Study compares effectiveness of inspection and test
Steve Scheiber, Contributing Technical Editor -- Test & Measurement World, 8/22/2007 10:43:00 AM
As part of his major new study on solder-joint quality, Stig Oresjo, formerly with Agilent Technologies and now principal of Test Strategy Consulting, compiled data on the effectiveness of each inspection and test step in a common “test” strategy. (See "Researcher looks at 3.7 billion solder joints to assess DPMO.")
In his analysis, Oresjo also looked at some of the 20 to 25 test-effectiveness studies that Agilent has conducted over the years. Such studies are time-consuming, each one taking four to five engineers about a week. They always take place at company manufacturing sites. Each company is asked to provide enough medium- to high-complexity boards to yield about 100 defects. No inspection, test, touch-up, or repair is done before the study begins. In a study, the boards undergo manual or automated optical and then automated x-ray inspection (the latter limited to 5DX systems for consistency), followed by conventional in-circuit (from Agilent as well as other vendors) and functional test.
Oresjo commented, “For all calls made by an inspection system, we optically inspected the boards, then asked the companies that supplied them to tell us which defects were real and which were false calls and logged the results. Then—without repairing the detected defects—we did the same thing with x-ray inspection. After x-ray we sent the boards through in-circuit test. We did repair any shorts that the in-circuit tester found before sending the boards on to functional test. We weren’t looking to test the smoke detectors. We simply wanted to determine which steps caught which failures, and which ones escaped. At ICT and functional test, we did very accurate fault diagnosis to verify the real defects.
“Analyzing the data from all of the studies we have done showed nine of them that produced enough data to generate a Venn diagram. These nine studies revealed a total of 865 defects on 421 boards. By not fixing the boards, we gave each test step the same chance to find all of the defects.”
Conventional wisdom suggested that each inspection method would find some defects while some would escape. In-circuit test would find most of the manufacturing defects, and functional test would catch nearly everything.
To Oresjo's surprise, that isn’t what happened. “It turned out that 3-D x-ray found almost 90% of all the defects that the manufacturers had identified, including insufficient solder and other solder-joint problems, missing components such as bypass capacitors or pull-up resistors, and opens on power pins. It did not find components that were the wrong part number, for example, or devices that weren’t working properly. It found a few but not all of the components that were inserted backwards.”
The other steps showed considerably less success. “Manual or automated optical inspection found only about 24% of all defects in these nine studies. In-circuit test covered about 26%. But the biggest eye opener came from functional test. If functional test were our only step, it would have caught no more than about 20% of the defects on those boards—and in most cases only around 10%. It couldn’t find solder problems or missing pull-up resistors, for example, because the boards usually still worked.
"Under normal circumstances, of course, many of the defects that functional test missed in our study would already have been removed in one of the earlier prescreening steps. Functional test did find some failures that the other techniques didn’t—performance-related issues and the like. Nevertheless, we determined that without the x-ray inspection step, fully 50% of the defects on those boards would have survived to reach customers.”
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