The first definition is the differences between the two classes of products where class 2 and class 3 differences are based upon system up-time. Class 2 products are defined as products where continued performance and extended life is required, and for which uninterrupted service is desired but not critical.
Whereas Class 3 products demand continued high performance or performance-on-demand is critical and equipment downtime cannot be tolerated, the end use environment may be uncommonly harsh, and the equipment must function when required, such as life support or other critical systems.
By reviewing the requirements of Class 3, within the IPC documents the differences are found in component placement for surface mount components, hole fill requirements for plated through holes, cleanliness requirements based upon residual contaminants on the surface of the product, plating thicknesses as defined in plated through holes and on the surfaces of printed circuit boards.
Although there are many other differences, these are ones which come to mind quickly. These individual issues can impact the performance requirements of Class 3 product, hindering their ability to function on demand.
A couple of examples that come to mind are a pacemaker, which has to work immediately and down time cannot be tolerated or a rocket on the wing of an aircraft has to fire when activated and its downtime cannot be tolerated.
Is there any data signifying the improvements in product functionality for these issues? The answer is yes, but would have to be dug out of the archives to get to the specifics of each one.
Class 3 informational criteria is only included in the documents when sufficient informational data has been supplied and approved by the various document committees.