High humidity, temperature fluctuations or even a combination of both: PCBs are often exposed to unfavorable environmental conditions that have a negative impact on the reliability of electronic assemblies. A suitable method for evaluating the resistance of components under such adverse conditions is the SIR test according to IPC. This enables the surface resistance (Surface Insulation Resistance) in a humid-heat climate to draw conclusions about the technical cleanliness and structural condition of the PCB surface.
Moisture in particular can cause migration paths to form more quickly on 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 ideal for checking placement processes and flux residues and evaluating the long-term effects of surface contamination.
Carrying out an SIR test
The SIR test according to IPC is one of the accelerated service life tests alongside the CAF test. This means that failure processes are accelerated in order to obtain information on the qualification of electronic assemblies under demanding climatic conditions at an early stage. For example, the SIR test is carried out at elevated temperatures and high humidity levels with bias voltages on specially designed PCB test circuits in order to induce electrochemical migration until failure. This is followed by a detailed data evaluation and analysis of the failed parts.
If a failure is detected, this is followed by a visual evaluation. The first step is to correlate the data with the existing test structure in order to localize the failure on the test circuit board. The second step involves a visual inspection using light microscopy or SEM/EDX analysis to identify dendrites or impurities.
The SIR test is most frequently carried out as a 21-day test (504 h) with a lower resistance limit of 500 MOhm as the failure criterion. The test parts typically have a test layout with comb structures and defined distances. As the opposing conductor runs are at opposite potentials, a defined electric field is created in which a directed migration of 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 an SIR test:
- Temperature: 40 °C
- Relative humidity 92 %
- Bias voltage: 10-100V
- Test duration: 96-504 h
Check unpopulated printed circuit boards
Unassembled PCBs have a carrier and wiring function. The cleanliness level of an unpopulated PCB is therefore decisive for the cleanliness of the assembly.
Reliability testing on an unassembled PCB allows statistically verified failure rate parameters to be determined experimentally and incorporated into calculation models for the reliability of assemblies. In addition, SIR tests enable a production-related evaluation of the cleanliness level as well as a separation of contamination causes from PCB production and further processing.
Avoid short circuits
The causes of short circuits in the SIR test according to IPC can be manifold. In order 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 that leads to shorter insulation distances
- Process residues in the form of particles that shorten distances
- Handling (finger perspiration)
- Water quality during final cleaning
- Cavities on the surface (e.g. release areas) in which residues can collect
Text: Dr. Swantje Frühauf; KSG GmbH
English 
German