a supplement to IPC – Hdbk-830 amd helps to determine the coating reliability of assembly surfaces and geometries. As mentioned above, an extremely clean surface is particularly necessary prior to coating assemblies. Therefore, a wide range of standards specifying the degree of cleanliness for coated assemblies is available.
Standards and cleanliness requirements for coatings
Three considerations must be taken into account to ensure the reliable coating of assemblies.
• Ionic contamination/residues from activators
• Resin residues and wet ability of the surface
• Varnish hardening and reliability
Ionic Contamination/residues from activators
Hygroscopic residues such as activators in fluxes attract humidity from the surrounding air. This moisture can lead to the determination of the protective coating due to the vapour permeability of the varnish. (Figure 1)
Test Unit Target Value
Ionic Contamination Pg/cm2 <0.4 pg/cm2
Surface Tension Mn/m >40mN/m
Zestron Flux Test - Residue free
Zestron Resin Test - Residue free
Polimerization Inhibitors - Residue free
Figure 1: Parameters for the evaluation of the surface cleanliness prior to coating.
In addition, electrolytes can occur, which trigger electrochemical migration.
The initial qualification of cleanliness should be completed by visual inspection with 10 to 40 times magnification. Examples of visual cleanliness are shown in IPC-A-610: “Acceptability of Electronic Assemblies” under section 10.4.
An indication of the presence of hygroscopic residues is provided by the measurement of ionic contamination and known as sodium chloride equivalent. This eqivalent is generally determined in conformance with the IPC-TM-650!Test Methods Manual”, number 2.3.25 using a 75 or 50% 2-propanol/water solution.
The evaluation is based on IPC J-STD-001D, 8.3.6.2. It should be noted that the indicated threshold of 1.56pg NaCL Eq/cm2 refers to ROL0 and ROL1 fluxes. Modern flux systems generate much less ionic contamination. For example, cleaned assemblies have a residual contamination of 0.1 to 0.4pg NaCL Eq/cm2 and 0.4 to 0.7pg NaCL Wq/cm2 for NoClean fluxes, due to the encapsulation of hydroscopic activators. The necessary requirement for reliable coatings is a contamination level <0.4pg NaCL Eq/cm2.
The measurement of ionic impurities does not permit any conclusion concerning the local distribution of these conductive contaminants. However, the precise distribution of the residues on the assemblies is extremely important when assessing the short or long term potential hazard from these residues.
Four years ago, the only option for detecting these impurities was to take a charge contrast images using a scanning electron microscope. Since this method is very time-consuming and expensive, it is not suitable for on-site monitoring during production. In the meantime, very easy to use analytical methods have been developed (such as the Zestron Flux Test), which selectively demonstrates the hygroscopic activator residues by means of a colour reaction. The Flux Test allows to reliably visualising the local distribution of residues directly on-site during production.
Resin residues
An additional obstacle to reliable coating is th amount of resin residues on the assembly. If a protective coating is applied on top of the resin residues, wet ability is impaired and adhesion insufficient. Due to the different expansion coefficients at temperature changes during operation, delamination or the formation of cracks in the coating can appear. The residual amount of resin is therefore specified in IPC J-STD-001D under section 8.3.6.1. For class 3 assemblies, a level <40pg/cm2 is required, which approximately corresponds to the amount of resin on a single solder joint. In IPC –TM-650, section 2.3.27.1 and extraction method is used followed by HPLC (high performance liquid chromatography) to determine the amount of resin.
Since this method is very time consuming and expensive, it is not suitable for quick process control during production. A simple, quick and more economical method of demonstration is the Zestron Resin Test. This quick analytical test selectively demonstrates the presence of rosin and synthetic resin on assemblies by means of a colour reaction. The analysis of surfaces for resin and activator residues as well as the determination of ionic cleanliness are hence the most important methods to ensure sufficient surface cleanliness for coatings.
In addition to residues on the surface, the curing of varnish also plays an important role for the coating process. Different factors can prevent the varnish from curing and thereby make reliable coatings impossible. There are different test methods available to check the curing and reliability of the varnish.
