What's "World Class" for Solder Joint DPMO?

Quality is a key differentiator when selecting a contract manufacturer, the standard question becomes, "What is good?" Arguably, the gold standard for SMT process quality is solder joint defects per million opportunities (DPMO).

High-value contract manufacturers aim for Six Sigma quality (3.4 ppm) for solder joints in an automated surface mount line. They key word here is "aim." Defect levels in an automated surface mount line operating within statistical control tend to vary between <50 to >500 DPMO, with 250 DPMO a reasonable baseline expectation for a repeat product running at least half of a shift.

Several articles point to 50 DPMO as world-class performance. An article by Don Revelino of Packard Bell, titled "Achieving Single Digit DPMO in SMT Processes," states that "world-class" is 50 ppm and Packard Bell achieved single-digit DPMO. An article by C. Mangin ("The DPMO: Measuring process performance for world-class quality") also states that approximately 50 or less DPMO is appropriate for continuous assembly of electronics.

In general, wave solder defect rate tends to be an order of magnitude larger then SMT, and manual solder operations tend to be two orders of magnitude larger than SMT. Realizing this, one can see the criticality of designing products that can be manufactured on a SMT line rather than by hand.

Other studies paint a more pessimistic viewpoint of process control over SMT manufacturing. An article by K. Walters ("Improving the Reflow Process with SPC") recommends a minimum Cpk value of 1.33 (600 ppm) for SMT. The report does state that three manufacturers achieved sub-200ppm.

When comparing manufacturers' performance, it is important to ensure that apples are compared to apples. The number of opportunities should be based on the number of solder joints, not the number of components. In addition, the number of defects should be based on physical defects that have or would lead to failure. Cosmetic defects should not be included. IPC 610 workmanship standards should guide the defect determination, for most products Class 2 is the applicable standard, Class 3 for high rel and defense products.

To illustrate, a fairly typical assembly with 100 BOM line items might have ~400 solder joints. At 250 DPMO about 10% of the boards may require rework, at 50 DPMO just 2% may require rework. For complex boards the DPMO becomes very critical. A product with 5000 solder joints and a 250 DPMO could see 100% of the boards go to rework, at 50 DPMO only 25% might see rework. (Note, these numbers assume perfect dispersion of the defects, or 1 defect per board).

The degree of process control required to obtain sub 100 DPMO can be appreciated by examining the scenario of one 144-pin quad flat pack (QFP) being misaligned so that all its solder joints are defective. If the SMT line had a 20 ppm defect rate before this defect, the next 7 million solder joints will have to be defect-free before the line can once again claim a 20 ppm defect rate.

DPMO is much easier to control when a SMT line assembles one product, or a family of very similar products. EMS companies running multiple products during a week that still achieve sub 100 DPMO should be congratulated as best in class.