Surface-insulation resistance test

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. 

A d.c. insulation resistance test or polarization index reveals only the surface condition of the insulation and does not allow a realistic assessment of internal condition. Loss tangent values are true reflections of the insulation condition to detect moisture content, voids, cracks or general deterioration. The tan 5 versus test voltage curve may be drawn and compared with the original curve provided by the manufacturer, and inferences drawn regarding the condition of the insulation. The different starting tan lvalues will reveal the condition of the insulation in terms of amount of contamination, as noted in Table 10.4 (See lEE, Vol. 127, May 1980). 

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

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... 

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... 

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. 

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. 

The materials used to fabricate circuit boards and component chip carriers have certain electrical properties which includes the resistivity of the surface layers. A test that measures the resistance to current flow between adjacent surface conductors is called a surface insulation resistance (SIR) test. When exposed to moisture, conductive ions present on the surface of the PCB dielectric layer will be dissolved into the surface moisture layer. When a biasing voltage (+ /—) power is applied to each surface conductor, the mobihzed metal or salt ions can link together to form dendrites or conductive bridges between the conductors. 

Because the surface of rubbers may conduct electricity more easily than the bulk of the material, it is usual to distinguish between volume resistivity and surface resistivity. Volume resistivity is defined as the electrical resistance between opposite faces of a unit cube, whereas surface resistivity is defined as the resistance between opposite sides of a square on the surface. Resistivity is occasionally called specific resistance. Insulation resistance is the resistance measured between any two particular electrodes on or in the rubber and, hence, is a function of both surface and volume resistivities and of the test piece geometry. Conductance and conductivity are simply the reciprocals of resistance and resistivity respectively. 

The arc resistance of an insulator may be defined as the time in seconds that an arc may play across the surface without burning a conducting path. A schematic of an arc-resistance test is shown in Figure 3.55. 

DC Resistance or Conductance of Insulating Materials. Primary Film Test Method ASTM D257 Additional Test Method lEC 60093. The test method ASTM D257 covers direct-current (DC) procedures for the determination of insulation resistance, volume resistance, volume resistivity, surface resistance, and surface resistivity of electrical insulating materials, or the corresponding conductances and conductivities. 

The insulation resistance between two conductors or plated holes is the ratio of the voltage to the total current between the conductors. Two measures of electrical resistance are volume and surface resistivities. Since these properties can vary with temperature and humidity, testing is normally performed at two standardized environmental conditions, one involving humidity conditioning, the other involving elevated temperature. Humidity conditioning subjects the sample to 90 percent relative humidity and 35°C for 96 hours (96/35/90).The elevated temperature conditioning normally subjects the sample to 125°C for 24 hours (24/125). 

Temperature, Humidity, Bias. These tests are designed to promote corrosion on the PCB surface and conductive anodic filament growth, either of which can cause insulation resistance failures. 

Surface insulation tests utilize two interleaved Cu combs with an imposed dc bias across the combs. These combs may be designed into existing boards or a coupon such as the IPC-B-25 test board shown in Fig. 57.24 may be used.The measured resistance (ohms) from the comb pattern can be converted to surface resistivity (ohms per square) by multiplying the measured... 

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