IPC-A-610 Criteria

How did the Class 3 pass/fail criteria in IPC-A-610 come about? Was there field testing and or life cycle testing to support these requirements?

If our products need to meet Class 3, but the heel fillets on one gull wing component do not comply with Class 3, but do meet Class 2, and if we do not rework the components, can we meet Class 3 if we conduct additional testing and the product passes the tests? What tests would we need to conduct?

B.P.

Expert Panel Responses

The criteria for all of the IPC standards are gathered from all over the industry. The data comes from extensive testing and years of practical experience.

To your question of, "can we meet class three criteria through testing?" the short answer is, "No". In order for an assembly to "meet the criteria of theIPC-A-610" it must do just that; have observable conditions that falls within the scope of the stated criteria.

Now that's not to say your customer can't take exception to the criteria. In the opening pages of the IPC-A-610 you'll find the order of precedence. At the top of the order of precedence is the customer requirements (contract). If the customer contract says that the assembly must be built in accordance with the IPC-A-610 criteria, then that is what you need to follow, no exceptions. If the customer states something along the lines of, "... built in accordance with IPC-A-610 unless testing can prove the reliability of the assembly", then you have room for testing.

If you customer allows testing for exceptions to the criteria, the customer will have to specify what tests are allowed.

If you'd like to be involved in the standards development process, Please join us at one of the committee meetings held periodically throughout the year. Anyone who uses the criteria can benefit from knowing where the criteria originates and how the standards are developed. Check out http://www.ipc.org/Committeepage.aspxfor a list of all the IPC committees and the documents they develop.

Kris Roberson
Manager of Assembly Technology
IPC

Kris Roberson has experience as a machine operator, machine and engineering technician and process engineer for companies including Motorola, and US Robotics. Kris is certified as an Master Instructor in IPC-7711 / 7721, IPC A-610 and IPC J-STD 001.

The heel fillet requirement has its history in MIL-STD-2000 and previous versions of military and NASA standards and has been verified by test. Unfortunately, those tests were done so long ago finding the data is probably impossible.

The heel fillet is accepted as the important part of the connections strength, so there is some concern depending on the expected operating environment. Thermal excursions and mechanical stimuli (vibration/shock) will stress the connection more than if the hardware resides in a benign environment.

Additional testing can be a way to reach a "comfort factor," but you must keep in mind that the additional testing can also shorten the life of the connection.

Garry McGuire
Sr. Engineer
NASA/Marshall Space Flight Center

Garry McGuire is a manufacturing process engineer and Chair of the IPC J-STD-001 and IPC/WHMA A-620 Space Addendum committees.

This response to this question could take forever; however, let me try to answer it this way.

The plated through hole and wire wrap sections are based upon historical and best manufacturing practices which have been in place for the last 50 years. Although the base laminate materials have changed over the years along with the introduction of lead-free material some changes had to be made to the manufacturing processes.

Those lead-free changes were incorporated after many tests were designed and round robin tests were conducted by member companies of IPC. Those results were submitted to the various task group membership committees, reviewed, expanded and finally accepted and incorporated into the documents.

As for the surface mount section which was introduced in the 80’s in revision A, all those examples were submitted with the appropriate test results defining their goodness and reliability. The Class 3 customers and users were part of this acceptance process and if the back up information was not sufficient to satisfy their requirements, additional testing was conducted to prove the technology. All of this was conducted along with the reliability people and reliability task groups who supported the various specifications.

It must be kept in mind the total quantity of solder joints created during the period of time where this document was created. The major companies contributing to the original document were companies like Bell Labs, IBM, Motorola, Martin Marietta, Northrup Grumman, Lockheed Martin, Hughes, JPL, NASA, Digital Equipment, Raytheon, Harris, Collins Radio, Rockwell, Texas Instrument, Tektronix, Boeing, AT&T, Northern Telecom, General Electric, Westinghouse, Magnavox, Honeywell, TraceLabs, Litton Guidance, E.I Dupont, Sanders Associates, Singer, Naval Avionics, Canadian Marconi, Nelco, Enthone, Unisys, Sandia Lags, SCI, Pace, Hexacon, Lawrence Livermore Labs, Computing Devices, plus many more. The number of solder joints created and the technology provided by these companies, has been the baseline for the 610 specifications.

So the bottom line is, the work was done to prove the point of reliability and goodness.

Now to answer the question as to what happens when one lead does not have the fillet height to meet the requirement of class 3 which is a heel fillet which is equal to thickness of the lead. Is this lead reworked or is it dispositioned to use as is?

This can only be answered by the customer who has the knowledge of where the product is going to be used and the environment where the product will be used. Would I generate a test to check it out would depend upon how much money do you have. We can tests these solder joints to failure and still not know how they will survive in their operational life.

My recommendations would be to check the manufacturing process, measure the amount of solder paste applied to those locations, define the solderability of the components, and if these things are acceptable then I would use the component as is with some ongoing monitoring.

Leo Lambert
Vice President, Technical Director
EPTAC Corporation

At EPTAC Corporation, Mr. Lambert oversees content of course offerings, IPC Certification programs and provides customers with expert consultation in electronics manufacturing, including RoHS/WEEE and lead free issues. Leo is also the IPC General Chairman for the Assembly/Joining Process Committee.

In addition to the excellent comments on the historical basis of reliability testing from which Class 3 criteria evolved, let me just make one more point regarding the heel fillet height. The reason this is so important as opposed to the toe and side fillets is because the original leadframes are typically stamped out of a flat piece of copper that was plated on both sides. Therefore the toe fillet and side fillets often have limited ability to form a good intermetallic bond because the solder joint there is made to exposed (non-plated) copper which typically has oxidized somewhat by the time the part is soldered. The bottom surface of the lead, from the point where it rises up from the pad to the top of the solder joint, presents the best place to form a solder fillet onto plated and non-oxidized copper. Hence the focus on heel fillets from a reliability standpoint.

Even if toe and side fillets are perfect and appear to display good wetting, it is hard to tell just how much of an intermetallic formation (IMF) was made to the bare oxidized copper, whereas the heel fillet is much more likely to have that critical IMF, and that is the reason the reliability experts want to see a heel fillet wetted up at least as high as the thickness of the lead. Two keys to achieving that are: 1) Provide sufficient pre-heat because solder wants to follow the heat and 2) Be sure the part is placed square on the pads. This assures maximum heat conductivity from the board and also ensures the largest possible path for molten solder to flow.

Richard D. Stadem
Advanced Engineer/Scientist
General Dynamics

Richard D. Stadem is an advanced engineer/scientist for General Dynamics and is also a consulting engineer for other companies. He has 38 years of engineering experience having worked for Honeywell, ADC, Pemstar (now Benchmark), Analog Technologies, and General Dynamics.

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