Automated Part Tracking Boosts Quality of Moisture-Sensitive Devices

Efficient RF-based systems replace burdensome, error-prone manual tracking.

To prevent PCB failures, manufacturers must carefully track and control moisture-sensitive devices (MSDs) during
the production process. This can be done manually, but the results of complex manual procedures vary greatly,
depending on the skills and training of the people involved. Even with the best personnel, the process will probably
be tainted by human error that allows some parts to exceed their assigned moisture-exposure levels. The end result:
defective boards that fail in the field.

To minimize the human element, many manufacturers are installing automated systems for tracking and controlling
MSDs. Automated systems boost PCB quality and reliability, while cutting material and manufacturing costs.

Package Problem

When surface mount devices made their debut, most manufacturers used hermetic packages that were impervious to
atmospheric humidity. But in the drive toward smaller, cheaper components, hermetic packages have largely been
replaced by plastic ones. So component manufacturers have been forced to find new ways of keeping moisture out of
their packages.

Moisture-resistant molding materials can help, but even these materials can’t stop moisture from building up at the
interfaces in some packages. Located near the center, where encapsulation compounds come in contact with the
lead frame and die, the critical interface is the weakest point of the package.

Moisture buildup at the interface can cause “popcorning,” cracks in the die, breaks in wire bonds, and delamination
during reflow. This means rework  — and sometimes even rejection of assemblies. To make matters worse,
manufacturers must also worry about invisible, latent defects that crop up in the field.

Standard Procedures

Several years ago, MSD problems led to the introduction of a standard that offers guidance in handling plastic
packages and determining the moisture sensitivity of components. J-STD-033, “Standard for Handling, Packing,
Shipping and Use of Moisture/Reflow Sensitive Surface Mount Devices,” is the result of a collaboration between IPC
and Joint Electronic Device Engineering Council. The standard includes many rules and tables based on the amount
of time components spend in dry storage and ambient conditions. Manufacturers use the rules and tables to
determine the remaining floor life of a part.

While the standard offers a solution to a serious assembly problem, the solution can be difficult to implement,
especially for manufacturers trying to do the job manually. The tasks involved — identifying MSDs, filling out log
sheets, entering date and time calculations, etc. — are time-consuming, and the results are likely to be tainted by
human error. The process gets even more complicated when the parts are on reels or in trays. In these cases, it’s
almost impossible to manually track individual components with varying degrees of moisture sensitivity, particularly in
high-mix production environments.

Good News

Now for the good news: manufacturers aren’t stuck with manual processes. Instead, they can use automated control
systems designed to track components on reels or mixed in trays. The primary purpose of these systems is to keep
components that exceed allowable moisture limits out of MSDs. To meet this objective, the systems automatically
track reels and trays from the time they’re removed from their original dry bags until all parts are mounted on the
substrate prior to reflow.

At the same time, automated systems reduce the risk of over-baking PCB components. By taking into account both
ambient conditions and J-STD-33 rules, the systems minimize the number and duration of bake cycles, resulting in
improved part reliability.

During PCB assembly, components are assigned a value called the moisture-sensitive (MS) level, which corresponds
to the maximum floor life of a part exposed to a temperature of 30°C and relative humidity of 60 percent. According to
J-STD-033, MS levels must be placed on the outside of dry bags. But this means the values are lost when individual
trays and reels are removed from the bags.

To automate component tracking, some kind of tag that allows automatic identification must be attached to the lowest-
level part container on the manufacturing floor. The tag must be ESD-safe and able to withstand the standard bake
temperatures of 125ºC for trays and 40ºC for reels. In addition, the tag must not interfere with machine feeders or the
stacking and unstacking of trays.

RF Identification

Most MSD packages lack a surface large enough for a barcode label. So many manufacturers are turning to Radio
Frequency Identification (RFID), a technology that allows precise automated tracking and meets J-STD-033
requirements. A basic RFID system consists of a control unit and radio frequency transponders that serve as tags.
RF tags house chips programmed with data that can be read at distances ranging from a few centimeters to several
meters. An integrated antenna lets the controller communicate with the tags.

Compared to manual MSD tracking, the RFID system is fairly simple. Operators attach the tags (they can be clipped
to the ends of trays or slipped in the adhesive pouches on reels) and then scan them whenever parts are moved from
one location to another. The tags can handle standard bake temperatures and won’t interfere with feeders or tray
stacking.

RF tags are more reliable than barcode labels and can be read at greater distances. They also offer more shape and
attachment options than barcodes. Though they’re more expensive than printed labels, the tags can be
reprogrammed and reused, which can make them less expensive than single-use printed labels.

