The SIR test (surface insulation resistance) is used to test the surface resistance in a humid/heat climate. In addition to temperature fluctuations, electronic assemblies are often subject to the effects of moisture during use. Moisture can lead to the accelerated formation of migration paths that eventually cause a short circuit. These are usually recognizable by the formation of dendrites (see picture). The reason for this lies on the one hand in corrosion processes and on the other hand in the diffusion of conductive impurities on the surface. This means that, in principle, the technical cleanliness and the structural condition of the PCB surface are evaluated.
The test parts have a test layout typically with comb structures and defined distances. The opposite conductors are at opposite potentials, so that a defined electric field is created.
The SIR test can be performed as a test under constant climatic conditions or as a cyclic test. The test specimens are additionally subjected to a bias loading voltage. Each test structure is connected by cable to the resistance measuring system located outside the climatic chamber.
Typical test conditions are:
- Temperature: 40°C
- Relative humidity 92
- Bias voltage: 10-100V
- Test duration: 96-504 h
Most commonly, the SIR test is performed as a 21-day test (504h) with the lower resistance limit of 500MOhm as the failure criterion.
The causes for short circuits in the SIR test 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
- 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.