|Ask the Experts|
July 16, 2021
What is the IPC Definition of Uncommonly Harsh?
IPC-A-610 and other documents define class 3 to include products where "the end use environment may be uncommonly harsh."
Does anyone have guidelines for how "uncommonly harsh" may be defined?
|Expert Panel Responses|
The language here may seem very vague, and there is, I think, actually a good reason for this. I don't think that the IPC intends to define what might fall into Class 3. That's up to the engineers familiar with the device's intended operating environment. In practical terms, however, the temperature range for both operation and storage for products where Class 3 is specified tend to be wider than for Class 2 or Class 1.
For instance, in aerospace applications, operational limits might typically be -40C to +55C, or even wider in certain circumstances. Most Class 2 or Class 1 items are only intended to operate down to 0C or so. This is not always the case, however.
Many automotive applications demand as wide or wider range of operating temperatures as aerospace, yet they are cost sensitive and may not be inspected to Class 3;reliability is ensured through robust design.
Uncommonly Harsh is anything greater than 10% more than Commonly Harsh.
Principal Materials and Process Engineer
When using the term "uncommonly harsh" it usually refers to meeting military specifications. For example, equipment used in airplanes, tanks, ships,etc. There is a military standard document (MIL-STD-810) which describes all these conditions and the testing involved.
Senior Manufacturing Engineer
The answers to your questions lie with your customer or end user's requirements,and not within any specification. This is because only the end user or agency can determine what "uncommonly harsh" really means for their products. A train control must be able to stand up to relatively harsh conditions. So must an avionics computer or control system. Ditto with the guidance system and control system for a military missile.
And likewise for a heart pacemaker. The inside of a human body is considered by many to be a very harsh environment, indeed. People's very lives depend on all of these devices operating reliably in the planned environment they are expected to see.
But the definitions of "harsh" are widely different for all four items I have described. A train control will see outdoors exposure to perhaps -50 C, and +88 C, but it must also be able to withstand rain, salt spray, snow and ice buildup thrown off by the train itself, plus plows, cars, trucks, and little old ladies with shopping carts and rancid coffee. Even earthquakes.
And you know what dogs and winos do around train controls, don't cha now? Not to mention I've seen your teenager with cans of paint, or plenty of rocks or quarters he needs crushed, and too much time on his hands. But an avionics computer may see some of the same temperature excursions as the train control, but not as likely to see the dog, the teenager, or the old lady, although I have known some pretty seedy pilots in my time.
Yet it must function just as reliably, even though it spends most of its life in a relatively mild environment, although exposed to hours of constant vibration and some pretty severe G-forces. But hardly ever rain, salt spray, snow, or really extreme cold or heat.
But a hot young military missile I happen to be closely acquainted with enjoys being shot out of a submarine deep in the ocean. She gets a real kick from the salt water battery that thrusts her at tremendous speeds through the ocean (imagine the changes in pressure!), and when she breaks the surface of the water the concussion would easily kill most people, yet her biggest thrill is when the rocket immediately fires, sending her through the Earth's atmosphere at tremendous G-forces into the alternate deep cold of space and extremely hot radiation from the sun in less than a couple of minutes.
Is that "harsh" enough for you? No? Well remember that this little gal orbits the earth a few times before re-entering. And you surely know what happens during re-entry. Still not harsh enough? Geeez. Now, what would we do if this babe decided to malfunction while in orbit? Do you think this is fun? Never mind about "don' shudder like my brudder"; this is some very serious business!
Hopefully you can see there can be no comprehensive interpretation by any standard as to what "harsh" environment really means, but I think you will begin to get some answers in the highlighted sections of IPC-7711/7721 below: the highlights are mine. But these are just guidelines; only your customer knows for sure. Well... maybe!
1.2.2 Requirements Flow down
The applicable requirements of this document must be imposed by each manufacturer or supplier on all applicable subcontracts and purchase orders. The manufacturer or supplier must not impose or allow any variation from these requirements on subcontracts or purchase orders other than those that have been approved by the user.
Unless otherwise specified, the requirements of this document are not imposed on the procurement of off the shelf assemblies or subassemblies. However, the manufacturer of these items may comply as deemed appropriate.
The user of the product is responsible for identifying the Class of Product. The procedure selected for action to be taken (modification, rework, repair, overhaul etc.) must be consistent with the Class identified by the user. The three Classes of Product are:
1. Class 1 - General Electronic Products Includes products for applications where the major requirement is the function of the completed assembly.
2. Class 2 - Dedicated Service Electronic Products Includes products where continued performance and extended life is required, and for which uninterrupted service is desired but not critical. Typically, the end use environment would not cause failures.
3. Class 3 - High Performance Electronic Products Includes products where continued performance or performance-on-demand is critical. Equipment down time cannot be tolerated, end-use environment may be uncommonly harsh, and the equipment must function where required, such as life support and other critical systems.
4. Repair The act of restoring the functional capability of a defective article in a manner that precludes compliance of the article with applicable drawings or specifications. Repairs are generally changes to an unacceptable end product to make it acceptable in accordance with original functional requirements. The control of repaired products should be by means of Material Review Board (MRB), or its equivalent, which may consist of Design Engineering, Quality Assurance, and User (customer) representatives.
The MRB, with technical support, should define the mutually acceptable repair method to be used and take the action necessary to ensure that all applicable procedures are adhered to. Repair of a failure in the field seldom includes an MRB,and typically is done in accordance with a contract,repair/service order or the user-activity maintenance program requirements. The maximum number of repairs per printed wiring board assembly should be determined by the using activity or agency.
1.7 Basic Considerations
1. Appropriate Approvals Appropriate approvals should be obtained before proceeding with PC board modification, rework or repair. Such approvals should include agreements as to acceptance criteria and limitations.(AABUS)
Thank you for your question about classification call outs in IPC standards. The three level classification has been used for many years. The purpose of the classification system is to provide a high level generic categorization for electronics hardware that allows users and suppliers to discuss specific performance requirements as identified in the standards.
In general Class 3 is IPC's highest category, being for defense and life support products; class 2 would be for industrial electronics and class 1 would be for consumer products. In some cases, certain industry segments have requested higher level performance than is called out in the IPC standard.
The aerospace industry has written several addendum documents to IPC standards that increase requirements above class 3 to meet unique application needs. However even with all of this detail, your specific question "how harsh is harsh" is not addressed.
It is up to the user to determine which class is appropriate for their hardware.
If you would like to see some additional helpful information on use environments and reliability, I suggest IPC-SM-785, Guidelines for Accelerated Reliability Testing of Surface Mount Solder Attachments.
VP International Relations
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