Each RF tag incorporates a unique identifier for recognizing and tracking the attached container. In addition, the tag’s
programmable memory contains all relevant material and process information, including part number (PN), MS level,
and expiration date/time. Tags also hold data such as component body thickness and categories for the bake and de-
rating tables mentioned in J-STD-33.

Information is entered into the tags when they’re attached to trays and reels. The PN can be scanned from an existing
barcode label on the outside of the dry bag. Usually, the MS level and body thickness aren’t available in barcode
format, so the data must be typed in or selected from a pull-down menu.

Life Change

After checking data such as sensitivity level and package body thickness, the RFID system can change the floor life
of a component. In accordance with J-STD-33, the system uses the de-rating factor due to factory environmental
conditions to automatically increase or decrease a part’s maximum floor life.

Remaining floor life can change many times, depending on the complete history of exposure. To track exposure and
apply the rules of the standard, the date and time must be recorded each time a tray or reel is moved from one
environment to another. Operators who tackle this task manually must add and subtract dates and times, which can
turn the process into an error-filled marathon.

The process is much easier for operators using the RFID system. When a reel or tray is moved, all the operator has
to do is scan the RF tags. Once the information is scanned in, the control system instantly performs all necessary
calculations.

RF tags should always be scanned during the loading and unloading of moisture-sensitive parts. After scanning, the
local reader/controller displays the remaining floor life of the parts. Audible or visual alarms can also alert the
operator to take action before a component reaches expiration.

Resetting the Clock

Today, many manufacturing procedures are based on the assumption that moisture diffusion in a package
temporarily stops when parts are returned to a dry environment. This assumption allows manufacturers to stop the
exposure-time “clock” while parts are in dry cabinets or resealed dry packs. But the assumption is wrong. While parts
are in dry storage, previously absorbed moisture continues to diffuse toward the critical interface at the center of the
package. This conclusion is supported by studies showing that some components exceed their critical moisture level
while in dry storage, after spending just a fraction of their floor life in ambient conditions.

Therefore, J-STD-33 includes no provision that lets manufacturers stop the exposure clock. But under certain
circumstances, the clock can be reset, according to the standard’s “short duration rule,” which accounts for the effect
of dry storage. This rule says that components can be adequately dried by room-temperature desiccation after less
than eight hours’ exposure to factory ambient conditions not exceeding 30°C and 60 percent relative humidity. When
components meet these criteria, their floor-life clock can be reset after a desiccating period at least five times longer
than the exposure time.

Needless to say, manual tracking of all these values can cause major headaches on the factory floor. But not for
users of the RFID system, which automatically applies the short duration rule based on exact measurements of
exposure and dry-storage times.

Baking Ingredients

Exposed components that fail the short duration test must be baked prior to reflow. Optimal bake cycles vary with the
package type, MS level, and body thickness of each component. In a manual process, operators must track
simultaneous bake cycles with different start and completion dates and times, taking care not to under- or over-bake
components.

Since manufacturers worry about reliability exposure, production processes tend to be based on worst-case
assumptions. In such processes, many components that don’t need to be baked are sent to the oven anyway. While
excessive baking might reduce moisture concerns, it can cause the growth of intermetallics. It also decreases lead
solderability, causing brittle solder joints that reduce reliability.

For these reasons, J-STD-033 limits components to one bake cycle. This means operators must be able to recognize
trays and reels that have already been baked.

Fortunately for operators, these baking complications melt away when RFID controls the process. Using information
scanned from an RF tag, the control unit determines the optimal bake cycle for the attached part. During the process,
the system provides a real-time listing of all parts in the bake oven, along with completion times. Warnings and alarms
insure that parts are removed from the oven at the proper time. After a successful bake cycle, the control unit records
the event on the part’s tag. This information triggers a warning if the part should mistakenly begin a second bake
cycle.

Control Options

While a small production environment may require only one RFID control unit, a large operation can include multiple
controllers networked together. The system’s modular design lets manufacturers add control units to meet changing
production requirements.

To eliminate repetitive data entry, the control unit stores part information in an MS component database. This
database also helps manufacturers detect moisture-sensitive parts that haven’t been properly packaged and labeled.
Without the component database, these parts could be treated as non-moisture sensitive and bypass the control
process.

In addition to the component database, the control system maintains a database of all process transactions. This
information can be used to determine average exposure time and other key metrics. These values help
manufacturers identify parts of the process that can be improved.

Conclusion

Automated RFID systems simplify the tracking and control of MSDs, while minimizing the human error that plagues
manual systems. RFID offers precise, efficient part monitoring from dry bag to bake oven, reducing waste, assembly
costs, reflow-related defects, and the likelihood of early field failure. By improving both PCBs and the process that
makes them, RFID should continue to win converts wherever people struggle to keep track of moisture-sensitive
devices.

I wrote this article for a public-relations firm. It was published under another person's byline.