Insulation Resistance Measurements
What is Insulation Resistance?
Insulation resistance is measured between two conductors. It is the ratio of the voltage to the total leakage current between the conductors.
Figure 1 demonstrates a typical short circuit on a PCB with a low resistance value.
Figure 2 demonstrates a potential area where insulation resistance may be affected by the environment etc.
Why Insulation Resistance Important in a PCB?
When a voltage is applied to a PCB under a condition of high humidity, a phenomenon called Electro-chemical migration occurs in which metal ions move from one metal electrode to the other, causing precipitation of metal or compounds. Short-circuiting between electrodes can result as the dissolution and precipitation progress.
The Conductive Anodic Filament (CAF) formation is a well-studied phenomenon that is driven by chemical, humidity, voltage, and mechanical means. It is characterized by a sudden loss of insulation resistance that happens internally in the PCB. CAF dendrites can form between adjacent Plated Through Holes (PTH), or between a plated through hole and a line on the PCB. Plating chemistry, material consistency, damage from multiple soldering steps, and excessive voltages (beyond designed voltages) accelerate the onset of CAF. The mechanism of CAF is an electro-chemical transport of ions across an electrical potential between anode and cathode.
The following have been reported as factors that influence the rate of Electro-chemical migration:
- Applied voltage.
- Insulation material type, structure, configuration, etc.
- Environmental conditions: Temperature, humidity, gas, etc.
- Other: Chemical residue left after PCB manufacture, impurities on insulation material, etc.
As PCB mounting densities have increased to meet demand for smaller and lighter electronic devices, there has been a rapid shift toward narrower-pitch electronic parts, finer-patterned PCBs, and integration of PCBs with other parts. The result has been smaller gaps between conductors, shorter insulation distances and higher average electric field strengths, as well as greater effects from local fields. And since the rise of mobile electronic devices has resulted in more diverse operating conditions, evaluating insulation reliability has become crucial.
Why Gardien’s Solution?
Gardien has implemented a simplified, and flexible solution into our G-Series Flying Probe machines and fixture-based machines. Gardien designed the process for ease of use on our service floors, allowing accuracy and reduced training (virtually push-button technology).
Parameters that can be adjusted to meet the PCB build requirements:
- Resistance
- Voltage within fixed tolerance
- Ramp and Dwell timeΩ
All information from the PCB fabrication drawing is transferred in the Tooling process via UCAMCO software to the job specific files. The information is than output into the file that used to perform the measurement.
