SIR and ECM Frequently Asked Questions

Below are two questions that we are frequently asked from clients on the subject of SIR and ECM testing. If you have questions on the subject of SIR / ECM that you would like to see our answers to on our site, please contact us.

Q1. What is SIR and ECM testing and are the two tests similar or different?

A1.  SIR stands for Surface Insulation Resistance and ECM stands for Electrochemical Migration.  Both tests are designed to determine the propensity for a material and/or process to lead to some sort of electrochemical event. Electrochemical events would include corrosion, electrical leakage and of course dendritic activity.  Both procedures utilize a test board that is placed under specified environmental conditions with an applied test bias for a period of time. The period of time portion is really where the two tests start to diverge.  In an SIR test, the duration of testing is typically 168 hours (7 days).  The typical ECM test lasts 25 days.  Additionally, the two procedures generally have different measurement intervals.  Older SIR test methods required measurements be taken at 24 hours, 96 hours and 168 hours. Newer SIR methods require frequent monitoring, which is defined at 20 minute intervals from the start of the test.  The start of an SIR test is usually one hour after the boards have equilibrated to the specified test environment.   In an ECM test, measurements are typically taken after the test boards have equilibrated at conditions for 96 hours and then again at the end of the test (21 days later).  Both techniques seek to evaluate similar affects of materials and the processing of those materials on product function, but they definitely have differences in the way they are performed.

Q2.  What are the factors that influence an electrochemical (ECM) reaction or event?

A2.  Generally, most industry experts focus on three primary factors that are significant drivers for ECM events. However, in truth there are really five influencing factors, but two are secondary factors that we may have little control over.  The three primary driving factors are as follows:

  • An applied bias
  • Some level of ionic contamination 
  • Some level of moisture 
The other two factors that will influence an ECM event are:
  • Temperature
  • Time (?)
To expound on each of these a bit more we will first discuss the three primary factors.  The first one listed (i.e. electrical bias) is obvious.  The electrical bias is a driving function for the product to work, but it also drives ECM events from the cathode to the anode. 

The second factor is the level of ionic contamination and this is going to vary by board design and application.  If you have read the FAQ for IC (if you haven't, then shame on you), then you will recall that we said, "Every board has its own threshold for how much residue it can tolerate".  So variations in the amount of ionic contamination are expected based on the boards design, the materials used for its production and its end-use environment.

The third factor is moisture, whether from humidity, water droplets coming in contact with the board or from submerging the board, this is a carrier for ionic contaminates.  Moisture allows dissociation of ionizable salts that are on the board surface and this allows them to mobilize once the voltage has been applied.  Higher concentrations of ionic residues often require less moisture to start the ECM process.  

Temperature is a consideration because it influences the rate of reaction.  Warmer temperatures will typically increase the rate of a chemical reaction, whereas colder temperatures typically slow it down.  The last factor is time, which is the hardest to predict or control, but everything is dependent upon it.  Ultimately, you want your designs to have the least amount of ionic residues AND the least amount of exposure to moisture.  If you minimize those two, you have done a lot to mitigate ECM events.  

If you have a need for SIR or ECM testing, then PAL has the experience to help you.  Please consider giving us a call to find out how we can help.  The consultation doesn't cost you anything but your time!  

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