Surface Insulation Resistance Testing

Surface insulation reistance testing

Surface Insulation Resistance (SIR) is defined as the resistance, across the surface of the base material, between adjacent conductors of a Printed Circuit Board (PCB). On a typical PCB surface there are primarily four paths for current leakage between adjacent conductors;

  • Through the air,
  • Through surface Contamination,
  • Along the surface, and
  • Through the substrate.

The Surface Insulation Resistance test method is used to quantify the deleterious effects of fabrication, process or handling residues on Surface Insulation Resistance (SIR) in the presence of moisture. The electrodes are parallel traces (printed comb patterns) on a standardized printed board or assembly. Samples shall be conditioned, and measurements taken at a specified interval under elevated temperature & humidity. Electrodes are electrically biased during conditioning to facilitate electrochemical reactions.

What is Insulation Resistance Testing?

Basically, we apply a voltage (specifically a highly regulated, stabilized DC voltage) across a dielectric, measuring the amount of current flowing through that dielectric, and then calculating (using Ohm’s Law) a resistance measurement. Let’s clarify our use of the term “current.” We’re talking about leakage current. The resistance measurement is in mega ohms. We use this resistance measurement to evaluate insulation integrity. Current flow through a dielectric may seem somewhat contradictory, but remember, no electrical insulation is perfect. So, some current will flow.

Purpose of Insulation Resistance testing

  • A quality control measure at the time a piece of electrical equipment is produced.
  • An installation requirement to help ensure specifications are met and to verify proper hook up.
  • A periodic preventive maintenance task and
  • A troubleshooting tool.

General Diagram insulation reistance testing

1 thought on “Surface Insulation Resistance Testing”

  1. In respect to the world of printed circuit boards (PCBs) and printed circuit assemblies (PCAs), SIR testing also commonly referred to as Temperature Humidity Bias (THB) testing is used to evaluate a product’s or a process’ ability to resist “failure” by means of current leakage or an electrical short (i.e., dendritic growth). SIR testing is typically performed under elevated temperature and humidity conditions—such as 85°C/85% RH and 40°C/90% with periodic insulation resistance (IR) measurements obtained.

    The primary purpose of SIR testing is to understand the impact of flux residues on the electrical reliability of a device, specifically the conductive and corrosive nature of the residue, regardless of whether the residue is removed or allowed to remain on the board post-reflow.
    It can be a little difficult to understand because it provides a measurement of conductivity of the residue in “ohms per square”.

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