Also known as insulation resistance (IR) or moisture and insulation resistance (M&IR) testing, Surface Insulation Resistance (SIR) testing is an industry-standard way of assessing the potential of a board to fail through corrosion and other processes associated with ionic contamination.
We come across this test regularly in any discussion about fluxes, soldering and cleaning processes, and the residues left behind, for the reason that increases in leakage current can effect the operation of most circuitry, particularly as operating currents are generally moving to lower values.
SIR is also used for evaluating board finishes since the mechanisms relating to the surface finish that can cause failure during the test reflect both the material choice and the process conditions, in particular any ionic contamination remaining after the plating process.
To maximise the sensitivity of the test, leakage current is measured between long parallel lines at different electrical potentials. Instead of making very long and narrow circuits, electrically equivalent inter-digitated ‘combs’ are typically used, an example of which is shown in Figure 1.
The standards depend on application: telecommunications comb patterns have 0.63mm (0.025") lines and 1.27mm (0.050") gaps; the IPC1 ‘B’ standard pattern has 0.31mm (0.0125") lines and spaces.
1 PDF files of the IPC standard Surface Insulation Resistance tests (IPC-TM-650 Method 2.6.3 series) may be downloaded free of charge at http://www.ipc.org/html/fsresources.htm
Traditionally, measurements are reported as insulation resistance (in ohms or Megohms) rather than as leakage current. This conversion is simple via Ohm’s Law, V = I x R, where V is the test voltage and I is the leakage current. The diagrams in Figure 2 show typical circuitry used to apply a 45–50VDC bias voltage and to measure the resistance with a reverse polarity 100VDC test voltage.
For most general cases, the SIR results will be a function of a number of parameters:
Although all these parameters will greatly influence the results, changes in SIR can be used validly to compare different processes by using repeatable settings which are representative of the application. Some tests may simulate equipment life, while other tests are carried out with a 100VDC test voltage and a 45–50VDC bias of reverse polarity, in an ambient with 90–98%RH (relative humidity), and at temperatures of 55ºC, 65ºC or 85ºC, to accelerate the ageing of samples. However, it is important to realise that an extreme environment may cause some surfaces to fail by mechanisms that do not apply under normal operating conditions.
Typical SIR test regimes will look for a minimum of 100MΩ insulation resistance, both at the outset and after a period of exposure to a defined combination of voltage, temperature and humidity. As with all humidity tests, accurate measurements in situ are difficult to make, and parts may be removed from the chamber for testing. Also, for higher levels of humidity, where water absorbed or condensed on the surface may affect the insulation resistance measurement, it is not uncommon to specify a defined period of ‘recovery’ after exposure and before retesting. Common cumulative exposure periods are 1, 4 and 21 days, with specimens that have acceptable insulation resistances being returned to the chamber after each retest, if they have been removed for testing.