MSD: What you don't know may be killing you: MSDs could be destroying your facility's yields at this very moment.

Ignoring moisture sensitive devices (MSDs) may be seriously detrimental to your printed circuit board (PCB) assembly process. Due to the material selected for the component substrate and/or overmolding compound, some integrated circuits (ICs) have a propensity to absorb moisture from the ambient atmosphere. This process becomes dangerous when the components have accumulated a critical level of moisture prior to solder reflow.

The root cause of this failure mechanism is the rapid heating of the moisture absorbed within the plastic encapsulant or organic substrate. During typical solder reflow cycles, any absorbed moisture is quickly turned into superheated steam. This sudden change in vapor pressure can cause the package to swell. If the pressure exerted exceeds the flexural strength of the plastic mold compound, the package can crack. Even if the package does not crack, interfacial delamination can occur.

The most familiar failure mode of MSDs is the dramatic cracking of the package--sometimes accompanied by an audible popping sound during reflow, hence the term "popcorn effect." In a few cases, if the component has a very low standoff, as with some thin small outline packages (TSOPs), the leads of the components will actually be lifted off the pads--resulting in solder opens. Most of the time, MSD failure is not that straightforward. Resulting cracks, and characteristic bubbles in the plastic, are quite small, difficult to see without proper lighting and magnification and are located underneath the package, which is not accessible for inspection after reflow.

You Don't Know What You're Missing

The most familiar failure mode associated with MSDs may be the dramatic cracking of the package, but this failure often goes undetected. A functional test or automated optical inspection (AOI) system may not detect the problem. In fact, x-ray and x-ray laminography seldom detect the failure. Currently, the only effective detection method is the use of a scanning acoustic microscope--a failure analysis tool, not an in-process inspection system. Many times the failure is recorded as a bad component and failure analysis is not performed.

Are You Vulnerable?

MSDs are classified by their propensity for moisture absorption. Visit your stockroom or visit the production floor--make your way to the feeders attached to the pick-and-place machine handling ICs. If the ICs are packaged in a sealed, nitrogen bag, the components inside have an MSD rating of at least three. Look for a label on the bag resembling a drop of water. The MSD rating will also be stated on the component data sheet. If your facility has components with an MSD rating of three, be afraid. If the components feature a rating of six, start working on a resume.

The guidelines for classification, handling, packing, shipping and use of MSDs are outlined in the industry standard J-STD-033, a joint publication of the IPC (Northbrook, IL) and JEDEC (Arlington, VA) (1).

What's an Engineer to Do?

Currently, most attempts at process control are performed through manual procedures, but, considering the advanced state of the industry, manual procedures seem archaic. Manual documentation includes maintaining identification of MSDs after removal from packaging. Once the packaging is opened, exposure time must be tracked as the components are loaded onto the pick-and-place machine, while the components reside on the machine and, if not placed on an assembly, the time they remain on the production line until finally placed in dry storage.

Time in dry storage, whether in a dry cabinet or in a resealed bag, must also be tracked. Rules from J-STD033 must be followed when calculating baking time, which is based on the component's cumulative exposure to ambient relative humidity. Ambient floor conditions, at the time of exposure, must also be tracked. Seem overwhelming? These issues concern only the MSDs on one side of the board--in applications where both sides are populated with MSDs, one side will be exposed to two thermal (reflow) cycles, absorbing moisture from the ambient atmosphere between cycles.

If your facility has been ignoring MSD issues, and blaming the component manufacturer for IC-related field failures, your days may be numbered. As more and more companies become aware of MSD issues and J-STD-033, related MSD problems will begin to appear on internal or customer audits as deviations from documented procedures. I'm aware of a recent case involving a top-tier contract electronics manufacturer (CEM) who was disqualified from building a new product for a major customer as a result of a production crisis related to MSD control. During a qualification build, nearly 100 percent of the printed circuit board assemblies (PCBAs) failed at electrical test--a reel of ball grid arrays (BGAs) was overexposed.

Graduate from the Stone Age

Remember, electronics is a high-tech industry-antiquated methodology is not the only option available to fight problems associated with MSDs. The application of existing material tracking methodologies, with appropriate software, can result in the proper tracking of all parameters. I prefer radio frequency identification (RFID) technology (2), which allows storage of more information than a barcode.

Incorporating an MSD tracking system offers a return on investment (ROI) similar to that of AOI systems. Good control procedures and systems will pay for themselves through direct improvements in yields and productivity. Considering that MSD components are typically the most expensive materials on an assembly, a reduction in the rate of defects can amount to large savings very quickly. Frankly, cost avoidance and competitiveness are the best motivations for implementing adequate control procedures and systems.

A solution exists--it's not rocket science and doesn't require a NASA budget. Soon, MSDs will be taken for granted, just as many now take electrostatic discharge (ESD) issues for granted. Remember, we're all in this together.

References

(1.) To examine J-STD-033, visit www.ipc.org. For more information regarding MSD, including translations/interpretations of J-STD-033, visit www.cogiscan.com.

(2). As discussed in the December 2000 "On the Forefront" column.

Phil Zarrow is president and surface-mount process consultant for ITM Consulting; Durham, NH; [phone]; www.ITMconsulting;org; [e-mail].