Surface insulation resistance

Insulation resistance measurements are important test data used to characterize printed wir- 

Insulation resistance is the integrated effect of both volume and surface resistivity according to the ASTM definition (Ref 8). The measured bulk resistance will depend upon the nature of the laminate, solder mask or conformal coating material under investigation, and the degree of cure. It will also be affected by soldering flux/paste 

Although the SIR readings are a combination of both bulk and surface resistance, for the case of FR4 epoxy glass laminate, 99.9% of the current leakage will occur on the surface of the laminate (the ratio of the surface resistivity to the volume resistivity is 1/10 (Ref 9). Test patterns for measuring volume resistance will either use electrodes on the top and bottom of the substrate (for Z axis measurements) or hole-to-hole patterns for measuring the XY resistance. Test patterns for surface resistance are typically inter-digitated comb patterns such as the IPC-B-24 coupon (Fig. 6). 

SIR tests are normally performed at accelerated conditions of temperature and humidity. These range from 35 °C (95 °F) and 95% RH to 85 °C (185 °F) and 85% RH with a test duration varying between 4 and 21 days. Pass criteria also vary from 10 Q to 10 Q depending upon the test pattern and the temperature/humidity conditions of the test. 

The electrochemical migration test requires that any degradation in insulation resistance value be less than a decade. The Telcordia Electrochemical Migration test method has also become an IPC test method (TM 2.6.14.1). Table 1 compares the variation in SIR and electrochemical migration test conditions for telecommunications test versus the non-telecommunication applications. 

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Figure 4.3 Surface insulation resistance (SIR) test pattern (IPC B-25).

Qualification and performance of electrical insulating compound for printed board assemblies, IPC-9201. Surface Insulation Resistance Handbook. IPC Jul. 1990. IPC-CC-830A(Oct. 1998). 

The level of surface cleanliness may also be evaluated by measuring the surface-insulation resistance (SIR) of a separate test pattern. The SIR test sample consists of an interdigitated comb pattern of conductor lines and spacings on a printed-circuit laminate representative of the actual circuit. The IPC B-25 or IPC B-24 test patterns (Fig. 4.3) are standard in the industry to evaluate the degree of cleanliness of a surface. If the test pattern is well cleaned and no ionic residues remain, the insulation resistances between conductor lines will be 10 ohms or greater and will not drop appreciably under moist ambients. If residues remain, leakage currents... 

The surface insulation resistance between two points on the surface on any insulating material is the ratio of the DC potential apphed between the two points to the total current between them. For surface resistivity, the measured current flows between electrodes 1 and 2, while stray current flows between electrodes 1 and 3, as shown in Fig. 8.16. Table 8.8 shows the surface resistivity values of some common fiberglass-reinforced materials. 

These designators were determined by a series of tests, including the copper mirror test, a qualitative silver chromate paper test for chlorides and bromides, a qualitative spot test for fluorides, a quantitative test for halides (chloride, bromide, and fluoride), a corrosion test for flux residue activity, and a surface insulation resistance test at accelerated temperatoe and humidity conditions. 

Do not use water to clean a no-clean soldered board unless the solder paste manufacturer recommends it, as it may result in a sticky, white residue on top of flux residues. This may impact electrical surface insulation resistance and corrosion. 

Do not use saponified water with no-clean unless the process has been tested for materials compatibility and subsequent corrosion. Surface insulation resistance (SIR) testing and electrochemical migration (ECM) testing are in order. 

Basic Surface Insulation Resistance (SIR) Test Pattern... 

Jachim,X A., Freeman, G. B.,Turbini, L. X, Use of Surface Insulation Resistance and Contact Angle Measurements to Characterize the Interactions of Three Water Soluble Fluxes with FR Substrates, IEEE Transactions on Components, Packaging, and Manufacturing Technology, PartB,Vo. 20, No. 4,1997, pp. 443-451. 

Electrochemical Failure Modes. The primary function of a printed circuit board is to provide electrical connections with the desired, stable, low-impedance and high-impedance insulation between them. A high surface insulation resistance (SIR) value is usually assumed by the circuit designer. Exposure to humidity, especially when ionic contaminants are present, is a common cause of insulation resistance failures that is accelerated by elevated temperatures and electrical bias. The impedance often decreases slowly over a long period of time. If the SIR value falls below the designed level, there will be cross talk between circuit elements that should be isolated, and the circuit may not function properly. Insulation... 

Cleaning and Cleanliness. Improper handling procedures and improper selection and application of solder paste and wave-solder fluxes and their associated cleaning processes can cause ionic residues to be left on the board that result in low surface insulation resistance. Low SIR values can cause failures in and of themselves for some sensitive circuits and in other cases set up the conditions for further corrosion that eventually result in short circuits. Sodium and potassium ions and halide ions are the most commonly quoted culprits for these failures. The major source of sodium and potassium ions is handling, i.e., fingerprints. The primary sources of halide ions are soldering fluxes. 

Organic-coated copper provides a consistent, flat, solderable metal finish. Exposed copper after printed circuit assembly has been a persistent reUabiUty concern, because it is generally not permitted on HASL boards. While exposed Cu on HASL boards is associated with poor solderabUity, which may be due to contaminants that were not removed before the HASL process, there is little evidence that exposed Cu on a properly processed OCC board causes reliability problems. Surface insulation resistance (SIR) testing shows that OCC boards have comparable or better performance than HASL boards in high-temperature, high-humidity storage tests. 

Temperature, Humidity, Bias. The primary purpose of these tests is to identify surface insulation resistance (SIR) degradation due to corrosive materiak left on the board from the assembly process or due to galvanic couples set up in the assembly process. The usual test procedure is to use SIR comb patterns on the PCB, and to subject the assembly to 85°C/85 percent relative humidity/-20Vac for 1000 h.The bias voltage k dependent on the test device or test vehicle chosen. 

Studies have shown that reliable lead-free solder joints, with proper grain structures and in-termetallics formation, can be produced using appropriate rework processes. Care must be taken to minimize any potential negative impact of the rework process on the reliability of the components and the PWB. Surface insulation resistance (SIR) tests must be performed to ensure the compatibility between the reflow/wave solder flux and the rework flux, i.e., to ensure that the rework flux and any products of reaction between the reflow/wave solder flux and the rework flux do not pose any unacceptable risk for electromigration and dendritic growth for noclean applications. 

E.J. Gorondy, Surface Insulation Resistance— Part 1 The Development of an Automated SIR Measurement Technique, IPC Technical Paper IPC-TP-518, IPC, 1984... 

Ag Migration. In the presence of water and an electric field, silver is anodically dissolved at its original location and moves toward the cathode where it is deposited. This migration phenomenon can lead to the growth of dendrites between adjacent electrodes and lower the surface insulation resistance (SIR) of the board. For many years, the short circuit due to Ag migration has been a nuisance to those using silver inks and similar materials. 

Fm Fourier transform infrared (spectroscopy) SIR surface insulation resistance... 

In 2003, a process qualification study was initiated with a test laboratory to qualify and validate the assembly process using Surface Insulation Resistance (SIR) electrical performance and Ion Chromatography testing on a test board. The study used IPC Class 3 level performance per IPC ANSI J-STD OOlC. The analysis was conducted using ionic cleanliness evaluation by Ion Chromatography IPC-TM-650, method 2.3.28 and SIR characterization IPC-TM-650, method 2.6.3.3A. 

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