Varnish hardening and reliability
Depending on the varnish system, organic tin salts, which can arise as a reaction product from solder and fluxes, can act as polymerisation inhibitors. Furthermore, sulphur and ammonia compounds, which are sometimes contained in fluxes, impair the curing of some coating systems. These impurities are generally identified by EDX using a scanning electron microscope. In this case, quick tests offer lest costly alternatives.
Various methods can be used to test the reliability of the coating after applying varnish. A complete life test can be performed according to IEC 68-2 standard. However, this test can only be performed after product development has been completed. In addition, the test duration of up to six months is still very time consuming and expensive. A quick, cost effective alternative, which can even be used in any stage of product development, is the so-called Coating Reliability (CoRe) Test. It is based upon the water immersion test described in the GfKORR “Guidelines for the Use and Processing of Protective Coatings”. By completely immersing the coated assembly in deionised water, the mechanism of electrochemical migration is strongly accelerated.
Therefore, conclusions concerning the reliability of the coating can be drawn within a few hours.
However, not only assemblies with protective coatings have to function reliably. The requirements for leakage current protection must also be fulfilled without the applied coating. Therefore, standards for uncoated assemblies, which describe methods for analysing the required surface cleanliness, are available.
Requirements for leakage current protection
To prevent leakage current on assemblies any residues may not exist, which lead to formation of electrolytes in conjunction with moisture. For this reason, the assembly should be visibly inspected and checked for hygroscopic compounds.
Another important criterion for leakage current protection is the surface insulation resistance (SIR). This should be as high as possible to prevent signal distortion on the assemblies. The surface insulation resistance is usually determined by means of a SIR measurement according to IPC-TM-650, whereas only specified comb structures are used for evaluating or estimating the situation on the assembly. The threshold according to the standard is 10 ohm. In this case, the Flux Test described above can also be used as an easy method for estimating the potential danger of leakage current.
Maximum surface cleanliness must also be achieved for wire bonding processes to ensure reliable bonds. Therefore, all requirements relevant to surface cleanliness prior to bonding will be described in the following.
Requirements for bonding processes
A visual inspection should also be made for bonding processes, which are mainly used with lead frames, DCB (Direct Copper Bond) substrates or hybrids on FC (Flip-Chip) assemblies.
The bonding process is primarily impaired by resin films and oxides on the bond pads. For example, resin can splash onto the bond pads during soldering. These resin residues lead to an insufficient wire bond since the bonds do not adhere properly on the residues. In addition, these impurities can reduce the adhesion of moulding materials. As described, time consuming extraction methods and HPLC analysis or quick analytical tests can be used to demonstrate resin residues.
Another major requirement affecting reliability of the bonds are intact and activated surfaces. Although surface activation is not a criterion specified in the standards, it represents an important variable for reliable bonds. To evaluate the oxidation status and check if metallically pure surfaces exist, the interference contrast is used. Metallically pure surfaces appear black and impurities, which could impair the bond ability, can be identified.
Thresholds for reliable coatings and stable bonds
Based on the provided criteria and empirical values with alternative test methods, parameters for the evaluation of reliable coatings were specified in the GfKORR working group “Corrosion protection of electronic assemblies”. In a combined study with F&K Delvotec, Zestron has measured similar values for reliable bond ability.
By means of these quick and easily determinable parameters, reliable conclusions regarding the coat ability and bond ability of electronic assemblies can be drawn.
Conclusion
If the described thresholds from the above standards are maintained, there will be no issues with leakage current protection and the reliability of coatings and wire bonds. To evaluate the cleanliness of electronic assemblies according to the standards, expensive and time-consuming test methods are not always necessary. By using the presented quick analytical tests, which can even be used on-site during production, reliable conclusions regarding surface cleanliness can be drawn
Zestron’s specially trained process engineers offer support to qualify the surface cleanliness of assemblies at its Analytic Centres.
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