Myth-Image

Modern Components Should Not Be Treated Like
Vacuum Tubes

A surprisingly large number of today's standard practices originated in the era of vacuum tubes. In other words, they were designed for the requirements of soldering wires to metal lugs. And, for the most part, they were created by technicians with no real scientific knowledge.

Today's electronics are vastly more complex and delicate than the primitive devices of 50 years ago. But much of the industry still believes in the antiquated beliefs from the primitive days.

The belief that touchup can make a connection more reliable is one of the antiquated beliefs. For some of the others, please go here.

Electronics Manufacturing Sciences helped us in many, often surprising, ways. The most significant contribution was the creation and implementation of a user–friendly reliability criteria document.

This was not a minor accomplishment. Nor was it a tool we had anticipated needing because our existing quality criteria formed the base for most standards used throughout the electronics industry. Our workforce was mature, extensively trained and very experienced.

The original standard filled hundreds of pages and covered every imaginable condition. The document produced by EMS was less than 40 pages, including large, clear computer graphics, yet it covered every important requirement. Important conditions with previously ambiguous wording were restated so effective evaluation became easy. For example, in place of evaluating solder balls on the basis of size relative to the spacing between leads, the new wording called for ignoring solder balls that could not be seen with the naked eye (or with 3x magnification on assemblies with fine pitch surface mount components).

The results were impressive. The amount of rework dropped dramatically because inspectors stopped rejecting reliable connections, test yields increased because assemblies were subjected to less handling, and production cycle time decreased.

If this had been the only improvement EMS provided us (and it wasn’t), they still would have been an excellent investment.

James H. Mosher
Manager, Switching & Network Products
Manufacturing and Engineering
Lucent Technologies, Inc.
Columbus, Ohio


The first step in reducing rejects and rework is not necessarily reducing the real defect rate. In fact, it is common for real defect rates to be completely eliminated without reducing the reject rate.

Rejection and rework of reliable connections is a plague in electronics manufacturing. Faulty inspection is often the single largest waste in electronics manufacturing.

EMS Reliability Criteria eliminates the rejects of good connections and increases the identification of real rejects.

Costs of Rejecting Reliable Assemblies

Rejecting reliable connections imposes several serious costs, including:

Why Inspectors Reject Reliable Assemblies

Why do inspectors perform so poorly? In electronics plants all over the world, we found the same cause: inspectors are confused. And who wouldn’t be confused? In the case of the most widespread workmanship standard, IPC–A–610, the inspectors are expected to memorize hundreds of pages of instructions, many of them:

Irrelevant Criteria

No electronics plant will ever see some of the defects contained in the pages of IPC–A–610. That’s a problem for users but not the main problem involves the inclusion of “acceptable” conditions such as “preferred” and “minimum acceptable”. That extra content overwhelms the inspector who ends up believing that any connection that is not the same as the "preferred" photograph must be defective. However, photographs don't look like real solder joints and the inspector ends up rejecting randomly.

Unclear Photographs

Solder connections are shiny 3–D metal formations. Light reflects off the solder in photographs and 2–D images never look like real world 3–D objects (especially when the 3–D object involves a component that is probably quite different from the component being inspected). These issues are further complicated by the tendency to show more than one condition in a single photograph.

Impossible Interpretation Specifications

IPC–A–610 also has impossible criteria. For example, no inspector can measure the size of a solder ball relative to the distance between component leads. Neither can an inspector know if the amount of solder in a partially filled through–hole meets the specification. There are many other impossible inspection requiremnents. (This doesn't mean hole fill should be ignored. It does mean inspectors can't tell the extent of flow in a partially filled hole. The real solution is to correct the process to achieve 100% fill; it isn't hard to do.)

Misleading Implications

Ultimately, perhaps the most serious problem with the cosmetic criteria is the implication that they apply to reworked solder connections. They do not. An experienced operator can make unreliable connections look perfect, but they are still just good looking defective connections.

The Problem With Classes

In the IPC world, there are three levels of acceptability. However, a scientifically designed and managed process will always meet the Class 3 requirements. The output is perfect without costing any more than what it costs to run an inferior process.

Of course, a connection that has been touched up to meet a more rigorous class cosmetic requirement is still just a flawed connection with a pretty exterior. So true Class 3 production should not allow touchup.

EMS RELIABILITY CRITERIA

It took us years of constant refinements to create criteria that meet the intent of IPC–A–610 without the communication problems. We replaced all the photographs with 3–D computer graphic images that clearly show the condition. The conditions themselves are only what an inspector may see. The criteria are customized for each client and available in both print and digital forms. There is no reference to the acceptable conditions. And the inspector is taught to reject only conditions in the Reliability Criteria — not to reject because the connection is different from the image of a perfect connection.

EMS Reliability Criteria eliminates the guesswork of inspection. And there are many other benefits that we’d like to tell you about. For more information, please write or call (001)727–866–6502, ext. 21

 

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