Franklin,

Here is some wording that I wrote for one of the IPC handbooks currently

being drafted.  Doing M&IR testing is more practical if you are looking for

electrochemical failure mechanisms.



Doug Pauls

Rockwell Collins





MIR vs. SIR



Many individuals who deal with solder masks and printed wiring board

materials and processes have expressed confusion over the terms “moisture

and insulation resistance (MIR or M&IR)” and “surface insulation resistance

(SIR)” when reading IPC-SM-840, the specification for solder masks.



In all practicality, they refer to the same thing, though term purists will

argue.  Both of these terms refer to the exposure of material systems to

elevated levels of temperature and humidity, often under electrical bias,

to examine propensities for degradation of insulating properties by

temperature and humidity.  The two terms can usually be used

interchangeable, though SIR testing has the greater acceptance as a term.



Surface insulation resistance is the measurement of a characteristic

property of a material set, such as solder mask over bare copper.  The

measurement of SIR can take place at ambient conditions, where it is

sometimes called insulation resistance (IR), or at some elevated conditions

of temperature or humidity, of varying test times or conditions..  In

practice, SIR testing has generically referred to these kinds of biased

temperature humidity tests.



The term Moisture and Insulation Resistance testing is usually only found

in the SM-840 document.  It is a measurement of insulation resistance at

ambient conditions prior to temperature-humidity testing, at elevated

temperature-humidity conditions during the cyclical humidity test, and

finally at ambient conditions at the end of the test.