High humidity, temperature fluctuations or even a combination of both: printed circuit boards are often exposed to unfavorable environmental conditions that have a negative impact on the reliability of electronic assemblies. A suitable method to be able to evaluate the resistance of the components under such adverse conditions is the SIR test according to IPC. This makes it possible to draw conclusions by testing the surface resistance (
Surface Insulation Resistance
) in a damp-heat climate to draw conclusions about the technical cleanliness and structural condition of the printed circuit board surface.

Moisture in particular can cause migration paths to form more quickly in printed circuit boards. The result: a short circuit. This can be recognized, for example, by the formation of so-called dendrites (see Fig. 1). Corrosion processes on the one hand and the migration of conductive impurities on the surface on the other are responsible for this. This is where the SIR test comes into play, as it is ideally suited for checking placement processes and flux residues and evaluating the long-term effects of surface contamination.

Carrying out a SIR test

The SIR test according to IPC is one of the accelerated service life tests, along with the CAF test. This means that failure processes are forced in order to gain early information on the qualification of electronic assemblies under demanding climatic conditions. For example, the SIR test is performed at elevated temperatures and high humidity levels with bias voltages applied to specially designed PCB test circuits to induce electrochemical migration to failure. This is followed by a detailed data evaluation and analysis of the failure parts.

If a failure is detected, a visual evaluation follows. The first step is to correlate the data with the existing test structure in order to localize the failure on the test PCB. In the second step, a visual inspection is performed using optical microscopy or SEM/EDX analysis to identify dendrites or even impurities.

Most commonly, the SIR test is performed as a 21-day test (504 h) with a lower resistance limit of 500 MOhm as the failure criterion. The test parts have a test layout typically with comb structures and defined distances. Since the opposing conductor paths are at opposite potentials, a defined electric field is created in which directional migration of the positively or negatively charged ions takes place. This diffusion in turn leads to the formation of a conductive path and ultimately to a short circuit.

Typical test conditions of a SIR test:

• Temperature: 40 °C
• Relative humidity 92
• Bias voltage: 10-100V
• Test duration: 96-504 h

Check unassembled printed circuit boards

Unpopulated printed circuit boards have a carrier and wiring function. The cleanliness level of an unpopulated PCB is therefore decisive for the cleanliness of the assembly.

With the reliability test on an unpopulated printed circuit board, statistically validated failure rate parameters can be determined experimentally and incorporated into calculation models for the reliability of assemblies. In addition, SIR tests enable a manufacturing-related evaluation of the cleanliness level as well as a separation of contamination causes from PCB manufacturing and from further processing.

Avoid short circuits

The causes of short circuits in the SIR test according to IPC can be manifold. To achieve good resistance to failure due to surface effects, the following main influencing factors should be carefully controlled:

• Process residues in the form of conductive impurities
• Layout offset leading to shortening of insulation distances
• Process residues in the form of particles that shorten distances
• Handling (finger sweat)
• Water quality during final cleaning
• Caverns on surface (e.g. exemption areas) where residues can collect

Blog post written by Dr. Swantje Frühauf; KSG GmbH.