EMS Reliability Criteria solves one of the most serious problems
of electronics manufacturing
"EMS Reliability Criteria eliminates the rejects of good connections and increases the identification of real rejects"
An interesting phenomenon is found in electronics manufacturing: it is common for the real soldering defect rate to go down significantly without any reduction in the amount of inspection and touchup.
Another interesting phenomenon concerns the correlation between rejects and defects. There isn’t any. Over the years, countless studies have found that the rejects reported if the same batch of assemblies is submitted to the same inspectors for reinspection (without letting the inspectors in on the plan) deviate markedly from the rejects reported the first time the assemblies were inspected.
In other words, inspectors are paid to find defects, so they find defects. Or, at least, they find what they think are defects. Also, the number of defects goes up when inspectors are given more time to evaluate each assembly. 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
Everyone agrees that rejecting reliable connections costs money for unneeded repairs (touchup). And many people agree that the touchup causes damage because components do not like the temperatures to which they are subjected by hand soldering. But a third serious problem is generally overlooked — the defect “data” those inspectors record is useless for statistical process management, making meaningful process improvement impossible.
Why Inspectors Reject Reliable Assemblies
So why do inspectors perform so poorly? We spent years studying the phenomenon in electronics plants all over the world. We found the same cause in every plant: the 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
- Impossible
- Unclear photographs
- Misleading
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. The real problem concerns the inclusion of “acceptable” conditions (often in the form of “preferred” and “minimum acceptable”); not only is that extra documentation too much for anyone to remember, inspectors begin thinking that any connection that is not the same as the photograph must be defective. However, no solder joints ever look like a photograph, so the inspector ends up rejecting randomly.
Impossible Interpretation Specifications
In addition to drowning the inspector in useless requirements, IPC–A–610 has impossible criteria. For example, how is an inspector to measure the size of a solder ball relative to the distance between component leads — and do it in the few seconds allowed for inspection? Or how can an inspector know if the amount of solder in a partially filled through–hole meets the specification? The answer is simple: they can’t; no one can.
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.
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 apply. While soldering machines do not generally produce cosmetically satisfactory solder connections when unsolderable materials are encountered. However, hand soldering operators have advantages — higher temperatures, the ability to push the solder around, or simply the ability to keep adding solder — over machines when it comes to meeting cosmetic requirements. So, a soldering operator (i.e., a touchup 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. The least demanding is Class 1, which accepts almost any condition aside from a bridge or no solder. Class 2 is more stringent and applies to most applications where lives do not depend on the reliability of the assembly. And Class 3 is the most rigorous of all, typically because a failure could cost lives.
The class system makes sense in an abstract manner (no one wants a pacemaker that is no more reliable than a child’s toy) but the reality is that the classes are generally counterproductive.
In many plants (especially automotive suppliers), there will be production of some assemblies that are specified to Class 2 and others that are classified to Class 3. There may also be some Class 1 products. People generally rotate from one production line to another (or a single production line may run more than one class of assembly). People being human, they gradually begin applying the same criteria to all output, typically Class 3 standards. This causes concern among the management because inspectors reject Class 2 assemblies for failing to meet a Class 3 standard.
The fascinating part about soldering, however, is that 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 requirement is still just a flawed connection with a pretty exterior. So a true Class 3 production would 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. There are no photographs, only 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 if one of the conditions is seen — 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
